A Western Rockweed

Rockweed Silvetia compressa, from here.

Silvetia compressa is a species of brown alga found on shorelines on the western coast of North America, from British Columbia to Baja California. It is a member of the wrack family Fucaceae that I covered in an earlier post. Silvetia compressa is found in midtidal habitats, generally higher up on the shoreline than other large seaweeds. Individual thalli can reach a maximum length of about three feet (90 cm) but are often smaller. This is a slow-growing species, so patches of Silvetia are slow to recover from damage due to trampling and other disturbance. Please try to avoid walking on the rockweed!

Thalli of Silvetia compressa are composed of thin strands a few millinetres in width with irregular, dichotomous branching. Strands of the thalli lack a midrib (distinguishing them from some other Fucaceae species found in the same area). The width of the strands and regularity of the branching varies with environmental conditions: for instance, individuals growing in locations with stronger wave action have more robust strands that branch more frequently. As with other Fucaceae, the reproductive structures a produced on swollen branch tips called receptacles, but these receptacles do not become inflated with gases and buoyant like those of other species. The exact size and shape of the receptacles is, again, variable.

In many older references, Silvetia compressa may be referred to as Pelvetia fastigiata. The supposed species 'Fucodium compressum' and 'F. fastigiatum' were originally distinguished on the basis that the latter was smaller than the former with more fastigiate branches (that is, the branches remained subparallel). As indicated above, these characters represent the effects of environmental conditions, not fixed differences (Silva 1996). They were eventually included in the genus Pelvetia, together with the Atlantic species P. canaliculata, on the basis of the thalli without a midrib, and the production of just two eggs from each oogonium in the receptacles. However, later analyses supported the separation of the Atlantic and Pacific species of Pelvetia. Not only did they not form a clade in molecular analyses, the eggs in the oogonia were separated by a horizontal division in the Atlantic species but a longitudinal or oblique division in the Pacific species (Silva et al. 2004). As such, the Pacific Pelvetia were transferred into a new genus Silvetia.

Further taxonomic complications involved subspecific variation in Silvetia compressa. A distinctive form of 'Pelvetia fastigiata' found at Pebble Beach in California's Monterey Bay, with smaller, finer thalli and more abundant, regular branching, was labelled as a separate forma gracilis. Similar individuals were also found on the islands off California's coast. However, when Silva (1996) examined the original type specimen of P. fastigiata, he discovered that it was an individual of this 'gracilis' form, not the more typical larger form. Later, Silva et al. (2004) examined genetic variation within the Silvetia compressa of California and Baja California. They found that the individuals of the offshore islands were indeed genetically distinct from continental individuals. As well as the differences in growth habit, there was also some difference in receptacle shape: the continental form had receptacles that tended to be linear and pointed whereas those of the island form were ellipsoidal and blunt. However, the island form still could not be labelled with either of the 'fastigiata' or 'gracilis' monikers, as individuals from the type locality of Pebble Beach did not align genetically with insular individuals but with other continental forms. As such, yet another name had to be coined for the insular form which now goes by the name of Silvetia compressa ssp. deliquescens. Let's see if it sticks this time.

REFERENCES

Silva, P. C. 1996. California seaweeds collected by the Malaspina expedition, especially Pelvetia (Fucales, Phaeophyceae). Madroño 43 (3): 345–354.

Silva, P. C., F. F. Pedroche, M. E. Chacana, R. Aguilar-Rosa, L. E. Aguilar-Rosa & J. Raum. 2004. Geographic correlation of morphological and molecular variation in Silvetia compressa (Fucaceae, Fucales, Phaeophyceae). Phycologia 43 (2): 204–214.

The Millenium Post

Apparently, this is the 1000^th post to appear at Catalogue of Organisms. When I first started this site, over ten years ago, I don't know if I had any idea when, if ever, I would reach this point and where I would be when it happened. I probably imagined I would be thinner.

