Carl Zimmer beat me to it. I was planning to announce the recent pettalid work after the paper arrived in the mail last week, but it seems I've been scooped. But because good work always deserves a second look, I'll write on it anyway. Besides, I was at least able to pinch the photo from Carl's site.
Pettalidae are a family of Cyphophthalmi, what are called the mite-like harvestmen. Cyphophthalmids are a fairly small group as far as is known, with probably less than fifty described species, but the number of species has been rapidly increasing in recent years. Though they are divided into about five families, cyphophthalmids are a fairly conservative bunch in appearance - the photo above is of Pettalus cf. cimiciformis*, but it is fairly typical of the group as a whole. They are quite distinct from other harvestmen (in fact, it is generally agreed that they are the sister-group to all others), and rather than having the spindly build of more familiar members of the order, cyphophthalmids are minute, stocky armoured tanks. If you look closely at the picture above, you may see a light spot on either side of the body that looks a bit like an eye, but it is in fact an ozophore - a raised mound bearing the opening of a stink gland. Except for members of the family Stylocellidae, cyphophthalmids have been described in the past as eyeless, but SEM studies of Pettalidae have revealed minute (often lens-less) eyes hidden on the side of the ozophore (Boyer & Giribet, 2007).
*For those who aren't already in the know, the 'cf.' in the name stands for the Latin confer (compare). In this case, it indicates that the animal in question is very similar to Pettalus cimiciformis, but is not definitely a member of that species.
The really interesting thing about cyphophthalmids (beyond their own intrinsic charm, of course) is their distribution patterns. Each of the various families has a definite, disjunct distribution (Boyer et al., 2007). The family Stylocellidae are restricted to south-east Asia. The Sironidae are found in what once was Laurasia - Eurasia and North America. The Neogoveidae are found in Florida, tropical South America and tropical West Africa - the tropical parts of what once was Gondwana. Two genera placed in their own families, Ogovea and Troglosiro, are found in West Africa and New Caledonia, respectively. And Pettalidae has a classic Gondwanan distribution, found in southern South America, southern Africa (including Madagascar), Sri Lanka, Australia and New Zealand (see Carl Zimmer's post for a map).
I think I should say something here about "Gondwanan" distributions. Science has a tendency to go through fads like any other aspect of human culture. For many years, most organisms showing what we would now call a "Gondwanan" distribution were interpreted as relicts of a former world-wide distribution. As acceptance of "continental drift" and recognition of the previous existence of Gondwana increased, more and more researchers considered its potential significance for modern biogeography. Needless to say, the significance was especially apparent to workers in the southern continents, doubtless not without some aspect of asserting the importance of the all-too-often neglected Southern Hemisphere biota relative to the Northern Hemisphere. Gondwanan origins became the next big thing for everything from birds (Cracraft, 2001) to beeches (Linder & Crisp, 1995) to butterflies (de Jong, 2003). In the last few years, the pendulum has begun to sway the other way, probably towards a more reasonable median.
The idea of a Gondwanan distribution for a given group of harvestmen particularly merits a critical look. The fossil record of harvestmen is pretty abysmal relative to the age of the group, but what record there is speaks of a remarkable degree of morphological conservatism. The Carboniferous long-legged harvestman Brigantibunum is almost indistinguishable from modern taxa (Dunlop & Anderson, 2005). The cyphophthalmid Siro platypedibus Dunlop & Giribet, 2003, from Bitterfeld amber (probably Oligocene or Miocene in age) is so similar to modern species that it is included in a modern genus.
In order to test whether the distribution of Pettalidae is an actual Gondwanan distribution as opposed to a relictual one, Boyer et al. (2007) tested the phylogeny of the family with just about every morphological and molecular method imaginable. They demonstrated that most of the cyphophthalmid families were monophyletic, with distribution matching phylogeny (the exception was the Laurasian Sironidae, which came out paraphyletic to the northern Gondwanan Neogoveidae and Stylocellidae).
To add another level of interest to the whole deal, most of the genera within Pettalidae each have separate geographic distributions (Boyer & Giribet, 2007). Chileogovea in South America, Purcellia and Parapurcellia in southern Africa, Pettalus in Sri Lanka, Karripurcellia in Western Australia, Austropurcellia in eastern Australia. The exception is New Zealand. New Zealand has a remarkable diversity of Pettalidae, with more described species than everywhere else combined, in three genera. But let's look a little closer. In the South Island of New Zealand, the genus Rakaia is concentrated in the east, while the genus Aoraki is found in the west from Mount Cook* north. New Zealand actually lies on the boundary between the Indo-Australian and Pacific plates, and if you were to map the distributions of the genera, you would see that Rakaia is mostly found on the Pacific plate, while Aoraki dominates on the Indo-Australian!**
*The Maori name for which just happens to be Aoraki. Not a coincidence - the genus was named after the mountain.
**I know, I said three genera. The third genus is a single species, Neopurcellia salmoni, in the southwest of the South Island.
Boyer, S. L., & G. Giribet. 2007. A new model Gondwanan taxon: systematics and biogeography of the harvestman family Pettalidae (Arachnida, Opiliones, Cyphophthalmi), with a taxonomic revision of genera from Australia and New Zealand. Cladistics 23: 337-361.
Cracraft, J. 2001. Avian evolution, Gondwana biogeography and the Cretaceous-Tertiary mass extinction event. Proceedings of the Royal Society of London Series B – Biological Sciences 268: 459-469.
It seems I've fooled at least one person into thinking I have some sort of intellect - Kevin Z at The Other 95% has passed on the "Thinking Blogger" award to me. Apparently I'm supposed to pass it on to five more people, but this being something that has been going around for a little while, there's a shortage of people to pass it on to. There's no mention of dire things happening to my relatives if I don't pass it on like there normally is with chain letters, but I will highlight five other writers out there that have caught my attention lately (in no particular order). I'd also recommend heading to Kevin's site - it's well worth the trip, even if he is currently sobbing over his lophophorates.
Brian Switek of Laelaps has doubtless already received one of these, but he has a fantastic site for anyone with an interest in study of vertebrate palaeontology, and the study of the study of vertebrate palaeontology.
Also in the palaeontology field is Julia of The Ethical Palaeontologist. I'm not sure why she has tagged herself "ethical" (though as far as I can tell her ethics are impeccable), but she certainly writes some excellent posts.
And in a double whammy for the day, I'll head straight into this week's highlight taxa, meaning the Dactylopodolidae (I can't help it - I love the name!) lose their seat faster than an Italian government. So let's welcome in the plant family Violaceae!
