Return to the Crinoids

For the second time at Catalogue of Organisms, I'm presenting a fossil crinoid family as taxon of the week. However, while the last crinoid family I covered belonged to the subclass Camerata, today's family, the Sostronocrinidae, belongs to the Cladida (figure above from Waters et al., 2003).

The Cladida first put in an appearance in the late Ordovician, and were the most successful of the three major crinoid divisions* of Cladida, Camerata and Disparida. The two further groups of crinoids commonly recognised as subclasses, the Flexibilia and Articulata, are now both known phylogenetically to be within the Cladida (Ausich, 1998). Through the Articulata, the Cladida are also the only one of the three major clades to have survived the Mesozoic. Cladids have three circles of plates involved in the cup (a condition known as "dicyclic"), as opposed to the monocyclic disparids with two circles of plates**.

*At least one small early family of crinoids, the Aethocrinidae, does not fall into any one of these three groups.

**Yes, I know the terms don't quite add up. The outermost (or uppermost, depending on how you're looking at them) radial plates are present in both groups and aren't counted. Dicyclic crinoids have both basal and infrabasal plates, monocyclic crinoids have basal plates only.

The family Sostronocrinidae was recognised only recently, being established by Lane et al. in 2001 (despite its being described as a new family in Waters et al., 2003, the latter paper's authors include the authors of the 2001 paper, and the description is almost word for word identical). Previously, sostronocrinids had been included in the family Scytalocrinidae, but they differ from that family in having twenty arms rather than ten. They have relatively large infrabasal plates that are clearly visible in side view (in many other taxa the infrabasals are smaller and generally hidden by the basals). The arms were pinnulate, and branched once. Members of the family ranged from the late Devonian to the early Permian.

Waters et al. (2003) included four genera in the Sostronocrinidae - Sostronocrinus, Amadeusicrinus, Haeretocrinus* and Tundracrinus. One notable trend over time in the family was the reduction in the number of primibrachials (the plates in the initial section of arm before it branches) and hence a reduction in the length between the base of the arm and the division into branches. The earliest Devonian genera, Sostronocrinus and Amadeusicrinus**, branched on the third or fourth primibrachial. Sostronocrinus survived into the Carboniferous, but species in that time period branched on the second or third primibrachial. By the time Haeretocrinus and Tundracrinus appeared in the Permian, there was only a single primibrachial between the base of the arm and the branches.

*Misspelled as Haertocrinus in both Lane et al. (2001) and Waters et al. (2003).

**Waters et al. (2003) established Amadeusicrinus as a new genus, moving its type species from its original position in the unrelated genus Pachylocrinus. However, it is not clear what, if any, features are supposed to distinguish it from Sostronocrinus.

The implied suggestion in all this is that there is a continuous series of Devonian Sostronocrinus ancestral to Carboniferous Sostronocrinus, itself ancestral to the Permian genera. Unfortunately, workers on fossil crinoids seem to largely eschew cladistic analyses, or even explicit phylogenetic proposals. If I may briefly channel the style of Toby White, maybe there is some hidden crinoid temple somewhere where, like the Eleusinian Mysteries of old, initiates have revealed to them the secrets of knowing the cyathocrinid from the pachylocrinid, or which crinoid group gave rise to what. To the uninitiated, unfortunately, it all looks decidedly unclear.

REFERENCES

Ausich, W. I. 1998. Early phylogeny and subclass division of the Crinoidea (phylum Echinodermata). Journal of Paleontology 72(3): 499-510.

Lane, N. G., C. G. Maples & J. A. Waters. 2001. Revision of Late Devonian (Famennian) and some Early Carboniferous (Tournaisian) crinoids and blastoids from the type Devonian area of north Devon. Palaeontology 44(6): 1043-1080.

Waters, J. A., C. G. Maples, N. G. Lane, S. Marcus, Liao Z.-T., Liu L., Hou H.-F. & Wang J.-X. 2003. A quadrupling of Famennian pelmatozoan diversity: New Late Devonian blastoids and crinoids from northwest China. Journal of Paleontology 77(5): 922-948.

Naming the Monad

Know you not that around the animalcule that sports in the water there shines a halo, as around the star (The Monas mica, found in the purest pools, is encompassed with a halo. And this is frequent amongst many other species of animalcule.) that revolves in bright pastime through the space? True art finds beauty everywhere.--Zanoni, Edward Bulwer-Lytton (1842).

The classification of microscopic organisms long lagged behind that of other organisms in attention and reliability, for the simple reason that the technology to distinguish a significant amount of features in said microscopic organisms took a lot longer to develop. The really minute flagellates posed a particular challenge due to their particularly small size, as well as a shortage of distinguishing features between taxa. A review in the recent issue of Protist (Boenigk, 2008) looks at the history of classification of the nanoflagellates through the lense of the history of the first genus named, Monas Müller, 1773. Monas was long a receptacle for almost all minute flagellates (and even if not specifically referred to Monas, such organisms were still referred to as 'monads'), only to eventually slip into disuse as it became increasingly unclear what the name Monas had originally referred to (figure below from Boenigk, 2008).



When Monas was first named in 1773, life was still firmly divided between 'plants' and 'animals', and the concept of organisms falling outside these classes had not yet developed (Hogg proposed 'Protoctista' in 1860). The two kingdoms were distinguished by the motility of animals, making most protists such as Monas animals (and fungi into plants). It was only later that some protists were reclassified as plants due to their autotrophy (or more accurately, lack of particulate food uptake - fungi were still plants). Perty summed up the problem when establishing his order Phytozoidia in 1852 (as quoted in Boenigk, 2008):

Die zweite Ordnung [der Infusorien] kann den Namen Phytozoidia erhalten, weil unter ihnen sehr viele Formen sich befinden, welche in ihrem Lebenscyklus in Wahrheit bald dem Thier- bald dem Pflanzenreiche angehören, zwischen beiden oscilliren, während andere, bei denen dies nicht der Fall ist, so sehr in Gestalt, Bau, Bewegung und sonstigem Verhalten mit ihnen übereinstimmen, dass an eine völlige Trennung nicht zu denken ist.

[The second order [of the Infusoria] may get the name Phytozoidia as it contains many forms, which in their life cycle truly belong at times to the animal — at times to the plant kingdom; they either oscillate between both, or, when this is not the case, are that similar to the former in shape, structure, motion, and other behavior, that they cannot be separated.]