I want to thank everyone that has followed Catalogue of Organisms over the years. I particularly want to thank those readers who have supported me on Patreon: Paul Selden, Sebastian Marquez, Rob Partington, William Holz. Your contributions have meant a lot to me. Apropos of that, some news: some of my readers may recall that my employment status has been a little up in the air for a large chunk of the last couple of years (though I was able to find casual positions for some of that time). A few months ago, though, an opportunity came up to work on a project looking at insect diversity in mangroves in Hong Kong. Though it means being away from my home, my partner and my dog for a couple of years, the opportunity was too good to pass up and for the next little while I'll be based in the city of the Fragrant Harbour (especially around the port district in high summer).

So on to the next 1000 posts, then? We'll just have to see. Certainly I'm not putting stuff up here as frequently as I did in the past, when I was a carefree post-graduate student. There have been times when I've wondered if I should keep going. People far more talented and perspicacious than I have had a great deal to say elsewhere about the apparent decline of science blogging as a format, and it certainly doesn't seem to attract the audience it once did. Nevertheless, I think I'll be going for a while yet. I've noted before that this blog functions as my own means and motivation for investigating things that I might find interesting, and there's certainly no shortage of things left to investigate. And as for the health of science blogging overall: a glance to the sidebar to the right of this page reminds me that there's still a lot out there worth following. There's Deep Sea News, there's Small Things Considered, Bug Eric, Tetrapod Zoology, Letters from Gondwana, Synapsida, Beetles in the Bush, and so many more. If you don't already know these sites, check them out!

For my part, the main indicator I see as to whether people are reading anything here is when people leave comments. A big thank you again to those who have contributed over the years. I'm needy, and need validation... And in that light, I'd like to specially ask my readers to comment on their general feelings (if any) about Catalogue of Organisms. Has there been anything you've particularly liked about the site over the years? Any favourite posts? Anything you'd like to see going forward? And once again...

The Shrinking World of Bandicoots

A bandicoot is a very disagreeable animal to clean, therefore it should be done as soon after killing as possible, and then the flesh can be left in strong vinegar and water for a few hours before dressing. Sweet potatoes and onion make a good stuffing for bandicoot, which is good either boiled or baked.--Mrs Lance Rawson, Australian Enquiry Book of Household and General Information.

Golden bandicoots Isoodon auratus barrowensis, copyright Kathie Atkinson.

Back when I used to work on Barrow Island in the north-west of Australia, one of the more noticeable animals to be seen around the place was the golden bandicoot Isoodon auratus. In the evenings, the place seemed to absolutely swarm with them. About the size of a guinea pig, with no tails to speak of (bandicoots are actually born with fairly long tails but tend to lose them in the course of their quite vicious fights with one another; few if any individuals reach maturity with their tails intact), there was no question about their qualifications when it came to cuteness.

Bandicoots are a group of twenty-odd species of marsupial found in Australia and New Guinea (one species, the Seram bandicoot Rhynchomeles prattorum, was described from montane forest on the Indonesian island of Seram to the west of New Guinea). Most are primarily insectivorous, but they also eat varying amounts of small vertebrates and plant matter such as bulbs and fruit. The largest bandicoot, the giant bandicoot Peroryctes broadbenti, has been recorded to reach close to five kilograms in weight. The smallest, the Papuan bandicoot Microperoryctes papuensis, weighs less than 200 grams. I suspect many people in Australia assume that the name 'bandicoot' comes from one of the the Aboriginal languages, but it is in fact Indian (specifically Telugu) in origin. The original bandicoot Bandicota indica is a large rat that is widespread in southern Asia and Australian bandicoots were named for their resemblance to this species. Personally, I have maintained in the past that Australian bandicoots look more like rats than rats do: with their relatively long snouts, bandicoots bear a distinct resemblance to the sort of cartoon figure that comes to most people's minds when they hear the word 'rat'.