Violaceae are a medium-sized family (about 800 or more species), about half of which belong to the single genus Viola (violets, pansies and small string instruments) (shown in the image above from Wikipedia. Most Violaceae are herbs, but a few are woody shrubs, trees or lianes - for instance, the South American tree Leonia triandra reaches 25m in height (see here). In fact, I get the impression that, taken genus by genus, there are actually more woody genera of Violaceae than herbaceous ones, and it is only the high diversity of the mostly herbaceous Viola that skews the ratio. Phylogenetically Violaceae are members of the rosid order Malpighiales that I've had cause to mention before as containing the gigantic-flowered holoparasite Rafflesia.
Violaceae don't appear to include anything as remarkable as Rafflesia (at least as far as I know), but they are certainly not devoid of interest. Many species of Viola produce cleistogamous flowers, i. e. the flowers never open and fertilise themselves. Often (as in Viola pubescens, shown here in a picture from Wikipedia) both cleistogamous and open flowers are produced (Culley & Wolfe, 2001), thus achieving the best of both options - the greater genetic variability obtained through outcrossing, as opposed to the more guaranteed success in setting seed of cleistogamy.
Also worth a mention are the ten or so species of Viola endemic to Hawaii, which are unique among the genus in their combination of woody stems (present in a few other species) and flowers borne in inflorescences (as opposed to singly in all other Viola). [The picture at left from the Hawaiian Plants website of Gerald Carr shows Viola chamissoniana var. tracheliifolia.] These distinctive features have lead to the suggestion that the Hawaiian species are quite basal in the genus (possibly relicts) and that they are related to basal South American Viola species that also have woody stems. However, a molecular study by Ballard & Sytsma (2000) indicates that, far from being an ancient group, the Hawaiian violets represent a single quite recent colonisation, not from South America, but from the Arctic! The sister taxon of the Hawaiian violets is the herbaceous Viola langsdorffii, found in the American Arctic and Japan. As circumstantial support for this result, Ballard and Sytsma pointed out the large numbers of migratory birds passing Hawaii from their breeding grounds in the Arctic, and that at least two Hawaiian birds appear to have recent Arctic origins - the goose Branta sandvicensis (from B. canadensis and the duck Anas wyvilliana (from A. platyrhynchos).
Culley, T. M., & A. D. Wolfe. 2001. Population genetic structure of the cleistogamous plant species Viola pubescens Aiton (Violaceae), as indicated by allozyme and ISSR molecular markers. Heredity 86 (5): 545-556.
Once again, Taxon of the Week has been delayed. But don't worry, it's here now, and it's a doozie - or it would be, if I was actually able to find much information on it. This week we dive underwater and scrabble in the mud in search of the gatrotrich family Dactylopodolidae. [Just kind of rolls off the tongue, doesn't it?]
Gastrotrichs are minute (usually less than 1 mm) aquatic 'worms' that are one of those horribly obscure animal phyla that usually get allocated half a page in hidden corners of the textbooks, if they're lucky. They are inhabitants of the interstitial - they live among and between the grains of sand and mud, where they hunt down microscopic algae and protozoa by crawling about on their ciliated bellies. There are two distinct orders of gastrotrichs - the illustration above from Hochberg & Litvaitis (2000) shows an idealised representative from each. The animal of the right belongs to Chaetonotida, which have a fairly consistent bowling pin shape, covering of spined scales and two long posterior furcae each bearing a single adhesive tube. The more varied Macrodasyida, on the left, generally have a more elongated body shape and a greater number of adhesive tubes. The Macrodasyida are simultaneous or alternating hermaphrodites, while Chaetonotida have a higher diversity of reproductive strategies, including a number of parthenogenetic species. Macrodasyida are almost exclusively marine, with only a couple of exceptions; Chaetonotida are both marine and freshwater.
The Dactylopodolidae are members of the Macrodasyida. Phylogenetic studies using both morphological and molecular data agree that the Dactylopodolidae are the basalmost family of macrodasyids, which makes them potentially very significant for gastrotrich phylogeny (Hochberg & Litvaitis, 2000, 2001; Todaro et al., 2003). They seem to have a fairly generalised body-plan - no extravagant ornamentation, relatively short body with a deeply lobed posterior, while the adhesive tubes are generally restricted to the posterior part of the body (Hummon, 1974). Their basal position is indicated by a plesiomorphic musculature and monociliated epidermis (Hochberg & Litvaitis, 2001).
The Dactylopodolidae contains three to five genera (depending on whether or not the contentious genera Xenodasys and Chordodasys are included). The largest genus is Dactylopodola - the picture above comes from the Senckenberg Forschungsinstitut und Naturmuseum and shows Dactylopodola typhle - the linked site also has a close-up of its head that I recommend taking a look at. Get a good look - these appear to be the only images of Dactylopodolidae sensu stricto available on the web!
For some time now, I've been contributing to Palaeos.org, the wiki site that was established last year to supplement Palaeos.com, and ideally to eventually supplant it when the original authors of Palaeos.com were no longer able to maintain it. Palaeos.com was originally founded as a site on palaeontology, but since then its mandate has expanded to cover all aspects of biology. It has gone offline on two occassions in the past - both times it has come back after overwhelming demand, and it was the latest disappearance that led to the foundation of Palaeos.org.
Palaeos.org is publicly editable, and I'd invite you all to take a look at it and add to it, correct errors, etc. as you may see fit. In particular, I'm going to start putting up notices on pages that I add to the site, in the hope that someone out there who knows more than I do may improve them. In recent times I've added pages on Combretaceae, Excavata, Eutrochozoa and Eoraptor. Tell me what you think!
I've noted some examples before (in posts here and here) from the bizarre world of deep-sea fish, where life gets really ugly (because where there's no light and no-one can see you, you can really let yourself go). I thought I'd put in a mention of what are arguably among the most bizarre of deep-sea fishes, the Saccopharyngiformes. I can't recall when I first came across an illustration of these incredible creatures, but they're not something you readily forget (the image above comes from Animal Diversity Web).
Saccopharyngiformes are deep-sea 'eels'. They're not real eels (i.e. they're not members of the order Anguilliformes), but they are closely related and like true eels are members of the clade Elopomorpha. Elopomorphs are united by a distinct planktonic larval form called a leptocephalus, with a leaf-shaped transparent form shown below in a photo from Wikipedia. Admittedly, this photo shows a true eel rather than a saccopharyngiform eel, but the general idea's the same - except that saccopharyngiform leptocephali have the unique feature that the myomeres (the muscle blocks) are V-shaped instead of W-shaped. Saccopharyngiformes have greatly elongate jaws, attached to the neurocranium by only a single condyle. Most of the other uniting features of the order are absences - no scales, no pelvic fins, no ribs (see Fishbase for a complete list).
