Salinella - what the crap was it?

In 1991, Haszprunar et al. published a brief book chapter in which they listed a collection of modern animals of exceedingly uncertain relationships. Any one of those organisms would make for an interesting blog - some of them have now been placed with reasonable confidence in the animal family tree. The worm-like Xenoturbella masqueraded as a mollusc for a while, but now appears to be a basal deuterostome (Bourlat et al., 2003 - see Palaeos.com for a section I wrote on this affair a couple of years ago). The also-worm-like Buddenbrockia has, rather spectacularly, been shown to be a very basal member of the parasitic Myxozoa, the least animal-like of all animals (Okamura et al., 2002). But today, I'm going to touch on perhaps the most mysterious of all Haszprunar et al.'s subjects - Salinella salve Frenzel 1892.

Salinella has only ever been found once, in a saline culture derived from salt beds in Argentina (Brusca & Brusca, 2003 - see Answers.com for further details). It was described as having a unique body plan, with a single layer of cells surrounding a hollow sac, open at both ends. All cells were densely covered by cilia both inside and out, and there were longer cilia around the openings (which were the mouth and anus). Other than that, there appears to have been no distinction into organs, tissues, whatever. Salinella moved by ciliary gliding, and reproduced asexually by fission.

If this description was accurate, then the affinities of Salinella become very difficult indeed. The presence of a through-gut of sorts gives Salinella a superficially bilaterian appearance, but there is no way it could be a bilaterian. All known bilaterians are triploblastic (i.e. possess three basic cell layers, with the possible exception of mesozoans, if they are reduced bilaterians), and even the outgroup of bilaterians, whether cnidarians or ctenophores, has at least a diploblastic organisation (two cell layers). Even sponges are essentially diploblastic. If there was a monoblastic stage in the evolution of animals, as has been suggested by some authors, then it would have been very early in their history. It is possible that Salinella might represent this stage, which has inferred from the blastula stage in embryonic development (Clark, 1922).

Salinella has been compared in the past to the simple animals Trichoplax and the Mesozoa. Trichoplax (generally placed in its own phylum, Placozoa) is a flattened organism with only four cell types, and has been referred to as the simplest-organised animal known (Syed & Schierwater, 2002). It basically comprises an upper epithelium, a lower epithelium and a central mass of cells. Digestion occurs through the formation of a hollow underneath the lower epithelium into which the animal exudes digestive juices and absorbs nutrients. Epithelial cells are ciliated, but externally only.

The Mesozoa are two groups of internal parasitic animals, the Orthonectida and Dicyemida (it is now thought quite likely that these two groups are not closely related to each other). Both have a basic structure of an outer layer of ciliated cells surrounding a central mass. In dicyemids, the central sector is a single long tube cell. In Orthonectida, the central area contains gametes. Before the formation of gametes, orthonectids are a multinucleate plasmodium without distinct cells.

Beyond the superficial similarity of undifferentiated cell layers, however, Salinella as little in common with these animals. It has long been suggested that, rather than being primitively simple organisms, mesozoans represent derived animals that have become secondarily simplified as a result of their parasitic lifestyles. For both groups, there is genetic evidence to support this (Hanelt et al., 1996; Kobayashi et al., 1999). Trichoplax has a better claim to be genuinely primitive. However, Salinella lacks Trichoplax's central cellular layer, and Trichoplax does not have Salinella's cilia on both sides of the cell. And certainly there is no similarity between Trichoplax's external digestion and Salinella's through gut.

Which brings us to the final possibility, the one that many zoologists have suspected - Salinella never actually existed. It is possible that Frenzel was mistaken in his description of Salinella's structure (I've never seen Frenzel's original description, and I'd be interested in seeing how likely this is). Could Frenzel have actually been looking at Trichoplax-like organism? Unfortunately, unless more specimens of Salinella are recovered, we are unlikely to ever know. And if Frenzel was severely mistaken, then even if the organism he was looking at is recovered, it may never be recognised as such.

REFERENCES

Bourlat, S. J., C. Nielsen, A. E. Lockyer, D. T. J. Littlewood & M. J. Telford. 2003. Xenoturbella is a deuterostome that eats molluscs. Nature 424: 925-928.

Brusca, R. C., & G. J. Brusca. 2003. Invertebrates, 2nd ed. Sinauer Associates, Inc., Publishers: Sunderland (Massachusetts).

Clark, A. H. 1922. Animal evolution. Proceedings of the National Academy of Sciences of the USA 8: 219-225.

Hanelt, B., D. Van Schyndel, C. M. Adema, L. A. Lewis & E. S. Loker. 1996. The phylogenetic position of Rhopalura ophiocomae (Orthonectida) based on 18S ribosomal DNA sequence analysis. Molecular Biology and Evolution 13 (9): 1187-1191.

Haszprunar, G., R. M. Rieger & P. Schuchert. 1991. Extant "problematica" within or near the Metazoa. In The Early Evolution of Metazoa and the Significance of Problematic Taxa (A. M. Simonetta & S. Conway Morris, eds.) pp. 99-105. Cambridge University Press.

Kobayashi, M., H. Furuya & P. W. H. Holland. 1999. Dicyemids are higher animals. Nature 401: 762.

Okamura, B., A. Curry, T. S. Wood & E. U. Canning. 2002. Ultrastructure of Buddenbrockia identifies it as a myxozoan and verifies the bilaterian origin of the Myxozoa. Parasitology 124: 215-223.

Syed, T., & B. Schierwater. 2002. Trichoplax adhaerens: discovered as a missing link, forgotten as a hydrozoan, re-discovered as a key to metazoan evolution. Vie Milieu 52 (4): 177-187.

Taxon of the Week #3: Rana

The taxon that has been chosen to receive the coveted Taxon of the Week spot today is the frog genus Rana. Rana is a large primarily Holarctic genus of frogs, and probably the inspiration for most depictions of frogs in the world (see the page on Wikipedia and linked pages for images). Well-known species are the edible frog (Rana × esculenta - actually not a true species but a hybrid) and the European common frog (Rana temporaria).

I thought I'd look up the info on Rana over lunchtime. Pretty soon, my head was swimming. The genus Rana has been hit with two major investigations in recent years, both of which have received some frosty responses. Frost et al. (2006) in their investigation of the 'Amphibian Tree of Life' divided Rana between more than fifteen smaller genera to remove its previous paraphyly (for instance, the above-mentioned Rana esculenta would become Pelophylax esculentus). As happens with any wholesale name change, there has been quite a bit of outcry at the idea of having to update the filing systems. Also, a number of authors have felt that the number of taxa sampled by Frost et al. was not enough to inspire confidence in their results. The review by Wiens (2007) was particularly vitriolic - the scientific equivalent of attempting to hold the subject down and kick them repeatedly in the teeth. Smith and Chiszar (2006) have suggested the more mollifying approach of treating Frost et al.'s various genera as subgenera, though unless one was willing to accept a paraphyletic genus this would also require sinking some well-established genera such as Staurois within Rana. Division of the genus Rana was also supported by Che et al. (2007).