The genus Monas was first established with three species, M. termo, M. lens and M. mica. Monas termo has since been reclassified to the chrysophyte genus Oikomonas while M. lens is now in the kinetoplastid genus Bodo, leaving M. mica as the effective type species of Monas* (Silva, 1960). Over the years, more than 100 species have been assigned to Monas with little more to unite them than being small, fairly nondescript and (usually) with flagella. Different authors have applied different definitions of the genus, often in complete contradiction to each other (for instance, Bory [1824] defined Monas as lacking caudiform appendages, while Dujardin [1841] defined it as possessing a single flagellum). As recounted by Boenigk, species attributed to Monas eventually turned out to belong to the entire range of protist groups - amoebozoans, fungi, chlorophytes, excavates, even a few bacteria.

*As with many if not most protists, it is a little unclear whether Monas should be treated as a name under the Zoological or Botanical Code. The typification of the genus by M. mica through removing the other two species is valid under the ICBN, but not necessarily so under the ICZN. Nevertheless, we'll accept M. mica as the type species of Monas because there's no reason to unnecessarily tick off the botanists.



So what does the name Monas ultimately belong to? Unfortunately, that's where we must draw a blank. Boenigk (2008) gives us the original description of Monas mica (as well as the original illustration, shown above):

Lenticula 3. microsc. simplicis puctulum lucidem conspicitur, aucta vero magnitudine animalculum ovale vel sphaericum, nam utramque figuram pro lubitu assumit, exhibetur. Hyalina est, circulo ovali intus inscripta; hic mobilis est, & in medio, vel versus antica vel versus postica videtur. Motus vacillatorius; saepe eodem in loco, assumata figura sphaerica, diu gyratur, tumque impressio reniformis oculo in medio corporis sistitur, animalculumque halone, absque dubio e ciliis vibrantibus invisibilibus orto, pulchre cingitur. In aquis purioribus passim.

[Under the lens 3 of a simple microscope it is seen as a bright dot, yet at higher magnification as oval or spherical animalcule, because it adopts at times the one, and at times the other shape. It is hyaline, with an oval shape inside, here it is motile and seen either in the centre or anteriorly or posteriorly. With trembling movement; often rotating in one place, then with a spherical shape but with a persisting kidney shaped depression near the centre of the body, and the animalcule is surrounded by a nice halo, which arises without doubt from trembling invisible cilia. In pure water.]

There's only a few thousand taxa to which that description could apply to choose between. In fact, the description is a little vague as to whether the organism in question was even a flagellate - the presence of flagella was only inferred from the 'halo', not demonstrated. Later descriptions of Monas mica in Pritchard (1834) and Hogg (1854) add little to the original description, though both repeat the description of a halo, as for that matter does the passage quoted at the top of this post*.

*Edward Bulwer-Lytton was an immensely popular Victorian author whom Mary Shelley described as the "first author of the age". Modern audiences have a slightly different opinion of him - San Jose University in California established its Bulwer-Lytton Fiction Contest for the worst beginning to an imaginary novel in memory of the opening words of Bulwer-Lytton's Paul Clifford - "It was a dark and stormy night...".

A general assumption has evolved that Monas is vaguely equivalent to the chrysophyte genus Spumella. However, Silva (1960) demonstrated that this association was not connected with any of the original species of Monas, but due to the species described as M. vivipara (now Spumella vivipara) by Ehrenberg in 1835. As a result, there is no reason to associate Monas with Spumella any more than with any other flagellate genus, and the suggestion by Cavalier-Smith & Chao (2006) that Monas could be used for one of the clades of the also polyphyletic Spumella lacks any taxonomic justification. Despite having a long and exalted history, the name Monas has fallen into obscurity, and there it must stay.

REFERENCES

Boenigk, J. 2008. The past and present classification problem with nanoflagellates exemplified by the genus Monas. Protist 159: 319-337.

Cavalier-Smith, T., & E. E.-Y. Chao. 2006. Phylogeny and megasystematics of phagotrophic heterokonts (kingdom Chromista). Journal of Molecular Evolution 62 (4): 388-420.

Hogg, J. 1854. The Microscope: its history, construction, and applications. Being a familiar introduction to the use of the instrument and the study of microscopical science. The Illustrated London Library.

Pritchard, A. 1834. The Natural History of Animalcules: containing descriptions of all the known species of Infusoria; with instructions for procuring and viewing them, &c. &c. &c. Whittaker and Co.: London.

Silva, P. C. 1960. Remarks on algal nomenclature. III. Taxon 9 (1): 18-25.

Carnivalia and Such

Two new carnivals out in the last week.

For palaeontology, see The Boneyard at The Dragon's Tales. The current edition features fossilised dwarfs and archosaurian teen sex.

For botany, see Berry Go Round at Greg Laden's Blog. This month - artichokes, borage and explosive liverwort spooge!

Also, I've been waiting nearly a year and a half for it, but the newest update has appeared at Mikko's Phylogeny Archive, which has also changed locations. Mikko Haaramo's site was actually quite influential in my own initial forays onto the interweb, and I'd been getting worried. If you're like me and can happily peruse phylogenetic trees all day, Mikko's site can keep you going for weeks.

Finally, a reminder that the next edition of Linnaeus'Legacy is coming in a little over a week. If you want to submit a post (and a number of you already have), you can send links to me, to Jim Lemire who's hosting at From Archaea to Zeaxanthol, or you can submit a post using the form at Blog Carnival. Come on, all the cool kids are doing it!

Standing the Heat

This week's highlight taxon is the Aquificae. Aquificae is a smallish group of thermophilic to hyperthermophilic bacteria whose main claim to fame is that ribosomal DNA phylogenies suggest that it is the earliest-diverging branch of the Eubacteria, making them a key player in the theory that life as a whole may be descended from a thermophilic ancestor. The type genus of the Aquificae, Aquifex (A. aeolicus shown above in an image from here), was only formally described in 1992, but the numbers have swelled since then to about fifteen genera divided between three families. Because their thermophilic habits make them difficult to culture, the diversity of Aquificae is almost certainly underestimated. Environmental molecular analyses, for instance, indicate that near neutral pH terrestrial thermal springs may be dominated by Aquificae, while examples of Aquificae have been isolated from deep-sea, shallow marine and terrestrial hydrothermal systems, subsurface mines, and even heated compost (Aguiar et al., 2004). Aquificae found in thermal springs form extensive microbial mats stained black or yellow by iron or sulphur mineral deposits, leading to their being referred to as "black filaments" or "sulphur-turf".