There are four families of Saccopharyngiformes. The most distinctive family is the bobtail snipe eels of the Cyematidae (image above of Cyema atrum from Animal Biodiversity Web again). Cyematidae are relatively small creatures with a distinctly cut-off appearance. Only two adult species are known, but apparently the known diversity of leptocephali attributable to this family suggests the existence of more. The long jaws bend away from each other and so can't be closed against each other - a feature shared by the unrelated but superficially similar true snipe eels of Nemichthyidae in the Anguilliformes.
The family Saccopharyngidae is the most familiar in the order (relatively speaking, of course), containing the gulper eels. Gulper eels can be extremely long, up to 2m in length, but the greater part of this (2/3 to 4/5 of the length) is taken up by the exceedingly long and filamentous tail. The remainder is dominated by the head - specifically the jaws - giving the appearance that these creatures are all mouth. How exactly that gigantic mouth is propelled by such a slender tail seems somewhat mysterious to me, and I'd love to know just how gulper eels spend their time. The tip of the tail bears an expanded, usually luminescent caudal organ - is has been suggested that this is used for a lure to attract prey, but without life observations this is mere speculation. Male gulpers have reduced jaws and an enlarged olfactory system relative to females. Eurypharynx pelecanoides, the pelican eel (the subject of the picture at the top of this post) is similar to the gulpers, but is separated as its own monotypic family. Eurypharynx has an even larger mouth than the Saccopharyngidae - over half the preanal length in the former as opposed to less than 40% in the latter.
The most bizarre of all the Saccopharyngiformes (and that's saying something) are undoubtedly the one-jawed eels of Monognathus, shown above in an image stolen from Smith (2002). Monognathus are the deepest-living of all elopomorphs, and have been found at depths of 5400m. The head is greatly reduced, and the common name refers to the complete absence of the upper jaw. A single venomous pronged fang sticks forward from the skull where the upper jaw should be - doubtless this is used to impale prey, but as Smith (2002) notes, " their odd morphology and their near total lack of sense organs make it difficult to imagine how they function and survive in their environment". Like other Saccopharyngiformes, Monognathus have a distensible abdomen, the posterior part of which oftens extends in a pouch that may go past the anus.
Though fourteen species of Monognathus have been described, only a single mature male specimen has ever been recovered. This specimen differed significantly from females. The lower jaw was almost absent, the fang was blunted, the olfactory organs were greatly enlarged, a layer of spongy tissue covered the head and the dorsal and anal fins were enlongated behing the tail into a notched fin. Obviously the males completely stop feeding on reaching maturity, and become totally dedicated to finding a mate. Their short life-span as a result probably explains why specimens are so rare.
Smith, D. G. 2002. Families Cyematidae, Saccopharyngidae, Eurypharyngidae, Monognathidae. In: FAO Species Identification Guides for Fishery Purposes, The Living Marine Resources of the Western Central Atlantic, Vol. 2.
Every couple of weeks or so I go into the Western Australian Museum library to look over the new journals and see if anything interesting has come out that I've missed. I did so this morning, and among the papers I noticed was van der Meijden et al. (2007) in the Biological Journal of the Linnean Society which established a new genus Leptosooglossus for the frog species previously known as Sooglossus gardineri from the Seychelles (shown above in an adorable image from the Nature Protection Trust of the Seychelles). A second species, Sooglossus pipilodryas, was also transferred into the new genus.
This was all well and good, until a few journals later I came across Nussbaum & Wu (2007) in Zoological Studies which established a new genus Sechellophryne for the frog species previously known as - yep, you know what's coming - Sooglossus gardineri (again, So. pipilodryas was also transferred). Oh dear. Two papers, published very close together in time, coining different names for the same thing.
Before anyone madly leaps to any suspicions, I can't find any obvious signs of plagiarism or claim-jumping in either paper. Both recognised the new genus on the basis of paraphyly of the genus Sooglossus, but van der Meijden et al. only used molecular data, while Nussbaum & Wu only used morphological data. It does seem somewhat incredible that there could be two separate groups of people both working on as small a group as Sooglossidae (only four species restricted to the Seychelles, a small group of islands in the Indian Ocean roughly the size of a postage stamp) and unaware of each other, but I can't find any obvious indications otherwise (if there is any sort of scandal, I'm chucking in a vote that it be referred to as 'Bubblegate'). It is good that the two papers using completely different methods agree so much in their results.
So the next question becomes - which is the correct name to use? The van der Meijden et al. paper was in the July issue of the journal it appeared in, while Nussbaum & Wu appeared in a May issue. So the first round would appear to favour Sechellophryne over Leptosooglossus. However, the cover date of a journal issue is not necessarily identical to the actual print release date, which is what is supposed to determine priority. The online release date for van der Meijden et al. (which may not be identical to the print release date, but is usually at least an indication) is given as 5th July at the journal website. Unfortunately, the website for Zoological Studies doesn't appear to list specific release dates, and there doesn't appear to be one on the paper. If anyone out there in the know is able to confirm the release date for me, I would be quite grateful (it suddenly occurs to me that I should have looked inside the cover or on the table of contents or such of the journal itself, but I'm no longer at the museum and can't do that now - d'oh!). Again, at the moment Sechellophryne appears to be the senior name unless proven otherwise.
Oh, and if you're wondering why Bubblegate, it's a reference to one of my partner's current favourite jokes (warning - PG rating):
Three frogs are brought before the court. As the first frog is taken to the stand, the judge asks the bailiff for his name and crime, to which the bailiff replies, "This is Frog, and his crime is blowing bubbles in the pond". The second frog is taken in, and again the judge asks for his name and crime. The bailiff replies, "This is Frog-Frog, and his crime is blowing bubbles in the pond". The third frog is then brought in, and the judge asks, "I suppose this is Frog-Frog-Frog?" "No," replies the bailiff, "this is Bubbles".
Meijden, A. van der, R. Boistel, J. Gerlach, A. Ohler, M. Vences & A. Meyer. 2007. Molecular phylogenetic evidence for paraphyly of the genus Sooglossus, with the description of a new genus of Seychellean frogs. Biological Journal of the Linnean Society 91: 347-359.
Nussbaum, R. A., & S.-H. Wu. 2007. Morphological assessments and phylogenetic relationships of the Seychellean frogs of the family Sooglossidae (Amphibia: Anura). Zoological Studies 46 (3): 322-335.
Apparently, I'm far too inoffensive for my own good:
This is actually a bit of a worry considering my supposed topic. Taxonomy, especially of invertebrates, is supposed to be all about filth. I once attempted to send my taxonomic paper on Pantopsalis to a colleague, only to hear from him later that he had had a fair degree of difficulty extracting it from his work e-mail - an overzealous filtering programme had noticed its repeated use of the word penis and marked it as spam.