The other cause of debate was perpetrated by Hillis & Wilcox (2005), who investigated the phylogeny of New World species of 'Rana' (most of which would belong to Lithobates in the Frost et al. system). The problem came when Hillis & Wilcox suggested a whole series of subgeneric taxa for nested groups of species that they defined according to the rules of the PhyloCode, but also allowed for use under the ICZN as subgenera. Debate promptly exploded about whether Hillis & Wilcox's names were validly published and usable (Dubois, 2006, 2007; Hillis, 2007). Compared to this argument, Frost et al.'s division appears quite simple. I may return to this in a later post, if my brain doesn't implode first. Check out the Dubois (2006) paper in particular - Dubois thinks that the answer to our problems is to make the ICZN more complicated. No. Thank. You.

REFERENCES

Che, J., J. Pang, H. Zhao, G.-F. Wu, E.-M. Zhao & Y.-P. Zhang. 2007. Phylogeny of Raninae (Anura: Ranidae) inferred from mitochondrial and nuclear sequences. Molecular Phylogenetics and Evolution 43 (1): 1-13.

Dubois, A. 2006. New proposals for naming lower-ranked taxa within the frame of the International Code of Zoological Nomenclature. Comptes Rendus Biologies 329 (10): 823-840.

Dubois, A. 2007. Naming taxa from cladograms: A cautionary tale. Molecular Phylogenetics and Evolution 42 (2): 317-330.

Frost, D. R., T. Grant, J. Faivovich, R. H. Bain, A. Haas, C. F. B. Haddad, R. O. de Sá, A. Channing, M. Wilkinson, S. C. Donnellan, C. J. Raxworthy, J. A. Campbell, B. L. Blotto, P. Moler, R. C. Drewes, R. A. Nussbaum, J. D. Lynch, D. M. Green & W. C. Wheeler. 2007. The amphibian tree of life. Bulletin of the American Museum of Natural History 297: 1-370.

Hillis, D. M. 2007. Constraints in naming parts of the Tree of Life. Molecular Phylogenetics and Evolution 42 (2): 331-338.

Hillis, D. M., & T. P. Wilcox. 2005. Phylogeny of the New World true frogs (Rana). Molecular Phylogenetics and Evolution 34 (2): 299-314.

Smith, H. M., & D. Chiszar. 2006. Dilemma of name-recognition: why and when to use new combinations of scientific names. Herpetological Conservation and Biology 1 (1): 6-8.

Wiens, J. J. 2007. Review: The Amphibian Tree of Life. Quarterly Review of Biology 82: 55-56.

Big Shout-Outs to my Homies

Last week, my blog was linked to by a couple of others, and it seems to me that the least I could do would be to return the favour.

First off, John Wilkins at Evolving Thoughts. John writes on scientific philosophy (or philosophy of science, whichever it is). I'd especially like to highlight his recent series of posts on the early chapters of Genesis, of which this is the most recent entry.

Second was Coturnix's A Blog Around the Clock. Coturnix's specialty is biological rhythms, but he blogs on other subjects as well.

I don't spend a great deal of time scanning other websites, I'm afraid. Time is a frustrating thing - there never seems to be enough of it. Mein Herr's explanation in Lewis Carroll's Sylvie and Bruno of how the people of his home country save up time from when they don't need it to be re-used later when they did never fails to fill me with envy. Of course, I'm not entirely sure when I'd take the excess time from, though - when I'm sleeping, perhaps. I'm sure a lot more would get done if I could just dispense with sleeping.

That said, I'd like to mention a couple of sites. Darren Naish's Tetrapod Zoology is one of the best, and was actually the inspiration for my starting this blog (I found his accounts endlessly fascinating, but was slightly frustrated by his self-imposed restriction to tetrapods when there are so many amazing inverts, fungi, bacteria, what-have-you out there). Darren's enhusiasm for his subjects is infectious, and his frequent distractions, tangents and subject-changes express just why we all love this complicated, never-ending subject.

It's not science, but I was somewhat saddened recently by the completion of David Plotz's Blogging the Bible. I was always entertained by David's sometimes bemused, sometimes confused but always respectful commentary on the Big Book. Especially Job, which was one that always confused me too.

Back at Scienceblogs, I've always been impressed the few times I've taken a look at Jason Rosenhouse's EvolutionBlog. Jason never fails to look at his subject in detail, and seemingly with endless patience. Creationism is probably not a topic I'm likely to cover here at the Catalogue (except that I wouldn't mind writing on the supposed creationism of influential figures such as Linnaeus and Owen) - there's other topics I'd rather cover, and there are people such as Jason to cover that topic far better than I ever could, for which I thank them.

Bouncing bristletails!

Last post I briefly mentioned my recent encounter with an archaeognathan (specifically, I found it mixed in a specimen vial with a bunch of harvestmen). Archaeognatha are one of the few living orders of wingless insects. In fact, under the current most-commonly-used definition for Insecta (which excludes the entognathous hexapods such as Collembola), Archaeognatha are the basalmost living order. As such, I was pretty excited to finally see one, even if only in the corpse. The Tree of Life page for this order has an absolutely fantastic photo of a live specimen. Archaeognathans are also referred to as bristletails in reference to the long cerci extending behing the abdomen.

The first feature that grabs the attention is the distinct hump that the back makes. This hump contains muscles for the archaeognathan to rapidly bend the abdomen downwards, pushing itself into the air and jumping up to 10cm high. Archaeognathans also have very large forward-facing compound eyes that actually meet in the middle. The maxillary palps are very large and could almost be mistaken for an extra pair of legs coming off the head.

If you want to see what makes archaeognathans really cool, though, you'll have to look a little closer. As befits the basalmost insect order, they retain a few uber-primitive features that have disappeared from other modern insects. They are the only modern insects with monocondylic mandibles - i.e. the mandibles have only one condyle (the socket where they attach and articulate with the head). All other insects have two. And if you were to look underneath the abdomen, you would see that each segment bears a pair of small pointed styles. These styles are moveable by muscles, and are thought to represent reduced legs. Let me repeat that in italics for emphasis - reduced legs. Archaeognathans are the only insects sensu stricto with abdominal styles, though they are also present in diplurans and proturans, two entognathous hexapod orders (I have not been able to find any indication that the styles in these orders are mobile, however).

While they have a fairly wide distribution worldwide, archaeognathans do not appear to be abundant and are fairly localised. I have heard that California is fairly well-blessed with them (big ones, too), but here in Australia they are most abundant in the eastern states.

Insects Never Fail to Amaze

Recently I saw my first ever specimen of Archaeognatha. I was going to write on that, so I picked up the lab's faithful copy of The Insects of Australia (CSIRO, 1991) to look up information. Before I found the Archaeognatha chapter, however, I came across something else that just blew me away so much that I had to share it with you all. Meet the freaky little marine midge Pontomyia (the only image I could find online was a rather blurry one here. Sorry).

There are very few marine insects, and only a single genus, the waterstrider Halobates, has species that are actually found on the open ocean (van der Hage, 1996). Other marine insects are restricted to inshore habitats. Pontomyia appears to be an inhabitant of tide pools and lagoons in the West Pacific. It belongs to the large family Chironomidae, mosquito-like (but non-parasitic) midges with aquatic larvae

Individuals of Pontomyia spend most of their lives as benthic larvae. After they emerge as non-feeding adults, they only live for a couple of hours (Soong et al., 1999). In this brief time, they must find a mate and produce eggs.