New Guinea spiny bandicoot Echymipera kalubu, copyright Michael Pennay.

Bandicoots are highly distinctive from all other marsupials in appearance. Their hind legs are noticeably longer than their forelegs and more or less specialised for cursorial locomotion (especially so in one example that I'll get to shortly). The fourth and fifth toes of the hind foot are much larger than the other three; the first toe in particular is reduced to a non-functional stub. The second and third toes of the hind foot, as in diprotodontian marsupials such as kangaroos and possums, are externally joined together with the two claws at the end forming a comb that is used in grooming more than in locomotion. The fore feet, in contrast, are mostly functionally three-fingered (with the first and fifth fingers reduced) and adapted for digging with the claws large and flat.

Many bandicoots are rapid reproducers with their gestation periods among the shortest of any mammal, less than two weeks between fertilisation and birth. Bandicoots also have the most developed placentas of any marsupial group (yes, most marsupials do have a placenta, albeit a much simpler one than found in placental mammals); it is presumably because of this that, despite their short gestation, bandicoot young are born at a more advanced stage of development than those of some other marsupials. When the young are born, they initially remain attached to their mother via the umbilical cord; this latter does not become detached and the placenta ejected until after the joey is firmly attached to a teat in the rearward-opening pouch. The young remain in the pouch for about two months and grow rapidly; they may reach full sexual maturity at the age of only three months. As a result, bandicoot populations may increase rapidly if conditions permit.

Greater bilby Macrotis lagotis, copyright Bernard Dupont.

In terms of classification, there is a general consensus that Recent bandicoots can be divided between four groups though there has been some disagreement about exactly these groups are interrelated (and hence exactly how they should be ranked). The most diverse, but probably also the least studied, group of modern bandicoots are the rainforest bandicoots of the Peroryctidae or Peroryctinae. These are about a dozen species found mostly in New Guinea with the aforementioned Rhynchomeles prattorum on Seram and the the long-nosed spiny bandicoot Echymipera rufescens extending its range to the northern tip of Queensland. Most of continental Australia is home to the dry-country bandicoots of the Peramelidae sensu stricto or Peramelinae, of which there are six Recent species (one of these, the northern brown bandicoot Isoodon macrourus, is also found in southern New Guinea). Peramelids tend to have shorter snouts and flatter skulls than peroryctids. The other two groups are both very small and also native to arid regions of Australia. Two Recent species are known of the genus Macrotis, the bilbies, though one of these is extinct and the other is endangered. Bilbies are larger than most other bandicoots, with long ears (hence their alternative vernacular name of 'rabbit-bandicoots') and a long, silky-haired tail.

Gerard Krefft's 1857 illustration of the pig-footed bandicoot Chaeropus ecaudatus, from here.

The final representative of the Recent bandicoots is unquestionably the weirdest of them all. Unfortunately, it is also now extinct, last recorded some time about the middle of the 20th Century, a fact that cannot be called anything less than a fucking tragedy. The pig-footed bandicoot Chaeropus ecaudatus was the most cursorial of all bandicoots. Its forelegs, rather than being adapted for digging as in other bandicoots, had only two functional toes on which the claws were modified into hooves. The hind legs went a step further and had only a single functional toe (raising the question of how this animal groomed itself without the aforementioned claw-comb of other bandicoots Edit: That was a bit of a blonde moment; a second look at the Krefft illustration above shows that the comb is definitely there). The most extensive observations of its habits seem to have been made by Gerard Krefft (1866) who kept a pair alive for about six weeks in 1857 on a trip to the Murray-Darling region before killing them to provide specimens because, you know, 19th-Century naturalist. Krefft recorded that his bandicoots subsisted primarily on plant foods such as lettuce, grass and roots, refusing all meat offered to them (Krefft also refers to providing grasshoppers for them but his account is unclear about whether they were ever eaten). A herbivorous diet was also indicated by the animals' droppings, which where dry and similar to a sheep's. The bandicoots constructed a covered nest from grass and leaves in the tin enclosure in which Krefft kept them in which they sheltered during the day, only becoming active after nightfall. Krefft notes that he acquired "about eight" specimens of pig-footed bandicoot during his six-month camp, admitting that some met a stickier end than others: "They are very good eating, and I am sorry to confess that my appetite more than once over-ruled my love for science; but 24 hours upon "pig-face" (mesembryanthemum) will dampen the ardour of any naturalist". Krefft also noted that several of the specimens found were female, and that despite being provided with eight teats the females never carried more than two joeys. A particularly interesting detail was that the fourth toe of the joeys' fore foot, rather than being reduced as in the adults, remained large so that the feet resembled those of other bandicoots. Presumably this was so that the fore-claws could still be used to allow the newborn joeys to climb from the birth canal to the pouch.