This week I've got something a little more recognisable to go on, at least in general - butterflies! I've referred to butterflies in the past as "honorary vertebrates", as they seem to be about the only group of invertebrates that receive as much attention and recognition as vertebrate groups seem to. What those of us in the know can tell you, though, is that really butterflies are just a flashy kind of moth. Specifically, today I'll be looking at butterflies of the genus Delias.
Delias, commonly known for no particular reason as 'jezebels', are found from southern and south-east Asia to the northern tip of Australia (the image above is of Delias aglaia and is from Answers.com). The bright colouration in the photo above is usually restricted to the underside of the wings, while the upper side is far plainer - most often white with black edging, as shown in the illustration below (from Wikipedia) of Delias aganippe (a notable exception in Australia is Delias aruna, which has the upper surface of the wings bright orange-yellow). Nevertheless, they appear to be among the more colourful members of the generally modest family Pieridae, which may be best known to many of you by the cabbage whites of the genus Pieris.
There are a large number of species of Delias (I couldn't be bothered actually counting them up) placed in 23 species groups. If you want to know exactly what they all are, I'd recommend looking at Les Day's exceedingly thorough site dedicated to Delias at http://www.delias-butterflies.co.uk/. The caterpillars feed on mistletoes (hence the title of this post), which makes them notable from a conservation point of view - many mistletoes are rare and/or endangered (their thick, fleshy leaves make them very attractive to browsers), and if a species of mistletoe goes extinct then its specialist herbivores go extinct as well. While most members of Pieridae lay eggs singly, Delias lay their eggs in large clusters. The caterpillars come in a range of colours, and have long white hairs - the photo here from Wikipedia of Delia eucharis caterpillars shows both the hairs and their gregarious habits. The chrysalis is brightly-coloured, usually bright yellow or orange.
Many species of Delias have seasonal varieties, with the dry-season or winter variety being darker above, or having the underside more cryptically coloured. Studies in other Pieridae have shown that rather than being genetically determined, these variations appear to be determined by the photoperiod the larva is exposed to during development, specifically during the third and fourth instars (Hoffmann, 1973). Experimental manipulation of photoperiod exposure has even been able to induce 'seasonal variation' in species that are univoltine (only one generation per year) instead of multivoltine (multiple generations per year - Shapiro, 1977).
Braby, M. F. 2004. The Complete Field Guide to Butterflies of Australia. CSIRO Publishing: Collingwood (Australia).
Hoffmann, R. J. 1973. Environmental control of seasonal variation in the butterfly Colias eurytheme. I. Adaptive aspects of a photoperiodic response. Evolution 27 (3): 387-397.
Shapiro, A. M. 1977. Evidence for obligate monophenism in Reliquia santamarta, a Neotropical-alpine pierine butterfly (Lepidoptera: Pieridae). Psyche 84: 183-190.
A paper appeared very recently in PLoS One by Philippe et al. on the phylogenetic position of Acoela in the animal evolutionary tree (freely available at the link). I promised last week that I'd comment on the paper when I'd read it, and that would be now.
Acoela are a smallish group (only a few hundred described species) of marine "worms", superficially similar to flatworms (Platyhelminthes) in appearance. Acoela lack a proper gut - the mouth in the diagram above (from Answers.com) opens into a pharynx leading to a shapeless mass of digestive cells, where individual cells take up food particles and digest them via phagocytosis. In the past they were included in Platyhelminthes, and still appear as such in textbooks (which are always out of date), the entire complex being regarded as the basalmost of all bilaterian phyla. However, while Acoela resemble flatworms in features such as absence of a coelom or through-gut (the features previously regarded as primitive for bilaterians), recent molecular investigations have positioned them well away from the Platyhelminthes. Meanwhile, the Platyhelminthes cluster within the Protostomia, implying that their supposed primitive features instead represent derivations from more complex organisms. I am not aware of any attempts so far to explain exactly why and how Platyhelminthes came to dispense seemingly integral features like an anus, but they seem to have done exactly that.
The Acoela are a different story, though. Molecular analyses have placed Acoela at the very base of the bilaterian tree, below the divergence of deuterostomes and protostomes. With this topography, the primitive features of Acoela might indeed be primitive, making this a potentially significant taxon in understanding how the triploblastic Bilateria arose from their diploblastic, radial ancestors (probably similar to modern Cnidaria). Another small marine worm group*, the Nemertodermatida, shares a number of features with Acoela and the two are usually united as Acoelomorpha. However, the monophyly of Acoelomorpha is not certain (Ruiz-Trillo et al., 2002), and could do with further investigation. The paper I'm dealing with today only deals directly with Acoela.
*If you're wondering if "small marine worm group" means "small group of marine worms" or "group of small marine worms", both are equally applicable.
The supposed position of Acoelomorpha as basalmost bilaterians is particularly interesting because Acoelomorpha are not unlike in appearance to a cnidarian planula larva (shown above in a photo from the site of Prof. Fiorenza Accordi). As such, they give credence to the idea that Bilateria may have originated from the development of sexual maturity in a planuloid larva (Baguñà & Riutort, 2004) [As an aside, the Wikipedia page for "planula" previously stated that planulae are incapable of feeding - this only appears to be true for Medusozoa, as Anthozoa have feeding planulae.]
Enter Philippe et al., who used a positively huge molecular data set derived from 68 protein-coding genes to test the position of Acoela, using the exemplar Convoluta pulchra. Because Acoela show a rapid evolutionary speed, making long-branch attraction potentially a significant problem, Philippe et al. used an analytical model that is supposedly more resistant to long-branch attraction. Their final tree agreed with previous studies that Acoela were not related to Platyhelminthes, but disagreed about their position outside the deuterostome + protostome clade. Instead, Convoluta appeared as the sister group to Deuterostomia, a previously unsuspected position.
Under an alternative model, Convoluta appeared in its more traditional position with Platyhelminthes, but this is almost certainly due to long-branch attraction. Platyhelminthes had the longest branches in the analysis (other than Convoluta itself), and their removal caused Convoluta to madly leap away and latch itself onto the chordate Oikopleura, possessor of the next-longest branch. Also, Philippe et al. were able to identify the presence in Convoluta of the gene for guanidinoacetate N-methyltransferase. This gene is found in all animals other than protostomes, but has been lost in the latter. Strangely enough, when the analysis was run without including non-bilaterians, Convoluta became the sister in Deuterostomia of the worm-like Xenoturbella, which is the sister of the Ambulacraria (hemichordates and echinoderms). Xenoturbella resembles Acoela in lacking a through-gut and in the ultrastructure of the epidermal ciliary rootlets, so this is not outside the bounds of possibility.