Pontomyia adults emerge at dusk or after sunset. At least one species, Pontomyia oceana, only emerges around the new and full moons (in combination with the specific emergence time, this probably ensures that the females end up laying eggs at low tide). Pupae swim to the surface and emerge as adults. The females are vermiform and structurally degenerate, with seemingly little activity as far as I can tell.

The males are the freaky ones. They skim the water surface film on the tips of the stout second and stilt-like, trailing third pairs of legs. The first pair of legs is immensely long and curve out on either side of the body as a pair of 'outriggers', barely skimming the surface and maintaining the animal's balance. The paddle-like wings propel the midge by flicking the air just above the water surface (Norris, 1991).

Females do not complete emergence from the pupa unless males are nearby (Soong et al., 1999). Males generally emerge up to an hour before females, and have been observed stripping the pupal skin from females to help them emerge. Once a male has found a female, he picks her up with the second legs and the base of the third legs and carries her while mating. Males are apparently quick movers, and I was especially taken by this sentence in Soong et al. (1999): 'They did not appear to slow down after catching females, sometimes climbing the vertical substrate up to 15 cm above the water level while dragging a female along'. One can't help wondering what Germaine Greer would make of the verbs in there.

After mating, the male drops the female. She lays her eggs on bits of dead coral or the like sticking up about the water surface in long interconnected strings. And that, as they say, is that.

REFERENCES

Norris, K. R. 1991. General biology. In The Insects of Australia (CSIRO, eds.), 2n ed., vol. I pp. 68-108. Melbourne University Press.

Soong, K., G.-F. Chen & J.-R. Cao. 1999. Life history studies of the flightless marine midges Pontomyia spp. (Diptera: Chironomidae). Zoological Studies 38 (4): 466-473. (Pdf here)

van der Hage, J. C. H. 1996. Why are there no insects and so few higher plants, in the sea? New thoughts on an old problem. Functional Ecology 10: 546-547.

Top Ten Follow-up

Paul W. complained that ten was too few for the previous post, so I'll add the runners-up to bring the total to twenty-four. They were:

Brachytrachelopan mesai: The short-necked sauropod of South America - a smashing little stunner that surprised us all. You've got to love the name, too.

Carnotaurus sastrei: Another real oddball. Combine the boxy horned skull with the ridiculously tiny forearms (it makes Tyrannosaurus look like a gorilla) and it's a mystery what this creature was doing for a living.

Diplodocus carnegii: Another classic sauropod. I think my scoring system favoured sauropods a little - they pretty much all scored highly in the 'impressiveness' box, but how are you going to call these giants anything else?

Microraptor gui: The 'four-winged' miniature marvel that inspired much passionate debate on its flying abilities (also, the first non-avian dinosaur to be claimed as a flier). Though there is still no agreement about whether the long feathers preserved on the legs formed functional wings, the evolution of flight in dinosaurs will never be viewed so simplistically again.

Opisthocoelicaudia skarzynskii: My favourite sauropod as a kid. Let's have that name again - Opisthocoelicaudia. Just kind of rolls off the tongue, doesn't it? Even better than Parasaurolophus.

Plateosaurus longiceps: Prosauropods are kind of the poor relative in the dinosaur family - never given much time, and about the only major group to not even have a cameo on a Jurassic Park movie. That said, Plateosaurus is the archetypal prosauropod (by definition, as it happens).

Stegosaurus ungulatus: Everyone knows this critter. As David Marjanovic has pointed out, the original thagomiser.

Turiasaurus riodevensis: Europe's largest known dinosaur, and type of a previously unknown group of sauropods. There's something vaguely funny about a previously unseen sauropod.

Centrosaurus apertus: It was either this one or Styracosaurus albertensis. Like Triceratops, but funkier.

Coelophysis bauri: When one is used to thinking of dinosaur finds as a few disarticulated remains, the Ghost Ranch deposit with more individuals of Coelophysis than can be counted are just stunning.

Herrerasaurus ischigualastensis: The prototype of later theropods. Herrerasaurus may no longer be the basalmost known dinosaur - recent workers favour a position very basal saurischian, plus there's always Eoraptor to trump it - but I still have a soft spot for the big lunk.

Pachyrhinosaurus canadensis: The ceratopsian that ditched the ceras, favouring a big ol' ugly boss instead. Still not to be messed with.

Psittacosaurus mongoliensis: I used this species as stand-in for the whole seemingly endless run of Psittacosaurus species. Every time I turn around there seems to be another one. They've been found huddled together in nests, they've been found with apparent long quills on the tail, they've been found halfway down a mammalian gullet. You can't escape them.

The Top Ten Dinosaurs - Triceratops beats Tyrannosaurus

Time it took me to do something hopelessly populist in order to try and draw more attention to the Catalogue of Organisms: 24 days. What can I do that's guaranteed to work up some steam?

In that light, I here present a list of my top ten dinosaurs, inspired by Don Robertson's Top 50 Birds (actually top eleven - some tied). Candidates for the list were rated completely subjectively on five factors: (1) Impressiveness - if I were to come across one of these critters, would the appropriate response be 'wow' or 'meh'? (2) Knowledge - how extensively the species has been studied and how well it is known. (3) History and Significance - if the discovery of this species had much significance, especially at the time it was discovered. This is also the category that was influenced by how much attention this species has received from the general public over time. (4) Controversy - has the species has inspired much debate over the years? (5) Special Factors - like Don Robertson did with his bird list, I also scored for an entirely subjective character of any special significance the species has that isn't really covered by the previous four categories. This category also reflected my own personal feelings about the animal in question (and whether or not I wanted it to win).

The list:

6th Equal:
Deinonychus antirrhopus: The little evil-looking buggers with the massive sickle claws on the feet. The description of Deinonychus has been directly credited with inspiring the renaissance in views on dinosaur metabolism. Older reconstructions of sluggish, low energy dinosaurs just made no sense when applied to this obvious speedster.

Euoplocephalus tutus: Everyone's favourite living tank. What more can you say about a creature so heavily armoured that even its eyelids would have clanged when it blinked? Not to mention the thagomiser at the end of the tail.*

*I'm not sure if the term thagomiser has ever been used formally, but it has a reasonable amount of informal currency as a term for an offensive structure at the end of a tail (like the ankylosaurid club, or the stegosaurid spike array). I believe it derives from a Far Side cartoon about cavemen, where it is named after the late Thag.

Falcarius utahensis: Apparently known from more specimens than you can shake a thagomiser at, this discovery of a couple of years ago represents the basalmost member of the therizinosaurs, gigantic (probably) herbivorous theropods with ridiculously oversized claws. Falcarius was a nice find because it perfectly slotted into the gap between derived therizinosaurs and their supposed relatives.

Iguanodon bernissartensis: The classic European dinosaur, one of the earliest discovered and possibly the earliest known from significant remains (this is the best known of the multiple Iguanodon species - while Iguanodon was one of the original three dinosaurs, the specific species involved there was Iguanodon anglicus, which is no longer regarded as identifiable).