Aquificae are all chemolithoautotrophs - that is, they produce their own energy directly from the reaction of inorganic sources. This is acheived through the oxidation of molecular hydrogen, which is a major component of emissions from deep-sea hydrothermal vents. Aquifex reacts hydrogen and oxygen to produce water, while other species of Aquificae use such substrates as elemental sulphur or nitrates as electron acceptors. Hydrogen oxidation is a common metabolic process in archaebacteria, but is unusual in eubacteria - the only hydrogen-oxidising eubacteria other than Aquificae belong to the ε-proteobacteria. Aquificae may be anaerobes or microaerophiles.


Microbial filaments containing Aquifex growing in water at 83°C (from MicrobeWiki).

Phylogenetically, Aquificae are actually something of a puzzle. As already noted, the ribosomal DNA phylogenies place the Aquificae basal to all other eubacterial groups. However, it has been fairly conclusively shown that the eukaryote section of the rDNA tree is quite severely compromised by long-branch attraction, and it is quite possible that the bacterial section of the tree suffers the same problem. In the case of the Aquificae, there is evidence that they may not be as basal as they appear.

Bacteria can be divided into two major groups, the Gram-positive and Gram-negative bacteria. While this originally referred only to the ability of the bacterium to be stained with Gram's iodine and crystal violet, the results actually reflect a far more fundamental distinction between the two groups. Gram-positive bacteria have a single cell membrane surrounded by a thick cell wall. Gram-negative bacteria, on the other hand, have a thinner cell wall with a second membrane overlying it. Many molecular phylogenetic trees also show a sort of rough division between the two groups, with Gram-positive bacteria tending to sit closer than Gram-negative bacteria to the archaebacteria and eukaryotes, which like Gram-positive bacteria have a single cell membrane. Gupta (1998) formalised this distinction by dividing prokaryotes between the Monodermata (Gram-positive bacteria and archaebacteria) and Didermata (Gram-negative bacteria), suggesting only a single loss or gain (depending on where exactly the root of the tree of life sits) of the second cell membrane.

Aquificae, however, are Gram-negative, complete with second cell membrane. If their position on the ribosomal DNA trees is accurate, this would require either that the outer membrane was gained or lost multiple times. If the membrane gain or loss was a unque event, then Aquificae must be closer to the other Gram-negative bacteria. Cavalier-Smith (2002) placed Aquificae in a position within or near the Epsilonproteobacteria, as suggested by RNA polymerase and a few other molecular phylogenies. As already mentioned, it is notable that ε-proteobacteria include the only other hydrogen-oxidising eubacteria (Takai et al., 2003). Insertions in the alanyl-tRNA synthetase and RNA polymerase β genes also support a position for Aquificae among the other Gram-negative bacteria, possibly close to the Proteobacteria. It seems possible (though it must be stressed that it is far from well-established) that the rDNA tree has been compromised by long-branch attraction, probably due to the high G+C content of the Aquificae genome, which is itself believed to be an adaptation to a thermophilic lifestyle.

REFERENCES

Aguiar, P., T. J. Beveridge & A.-L. Reysenbach. 2004. Sulfurihydrogenibium azorense, sp. nov., a thermophilic hydrogen-oxidizing microaerophile from terrestrial hot springs in the Azores. International Journal of Systematic and Evolutionary Microbiology 54: 33-39.

Cavalier-Smith, T. 2002. The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification. International Journal of Systematic and Evolutionary Microbiology 52: 7-76.

Gupta, R. S. 1998. Life's third domain (Archaea): an established fact or an endangered paradigm? A new proposal for classification of organisms based on protein sequences and cell structure. Theoretical Population Biology 54 (2): 91-104.

Takai, K., S. Nakagawa, Y. Sako & K. Horikoshi. 2003. Balnearium lithotrophicum gen. nov., sp. nov., a novel thermophilic, strictly anaerobic, hydrogen-oxidizing chemolithoautotroph isolated from a black smoker chimney in the Suiyo Seamount hydrothermal system. International Journal of Systematic and Evolutionary Microbiology 53: 1947-1954.

Tiny Flowers of the Sea

The greatest feeling in biology is undoubtedly that which comes with seeing a particular organism for the first time. You can read on it, study specimens of it, think you know it inside out - and then you actually see one, and suddenly you realise that it's so much more than you ever imagined. This is the feeling that drives bird enthusiasts to spend their life's savings on trips to some malaria-infested corner of the third world to try and catch a brief glimpse of the Lesser Spotted Turntwick or some such. This is the feeling that inspires deep-sea biologists spending whole days lying on the bottom of the ocean in miniature submarines in which they can't move more than a few centimetres in any direction, just so that they can watch a tube-worm wave its tentacles at them. It can only be described as a mystical experience, this moment in time that really brings it home to you that this is why you study biology.

I had a brief such moment yesterday, when I encountered representatives of an animal phylum that I'd never seen before. I was tutoring a lab for which some live specimens of encrusting bryozoans had been brought in. Unfortunately, the bryozoans were being very unco-operative and refusing to emerge from their protective skeleton. While scanning the colony under a microscope to try and find some emerging zooids, I saw some soft white structures overgrowing the colony. A quick change of focus brought them into view, and I was presented with my first ever view of an entoproct.



It seems to be quite impossible to find an image available online that does these animals justice. The one at the top of the page comes from here, while the picture above from here shows part of an entoproct colony on the left (the other two animals are a gnathostomulid and a myxosporidian). Entoprocts are minute colonial animals with a body shaped like a wine-glass, topped by a ring of feeding tentacles. The relationships of entoprocts are rather uncertain at the present point in time. In the past they were included in the Bryozoa, but it is now widely agreed that the apparent similarities to Bryozoa in the stricter sense are only superficial, and the two groups are probably not closely related. It is accepted that entoprocts belong somewhere within the Spiralia, the large group of animals that includes, among others, molluscs, annelids and flatworms (and probably bryozoans), but exactly where in this group they sit is very much an open question. The specimens I saw each arose from the substrate on their own individual stalks, which according to the guide book on Australian marine life that was lying in the lab meant that they belonged to the family Loxosomatidae, as opposed to the Pedicellinidae which have multiple zooids budding from a single stalk.

Personally, though, I was stunned by just how beautiful these little animals were. They looked like minute, frilly tulips, balanced ethereally on their long slender stalks. If I looked closely, I could see a line of fine white filigree connecting each individual to its neighbours in the colony, forming delicate tracings over the substrate. I was enthralled, and the thought crossed my mind - "This is why I study biology".

Nettle, Where Is Thy Sting?