Nevertheless, I remain somewhat skeptical of the overall result. Bootstrap support for the result was low (only 34%), but a position for Convoluta outside the deuterostome + protostome clade (Eubilateria) got even less support (only 7%). In fact, bootstrap support was pretty low overall within the Deuterostomia (in contrast, Protostomia [including Chaetognatha as basalmost branch] had a bootstrap result of 99%). I feel compelled to chant the usual mantra of all studies - "we need more data!" (though the authors of a study that did, after all, compare 11,959 positions might reply with "for the love of - how much bloody data do you want?"). One particular comment in the paper got my attention:
In more conceptual terms, the position of acoels out of the Platyhelminthes should warn us against the naive view that considers some features as ‘lost’, ‘absent’, or ‘reduced’ in clades (e.g. acoels) than might never have had them in the first place.
However, if Acoela are indeed related to Deuterostomia, then I would feel that they are almost certainly secondarily reduced. It is much easier to regard characters shared between eubilaterians, such as fully formed brain ganglia and the through-gut (Baguñà & Riutort, 2004), as lost in Acoelomorpha rather than gained independently in deuterostomes and protostomes (or even protostomes, chordates and ambulacrarians, if Acoelomorpha are together with Xenoturbella[which also lost these features] the sister to Ambulacraria).
I was going to mention Hox genes here too. Baguñà & Riutort (2004) had a nice tree to support the basal position of acoelomorphs that showed Acoelomorpha with only one central Hox gene and one posterior Hox gene, while the basal number for Eubilateria was four central and at least two posterior Hox genes. However, I then noticed that the author list for the current paper included the same two authors, Baguñà and Riutort. Now, the ideal rule in science is to "believe the data, not the reporter", but I couldn't help wondering if they now knew something that I didn't (quite possible, after all). A quick search doesn't reveal much change for Acoela - Nemertodermatida do have two central Hox genes, but as the second cannot be readily attached to any of the missing eubilaterian genes, it may represent an independent duplication (Jiménez-Guri et al., 2006). However, an apparently not-yet-published study available online by Fritzsch et al. found that Xenoturbella also had only two central Hox genes (though sequence analysis of said genes united them with other deuterostomes rather than acoelomorphs). As a result, if acoelomorphs are closer to deuterostomes than protostomes, I feel that the position in the restricted analysis as sister to Xenoturbella seems more likely than the position as sister to all Deuterostomia in the total analysis. At least that only requires one loss of the characters mentioned.
Brian Switek at Laelaps has written an absolutely fantastic post on the history of scientific and philosophical theories on the origin of humans. If you haven't seen it yet, you go, read now! I found Alfred Romer's quoted comment on the diversity of named hominin 'genera' very informative, correlating as it did with my suspicions that the names were used almost as a shorhand to refer to individual specimens rather than representing actual biological taxa.
This is the first of the commentaries I promised yesterday. While the rest of the world seems to have become bizarrely fixated on some fossil find from some minor mammalian clade, yesterday's Nature also included two far more interesting papers on the distribution of herbivorous insects in tropical rainforests.
"Short-range endemics" is a bit of a buzzword here in Australia at the moment, referring to the pattern in a number of taxa, especially invertebrates, of large numbers of closely-related species of exceedingly restricted distributions (a study one of my supervisors recently conducted of subterranean arachnids called schizomids found that almost each individual mesa that housed schizomids housed its own individual species). The current papers could be very interesting in light of short-range endemism. They are also very interesting in light of the overall question of why the tropics are so hyperdiverse compared to higher latitudes.
As I said yesterday, the two papers differed somewhat in their conclusions (but more on that later). First off, the paper by Novotny et al. looked at diversity within a 75,000 square kilometre area of lowland rainforest in Papua New Guinea. While the area of rainforest was continuous, the Sepik River does cut through it, and some of the plant species compared had quite restricted distributions. Novotny et al. looked at Lepidoptera (caterpillars), ambrosia beetles (Scolytinae and Platypodinae) and Tephritidae (fruitflies) and compared the species found on each host plant genus investigated between eight sites. They found that there did not appear to be a significant change in species composition from one area to another - the species that were found on Ficus at one site were pretty much the same as those found on Ficus at another, over the entire area investigated. The Sepik River did not appear to be a major barrier to dispersal.
At the same time, Dyer et al. looked at average host specificity of herbivorous caterpillars at different latitudes in the Americas. They found that tropical species tend to have much higher host specificity than temperate species. This is in direct contrast to a paper Novotny et al. published last year, that found no significant difference in host specificity between taxa in Papua New Guinea and Europe. Instead, Novotny et al. attributed the increase in insect diversity in the tropics to the shear increase in number of potential host plant species.
So on the one hand we have a paper that seems to argue for wide distributions of tropical taxa, on the other we have one that argues for high host specificity (and hence, one suspects by implication, more restricted distributions). After reading through the papers, I don't think the conflict is actually that strong, as I'll explain in a moment.
Dyer et al. do offer some suggestions for why their results were different to Novotny et al.'s last year. One is that there may be actual difference between the Old World and the Americas. I just can't see that being significant - while there are some differences in which families are dominant in each hemisphere, there are many families that are present in both, and the latitudinal influences are still similar in each - far north it's still colder. The other factor that I think is far more likely to be significant is that Dyer et al. looked at a far greater range of host species than Novotny et al - the latter looked at 18 species in each area, while Dyer et al. looked at up to a maximum of 281 species in Costa Rica. Most significant of all, though, is that Dyer et al. looked at only one potential host species per genus per area. This would tend to bias their results towards higher measurements of host specificity, but is arguably more informative. If you compare a tropical species that feeds on three species of Ficus to a temperate species that is recorded feeding on one species each of Euphorbia, Quercus and Fagus, the temperate species should obviously be regarded as far less host-specific in light of the far greater phylogenetic distance separating its hosts. Unfortunately, a solely numerical metric will not distinguish the two.
Which brings us back to my point that the two Nature papers are not as contradictory as they first appear. The paper from Papua New Guinea compared species from different areas of the same genus. It looked at a different level of resolution than the Dyer et al. paper. As for the implications of the Novotny et al. paper for short-range endemism, the obvious point seems to be that most short-range endemics appear in taxa such as arachnids, myriapods and troglobites - taxa with relatively low dispersal capabilities. In contrast, Novotny et al. looked at insects - winged, and therefore one would expect able to disperse over greater distances more easily, so long as a suitable host plant was present when it got there. In support of this, spiders that disperse by a 'ballooning' stage when young (such as Nephila, the golden orb weaver) tend to be far less diverse with individual species found over much greater areas.
Of course, the number of potential host species in the tropics is still doubtlessly a factor. But this just begs a further question. If insects are so much more diverse because the plants are so much more diverse - then why are the plants more diverse?