Mononykus olecranus: Arguably the wierdest of all dinosaurs, with still no real idea about its lifestyle. A small bird-like theropod, Mononykus has greatly shortened yet very stout single-clawed forelimbs. The structure of the forelimbs appears suited for digging, yet the light cursorial form of the the rest of the body doesn't appear suitable for this.

Tyrannosaurus rex: Undoubtedly the best known of all dinosaurs. To be honest, I was kind of hoping old Tyrannosaurus would fall off the list - I think she's hogged the limelight for long enough. If you want to know more, a quick Google search will tell you more than you ever wanted to know - just pay no attention to the bit about coconuts.

2nd Equal:
Archaeopteryx lithographica: The Urvogel, the Missing Link (though obviously it's not missing anymore, is it?). Archaeopteryx was the original inspiration in recognising the connection between Cretaceous dinosaurs and modern birds. While later discoveries mean that Archie (as he is known to his friends) is now but one of many fossils demonstrating this link, he still retains one of the best represented by specimens, and his historical significance will never be lost.

Oviraptor philoceratops: The name means 'egg thief and lover of ceratopsians' - when the original specimen was found it was lying on a nest of eggs that were then believed to belong to Protoceratops, and it was thought to have died while attempting to predate them. It has since been found that the eggs belonged to Oviraptor itself, and not only was it not eating them, but it would have been sitting astride them bird-style to keep them warm - history's ugliest broody chicken. The true diet of the strange-looking, beaky Oviraptor is a cause of great debate.

Triceratops horridus: Another classic. There are few people who would not recognise this beast with its broad frill and intimidating spiky bits. You know you love him, just don't stick your hand near his mouth if you want to keep it.

And my choice for the Greatest Dinosaur Ever:

Brachiosaurus brancai (or Giraffatitan brancai, depending on whom you ask): What can one say when faced with a giant sauropod except WOW! There may be bigger sauropods than Giraffatitan, there may be prettier, but this is still the classic giant and one of the best-known. Besides, the difference between unbelievably mind-blowingly HUGE and stupidly unbelieveably mind-blowingly HUGE is not that great when you consider that both can reduce you to a small greasy puddle underfoot and barely even break their stride.

Taxon of the Week #2: Trachinoidei

I'm continuing the fishy theme for the Taxon of the Week. The Trachinoidei are what are known as the weevers and allies - mostly elongate, often dorsoventrally flattened, regularly benthic fish. In keeping with the benthic habitat (most species bury themselves in the sand for camouflage), the eyes are generally placed on top of the skull with the mouths pointing upwards. This suborder includes the stargazers and sand lances.

To be honest, I'm having a hard time finding what, beyond a vague overall similarity, are the actual uniting characters of this group. Bond (1996) emphasises the jugular pelvic fins (placed under the throat), but this feature is not unique to trachinoids - the Blennioidei (blennies), Gobiesocidae (clingfishes) and Callionymoidei (dragonets) all also have jugular pelvics. Pietsch & Zabetian (1990) gave two characters - presence of a pelvic spur and pectoral radials being small, short or wide - uniting a collection of families that they referred to as the core of, but not necessarily delimiting, the Trachinoidei. Not surprisingly, all the molecular studies that have addressed the issue appear to recover the trachinoids as polyphyletic (Stankovic et al., 2005), and authors have disagreed significantly about which families should be included in the suborder. Being a trachinoid, it seems, is not so much a reality as a state of mind.

Rather than give an entire list of families, I'll simply direct you to Mooi and Johnson's page on the subject. A number of trachinoids have poisonous spines, and the name "weever" for members of the Trachinidae is supposedly derived from the Anglo-Saxon word for "viper". Uranoscopus (stargazers) also have electric organs behind the eyes - Bond (1996) credits them with producing 50 volts.

Cheimarrichthys fosteri (the torrentfish) is a freshwater fish unique to fast-running streams in New Zealand that is sometimes placed in its own family (the page I've linked to includes it in a family with other marine species - note that the 'blue cod' mentioned is no relation to the Atlantic cod).

My definite favourite among the families assigned to this suborder, however, has to be the Chiasmodontidae (black swallowers), another addition to the mesopelagic freakshow. Black swallowers get their name from their ability to distend their stomach to three times their own size and so swallow fish larger than themselves whole (memories of snork-eater-eaters...). Doubtless this is a very handy adaptation in the fairly sparse environment of the mesopelagic, where meals may be few and far between.

REFERENCES

Bond, C. E. 1996. Biology of Fishes, 2nd ed. Saunders College Publishing.

Pietsch, T. W., & C. B. Zabetian. 1990. Osteology and interrelationships of the sand lances (Teleostei: Ammodytidae). Copeia 1990 (1): 78-100.

Stankovic, A., K. Spalik, P. Golik, A. V. Balushkin, P. Borsuk, M. Koper, S. Rakusa-Suszczewski & P. Weglenski. 2005. Polyphyly of Scorpaeniformes and Perciformes: new evidence from the study of notothenioid's mitochondrial and nuclear rDNA sequence data. Journal of Ichthyology 45 (Suppl. 1): S171-S182.

Products of Kinky Inter-species Sex

Um, maybe I don't want to know what sort of Google search will hit that post title, or who's doing the searching. I can assure you, the following post is both PG and work-safe.

I came across this post today on identifying a hybrid passerine bird. The bird in question is an entirely different individual from the one discussed here, which was also revealed not too long ago. (Offhand, the site linked to via the latter, Don Roberson's Creagrus, is well worth a look for anybody interested in birds.)

Both of these birds belong to the family Parulidae, the so-called American 'warblers' - an entirely distinct and unrelated family from the various 'warblers' of the Old World (previously Sylviidae, but see here - Don Roberson again - for a good summary of the collapse of that family) and from the Australasian 'warblers' of the family Acanthizidae. Interestingly, Parulidae seems to have produced a large number of recorded hybrids over the years (enough that many have been awarded their own common names), and attention has been drawn to the fact that a greater number of recorded hybrids have been between members of different genera than members of the same genus (those that are members of the same genus have invariably been very closely-related species). The same pattern has been recorded in South American manakins (Pipridae) (Stotz, 1993). Even if we admit the point that a 'genus' is simply a grouping defined by the author and has no objective reality, it still remains arguable that hybrids are always between very closely or distantly related species, never between fairly closely related species.*

*My apologies for the revoltingly turgid sentence.

Parkes (1978) commented on this phenomenon, and suggested that barriers to hybridisation may be more heavily selected for between closely-related and sympatric species for which hybridisation may be more of a risk. This theory has also been supported by work on courtship songs in insects where sympatric species of lacewing have very different songs, while different species from Asia and North America have very similar songs, and members of one species will actually respond if they hear songs from the other 'wrong' species (Henry et al., 1999).

Perhaps the best comment on all this, though, comes from my partner - when I told him that I was posting on an interspecific hybrid, he seemed rather incredulous that a bird had mated with a member of a different species, and when considering why commented, "that must be one ugly bird".

REFERENCES

Henry, C. S., M. L. Martínez Wells & C. M. Simon. 1999. Convergent evolution of courtship songs among cryptic species of the carnea group of green lacewings (Neuroptera: Chrysopidae: Chrysoperla). Evolution 53 (4): 1165-1179.