Our highlight taxon for this week is the Urticaceae, the so-called nettle family. I say "so-called" because, as with many plant families, the supposed representative member is not necessarily that representative at all, but just happens to be the best-known temperate member of a mostly tropical family. Urticaceae includes about 2500 species in a bit under 80 genera found world-wide (the image above, from here, shows the North American species Laportea canadensis). Members encompass all growth habits from herbs to lianas to trees, though tree species are relatively few (depending on whether or not the tropical tree genus Cecropia is included in Urticaceae or in a family of its own). They are wind-pollinated, and like other wind-pollinated plants the flowers are quite small and reduced. The stinging hairs for which the nettles are so well-known are actually restricted to a single tribe of Urticaceae, the Urticeae (also known as Urereae - Hadiah et al., 2003), most members of which belong to one of the two large genera Urtica and Urera.

Relationship-wise, the Angiosperm Phylogeny Group classification (Angiosperm Phylogeny Group, 2003) includes Urticaceae within the Rosales. However, phylogenetic analyses have established the monophyly of a smaller sub-group of the Rosales including Urticaceae, Moraceae (the mulberry and fig family), Ulmaceae (the elm family) and Cannabaceae (cannabis and hops) that corresponds to the Urticales of previous classifications (Hadiah et al., 2003). Relationships within the Urticaceae are a little more unsettled, a situation not helped by the fact that the family seems to have received relatively little monographic treatment since the work of Weddell in the mid-1800s. Monro (2006), in a molecular analysis centred on the largest genus of Urticaceae, Pilea, suggested that the five tribes originally established by Weddell fell into two clades, one containing the tribes Boehmerieae, Parietarieae and Forskohleae, and the other containing the Lecantheae and Urticeae, though the genus Myriocarpa (previously in Boehmerieae) fell into the Lecantheae-Urticeae clade. Monro (2006) also supported the inclusion of the genus Poikilospermum with the Urticaceae, specifically within the Urticeae (though Poikilospermum lacks stinging hairs). Other authors have included Poikilospermum in the Cecropiaceae, but Monro (2006) placed Cecropia distant from Poikilospermum in a trichotomy with the two Urticaceae clades (so unresolved as to whether or not it should be included within Urticaceae).



Economically, the Urticaceae are not a significant family. Species of Boehmeria, particularly B. nivea (shown above in an image from here), have been used for many years as a source of the fibre known as ramie. Ramie cloth was one of the textiles used for wrapping mummies in the Egyptian pre-dynastic period, but before mechanised methods became available extraction of fibres required a laborious process of repeated soaking and scraping. Industrial-scale production of ramie did not become practical until about the mid-1900s (Cook, 1984).

Some species of nettles of the genus Urtica are collected as edible herbs - the venom from the stinging hairs is destroyed by heat, so cooked nettles are quite harmless. An Indian species, Urtica tuberosa, also produces edible tubers. According Botanical.com, nettles can also be used to produce a fibre in a similar manner to ramie - however, the inferiority of nettle fibre to linen resulted in the decline in its use (though it did have something of a renaissance during the height of shortages in the Second World War). For the most part, nettle stings are more of an irritant than a significant medical issue, though some species are significantly more toxic than others. The New Zealand Urtica ferox (ongaonga or tree nettle, shown below in an image from Trek Nature) is a shrub of up to five metres in height that produces a strong enough sting to sometimes be fatal. At least one traditional chant of the Ngati Kahungunu iwi indicates that the ongaonga, along with other spined plants, was originally planted by Kupe, the discoverer of New Zealand, to protect the new land (Cowan, 1930):

Nau mai, e Tama,
Ki te tai ao nei.
Kia whakangungua koe
Ki te kahikatoa.
Ki te tumatakuru.
Ki te tara-ongaonga;
Na tairo rawa
Nahau e Kupe
I waiho i te ao nei.

Thou'lt be a powerful shield against
The weapons of the world;
The sharp and deadly spears,
The pricking darts and stings
That fill the foeman's armoury;
Thou'lt conquer e'en the barriers
Which Kupe the explorer raised
To guard this new-found land.

I haven't been able to find anything to support the rather more dramatic version recounted in Wikipedia that claims he placed these obstacles to evade the men whose wives he had stolen, so (unfortunately) I suspect this version to be a little dubious.

REFERENCES

Angiosperm Phylogeny Group (APG). 2003. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II. Botanical Journal of the Linnean Society 141: 399-436.

Cook, J. G. 1984. Handbook of Textile Fibres. Woodhead Publishing.

Cowan, J. 1930. The Maori: Yesterday and To-day. Whitcombe and Tombs.

Hadiah, J. T., C. J. Quinn & B. J. Conn. 2003. Phylogeny of Elatostema (Urticaceae) using chloroplast DNA data. Telopea 10(1): 235-246.

Monro, A. K. 2006. The revision of species-rich genera: a phylogenetic framework for the strategic revision of Pilea (Urticaceae) based on cpDNA, nrDNA, and morphology. American Journal of Botany 93 (3): 426-441.

The Whale that Looked Like a Walrus

Odobenocetops (shown above in an illustration from de Muizon & Domning, 2002) was definitely one of the odder forms of whale. It was found off the coast of Peru in the early Pliocene, about 3 or 4 million years ago, where it seems to have converged in a number of details on the modern walrus (Odobenus rosmarus). Or perhaps it is more accurate to say that the modern walrus converged on it - while Odobenus also appeared in the early Pliocene (Berta et al., 2005), Odobenocetops may have had a slight head start on it - the records in the Paleobiology Database suggest that the earliest records of Odobenus (from Belgium and New Jersey) are of somewhat uncertain age.

The walrus feeds on molluscs, particularly soft-shelled bivalves. The thick upper lip bears a strong array of sensitive bristles for finding molluscs buried in the bottom sediment. However, unlike most molluscivores the walrus doesn't directly crush the shells of its prey. Instead, the shell of the mollusc is held in the thick lips while the tongue is used like a piston to remove the animal from its shell by suction. Odobenocetops had a broadened palate and evidence of a thickened upper lip (probably with similar bristles) like a modern walruses, and probably fed in the same manner. The interesting thing is that Odobenocetops also evolved enlarged tusks like the walrus. Before the discovery of Odobenocetops, most authors had believed that the walrus' tusks were unrelated to its unusual feeding method, having probably evolved instead through sexual selection. That Odobenocetops also combined suction feeding and tusks suggests that their combination might not be as accidental as previously thought. What exactly the role of the tusks may be in feeding is more uncertain. De Muizon & Domning (2002) suggested that they might function as guides for the sensory bristles, or they may have been used to guide prey animals towards the mouth.