PS. I really feel that I should mention that the study by Novotny et al. had a large proportion of the fieldwork conducted by locally trained staff, a number of whom are in the author list below. With the low levels of scientific education available in third world countries, the organisers of this study are to be commended on this front.
Dyer, L. A., M. S. Singer, J. T. Lill, J. O. Stireman, G. L. Gentry, R. J. Marquis, R. E. Ricklefs, H. F. Greeney, D. L. Wagner, H. C. Morais, I. R. Diniz, T. A. Kursar & P. D. Coley. 2007. Host specificity of Lepidoptera in tropical and temperate forests. Nature 448: 696-699.
Novotny, V., P. Drozd, S. E. Miller, M. Kulfan, M. Janda, Y. Basset & G. D. Weiblen. 2006. Why are there so many species of herbivorous insects in tropical rainforests? Science 313: 1115-1118.
Novotny, V., S. E. Miller, J. Hulcr, R. A. I. Drew, Y. Basset, M. Janda, G. P. Setliff, K. Darrow, A. J. A. Stewart, J. Auga, B. Isua, K. Molem, M. Manumbor, E. Tamtiai, M. Mogia & G. D. Weiblen. 2007. Low beta diversity of herbivorous insects in tropical forests. Nature 448: 692-695.
I opened up my e-mail and Reader this morning, and found so many interesting things, from the fascinating to the tragic, I barely know which to comment on. I'll just introduce a few below:
First, the tragic: The Yangtze River Dolphin is Officially No More. Lipotes vexillifer thus becomes the first known cetacean to become extinct due to human activity, and probably only the third taxon recognised at family level to do so (after Thylacinus and Raphidae). This is truly a sad moment in ecological history.
Second, Kevin Z has written a good piece on the negative effect measurement of "impact factors" has had on biology, and on taxonomy in particular. I ranted on a similar subject yesterday, but Kevin's done a better job than I did. His comments on unreliable species IDs dovetail nicely with what I was saying.
A couple of things are going to require a bit more commentary on my part, but I'll mention them now in case I don't get back to them. First, a paper in PLoS One on the phylogenetic position of Acoelomorpha by Philippe et al. Acoelomorpha are gutless flatworm-like animals that were once including in Platyhelminthes (the flatworms proper), but in recent years have been regarded as the sister group to all other bilaterian animals. Philippe et al. agree with the removal of acoelomorphs from Platyhelminthes, but also disagree with their basalmost position in Bilateria, instead finding them close to Deuterostomia. I'm feeling a bit sceptical, but I'll have to actually read the paper in detail and get back to you.
There's also a couple of very interesting-looking papers in today's Nature on the distribution of biodiversity in tropical rainforests. The "News and Views" article by N. E. Stork for them is here, if you can access it. My apologies to those of you who can't - I haven't seen a publicly available release. I'll quote the two paragraphs from the N&V summarising the results:
With the help of a team of locally trained parataxonomists, Novotny et al. have compiled such a database of records for three groups of rainforest insects: those that feed on foliage, wood and fruit. They show that there is a low rate of change in species composition, or 'β diversity', across 75,000 km2 (an area equivalent to that of South Carolina or Ireland) of continuous lowland rainforest in Papua New Guinea. This contrasts with the previous evidence, as discussed by Novotny et al., of high β diversity for insects in the forest canopy and with changes in β diversity with latitude, altitude and climatic gradients.
In the second new paper discussed here, Dyer et al. describe how they carried out an equivalent analysis in the New World and have come to a different conclusion. Their approach required examination of hundreds of thousands of host-specificity feeding records for butterfly and moth caterpillars, from as far back as 1936 and from areas ranging from Canada to Brazil. In contrast to Novotny and colleagues, they find that, on average, the number of tree species on which an insect species feeds is fewer in the tropics than in temperate parts of the New World. They suggest that higher specialization in the tropics might be because of more intense interactions between an insect and its food source, as might be caused by more distinct secondary chemicals in tropical plants than in temperate plants.
Again, I'll get back to you once I've read the papers fully.
"An expedition led by the Wildlife Conservation Society (WCS) to a remote corner of the eastern Democratic Republic of Congo has uncovered unique forests which, so far, have been found to contain six animal species new to science: a bat, a rodent, two shrews, and two frogs." (My thanks to Coturnix for the link.)
There a mention of plants later, but nothing about any invertebrates from the area - I do hope they collected some. And even more to the point, that they have someone to look at them.
I've briefly mentioned before the vast number of species that probably remain to be described on this planet - to be honest, I would be surprised if an expedition to such an area in central Africa didn't recover new species. However, the limiting factor in describing new species isn't the availability of specimens - it's the availability of researchers to describe them. Every museum I've ever been to or heard about has vast collections of unidentified and unsorted specimens. And I'm not talking about places like Africa or South-East Asia that have a relative shortage of researchers - I'm talking Australia or New Zealand or even North America. You wouldn't need to travel far here in Australia to find an undescribed species. Heck, if you want to go down the bacterial level (and goddammit, I do) you probably have an undescribed species of bacterium - maybe even a whole host of undescribed species - inhabiting some crevice of your person at this very moment. Environmental DNA samples suggest that less than 1% of bacterial taxa have so far been described.
But there just aren't enough people working on taxonomy. I work on Opiliones - worldwide there are about six and a half thousand described species of Opiliones, with many more yet to come judging by the rate at which new species appear. All the same, all the Opiliones researchers in the world together could probably fit into a not overly large living room, and it wouldn't even feel that crowded.
And before anyone asks, "Why does it matter?" - if you don't have a solid taxonomic framework to work from, you can't really do anything else. Any sort of comparative work in ecology, physiology, even genetics, is ultimately dependent on accurate identification of the organisms concerned, and on a proper indication whether the subjects chosen are representative of what the researcher is trying to investigate. A study has been done on the visual abilities of cave-dwelling harvestmen in New Zealand (Meyer-Rochow & Liddle, 1987), using one representative each of the long-legged and short-legged suborders. Unfortunately, as regards the long-legged harvestmen, I have seen specimens from the cave system investigated, and there are potentially two species involved, neither of which is the species identified in the paper. This in no way reflects on the paper's authors, but entirely on the inadequacy of the taxonomic framework they were using. Despite the potential interest of the results found in this paper, the uncertainty about which species was investigated poses a severe impediment to their usefulness.
Anyway, I should leave off my rant for now - best I stop moaning about people not describing enough species, and go and actually describe some species.
Meyer-Rochow, V. B., & A. Liddle. 1987. Structure and function of the eyes of two species of opilionids (Megalopsalis tumida: Palpatores, and Hendea myersi cavernicola: Laniatores) from New Zealand glowworm caves. Proceedings of the Royal Society of London B 233: 293-319.