Parkes, K. C. 1978. Still another parulid intergeneric hybrid (Mniotilta × Dendroica) and its taxonomic and evolutionary implications. The Auk 95: 682-690. Pdf here.

Stotz, D. F. 1993. A hybrid manakin (Pipra) from Roraima, Brazil, and a phylogenetic perspective on hybridization in the Pipridae. Wilson Bulletin 105 (2): 348-351. Pdf here.

Taxon of the Week #1: Gonostomatidae

I had intended for this to appear earlier in the week, but it's been a fairly busy one for me. Here's how this is going to work, barring accidents - every week I'll do an introduction to a (somewhat) randomly-chosen taxon. As this is my first entry, you can be sure I've selected something exciting, cutting-edge, mind-blowing, insert your choice of superlative adjective here - the Gonostomatidae or bristlemouths.

If the names not ringing any immediate bells, then shame on you. Bristlemouths are possibly the most abundant vertebrates in the world (or so every source I've looked at [e.g. Bond (1996), Craddock & Hartel (2002)] says, though I've yet to find an actual figure). They are mesopelagic or bathypelagic fish (or 'feesh' as they say here in Western Australia), mostly quite small (according to Craddock & Hartel [2002], some species mature at less than 20 mm) and, like oh so many mesopelagic fish, not overly attractive (see images at wikipedia). Gonostomatidae have an elongate body form with relatively big mouths (according to the afore-linked wikipedia page, the name 'bristlemouth' refers to the evenly-sized bristle-like teeth, a description that appears more true for some genera [Cycothone] than others [Gonostoma]). There are one or more rows of photophores along the length of the body - some like Triplophos have multiple rows almost covering the animal, while others such as Bonapartia have only a single row on the lower edge of the body, plus a few scattered over the head (Harold, 1999). They are micropredators of small crustaceans and such (Craddock & Hartel, 2002).

The Gonostomatidae belong to the order Stomiiformes, sometimes known as dragonfishes (some of the other members of the order reach a reasonable size, and you may have seen illustrations of them before - they're the elongate deep-sea fish with the mouths full of enormous teeth). As an aside, the intro for Stomiiformes in Collette & Klein-MacPhee (2002) refers to it as a 'very large group'. Only vertebrate workers would consider a few hundred species a 'very large group'. The Gonostomatidae itself includes seven or eight genera in two subfamilies, the Gonostomatinae (Gonostoma, Sigmops, Margrethia, Bonapartia, Cyclothone) and Diplophinae (Diplophos, Manducus, Triplophos).

Unlike many mesopelagic fish, bristlemouths do not appear to engage in daily vertical migrations (McClain et al., 2001). Their small size and deep-water habitat mean that, despite their abundance, they are rarely seen except by researchers.

REFERENCES

Bond, C. E. 1996. Biology of Fishes, 2nd ed. Saunders College Publishing.

Collette, B. B., & G. Klein-MacPhee (eds.) 2002. Bigelow and Schroeder's Fishes of the Gulf of Maine, 3rd ed. Smithsonian Institution Press: Washington.

Craddock, J. E., & K. E. Hartel. 2002. Bristlemouths. Family Gonostomatidae. In Bigelow and Schroeder's Fishes of the Gulf of Maine, 3rd ed (B. B. Collette & G. Klein-MacPhee, eds.) pp. 181-184. Smithsonian Institution Press: Washington.

Harold, A. S. 1999. Gonostomatidae: Bristlemouths. In Western Central Pacific Identification Guide for Fishery Purposes (K. Carpenter & V. H. Niem, eds.) pp. 1896-1899. FAO Species Identification and Data Programme vol. 3.

McClain, C. R., M. F. Fougerolle, M. A. Rex & J. Welch. 2001. MOCNESS estimates of the size and abundance of a pelagic gonostomatid fish Cyclothone pallida off the Bahamas. Journal of the Marine Biology Association of the United Kingdom 81: 869-871.

Another word on arachnid phylogeny

Shultz (2007) has just published a new paper on arachnid phylogenetics, based on morphology. As you can see if you scan my profile blurb, I'm currently working on arachnids - specifically on harvestmen (Opiliones), so I'm always happy to see work on them. Shultz (2007) sits in couterpoint to the most recent other publication on arachnid high-level phylogeny, Giribet et al. (2002), which used combined molecular and morphological data.

I do feel the need to make a few comments on character coding. One of the issues with high-level morphological phylogenetics is that it becomes increasingly difficult to code characters without interpretative bias. One example of this that I can spot in Shultz (2007) is his coding for the tracheal system (character 126). In the past, presence or absence of a tracheal system has generally been treated as a single character. Shultz argues on the basis of differences in the layout of the tracheal system that it has probably evolved independently a number of times (an idea that does gain some support from the definitely independent evolution of a tracheal system in some spiders), and codes the different tracheal systems as different character states. While Shultz is likely to be correct that the different tracheal systems have evolved independently, his coding of the systems separately a priori excludes the possibility of their homology. Also, in his comments on presence of a penis (character 160), a synapomorphy of Opiliones, Shultz notes that 'A clearly homologous structure is present in Cyphophthalmi (Opiliones) and apparently functions in depositing a spermatophore in the female’s genital chamber', then follows with 'The ‘penis’ in Oribatida is really a spermatopositor; it functions in construction of a spermatophore'. Is it really justifiable without prior phylogenetic expectations to code the cyphophthalmid structure as a 'penis', but not the oribatid structure?

Shultz uses a few exemplars from each of the living arachnid orders, as well as a fossil exemplars of a number of them (he includes more fossil taxa than Giribet et al.) plus the fossil Eurypterida, Trigonotarbida and Plesiosiro. An analysis is run without fossil taxa, then one with. At first glance, the inclusion or exclusion of fossil taxa has a significant impact on topology. Without fossils, and using Xiphosura (horseshoe crabs) as an outgroup, the recovered topology is (Palpigradi ((Ricinulei (Anactinotrichida, Actinotrichida)) ((Araneae (Amblypygi, Uropygi)) ((Scorpiones, Opiliones) (Pseudoscorpiones, Solifugae))))). However, through in a few fossil taxa and you get ((Scorpiones, Opiliones) (Palpigradi, (Actinotrichida (Ricinulei, Anactinotrichida), ((Pseudoscorpiones, Solifugae) (Araneae (Amblypygi, Uropygi)))))*.

*I wanted to use actual trees for this section, if only in ASCII format, because that would be one hell of a lot easier to read, but it looks like they won't show up properly in the final page. if anyone knows of a way I can put in trees on this site, I'd be ever so grateful to hear it).

Actually these two topologies are nowhere near as different as they appear - the support values for most supraordinal clades are ghastly. If we collapse all nodes in the final tree with less than 50% support, we get (Scorpiones, Opiliones, Palpigradi, (Ricinulei, Anactinotrichida, Actinotrichida), Solifugae, Pseudoscorpiones, (Trigonotarbida, Araneae (Plesiosiro (Amblypygi, Uropygi)))). To somewhat mitigate the drawbacks of this low support, however, Shultz does test his results against other past theories.