One of the most unusual features of toothed whales (in a group well-provided with unusual features) is a tendency towards a loss of bilateral symmetry, with one side of the skull being more developed than the other. The reasons for this asymmetry are uncertain, but the most popular theory is that it is related to the development of the sonar system. Odobenocetops possessed one of the most dramatic examples of skull asymmetry in the cetaceans. The left tusk of the type specimen of O. peruvianus is estimated to have been about 20 cm in length (because the tusks were quite fragile, fossilised specimens are invariably broken). However, the right tusk was over twice as long, at least 50 cm. The type of the other known species, O. leptodon, is even more dramatic - the left tusk is about the same size as known for O. peruvianus at 25 cm, while the right tusk was over one metre! Oddly enough, the remainder of the skull was rather less asymmetrical than in other odontocetes, as Odobenocetops had lost the melon and therefore the sonar of other toothed whales. Sonar was probably unnecessary for a diet of less mobile animals than fed on by other whales.



Relationship-wise, Odobenocetops is included in its own family, but was closely related to the Monodontidae, the family including the narwhal and white whale (also known as beluga - image above of white whale from Whale Trust). Monodontids also have rather mobile lips compared to other whales. Even more significantly, Odobenocetops and monodontids both have a completely mobile neck with unfused vertebrae, in contrast to other cetaceans which have the first two to five vertebrae fused into a single mass. This extra head motility would have doubtless been critical in allowing Odobenocetops to become an efficient sediment feeder.

REFERENCES

Muizon, C. de, & D. P. Domning. 2002. The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications. Zoological Journal of the Linnean Society 134: 423-452.

Slime Nets: Another Group of Not-Fungi

The subject of this week's Taxon of the Week post is another example of the under-rated nature of protist diversity. Labyrinthuleans, commonly referred to as 'slime nets' are one of those organisms that, being neither animals nor plants, have been shuffled back and forth between and within the nomenclatural codes over the years, resulting in the same taxon being referred to by multiple different names. Labyrinthulea, Labyrinthista and Labyrinthulomycota are just three options that might be encountered. They are one of the protist groups that have been described as 'slime moulds', though they lack the dramatic life cycles of the Mycetozoa, the slime moulds proper. Most labyrinthuleans are found in aquatic habitats, but some species are terrestrial.

Labyrinthuleans may be divided into three groups, the Thraustochytriales, Labyrinthulales and the Diplophrys group. Note that these may not be phylogenetically separate groups - the molecular analysis of Cavalier-Smith & Chao (2006), for instance, doesn't separate the three - but they are still useful form groups. The Labyrinthulales and Thraustochytriales are united by the possession of an organelle called a bothrosome or sagenogen(etosome) that produces large amounts of filamentous net-like ectoplasmic membrane that the individual cells move along and absorb nutrients through, hence the name of 'slime nets'. The image at the top of this post (from here) shows a colony of Labyrinthula on the left and a close-up of individual cells on the right. Many labyrinthuleans are parasitic and invade and break down cells of host organisms, absorbing nutrients released by the decomposing cells. The individual cells form aggregative masses during reproduction, within which enlarged cells undergo meiosis and release flagellated zoospores (Barnes, 1998). The most obvious difference between the two groups seems to be the mode of colony formation - while Labyrinthulales form dispersed colonies of loosely connected cells surrounded by ectoplasmic matrix as seen in the top photo, Thraustochytriales (as exemplified in the photo of Schizochytrium limacinum below, again from here) form more compact colonies with the ectoplasmic net growing as "roots" from the base of the colony. The small inset photo shows the zoospore of Schizochytrium.



The genera Diplophrys and Sorodiplophrys are associated with the labyrinthuleans by molecular (Cavalier-Smith & Chao, 2006) and ultrastructural (Dykstra & Porter, 1984) data. However, while they do produce and move on ectoplasmic outgrowths, they lack a bothrosome for the production of said ectoplasm. Zoospore production has also never been recorded for these genera. The terrestrial Sorodiplophrys has an aggregative stage in its life cycle, but the aquatic Diplophrys marina does not (the actual type species of Diplophrys, D. archeri, has not been observed since 1902, and D. marina is only tentatively included in the same genus). Interestingly, the analysis of Cavalier-Smith & Chao (2006) places Diplophrys marina within the Labyrinthulales, which if correct implies that it lost the labyrinthulean characteristics during its evolutionary history.

As 'slime moulds', the labyrinthuleans were originally regarded as fungi (hence some publications refer to them as labyrinthulomycetes). However, it is now universally agreed that they are in fact members of the heterokonts (Chromista) based on the ultrastructure of the zoospores, as well as molecular data. Within the heterokonts, Cavalier-Smith & Chao (2006) place labyrinthuleans in a basal heterotrophic clade that also includes bicoecids and opalozoans and is sister to the remaining heterokonts. Labyrinthuleans are therefore not even close relatives of the other "ex-fungal" chromists in the Pseudofungi.

Labyrinthuleans have relatively little economic significance to humans. Some labyrinthuleans attack hosts of economic significance to humans, such as bivalves or golf course turf. Oils from the thraustochytrialean Schizochytrium contain one of the current dietary buzzwords, omega-3 fatty acids, so commercial growth of Schizochytrium is used to produce dietary supplements and alternatives to fish oils. Interestingly, webpages, patents, articles, etc. referring to such uses of Schizochytrium seem to invariably refer to it, somewhat misleadingly, as an 'alga', and the product as 'algal oil'. This strikes me as only a marginal improvement over 'fungus'.

REFERENCES

Barnes, R. S. K. 1998. The Diversity of Living Organisms. Blackwell Publishing.

Cavalier-Smith, T., & E. E.-Y. Chao. 2006. Phylogeny and megasystematics of phagotrophic heterokonts (kingdom Chromista). Journal of Molecular Evolution 62: 388-420.

Dykstra, M. J., & D. Porter. 1984. Diplophrys marina, a new scale-forming marine protist with labyrinthulid affinities. Mycologia 76 (4): 626-632.

Separating Segments

The marine worm Perinereis amblyodonta (image from here).

Struck, T. H., N. Schult, T. Kusen, E. Hickman, C. Bleidorn, D. McHugh & K. M. Halanych. 2007. Annelid phylogeny and the status of Sipuncula and Echiura. BMC Evolutionary Biology 7: 57.

This was published last year, but I only discovered it yesterday, and had a serious "Why didn't anybody tell me about this earlier?" moment. I would of liked to have known that a significant contribution had been made to one of the harder questions in animal phylogeny - the interrelationships of the annelids.