Yesterday, after much grousing, I introduced you to last week's belated taxon of the week. So as not to get caught out like that again, today I'm introducing this week's title-holder - the amphipod genus Crangonyx. The image at left comes from here.
Crangonyx is the type genus of the family Crangonyctidae. Crangonyctidae is an entirely Holarctic family (Holsinger, 1986) that shows an interesting tendency towards a subterranean lifecycle. When Holsinger reviewed the family in 1986, 126 of the 154 known species where stygobionts (exclusively cave-dwelling). More species have been described since then, but the proportion of stygobionts is probably still roughly the same. Even those species that are not stygobionts are cold-stenothermal (only able to tolerate cold water temperatures) and photonegative (keep away from light). Many are stygophilic (not exclusively cave-dwelling, but often found in cave habitats). Crangonyx, with 47 species, is one of only three genera of crangonyctids with epigean (surface-dwelling) species (the others are Synurella and the monotypic Lyurella hyrcana, which may be a species of Synurella - Holsinger 1986). Epigean species of Crangonyx have small but distinct eyes and pigmentation between light brown and pale green in colour, while stygobiont species may retain vestigial eyes and traces of pigmentation (in contrast to exclusively stygobiont crangonyctid genera, which are invariably eyeless and colourless). The greater number of known Crangonyx species come from North America, with only a few from Eurasia - however, I would be inclined to suspect (admittedly without real evidence) that a certain degree of researcher bias may be at fault here.* Perhaps the most widely distributed species is Crangonyx pseudogracilis, shown above in a photo from Bioimages (copyright Malcolm Storey, 2004). Originally native to North America, this species has been introduced to Europe.
*The vast majority of species of organisms on this planet remain undescribed - there are probably about 1.5 million described species, and while it is difficult to estimate how many species remain to be discovered, estimates of 20 million are not impossible (Harrison et al. have a brief review here). As a result, there are many higher taxa for which the current known species distribution does not accurately reflect reality. Major factors influencing this discrepancy will be collection and study bias - for instance, if most of the researchers on a given taxon have been North American, then there might be expected to be an inflated view of North American diversity as opposed to elsewhere. In the case of Crangonyx, 24 of the known species were erected in a recent review of the North American fauna (Zhang & Holsinger, 2003), and one can't help wondering what would be uncovered if, say, the Siberian fauna received the same treatment.
One species of Crangonyx, C. islandicusSvavarsson & Kristjánsson, 2006 was recently described as part of the Icelandic subterranean fauna. This has interesting implications for the biogeographical history of Crangonyctidae - due to the family's exclusively freshwater, stringent habitat requirements, it is believed to have originated prior to the tectonic separation of Europe and North America and spread by vicariance as the continents divided. The Crangonyctidae are certainly very old - Palaeogammarus from Baltic amber is essentially indistinguishable from modern crangonyctids. Iceland is far younger than the opening of the Atlantic. However, Kristjánsson & Svavarsson (2007) indicate that Crangonyx, as well as the ancestors of the endemic Icelandic Crymostygiidae (Kevin Z has already commented on this here) may have spread along the Greenland-Iceland ridge as the geological hotspot that is responsible for the formation of Iceland drifted east, if at least a part of the ridge was above sea-level and held groundwater during that time (note that it does not have to have always been the same part - if extra land was raised above sea-level in the east at about the same rate as land eroded below sea-level in the west, that'd do).
Holsinger, J. R. 1986. Holarctic crangonyctid amphipods. In Stygofauna Mundi: A faunistic, distributional, and ecological synthesis of the world fauna inhabiting subterranean waters (including the marine interstitial) (L. Botosaneanu, ed.) pp. 535-549. E. J. Brill/Dr. W. Backhuys: Leiden.
It has been a very upsetting morning. I have been planning for some months now to attend the International Conference of Arachnology in Brazil, booked my plane tickets ages ago, had my vaccinations... and discovered this morning that I have spent the last two months wandering about with the wrong leaving date in my head, and that I was due to leave a week earlier than I thought. So I have missed my plane flight, missed the conference.... There is one word for this. It begins with f and rhymes with 'duck'.
Anyway, if there's any readers who have been paying attention to what goes on here, you may have noticed that there was no 'Taxon of the Week' last week. That was due to things being rather hectic as I tried to organise things for the conference that isn't going to happen in my existence. So I'm introducing last week's taxon today, and the subject of today is the Arthrotardigrada.
Tardigrades are microscopic invertebrates commonly referred to as 'water bears'. I have elsewhere referred to tardigrades as possibly the cutest of all invertebrates, and I see no reason to retract that statement. Some forms put me in mind of nothing so much as little eight-legged versions of Winnie-the-Pooh, bumbling their way through an aquatic Hundred-Acre Wood. The image above shows Renaudarctus psammocryptus Kristensen & Higgins, 1984, and is from the original description. Tardigrades are divided into two main classes, the Heterotardigrada and Eutardigrada - the Heterotardigrada possess cephalic appendages which are lacking in the Eutardigrada, and lack the Malpighian tubules present in eutardigrades (Nelson, 2002). Heterotardigrades have a separate gonopore and anus, while eutardigrades have a single cloacal opening. Most Heterotardigrada also have ventral and dorsal segmental plates, though these have been lost in some families. A third class, Mesotardigrada, has been named for a single species which has unfortunately not been found since its original discovery. In a comment that just makes the reader beg for the back-story, Jørgensen & Kristensen (2004) note that "Mesotardigrades were described from a hot spring in Nagasaki, Japan... however the monotypic class has never been recovered since its original description, and the type locality disappeared just after the Second World War". 'Holotype lost' is an unfortunately not uncommon complaint in taxonomy, but 'type locality disappeared' is definitely unusual.
Arthrotardigrada is one of the two orders of Heterotardigrada (the other is the Echiniscoidea). They possess a median cephalic cirrus, three pairs of lateral cephalic cirri and two or three pairs of clavae (club-shaped appendages on the head) (Kristensen & Higgins, 1984). The suggestion has been made that Heterotardigrada may be paraphyletic with regards to Eutardigrada, and Arthrotardigrada may be paraphyletic within Heterotardigrada (making it the basal assemblage of all Tardigrada), but this remains uncertain. The molecular analysis of Jørgensen & Kristensen (2004) found weak support for a monophyletic Heterotardigrada, but this was not significantly statistically superior to a paraphyletic Heterotardigrada and the authors were only able to test a small number of species. Morphological analysis by Nichols et al. (2006) found a monophyletic Heterotardigrada, but not Arthrotardigrada. Almost all Arthrotardigrada are marine - only a single species is known from freshwater, Styraconyx hallsi (Nelson, 2002).