Shultz runs his neontological data set through a number of analyses constrained to recover particular clades. Clades suggested in the past that appear in trees only one step longer than Shultz's most parsimonious tree are Ricinulei + Anactinotrichida, Megoperculata (Palpigradi + Araneae + Amblypygi + Uropygi) and Rostrosomata (all arachnids except Scorpiones and Opiliones). The last one, notably, is what is recovered when palaeontological data are included. Scorpions sister to all other arachnids is only two steps longer, as is Micrura (arachnids except Scoropiones, Opiliones, Pseudoscorpiones and Solifugae). It is a little disappointing that these comparisons are run on the neontological data set alone rather than the complete data set, considering that the fossil taxa are not without influence on the result. Two comparisons are made using the full data set, testing a scorpion + eurypterid clade and a trigonotarbid + ricinuleid clade (the latter possibility was found by Giribet et al., 2002). Both possibilities are noticeably longer than the preferred tree.

A notable absence from Shultz (2007) is the Pycnogonida. Pycnogonids or 'sea spiders' are patently bizarre marine animals of very obscure relationships. Traditionallly they have been regarded as basal chelicerates owing to their possession of chelate pre-oral appendages, and many authors still support this view. Other authors regard pycnogonids as the sister group to all other living arthropods. When pycnogonids were included in the analysis of Giribet et al. (2002), they appeared in a completely unexpected position as sister to Palpigradi, within Arachnida. My impression on reading Giribet et al. is that the authors themselves are extremely sceptical of this result, and seem more inclined to attribute it to the high level of autapomorphy in pycnogonids. While it would have been interesting to see Shultz test the position of pycnogonids, it is possible that said degree of autapomorphy may have simply blown Shultz's analysis out of the water.

REFERENCES

Giribet, G., G. D. Edgecombe, W. C. Wheeler & C. Babbitt. 2002. Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data. Cladistics 18: 5-70.

Shultz, J. W. 2007. A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150 (2): 221-265.

Filling in the gaps

I'm going to continue on with the algal theme here, because I keep getting reminded lately of neat examples. However, I'm going to take a great leap sideways and deal with a different group from rhodophytes. I'm moving towards the brown algae (sort of...)

It is universally accepted these days that the algae are a polyphyletic grouping, at least from the viewpoint of nuclear and cytoplasmic ancestry. Chlorophyll originally developed within the blue-green algae, actually a clade of bacteria (Cyanobacteria). Chloroplasts in eukaryotes then arose through endosymbiosis between a non-photosynthetic protist and a cyanobacterium. However, many authors now agree that there was probably only one such primary endosymbiosis event that led to the majority of modern chloroplasts (there is a lonely cercozoan*, Paulinella, that appears to have derived its chloroplast independently). The direct descendants of this lucky protist today are the green plants and algae (Viridiplantae), the red algae (Rhodophyta) and a small group of unicells called the blue-green algae (Glaucophyta). Red and blue-green algae both have only a single chlorophyll type, chlorophyll a, while green algae possess a second form as well, chlorophyll b (interestingly, a small handful of cyanobacteria also possess chlorophyll b, and molecular phylogenies show that these oddjobs are not closely related within the cyanobacteria). The remaining algae are derived from secondary symbioses, where a eukaryotic alga has become an endosymbiont of another eukaryote followed by loss of the endosymbiont's independence and genetic material. This is rather spectacularly demonstrated by two secondary algal groups, the chlorarachneans and cryptophytes, whose chloroplasts retain a highly-reduced eukaryotic nucleus between the membranes surrounding the chloroplast. Two groups, the amoeboid chlorarachneans and flagellate euglenoids, have chloroplasts derived from green algae as shown by their possession of chlorophyll b. Four groups, the dinoflagellates, cryptophytes, haptophytes and ochrophytes, have chloroplasts seemingly derived from red algae. These four groups also share a third chlorophyll type, chlorophyll c, as well as the ancestral chlorophyll a, and on this basis it has been suggested that they all derive from a single endosymbiotic ancestor (though this seems likely, the case is not airtight as there are a number of non-photosynthetic protists without chloroplasts that seem to be closely related to one or another of the chlorophyll c groups). Some dinoflagellates have replaced their ancestral chloroplasts with chloroplasts derived from haptophytes in a tertiary endosymbiosis.

*Originally I had identified Paulinella in this post as an amoebozoan, but it belongs to a different amoeboid group, the Cercozoa.

Multicellularity has evolved a number of times within algae. Viridiplantae became multicellular multiple times, while red algae probably evolved multicellularity once at the base of the Bangiophyceae + Florideophyceae clade (see the previous post). Ochrophytes include two groups of multicellular algae, the brown algae (Phaeophyceae) and some members of the yellow-brown algae (Xanthophyceae).

Ochrophytes are the clade of photosynthetic heterokonts. Heterokonts are a well-supported clade of protists distinguished in most members by, among other features, a shorter posterior flagellum and a longer anterior flagellum with numerous side bristles (mastigonemes). At cell division, the anterior flagellum moves backwards and loses the mastigonemes to become the posterior flagellum, while a new anterior flagellum is generated (Andersen, 2004). As well as the two above-mentioned classes, ochrophytes include diatoms and a whole bunch of unicellular algae previously united as the golden algae (chrysophytes). The chrysophytes have proven to be paraphyletic with regard to the other ochrophytes, and have been divided into a whole host of smaller classes.

Now we've gotten through all that, I'll finally introduce the star of today's post, Schizocladia ischiensis Henry, Okuda & Kawai in Kawai, Maeba et al., 2003. The position of the brown algae in relation to other ochrophytes has been obscured by the absence of clear connecting features between the multicellular brown algae and the various unicellular golden algae. The significance of Schizocladia is that it goes some way towards filling that gap. Schizocladia is a small marine ochrophyte that grows as filaments of cells in single file. Like phaeophytes, Schizocladia has cell walls impregnated with alginates. Unlike brown algae, Schizocladia lacks cellulose or plasmodesmata (cytoplasmic connections between cells). Propagation in Schizocladia was via zooids produced in individual compartments in swollen cells at the end of the filaments.

The molecular phylogenies presented in the original description of Schizocladia agreed in positioning it as the sister group of Phaeophyceae. They also agreed with the result found by other studies that there is a clade composed of Phaeophyceae (+ Schizocladia), Xanthophyceae and the unicellular Phaeothamniophyceae (the unicellular Chrysomeridales may also belong to this clade, but do not appear to have been investigated molecularly). While the Xanthophyceae do include some multicellular members, it also includes unicellular forms, and multicellularity was probably evolved independently of Phaeophyceae. Xanthophyceae do possess cellulose in the cell walls, and the presence of alginates has also been demonstrated in some species.

Due to the absence of some supposed key phaeophyte characters, Schizocladia was not included in Phaeophyceae but placed in its own independent class Schizocladiophyceae. Nevertheless, its simple morphology provides a nice connection between the unicellular ochrophytes and multicellular phaeophytes.

REFERENCES

Andersen, R. A. 2004. Biology and systematics of heterokont and haptophyte algae. American Journal of Botany 91 (10): 1508-1522.

Kawai, H., S. Maeba, H. Sasaki, K. Okuda & E. C. Henry. 2003. Schizocladia ischiensis: a new filamentous marine chromophyte belonging to a new class, Schizocladiophyceae. Protist 154: 211-228.