Annelids have long been victim to a certain chauvinism in systematics. They've been treated as kind of the poor cousin to the other major animal phyla*, coupled with an idea that they were in some way "primitive". A number of other phyla, most notably the arthropods and molluscs, have at various times been explicitly or implicitly regarded as derived from annelid ancestors. It must be stressed that in very few of these cases of proposed annelid ancestry was a direct connection made to any specific annelid subgroup. Annelid ancestry was less of a rigorous hypothesis and more of a vague assertion, in the same vein as suggestions of a "thecodont" ancestry of birds.

*Though they still had the flatworms to look down on, at least.


Christmas tree worm (Serpulidae - image stolen from here).

Things changed somewhat with the advent of recent molecular or molecular-influenced phylogenetic studies. Three phyla in particular, the annelids, onychophorans and arthropods, had been united by their metameric segmentation, the regular repetition of separated, similar body segments (this pattern of segmentation has become obscured in many arthropods by specialisation of the separate segments, but is still recognisable in groups such as centipedes and millipedes). More recent analyses have shown the onychophorans and arthropods to have evolved their segmentation separately from annelids, forming a clade with the nematodes and other smaller phyla (the Ecdysozoa) while annelids sit in a clade called the Lophotrochozoa with such phyla as molluscs and brachiopods. Within the trochozoans, metameric segmentation might have been reborn as a defining character of annelids, but the spectre of "ancestral annelids" still didn't quite go away. For instance, while molluscs are mostly unsegmented, one supposedly basal taxon, the Neopilinida, possesses serial repetition of some organs and was suggested to demonstrate the origin of molluscs from a segmented ancestor, with the implication that "segmented" equaled "annelid" generally not too far behind.

Within the annelids, things have not been much better. Traditionally, annelids have been divided into three classes, the Polychaeta (marine worms), Oligochaeta (earthworms) and Hirudinea (leeches), but it has long been recognised that this is not a satisfactory situation. It is well-established that the earthworms and leeches form a single clade, the Clitellata, but within the Clitellata the "oligochaetes" are united only by the absence of the derived features of leeches. Relationships between the Clitellata and Polychaeta have been even more contentious - authors have differed on whether the polychaetes form a monophyletic group that is sister to the Clitellata, or whether the Clitellata is nested within the polychaetes. The polychaetes in turn have been divided between about 80 families, but relationships between those families have been almost completely unresolved.


Giant tube worms (Riftia [Pogonophora] - image from here).

An influential study in annelid phylogenetics was that of Rouse & Fauchauld (1997) which undertook a morphological analysis of the polychaetes. Rouse & Fauchauld found a monophyletic Polychaeta with Clitellata as sister group, and division of the polychaetes into three major clades, named Aciculata, Canalipalpata and Scolecida. They also found that the worms previously regarded as the separate phylum Pogonophora were actually highly derived annelids, as had been suggested by some authors previously. Unfortunately, support for any of the clades found was relatively low, and homoplasy was rampant. The benefits of hindsight allow us to quibble with their choice of outgroups, as well - Rouse & Fauchauld rooted their tree using the small non-segmented worm clades Sipuncula and Echiura (on which see more below) and the arthropods and onychophorans, for which many of the supposedly shared characters were probably homoplasies. A recent major molecular study (Rousset et al., 2007), despite including some 217 taxa, was unable to even demonstrate annelid monophyly, finding many of the supposed 'outgroups', including molluscs, brachiopods and nemerteans, scattered around within the annelids, and recovered almost none of the major clades of Rouse & Fauchauld (1997). However, support over the entire analysis was low, and large chunks of data were missing for many of the taxa included in the analysis.


The echiuran Urechis caupo (image from here).

And so we finally get to Struck et al. (2007). While Struck et al. did not cover quite as many taxa as Rousset et al. (2007), they included more genes and more complete data for the taxa included. Outgroup taxa were drawn from a number of other lophotrochozoan phyla, and the first major result of Struck et al. was the resolution of Annelida as a coherent clade, in contrast to earlier molecular studies. Within Annelida, polychaetes were paraphyletic with regard to Clitellata, and the closest relatives of the Clitellata were the Aeolosomatidae, previously suggested as such on morphological grounds. As for the major morphological clades of Rouse & Fauchauld, while none were strictly monophyletic, the conflict between morphological and molecular results was much reduced. Rouse & Fauchauld's Aciculata was largely monophyletic except for the inclusion of one taxon that had been included with the Scolecida, while the majority of the Scolecida formed two branches of an unresolved trichotomy with the clade including the Clitellata.


(From Struck et al., 2007) ML analysis and BI of Nuc data set with 81 OTUs (-ln L = 66,627.30). 1 of 2 best trees is shown. In the other tree the trichotomy of Nephtyidae, Syllidae, and Pilargidae is resolved with Syllidae being sister of Nephtyidae. OTUs with just the genus names (e.g., Lumbricus) indicate that the sequences from different species of that genus were concatenated. Nuc consisted of 9,482 characters, from which 4,552 (28S rRNA – 2,504; 18S rRNA – 1,375; EF1α – 673) unambiguously aligned and non-saturated ones were included. BS values above 50 shown at the branches on the left; PP's on the right or alone. The branch leading to Ophryotrocha labronica is reduced by 90%. Ophryotrocha individuals have been sampled from a long time culture, which got bottlenecked several times. For 28S rRNA, Capitella forms a long branch and does not cluster with the two other Capitellidae in the analyses. ML settings: Base frequencies: A = 0.2727, C = 0.2495, G = 0.2586, T = 0.2192; Rate matrix: AC, AT, CG, GT = 1.0000, AG = 2.5097, CT = 3.7263; α = 0.4830; Proportion of invariant sites = 0.3103. Models in BI: 28S rRNA, 18S rRNA, EF1α : GTR+I+Γ. Clitellata, Echiura, Siboglinidae, Sipuncula highlighted with gray and bars indicate polychaete groups: orange = outgroup; A, blue = Aciculata; C, green = Canalipalpata; S, red = Scolecida; Ca = Capitellida; Eu = Eunicida; Ph = Phyllodocida; Sa = Sabellida; Sp = Spionida; Te = Terebelliformia; Aph = Aphroditiformia.

Three small groups of worms previously classified as separate phyla were also included among the annelids. The annelid nature of the Pogonophora (corresponding to the Siboglinidae in the tree above) was confirmed, as was its position in the order Sabellida as proposed by Rouse & Fauchauld. The Echiura had also been previously suggested to be derived annelids - while the adults are non-segmented, echiurans do possess chaetae (bristles) like those of annelids and characters related to segmentation have been demonstrated in their larvae. Struck et al. found a relationship between Echiura and the polychaete family Capitellidae, as had been found in previous molecular studies.