Krefft also noted that the pig-footed bandicoot was already declining in abundance, blaming its increased rarity on competition with introduced grazing livestock. Sadly, changing habitats and introduced predators have caused other bandicoot species to also become endangered since Krefft's time. Please, don't let them go the way of the pig-footed bandicoot.

REFERENCES

Gordon, G., & a. J. Hulbert. 1989. Peramelidae. In: Fauna of Australia vol. 1B. Mammalia. Australian Biological Resources Study: Canberra.

Krefft, G. 1866. On the vertebrated animals of the lower Murray and Darling, their habits, economy, and geographical distribution. Transactions of the Philosophical Society of New South Wales 1862–1865: 1–33.

A Second Look at Scallops

In a post that appeared at this site over eight years ago, I described some of the distinctive features of the Pectinoidea, the group of bivalves commonly known as scallops. It's time to look in a bit more detail at some of the points mentioned in that post.

Fossil of Pernopecten, the earliest scallop genus, from ammonit.ru.

Pectinoidea, in the sense recognised by Waller (2006), first appear in the fossil record way back in the late Devonian. They were probably derived from earlier members of the Aviculopectinoidea, an extinct group of bivalves that closely resemble scallops in their overall appearance and were included in the Pectinoidea by many earlier authors (such as in the 1969 Treatise on Invertebrate Paleontology volume on bivalves). However, the shell ligament of aviculopectinoids was reinforced by aragonite fibres (a primitive feature for bivalves) rather than having the specialised rubbery core found in pectinoids. As such, aviculopectinoids would have lacked the swimming abilities of true scallops. The Palaeozoic pectinoids belong to a single genus, Pernopecten, that possesses a number of features such as details of the shell crystalline structure that indicate a position outside the pectinoid crown group. In the early Triassic, Pernopecten begat the family Entolioididae that includes the ancestors of living pectinoids.

As mentioned in the previous post, four pectinoid families survive to the present day: the Pectinidae, Propeamussiidae, Entoliidae and Spondylidae. The first three families diverged in the early Triassic. Spondylids (usually classified in a single genus, Spondylus) were not to appear until the mid-Jurassic and Waller (2006) argued for their derivation from within the Pectinidae. The Pectinidae are otherwise distinguished from other pectinoids by a structure called the ctenolium. This is a row of teeth that develops on the shell in the gap between the disc and one of the auricles (the triangular 'wings' at the top of the shell). During the earlier part of the scallop's life, when it lives attached to the ocean bottom by a byssus (what in mussels we call the 'beard'), the ctenolium functions to hold the byssus threads in place and help stop the shell from twisting. In those pectinid species that lack a byssus in the latter part of their life, the ctenolium may end up getting overgrown by the expanding shell and disappearing, but all pectinids (ignoring the aforementioned Spondylus question) have a ctenolium for at least part of their life.

The propeamussiid Cyclopecten secundus, copyright Museum of New Zealand Te Papa Tongarewa.