A Parasite in the Family

In the previous post, I wrote about an epiphytic red alga, and mentioned in passing the interesting phenomenon of adelphoparasitism, where a parasite is very closely related phylogenetically to its host. Since then, I've been wondering how such a situation arose, and specifically whether there was a connection between red algal adelphoparasitism and the complexities of red algal life cycles (see here for a summary).

Red algae fall into three to seven classes - Rhodellophyceae (which Yoon et al., 2006, divide into five, but as I haven't yet seen the paper I'll let it slide), Bangiophyceae and Florideophyceae. Rhodellophyceae are unicellular, and I confess I don't know the details of their life cycles. Bangiophyceae (which include Porphyra, the nori used in making sushi) alternate between distinct haploid and diploid generations. Florideophyceae include the vast majority of red algae, and verge on the completely insane in life style complexity. The basic florideophycean life cycle (which, as shown in the previous post, not all members of the class go through) involves no less than three alternating generations (I linked to the diagram here in the previous post, but I'll do it again because it's a good'un). Starting with the diploid tetrasporophyte, the tetrasporophyte releases haploid spores that settle and grow into gametophytes. Male gametophytes release spermatia (aflagellate sperm) that are captured by the female gametophytes and fertilise the carpogonia. The carpogonium (and this is the interesting part for this post) then grows into a carposporophyte, which remains attached to the parent gametophyte, releasing diploid spores that grow into new tetrasporophytes. So in effect, parasitism is already part of the florideophycean life cycle. Is it somehow possible that this parasitism is behind the rise of adelphoparasitism?

It's worth noting here that similar patterns to "adelphoparasitism" are not unique to red algae. They have also been recorded among social Hymenoptera as well as mistletoes. Red algal parasites have traditionally been divided between adelphoparasites (which are closely related to their hosts) and alloparasites (not so closely related). The two classes are also supposedly distinguished by the mode of parasitism. In both, after the parasite rhizoid invades the host it adheres to and fuses with the host cells, injecting parasite nuclei and mitochondria. In adelphoparasites, the parasite nuclei then multiply within the host cell, hijacking it and causing the formation of growths which release spores of the parasite species (Goff et al., 1997), In alloparasites, the parasite nuclei do not divide in the host cytoplasm, though they do alter its physiology to facilitate the transfer of nutrients from host to parasite, and (I assume) the parasite reproductive bodies grow from the parasite rhizoid itself. Goff et al. (1997) demonstrated that one 'genus' of adelphoparasites had actually arisen polyphyletically from the host 'genus'. Zuccarello et al. (2004) demonstrated the same thing for a 'family' of alloparasites. The latter authors therefore suggested that the terms 'adelphoparasite' and 'alloparasite' were not useful. However, this does still leave the question of the different cytoplasmic interactions (Zuccarello et al. implied that this might be due to the taxa studied belonging to different orders).

Goff et al. (1997) give two possible scenarios for the origin of parasitic red algae. In one, the parasites are ancestrally epiphytic, later becoming endophytic and eventually parasitic. In the second, the parasites derive directly from spores that lose the ability to survive independently of the parent. The existence of the carposporophyte, in my opinion, gives a lot of support to this option. One possibility is that adelphoparasites arose by the second method while alloparasites arose by the first.

Goff et al. also examined the main complaint towards the second origin - even if some mutant parasitic individual does arise, what is to stop it backcrossing to the parent population? How does the parasite become established as a new species? At present, there is no really satisfying answer to this question. Goff et al. do point out that parasitic taxa have life cycles taking a fraction of the time of the host species. At any given time, only a small percentage of the individuals in a population of algae are reproductive - perhaps the difference in timing of life cycles simply meant that the chance of backcrossing between parasite and non-parasite was too low to prevent speciation?

REFERENCES

Goff, L. J., J. Ashen & D. Moon. 1997. The evolution of parasites from their hosts: a case study in the parasitic red algae. Evolution 51 (4): 1068-1078.

Zuccarello, G. C., D. Moon & L. J. Goff. 2004. A phylogenetic study of parasitic genera placed in the family Choreocolacaceae (Rhodophyta). Journal of Phycology 40: 937-945.

Little Discs of Doom

Okay, total hyperbole in the title, but I wanted to get your attention. Today I'll be looking at Pihiella liagoraciphila, a very distinctive member of the red algae that was only described a few years ago (Huisman et al., 2003).

Pihiella is an endo/epiphyte found on members of the red algal family Liagoraceae, but not a parasite as far as I can tell (red algae are notable for the range of associations between different taxa, most interestingly the occurrence of what is call 'adelphoparasitism', where parasitic species are closely related to their hosts - I'll have to write on that some day). It has a quite simple disc-shaped or subspherical morphology with rhizoids to attach it to the host and long hairs and trichogynes (hair-like appendages of the female carpogonia that catch the male gametes). Mature discs are very small, up to 400 μm in diametre and 150 μm thick, though the hairs can be up to 800 μm long. Specimens were first observed as long ago as 1858, but were interpreted as buds of the host plant. Authors thereafter disagreed as to whether the so-called 'monosporangial discs' were asexual reproductive organs of the host or an independent organism. All authors agreed that the discs were asexually reproductive.

Sexually reproductive organs on the discs weren't recorded until 2003, when Huisman et al. established that the discs were indeed a separate organism from the host. Pihiella seems to lack the obscenely complicated triphasic life cycles of other red algae (see here for an example). As already mentioned, the carpogonia (sexual organs) possess a long hair-like trichogyne, and Huisman et al. did observe examples with spermatia (the aflagellate male sex cells) attached. Nevertheless, Huisman et al. were unable to conclude whether the mature sporangia observed were asexually produced monosporangia, sexually produced zygotosporangia, or both (I feel the last option seems most likely, but what do I know?). No carposporophytes or tetrasporangia were observed (see the link above to find out what these are).

The morphology of Pihiella was too distinct from any other red alga to be phylogenetically informative, but Huisman et al. were able to assess the phylogeny molecularly. Pihiella turned out to be quite isolated from other red algae, enough that Huisman et al. established a new monotypic order for it. Interestingly, the trees recovered Pihiella as sister taxon to Ahnfeltia, another phylogenetically isolated taxon, with a high level of support. Morphologically, Ahnfeltia is very distinct from Pihiella, being a large cartilaginous plant with a triphasic life cycle found in cool waters (the host family of Pihiella, Liagoraceae, is a mostly warm-water group). Though Ahnfeltia and Pihiella are each other's closest relatives, the relationship is not close. Liagoraceae, in contrast, was in a quite distant part of the tree.

REFERENCES

Huisman, J. M., A. R. Sherwood & I. A. Abbott. 2003. Morphology, reproduction, and the 18S rRNA gene sequence of Pihiella liagoraciphila gen. et sp. nov. (Rhodophyta), the so-called 'monosporangial discs' associated with members of the Liagoraceae (Rhodophyta), and proposal of the Pihiellales ord. nov. Journal of Phycology 39: 978-987.