Perhaps one of the most remarkable features of tardigrades is the ability to enter cryptobiosis, which is a dormant state in which they can sometimes survive long periods of unfavourable conditions. Dormant tardigrades often form a shrivelled cyst called a "tun". All sorts of hyperbolic claims can be found on the internet for the survival abilities of a tardigrade tun, but I haven't yet been able to find a proper source for any of these claims, so they should probably be taken with the contents of a small Siberian salt mine (to steal a phrase from Alan Kazlev).
Jørgensen, A., & R. M. Kristensen. 2004. Molecular phylogeny of Tardigrada—investigation of the monophyly of Heterotardigrada. Molecular Phylogenetics and Evolution 32 (2): 666-670.
Kristensen, R. M., & R. P. Higgins. 1984. A new family of Arthrotardigrada (Tardigrada: Heterotardigrada) from the Atlantic coast of Florida, U.S.A. Transactions of the American Microscopical Society 103 (3): 295-311.
Nelson, D. R. 2002. Current status of the Tardigrada: evolution and ecology. Integrative and Comparative Biology 42 (3): 652-659.
Nichols, P. B., D. R. Nelson & J. R. Garey. 2006. A family level analysis of tardigrade phylogeny. Hydrobiologia 558 (1): 53-60.
...and nothing much seems to be working as it should. Here are a couple of photos to while away the time that were taken last year up at Lorna Glen, a station-turned-into-a-reserve in central Western Australia. The creature above is an absolutely massive mantis that we came across - I can't give you a more specific ID, I'm afraid. Hopefully the hand gives you some idea of the scale of the thing - it was at least four inches in length, possibly longer. And if you look really closely, you may be able to make some of the relatively minute ants that were making its life difficult when we found it - they were busily attacking the sensitive joints between leg segments.
Moloch horridus, the spiny devil or moloch, is arguably the strangest-looking of all reptiles, and I can assure you that they look even stranger in the flesh. They have an odd jerky way of moving, the closest thing to it in appearance being old stop-motion model animation. And they are perhaps the most docile animals in all existence - an attempt to pick one up will spark an instant outburst of absolutely nothing. The picture below, I think, gives an idea of how energetic and fractious molochs aren't.
Perhaps the most abundantly obvious group of animals in the area were grasshoppers. One of the common species was a spotted, brachypterous form (Greyacris picta or something similar*) that I thought was a nymph until one day we found this mating pair (the little guy on top is the male).
*I originally IDed them on this post as Monistria pustulifera. A comparison of the excellent photos in Rentz et al. (2003) (a generally excellent book) set me right.
Update: A reader has suggested that the mantis may be a species of Archimantis. He also confirmed my ID of the grasshopper as probably Greyacris, though not necessarily G. picta itself.
Rentz, D. C. F., R. C. Lewis, Y. N. Su & M. S. Upton. 2003. A Guide to Australian Grasshoppers and Locusts. Natural History Publications (Borneo): Kota Kinabalu.
I've covered Salinella and Buddenbrockia, now I'll move onto another of the 'living problematica', though today's subject is arguably not as problematic. Let me introduce you (assuming you've not already met) to the giant planktonic larva Planctosphaera pelagica Spengel, 1932.
'Giant' is, of course, a relative term. The roughly spherical Planctosphaera reaches about 10mm in diameter (van der Horst, 1936) or even 25mm (Williamson, 2001). A diagram of the internal anatomy above comes from van der Horst (1936). Compared to its generally believed closest relative, the tornaria larva of Enteropneusta (acorn worms), this is huge - tornariae may be about a millimetre in size (Bourne, 1889). The intriguing point about Planctosphaera is that the adult form has never been identified. The similarity between Planctosphaera and tornariae means that it is almost universally accepted as a member of the Hemichordata (see the comparison to the left between the two, again from van der Horst), but it is different enough that the adult may not be a typical acorn worm (not to mention the size...) Williamson (2001) provides one exception - he maintains that Planctosphaera as currently known is the adult form. However, this interpretation is connected with Williamson's unusual theory of 'larval transfer', which is not widely accepted*, and jibes with the fact that known Planctosphaera do not have any sort of gonads or other reproductive structure (van der Horst, 1936).
*Williamson maintains that distinct adult and larval forms in various animals result from hybridisation between animals with distinct bauplans, with one stage in the life cycle resembling one parent and one resembling the other. For instance, caterpillars and other insect larvae would be derived from a hybridisation between a direct-developing winged insect and an onychophoran-like animal. First, try to imagine a butterfly mating with an onychophoran. Then, try to stop imagining a butterfly mating with an onychophoran.
One imaginative interpretation of Planctosphaera that does have a lot going for it is the idea that Planctosphaera is a normal tornaria larva that has become hypertrophied by a long planktonic period. The reference for this idea is Hart (1994) - unfortunately, I haven't been able to obtain the paper in question and I am unclear whether this is meant to be an adaptive change, or whether Planctosphaera represents a pathological form of a normal tornaria that has failed to develop in the normal way. Such pathologies are not unknown - Temereva et al. (2006) describe a giant phoronid larva (which even possesses rudimentary gonads!) that they interpret as such, and note the existence of giant larvae of ceriantharians, sipunculids and even fish.
Of course, the adult form of Planctosphara could still be out there somewhere, lurking in the ooze at the bottom of the oceanic abyss. As the relatively recent description of Torquarator Holland et al., 2005 demonstrated, we may have only scratched the surface of enteropneust diversity.
Bourne, G. C. 1889. On a tornaria found in British seas. Journal of the Marine Biological Association 2 (1): 63-68, pl. 7, 8.
Hart, M. W., R. L. Miller, & L. P. Madin. Form and feeding mechanism of a living Planctosphaera pelagica (phylum Hemichordata). Marine Biology 120: 521-533.
Holland, N. D., D. A. Clague, D. P. Gordon, A. Gebruk, D. L. Pawson & M. Vecchione. 2005. 'Lophenteropneust' hypothesis refuted by collection and photos of new deep-sea hemichordates. Nature 434:374-376.
Horst, C. J. van der. 1936. Planctosphaera and Tornaria. Quarterly Journal of Microscopical Science: 605-613.
Temereva, E. N., V. V. Malakhov & A. N. Chernyshev. 2006. Giant actinotroch, a larva of Phoronida from the South China Sea: the giant larva phenomenon. Doklady Akademii Nauk 410 (5): 712-715 (transl. Doklady Biological Sciences 410: 410-413).
Williamson, D. I. 2006. Hybridization in the evolution of animal form and life-cycle. Zoological Journal of the Linnean Society 148: 585-602.