The Pectinidae is the largest scallop family in the present day, followed by the Propeamussiidae. The Entoliidae were diverse during the Mesozoic but declined dramatically after the end of the Cretaceous (I'm not clear whether or not their decline was a direct part of the end-Cretaceous mass extinction). Indeed, entoliids are completely unknown from the fossil record between the Palaeocene and the late Pleistocene; like the tuatara, it might be that the post-Mesozoic survival of entoliids could have gone completely unrecognised were it not for the single surviving relictual genus.

In the earlier post, I implied that propeamussiids lack the eyes and guard tentacles of other pectinids; it turns out that this was a mistake on my part. Many propeamussiids found in the deep sea do indeed lack these features but they are present in shallow-water propeamussiids. It appears that these features are ancestrally common to all crown-group pectinoids but have been lost as an adaptation to life below the photic zone. The anatomy and lifestyle of many propeamussiids remains poorly known but those species that have been investigated have simplified gills compared to pectinids. The filaments of the gills are free rather than being connected by ciliary junctions. The lips of the mantle are also simplified, lacking the complex lobes found in pectinids. These features may be related to the carnivorous diet of many propeamussiids that feed on zooplankton rather than smaller phytoplankton and organic particles.

REFERENCE

Waller, T. R. 2006. Phylogeny of families in the Pectinoidea (Mollusca: Bivalvia): importance of the fossil record. Zoological Journal of the Linnean Society 148 (3): 313–342.

Trichosternus

Trichosternus vigorsi, copyright Udo Schmidt.

Many of the carabid ground beetles tend to attract a lot of attention from amateur entomologists due to their size and striking appearance, but it must be admitted that they are often not the easiest of animals to work with from a taxonomic perspective. The larger species tend to fall into the category of 'big, black, massive sharp mandibles' and it can require a lot of practice to reliably identify which genus a specimen belongs to, let alone species.

Trichosternus is a genus of ground beetles found in far eastern Australia, from the base of Cape York in Queensland to a bit north of Sydney in New South Wales, in the band of land between the coast and the Great Dividing Range. There is also a single isolated species T. relictus in the southwest corner of Western Australia, and apparently another in New Caledonia (Darlington 1961). However, considering the difficulty that many authors have had in the past in providing an exact definition for Trichosternus relative to other closely related genera, it would be interesting to see if future studies corroborate the inclusion of these outlying species. By way of contrast, a reasonable number of New Zealand species assigned at one time or another to Trichosternus have all long since been moved elsewhere.

Trichosternus species are all flightless and in most the elytra are fused and cannot open (the exception is T. relictus). Most species have a distinctive male genital morphology, with the genital opening deflected to the right and the right paramere (the parameres are two sclerotised 'arms' on either side of the genitalia) modified into a specialised falcate shape, the exact functional significance of which seems to remain unknown. Again, the outlier in this regard is T. relictus in which said paramere retains a primitive styloid shape. Similar falcate parameres are also known from members of related genera such as Megadromus and Nurus; the latter is particularly similar to Trichosternus with the only real difference between the two being that Nurus is more robust with longer mandibles. Trichosternus relictus also has a distinctive female genital morphology, in which the internal passage between the median oviduct (where emerging eggs are fertilised by sperm stored in the spermatheca) and the vagina is remarkably extended and concertina-like. Again, the function of this structure is unknown though Moore (1965) suggested that it might be related to viviparity.

Northern Trichosternus species found in tropical Queensland are all inhabitants of rainforest (hence the restriction of the genus to east of the Great Dividing Range: on the western side of the range, rainforests are absent and the arid zone begins). Southern species are found in upland temperate rainforests or in savannah woodland (Darlington 1961). Some species have very restricted ranges: T. montorum, for instance, is known from two mountains on the Spec Plateau, Mts Bartle Frere and Bellenden Ker.

REFERENCES

Darlington, P. J., Jr. 1961. Australian carabid beetles VII. Trichosternus, especially the tropical species. Psyche 68 (4): 113–130.