TAFKAMI

I was at a bit of a loss as to what to post on next, so I asked my partner to "name an organism, any organism". He suggested "amoeba", so I'm following his instructions. Besides, the subject of today's post was bound to raise its pseudopodia somewhere along the line. I've decided to write on what I'll informally dub TAFKAMI - The Amoeba Formally Known As Mastigamoeba invertens. TAFKAMI is an anaerobic amoeboflagellate with a cilium shorter than the body.

A little backgroud, first. Protist phylogeny has always been a contentious, uncertain world - compared to multicellular organisms, unicells have relatively few obvious characters to unite various groups of taxa. However, the availability of better and better electron microscopy and the continued improvements in molecular phylogenies mean that in recent years, a growing consensus has developed that the majority of eukaryotes fall into a few large "supergroups" (Simpson & Roger, 2004) - the opisthokonts (including fungi and animals), amoebozoans, excavates (mostly flagellates), rhizarians (including radiolarians and foraminiferans), chromalveolates (including ciliates, brown algae and dinoflagellates) and plants (including green and red algae). There are still a few random taxa that don't necessarily fall into any of these groups.

The supergroup Amoebozoa includes the majority of amoebae with lobose pseudopodia, as well as most slime moulds and the Archamoebae, a group of amitochondriate anaerobic amoebae (take a moment to appreciate the assonance). The Archaemoebae include Mastigamoeba proper (as well as Entamoeba, the causative organism of amoebic dysentery).

According to Walker et al. (2006), TAFKAMI was first isolated back in 1992. Since then, it has been what is technically referred to as a right pain in the khyber. Comparisons using both microscope and molecular techniques between TAFKAMI and other supposedly related organisms increasingly indicated that it was not Mastigamoeba, or any other known amoeba. In molecular studies (such as Cavalier-Smith & Chao, 2003) TAFKAMI leapt about madly, sometimes with amoebozoans, sometimes with apusomonads (another small group that doesn't fit into any of the supergroups), sometimes entirely elsewhere.

Walker et al. (2006) recently established TAFKAMI as a new taxon, Breviata anathema (the original description of Mastigamoeba invertens from 1892 does not allow reliable identification of what that species is). They also presented a detailed comparison of Breviata with the main contenders for close relationship.

Cavalier-Smith et al. (2004) felt that TAFKAMI was the basalmost member of the Amoebozoa, placing it in its own class Breviatea. The main reason for doing so was that TAFKAMI was supposed to possess a single ciliary basal body (the organelle that the flagellum emerges from). However, Walker et al. found that Breviata actually had double basal bodies. There is reasonably good evidence (Cavalier-Smith, 2002; Simpson & Rogers, 2004) that eukaryotes with double basal bodies form a single über-clade, the bikonts (including the excavates, rhizarians, chromalveolates and plants). It seems quite believable that Breviata is a member of this clade. Breviata also lacks the molecular markers of Amoebozoa proper (Cavalier-Smith et al., 2004).

Under certain parameters, molecular phylogenies supported an association of Breviata with the afore-mentioned apusomonads, which are also bikonts. Breviata also has similar pseudopodia to apusomonads. However, I would be just as sceptical of a direct apusomonad-Breviata connection. At the present, I don't feel that Breviata can be placed as anything more that "basal bikont". (It is worth noting, too, that one amoebozoan group, the Myxogastrea, has independently evolved double basal bodies.)

Perhaps most interestingly, Walker et al. identified a large organelle overlying the nucleus that they suggested as a possible hydrogenosome. Hydrogenosomes are respiratory organelles that are generally regarded as having been independently derived from mitochondria in a number of anaerobic groups. As such, Breviata potentially joins the growing list of supposedly amitochondriate taxa retaining mitochondrial remnants.

REFERENCES

Cavalier-Smith, T., & E. E.-Y. Chao. 2003. Molecular phylogeny of centrohelid Heliozoa, a novel lineage of bikont eukaryotes that arose by ciliary loss. Journal of Molecular Evolution 56 (4): 387-396.

Cavalier-Smith, T., E. E.-Y. Chao & B. Oates. 2004. Molecular phylogeny of Amoebozoa and the evolutionary significance of the unikont Phalansterium. European Journal of Protistology 40: 21-48.

Simpson, A. G. B., & A. J. Roger. 2004. The real ‘kingdoms’ of eukaryotes. Current Biology 14 (17): R693-R696.

Walker, G., J. B. Dacks & T. M. Embley. 2006. Ultrastructural description of Breviata anathema, n. gen., n. sp., the organism previously studied as ‘‘Mastigamoeba invertens’’. Journal of Eukaryotic Microbiology 53 (2): 65-78.

A Frustrating Giant Bird

Darren Naish in a recent post on his most excellent Tetrapod Zoology blog on a completely different subject mentioned the giant fossil bird Eremopezus, which inspired me to look it up (I was nearly inspired to change subject by watching a bagmoth in the lab here sealing itself into its bag in preparation for pupating, but another time, perhaps...)

Eremopezus is known from leg bones from the upper Eocene of the Fayum of Egypt. The most recent review is by Rasmussen et al. (2001), but it was first described in 1904. Lambrecht later divided the then-available material into two genera, Eremopezus Andrews 1904 and Stromeria Lambrecht 1929, but there is little significant difference between material assigned to the two and they are now regarded as synonymous.

Being a giant landbird, Eremopezus was originally thought to be related to modern giant landbirds, the ratites. Ratites are a group of flightless birds distributed between the southern continents - the ostrich (Africa), rheas (South America), emu, cassowaries (Australia), moa and kiwis (New Zealand). Arguments have run back and forth about whether the ratites are monophyletic, or have arisen independently from different ancestors. Recent molecular phylogenies have been pretty much unanimous that the ratites are indeed monophyletic, and together with the flighted tinamous (Tinamidae) are the sister group to the remaining modern birds. On the basis of a prominent ridge on the tarsometatarsus, Lambrecht (1933) suggested that Eremopezus was related to the elephant birds (Aepyornithidae) of Madagascar.

The problem is that this is simply not very significant evidence, as pointed out by Rasmussen et al. (2001). Large flightless birds show a great deal of similarity in the form of the leg bones, due to similar functional requirements. The flightless carnivorous bird Diatryma has hindlimb bones indistinguishable from those of ratites, despite being more closely related to the modern Anseriformes (ducks and geese). Rasmussen et al. concluded that Eremopezus could not be reliably associated with any other known group of birds.

To add to this, Eremopezus showed a number of distinct features all of its own. It appears to have been a fairly lightly-built bird, but slightly larger than a cassowary or rhea. The distal end of the tarsometatarsus is markedly flattened dorsoventrally, and the trochleae (and hence the toes) are quite widely splayed (an attachment scar indicating the presence of a hallux - the rear-pointing toe - is present, but this was probably small as an adaptation for terrestriality). The trochleae on either side have relatively light grooves, suggesting that the toes were quite mobile. The modern birds with the most similar morphologies are Sagittarius serpentarius (secretarybird) and Balaeniceps rex (shoebill). Both these birds use their feet for manipulation - Sagittarius is a ground predator that catches prey with its feet, while Balaeniceps uses its feet to grasp floating vegetation in swampy habitats. The Fayum of the Eocene also appears to have been a quite swampy habitat, but the appeal of a gigantic secretarybird is not to be denied. In the meantime, we simply have to wait on further remains to turn up before we can say more on the subject.