Something that is likely to cause more debate, though, is Struck et al.'s finding the Sipuncula (examples shown above in a photo from here) within the annelids. The relationships of the Sipuncula or peanut worms have long been debatable. Some authors have favoured a relationship with annelids, while others have placed them closer to molluscs. Unfortunately, Struck et al. included only one representative of the Sipuncula, and while it was nested well within the annelids its position therein was quite unstable, moving about a lot between analyses. It is worth noting here that the large-scale analysis of animal interrelationships by Dunn et al. available from yesterday as an advance online publication at Nature also positioned Sipuncula within the Annelida. Nevertheless, I can see a lot of further study being done on this result in the future.

A lot remains to be done before we can fully understand the evolution of the annelids, but Struck et al. have certainly made an important contribution. Hopefully, the exorcism of the spectral "ancestral annelid" will encourage the study of annelids not as some relictual halfway-house on the way to somewhere else, but as a specialised and diverse grouping in their own right.

REFERENCES

Rouse, G. W., & K. Fauchald. 1997. Cladistics and polychaetes. Zoologica Scripta 26 (2): 139-204.

Rousset, V., F. Pleijel, G. W. Rouse, C. Erséus & M. E. Siddall. 2007. A molecular phylogeny of annelids. Cladistics 23: 41-63.

Indian Entomologists Cut Off

A worrying piece of news in this week's Nature:

Jayaraman, K. S. 2008. Entomologists stifled by Indian bureaucracy. Nature 452: 7.

An international collaboration to study insects in the Western Ghats mountains in southern India has been unable to get off the ground because of government concerns over biopiracy.

The story in a nutshell - an Indian entomology team working for an organisation called the Ashoka Trust for Research in Ecology and the Environment collected some 200,000 specimens for an ecological survey of the Western Ghats, but needed to send the specimens to experts in different countries to be identified (and potentially described if and when some turned out to be undescribed species) because India lacks the experienced persons to do so. However, sending specimens out of the country required that the Trust obtain a licence to do so from the governmental National Biodiversity Authority, and NBA says no.

The reason for refusing the permits was tied up in legislation designed to stop biopiracy, a legitimate concern for many developing countries. Specifically, the legislation requires that specimens be retained in Indian institutes and not deposited out of country (the Ashoka Trust specimens were to be returned to India after identification). Unfortunately, such laws cause more harm than good if they cripple legitimate research. The secretary of the NBA responded to Nature's questions:

“There is no restriction on collection or export of a few specimens for research.... But exporting 200,000 specimens is not permissible.” The NBA encourages Indian scientists to send photographs or digital images to collaborators abroad instead of actual specimens, he says.

Unfortunately, this attitude shows complete ignorance of the requirements for research. Many invertebrates simply cannot be identified from external photographs along. Genitalia may need to be dissected out for many if not most insects. The news item does not mention if specimens of any groups other than insects are concerned, but many soft-bodied animals such as worms may require fixation on slides to examine internal anatomy. Experts on identification of a given invertebrate group will usually be few and far between, and it is quite believable that India (which does not have many well-funded research centres) lacks the expertise to identify many groups. This is especially true if costly techniques such as electron microscopy are required in identification, which are probably of limited availability in developing countries.

I also suspect, reading between the lines, that this might be a case of interference from the vertebrate-invertebrate divide, where policies that are developed with vertebrates in mind are unwisely applied to invertebrates. 200,000 specimens would be an extraordinary amount in a study on vertebrates, and would be quite rightly regarded as inordinate. One could also expect that most of the specimens would represent known species, and possibly could be identified from photos only. In a large-scale invertebrate survey, 200,000 specimens is surprisingly little, and one can feel certain that a significant proportion of those would represent undescribed species. The problem is especially acute when, as with many micro-invertebrates, the distinctiveness or otherwise of a given specimen can't be estimated prior to expert inspection.

And by essentially cutting off external input from better-funded countries (by making outside experts leap through too many hoops for it to be worth their time), the Indian government is effectively preventing its local researchers from improving their situation. For researchers in developing nations to improve their skill and knowledge base to the point where they don't need to seek outside assistance will almost certainly require a fair amount of initial input and training from said outside assistance. Also, as a significant amount of taxonomic work on Indian taxa has been conducted in other countries in the past, before principles of returning specimens to countries of origin were considered, many types and other critical specimens are held in non-Indian institutes. If Indian institutes are prevented from sharing their own specimens, then it forms a disincentive for non-Indian specimens to share specimens in turn, which could prevent Indian researchers from obtaining specimens critical to their own research.

It would be absolutely fantastic if Indian researchers were able to do all the required work on their own, without having to call in outside help. But to insist on such a state of affairs is to ignore the realities of current invertebrate taxonomic research, and to prevent conditions from ever improving. For shame!

Linnaeus' Legacy #5: You Can't Stop the Beat

Welcome to the fifth edition of Linnaeus' Legacy, the monthly taxonomy and biodiversity carnival. Yes, it's now been five months since I started this event, and I've been really happy with the response it's gotten so far. Thank you so much to everyone who's played a part in making it happen, and let's keep making it happen for as long as there's an interweb for it to happen on!

In that congratulatory air, I'm going to prefix this carnival with... a link to another carnival! Carnival of the Blue, which covers all things oceanic, has been up at Switchboard for a couple of days, so I'll take the opportunity to bring your attention to it.

Theory

We are, unfortunately, in the midst of a Great Taxonomy Crisis. The number of workers in taxonomy has been steadily declining, and a significant proportion of those workers still active are approaching or even past retirement age. This is a Crisis (with a capital C) in the making because taxonomy is one of the lynchpins of all other biological research - you can't do any sort of productive communication about your studies on an organism if you don't know what to call it. How are we going to revitalise the study of taxonomy? Kevin Z thinks Open Access publishing might be the way to go.

Others think that a change in the way we do taxonomy is called for, but that immediately inspires the question of how did we do taxonomy before? John Wilkins examines one of the oft-repeated stories about what inspired Linnaeus' taxonomic work, and wonders if the story has any merit.

The Encyclopedia of Life is a website that may increase the perception of taxonomy through its aim to have an information page available for ecery species on the planet. Rod Page took a look at it, and feels it has some way to go yet.

Protists and Plants



Yours truly got quite excited over the description of Chromera, a seemingly featureless blob of a unicellular alga that actually goes some way to answering some very significant questions - as well, of course, as bringing up a whole bunch of new ones.