Moore, B. P. 1965. Studies on Australian Carabidae (Coleoptera). 4.—The Pterostichinae. Transactions of the Royal Entomological Society of London 117 (1): 1–32.

The Lonely Life of the Cave Collembolan

For a few weeks last year, I had the job of sorting and identifying a collection of Collembola, springtails. Prior to doing this work, I had only the vaguest of understandings of springtail diversity: I knew that there were the round blobby ones, the long thin ones, and the ones that look a bit like sausages, but that was about as far as it went. Needless to say, there's a bit more to it than that.

Pseudosinella immaculata, copyright Andy Murray.

Pseudosinella is the largest genus of Collembola currently recognised, with over 280 described species. The greater number of those species are in Europe and North America, but various Pseudosinella have also been described from other regions of the world (there don't appear to be any from South America, but then I don't know how thoroughly anyone's looked). Pseudosinella species are mostly associated with subterranean habitats, from soil and litter to deep caves, with the highest diversity in the latter. According to a key at collembola.org, Pseudosinella are distinguished from related genera by having reduced eyes (with six or fewer ommatidia, as opposed to the eight ommatidia of other genera), and a bidentate mucro lacking a projecting lamella (the mucro is the claw-like structure at the end of the furcula, the posteroventral prong that forms a springtail's 'spring'). The key also distinguishes Pseudosinella from the similar genus Rambutsinella by it's not having the fourth antennal segment swollen as in the latter, but Bernard et al. (2015) described the species Pseudosinella hahoteana as also having the fourth antennal segment swollen so I'm not sure how reliable that feature is. Pseudosinella is very similar to another genus Lepidocyrtus, the main difference between the two being Pseudosinella's reduced eyes, and more than one author has raised the possibility that Pseudosinella may be a polyphyletic assemblage derived from Lepidocyrtus adapted for life underground.

As well as the reduced eyes, Pseudosinella tend to show a number of other features commonly associated with a subterranean lifestyle, such as a pale coloration and relatively elongate appendages. The claws of the feet also tend to become modified, with the larger of the two becoming longer and progressively narrower (Christiansen 1988). This latter feature is probably an adaptation to movement on the wet surfaces that predominate in caves. At a moderate length, the claws dig into the substrate surface more than those of surface-dwelling forms, allowing greater grip. At longer lengths, the claws are suited to allow the springtail to walk over the surface of the water itself (most springtails float on water surfaces due to their small size and low density, but not all can move with purpose in this position).

Pseudosinella hahoteana, from Bernard et al. (2015). Scale bar = 200 µm.

The aforementioned Pseudosinella hahoteana is worthy of extra attention, as it is one of a half-dozen springtail species endemic to caves on Rapa Nui, the landmass previously known as Easter Island. Many of you will be aware of the ecological catastrophe that beset Rapa Nui following human settlement, as its entire forest covering was cleared away. As a result of this clearing, the native fauna was also all but wiped out; no vertebrates survive, and of about 400 arthropods known from the island only about twenty are indigenous (Bernard et al. 2015). As such, the handful of minute animals clinging to survival in patches of ferns and moss at the entrance to caves represent a significant proportion of Rapa Nui's surviving native fauna.

REFERENCES

Bernard, E. C., F. N. Soto-Adames & J. J. Wynne. 2015. Collembola of Rapa Nui (Easter Island) with descriptions of five endemic cave-restricted species. Zootaxa 3949 (2): 239–267.

Christiansen, K. 1988. Pseudosinella revisited (Collembola, Entomobryinae). Int. J. Speleol. 17: 1–29.