Taxonomy also has an essential role to play in the Biodiversity Crisis, as increasing numbers of species become endangered or extinct due to human activities. We need to know what's there before we can hope to protect it! Jamie McIntosh tells us about a recent project to insure some of the world's plant biodiversity by starting a global seed bank on icy Svalbard, not far south of the North Pole.



Meanwhile, Podblack Cat describes her astonishment at discovering that cauliflowers, kale, cabbages and kohl rabi were all different forms of the one species - the protean Brassica oleracea! After waxing poetical about the potential of a Brassica bouquet, though, she still has some preference for the more traditional roses she had just received.

Invertebrates



Bob Crean has started an online guide to all matters scorpion, with a series of detailed explanations of scorpion descriptive terms. If you don't know how to tell your prosoma from opisthosoma, this is the place to find out (before you get into serious trouble).

Sam Heads has some steamy photos for us of some very stick-insect-like grasshoppers caught in flagrante. Not that they were probably that difficult to catch - some insects can remain attached to each other for days!

Vertebrates



Among the discoveries published in the last month was one of the smallest pterosaurs yet known. Mike Keesey has more, in a post aptly named "Adorable Science". GrrlScientist, meanwhile, turns her attention to the question of when birds first appeared on the scene.

Everyone knows the value of a lemur for getting people's attention - just ask John Cleese. Greg Laden has a post on a recent study that aims to resolve the phylogeny of the lemur families. Afarensis wrote more on the same. Funnily enough, no-one mentioned Bugtilemur - maybe they're hoping it'll go away and quit sticking it's rhinarium where it's not wanted.

Regrettably, the aetosaur issue reported on in the last edition still continues to haunt us. Mike Taylor has links to the available coverage, and there have been recent posts from Julia Heathcote, Paul Anderson, The ReptiPage, ReBecca Hunt, Mike Keesey, Chris Rowan, Brian Switek and Janet Stemwedel.

Thank you for reading this month's Linnaeus' Legacy. April's instalment will be coming courtesy of From Archaea to Zeaxanthol. After that, we're still looking for hosts. If you would like to host an instalment of one of the net's best blog carnivals, let me know! If you don't, I'm going to start calling in favours from people, and they're going to feel like real heels if they have to say no. So for their sake, volunteer! Linnaeus' Legacy wants YOU!

Image Credits: Primate phylogenetic "tree" from Chinh Hoang. Cathedral cauliflower from Botany Photo of the Day. Brassica oleracea variation slide from Dr. Bruce Railsback. Emperor scorpion from Ugly Overload. Nemicolopterus reconstruction from Scientific American.

The Life of an Ostrich Foot

Before I start, just a reminder that Linnaeus' Legacy is happening in two days' time, so get your posts in to me quick!

This week's Taxon of the Week is the Struthiolariidae, a family of marine gastropods (specifically caenogastropods, a large clade characterised by the possession of a feeding proboscis) commonly known as "ostrich foot shells". I have to confess complete ignorance as to why they are supposed to resemble an ostrich's foot. If you look at the photos above of Struthiolaria papulosa (from gastropods.com), I suppose you might be able to see a resemblance if you exercise a significant amount of imagination, but I doubt it would be the first thing I would think of.

Ostrich foot shells are an entirely Southern Hemisphere family, found in cooler waters. Despite a fossil record extending back to the Cretaceous, the Struthiolariidae is currently not a very speciose family - gastropods.com lists only seven species. New Zealand is the current centre of diversity (four species), and also the source of the oldest known fossil struthiolariids of the genus Conchothyra. A single species is known from Australia, one from South Georgia, and one from South Georgia and Kerguelen. Struthiolariids can reach a respectable size - Struthiolaria papulosa can be nearly 9 cm long. The thickened lip that can be seen around the shell opening in the photo above is considerably stronger than the rest of the shell, and often remains intact long after the remainder has worn away - they are a common find on sandy New Zealand beaches.



Struthiolariids show two major modes of movement. They may crawl about on the foot like most gastropods, or they may use a more dramatic mode of movement in which fluid is drawn from the foot and the small operculum (visible in the photo at the top) is directed downwards. The operculum ends in a sharp hook, and this is used to push against the sediment and lunge the animal forward in a clumsy leap (in the related Strombidae, the conches, the sole of the foot has become permanently reduced and this second method is their only way of moving). This opercular movement can also be used to right the animal when it becomes tipped upside-down. Crump (1968) recorded that opercular movement was used when the animal came in touch with a sea star to throw itself into a series of rapid somersaults to carry itself away from the potential predator (figure above from Crump, 1968). Morton (1951) also suggested a defensive function for the operculum - when a live specimen was held away from the substrate, the operculum would be extended out in an attempt to gain a purchase. Even if this was primarily the usual righting action, the sharp, hooked operculum being waved about could quite possibly deter a potential predator.



Despite being quite capable of moving about, struthiolariids are actually sessile filter-feeders by habit. The above figure from Morton (1951) shows a specimen of Struthiolaria papulosa buried beneath the sand in feeding position. The animal digs itself into the sand using its foot, and the long proboscis is used to construct two mucus-lined tubes that house the inhalent and exhalent siphons. A continuous stream of water and particulate matter is taken in through the inhalent siphon, filtered through the mucus-producing endostyle and the gills, and the filtered water is expelled through the exhalent siphon while the particulate matter is trapped in a string of mucus and carried along a specialised food grove to the front of the animal, where it can be ingested through the proboscis.

The gastropod family most closely related to the Struthiolariidae, the pelican's foot shells of the Aporrhaidae, share the characters of the paired siphons and infaunal living habit. However, the aporrhaids do not have the gills specialised into feeding structures as in the struthiolariids. Instead, the aporrhaids use the strong current produced the movement of water over the gills to draw particulate matter towards the front of the animal, where it is picked up by the proboscis in the normal caenogastropod fashion. In 1937, before the lifestyle of the Struthiolariidae had ever been investigated, Yonge suggested that the mode of feeding found in Aporrhaidae was a possible precursor to ciliary feeding as found in many mollusc groups. It therefore came as significant support to Yonge's speculations when the exact development in feeding mode he had suggested was discovered by Morton (1951) in the aporrhaids' closest relatives.

REFERENCES

Crump, R. G. 1968. The flight response in Struthiolaria papulosa gigas Sowerby. New Zealand Journal of Marine and Freshwater Research 2: 390-397.

Morton, J. E. 1951. The ecology and digestive system of the Struthiolariidae (Gastropoda). Quarterly Journal of Microscopical Science s3-92: 1-25.