Define 'Trichostomum'

The moss in the above photo Icopyright Hermann Schachner) generally goes by the name of Trichostomum crispulum. Trichostomum is a cosmopolitan genus in the Pottiaceae, the largest recognised family of mosses with about 1500 species overall. But with great diversity comes great difficulty of identification. Pottiaceae tend to be small mosses that are common in harsh habitats. Features of pottiaceous mosses are often hard to distinguish and may be quite variable, making it difficult to confidently define taxa. As a result, Pottiaceae is a prime example of what I like to call 'taxonomic blancmange': something that tends to just get prodded nervously then backed away from when it wobbles ominously.

Characteristic features of Trichostomum as it is commonly recognised tend to include symmetric leaves with more or less plane margins, and with the basal cells of the leaf differentiated straight across the blade or in a U-shape. The peristome of the capsule also tends to be short and straight, and the sexual system is usually dioicous (with separate male and female plants) (Flora of North America). However, none of these features are entirely reliable, and some species have been the subject of extensive disagreement about whether they should be placed in Trichostomum, or in a related genus such as Weissia or Tortella.

To date, only a selection of Pottiaceae species have been subject to molecular analysis, but these analyses have confirmed the unsatisfactory nature of the current system. A molecular phylogenetic analysis of the pottiaceous subfamily Trichostomoideae by Werner et al. (2005) did not identify Trichostomum species as a monophyletic clade; instead, various representatives of the 'genus' were scattered throughout the subfamily. The type species of Trichostomum, T. brachydontium, was associated with a few close relatives such as T. crispulum in a broader clade containing numerous species of the genus Weissia. As a result, it has been suggested that the two genera should perhaps be synonymised, in which case the name Trichostomum would be absorbed by the older Weissia. But first, someone would need to work out just how such a genus could be recognised...

REFERENCE

Werner, O., R. M. Ros & M. Grundmann. 2005. Molecular phylogeny of Trichostomoideae (Pottiaceae, Bryophyta) based on nrITS sequence data. Taxon 54 (2): 361–368.

Cryptophytes: Four Genomes for the Price of One

Sometimes, the little things really do make a difference. Cryptophytes (or cryptomonads) are one of the many groups of minute flagellate protists to be found around the world whose role in our lives tends to get dismissed because of their microscopic size. Nevertheless, cryptophytes make up a large part of the photosynthetic phytoplankton in both freshwater and marine habitats and so ultimately are a starting point for many of the food chains that we depend on. They also had an important role to play in our developing understanding of how modern eukaryote cells have evolved.

Structure of a typical cryptophyte, from here.

As well as occurring in the phytoplankton, cryptophytes have also been found in damp soil and snow. They have a distinctive, slightly lop-sided cell morphology with two haired flagella of unequal length inserted in an invaginated gullet towards the right side of the front of the cell. This invagination is also lined on the ventral side by organelles called ejectosomes (sometimes spelled 'ejectisome'). When the organism is threatened, these ejectosomes shoot out a proteinaceous ribbon that propels the cell rapidly away from the source of irritation. Some of the references to ejectosome function that I've found seem to imply that the expelled ribbon is itself toxic, but I'm not sure if I've understood correctly. Smaller ejectosomes may also play a role in capturing bacteria and the like for the cryptophyte to feed on. Cryptophytes have a distinctive way of moving through the water column, resulting from the uneven lengths of their two propellent flagella, that has been reffered to as 'recoiling'. Essentially, they move in a series of circular tumbles while the cell itself corkscrews around its axis. This movement is distinctive enough that cryptophytes have been dubbed with the Dutch vernacular name of 'rekylalger', 'recoiling algae' (Novarino 2003).

Diagram of typical cryptophyte movement, from Novarino (2003).

The majority of cryptophytes are heterotrophic: one or more large chloroplasts provide much of the cell's energy, but they are also capable of ingesting particulate matter through the gullet. As alluded above, the cryptophyte chloroplast has been significant in the study of how chloroplasts evolved. The 1960s and 1970s saw an increasing acceptance of the concept that some organelles, most notably mitochondria and chloroplasts, had originally appeared through a process of endosymbiosis: bacteria had become intimately associated with eukaryote cells, becoming embedded in the host cell and eventually ceding enough of their vital functions to the host to be unable to function as independent organisms. The chloroplasts of the ancestors of land plants arose in this manner from cyanobacteria, as indicated by the presence of a remnant but reduced bacterial genome within the chloroplast itself, and the presence of a double membrane around each chloroplast (corresponding the cyanobacterium's original cell membrane, plus the vacuoule membrane in which it had been enclosed by the host eukaryote). In the early 1970s, however, it was found that cryptophyte chloroplasts have not two but four surrounding membranes. What is more, wedged between two of those membranes was a tiny remnant cell nucleus, dubbed the nucleomorph. The nucleomorph was a crucial piece of evidence in demonstrating that cryptophyte chloroplasts had arisen by a process of secondary endosymbiosis. A eukaryote cell containing a chloroplast that had arisen in the manner described above was itself engulfed and converted to a chloroplast by another eukaryote. The four membranes around the cryptophyte membrane were therefore, from the inside out, the original cyanobacterium cell membrane, the vacuole membrane containing the cyanobacterium, the cell membrane of the primary host cell (with the nucleomorph between this and the last), and the vacuole membrane in which that had been contained in turn. Other groups of eukaryotes also have chloroplasts that arose in this way, such as brown algae and dinoflagellates, but in these the nucleus of the captured eukaryote cell has entirely disappeared.

Another cryptophyte structural diagram of the species Guillardia theta, showing the arrangement of the chloroplast, from here. This also shows the sites of the four genomes contained in the typical cryptophyte cell.

Exactly when the cryptophyte chloroplast arose remains a contentious subject. Various lines of evidence point to the captured chloroplast donor being a red alga, as is also the case with the aforementioned brown algae and dinoflagellates. As such, some have argued for the chloroplasts of all such algae being descended from a single capture event. However, there are also a number of protists related to such taxa that lack chloroplasts. In the case of cryptophytes, there is strong evidence that the sister clade to the the photosynthetic cryptophytes is the chloroplast-less genus Goniomonas. The subsequent sister to these two clades together is less certain but a number of recent studies have pulled forward another chloroplast-less group, the katablepharids. If the cryptophyte chloroplast shares an origin with that of brown algae, then it must have somehow been lost in the ancestors of both Goniomonas and katablepharids. So far, an author's preference for a single or multiple origins of red alga-derived chloroplasts tends to come down to whether they think it is easier for chloroplasts to be lost or gained, a question whose answer is still unclear.

The diversity within cryptophytes is still not that well understood, largely due to difficulties in observing significant characters. Prior to the advent of scanning electron microscopy, some authors had gone so far as to dismiss cryptophytes as essentially unclassifiable. Nevertheless, not everything was as bleak as the pessimists would have it. Cryptophyte taxa may differ from each other in overall size and shape. They may also differ in cell colour, due to the presence of various accessory pigments in addition to chlorophyll. The primary accessory pigments found in cryptophytes are known as phycocyanin and phycoerythrin; species containing the former are a blue-green colour whereas those containing the latter are reddish, golden or a greenish yellow. The use of scanning electron microscopy has led to the discovery of other useful features such as those relating to the periplast, a protein envelope that covers the inside and outside of the cryptophyte cell membrane. Electron microscopy has shown that the outer periplast layer is often ornamented, such as by being divided into scales. And even more recently, of course, researchers have recognised the value that molecular tools may have to offer cryptophyte taxonomy, though said tools have also complicated matters by, for instance, giving hints that previously recognised 'taxa' may represent different life cycle stages of a single organism. Whatever the eventual result, there is no question that we still have a lot to learn about cryptophytes.

REFERENCE

Novarino, G. 2003. A companion to the identification of cryptomonad flagellates (Cryptophyceae = Cryptomonadea). Hydrobiologia 502: 225–270.