Field of Science

Bits of Cucumber in the Fossil Record

Aggregation of Eocaudina septaforminalis sclerites from Boczarowski (2001). Scale bar = 200 µm.


Echinoderms are a dream group of animals for invertebrate palaeontologists (that's palaeontologists who study invertebrates, not palaeontologists who are invertebrates). Their calcified skeletons mean that their fossil record is extensive and detailed. When most of the body is covered in plates, looking at the fossil can give you an instant idea of what the animal looked like when alive. But as with all things in biology, there are notable exceptions. The sea cucumbers are one group of echinoderms that has significantly reduced the original skeleton, sacrificing a hard outer skeleton for increased flexibility. Instead of solid plates, the sea cucumber skeleton is made up of many minute sclerites embedded in the skin. And while this may be all well and good for the sea cucumber, it is not so convenient for the palaeontologists. In the fossil record, these minute sclerites become separated, and one separated sclerite does not tell you much about the appearance of the sea cucumber as a whole.

As a result, palaeontologists looking at sea cucumber remains have found themselves presented with a conundrum. The classification of modern sea cucumbers is largely based on features of the soft body that are usually not preserved in the fossil record, making comparison of living and fossil cucumbers difficult. Also, the skeleton of a single sea cucumber may include different forms of sclerite, performing different roles. If two different types of sclerite are found close together in the fossil record, did they come from a single sea cucumber or from two different sea cucumbers that died close together? To bypass these barriers, palaeontologists have often used what are referred to as 'parataxa'. A single type of sclerite is treated as a single 'parataxon', with the recognition that there may not be a perfect correlation between the parataxon and the theoretical taxon that it originally came from.

Individual calclamnine sclerite, Priscocaudina crucensis, from Boczarowski (2001).


The Calclamnidae has been recognised as one such 'parafamily' of sea cucumber sclerites. As defined by Frizzell & Exline (1966), the Calclamnidae grouped together rounded or polygonal sclerites that are perforated with holes like a sieve, and that don't have any sort of stalk or other ventral protrusion. This is a very common sort of sclerite for echinoderms: 'calclamnid' sclerites have been identified as far back as the Ordovician (Boczarowski 2001), and sclerites of this sort are still found in sea cucumbers today (just to confuse matters, the skeletons of some brittle stars also include very similar sclerites, raising the spectre of misidentification). Boczarowski (2001) recognised two subfamilies of Calclamnidae: in one, the Eocaudininae, the perforations of the sclerite are all more or less even in size, while in the other, Calclamninae, the pores towards the centre of the plate are larger and arranged in a cross-shape. The eocaudinines include the earliest calclamnid plates, with the calclamnines appearing during the Devonian.

Recognition of parataxa is a convenient tool for keeping records of things like biostratigraphy without getting bogged down, but what sort of sea cucumber did calclamnids actually come from? The calclamnids resemble sclerites found in the group of modern sea cucumbers called the Dendrochirotacea, so they have often been classified with this group. However, a number of features of the dendrochirotaceans, including perforated calclamnid-like sclerites, have been suggested to be primitive for sea cucumbers, so similarities between calclamnids and dendrochirotaceans may represent shared ancestral features rather than true affinities. Haude (1992) commented on a number of cases of sclerites found preserved in assemblages that he believed represented original life associations, including some containing calclamnids. One of these contained sclerites that Haude identified as similar to Calclamna germanica, the type species of the family, in association with large hook-shaped sclerites. Hooks are not characteristic of dendrochirotaceans, but of Apodacea, a different group of sea cucumbers characterised by the loss of tube feet (with the hooks working to provide mobility in their place). Haude suggested the possibility that Calclamna might represent a stem-group apodacean that retained some primitive sclerite features. In other fossil groups such as conodonts, the identification of preserved assemblages has allowed palaeontologists to progress beyond the use of parataxa and integrate more recognition of evolutionary relationships. Hopefully we get the same opportunity with sea cucumbers.

REFERENCES

Boczarowski, A. 2001. Isolated sclerites of Devonian non-pelmatozoan echinoderms. Palaeontologia Polonica 59: 1-219.

Frizzell, D. L., & H. Exline. 1966. Holothuroidea—fossil record. In: Moore, R. C. (ed.) Treatise on invertebrate Paleontology pt U. Echinodermata 3 vol. 2, pp. U646-U672. The Geological Society of America, Inc., and The University of Kansas Press.

Haude, R. 1992. Fossil holothurians: sclerite aggregates as 'good' species. In: Scalera-Liaci, L., & C. Canicatti (eds) Echinoderm Research 1991, pp. 29-33. Balkema: Rotterdam.

The Mighty Limpets

The limpet Nacella concinna on bull kelp Durvillea, copyright David Cothran.


I am Clamp the mighty limpet,
I am solid, I am stuck.
I am welded to the rockface
with my super-human suck.


The hero of Pam Ayres' "Clamp the Mighty Limpet" is a misanthropic Napoleon, threatening would-be harassers with lost fingernails and the danger of attachment (if they would only be willing to stand in one place for two weeks). In reality, 'limpet' is a name that has been applied to a number of unrelated groups of gastropod with cap-like, more-or-less uncoiled shells. The most prominent group of limpets, however, and the one that Ayres almost certainly had in mind, is the 'true limpets' of the Patellogastropoda.

Owl limpet Lottia gigantea, copyright Jerry Kirkhart.


The Patellogastropoda are a mostly marine group of molluscs, though one species in south-east Asia, Potamacmaea fluviatilis, is found in rivers and brackish waters adjoining the Bay of Bengal (Lindberg 2008). The group takes its name from the north-east Atlantic genus Patella, which has been featured on this site in an earlier post. Patellogastropods are particularly prominent in the intertidal zone, where they can be found clinging to rocky surfaces, but there are also many subtidal or deep-water limpets. Some limpets are specialised for living on macro-algae or marine vegetation; one such species, the North Atlantic Lottia alveus, has previously been mentioned on this site due to its unfortunate extinction as a result of the wasting epidemic that devastated sea-grass populations in the 1930s. Deep-water patellogastropods may be found on sunken wood, or they may live in association with hydrothermal vents or cold seeps. One such genus, Serradonta, is restricted to the tubes of vestimentiferan worms (Nakano & Sasaki 2011).

Whitecap limpet Acmaea mitra, copyright Mary Jo Adams.


Phylogenetically speaking, patellogastropods are a very interesting group indeed. Morphological studies have identified them as the sister group to all other living gastropods. They have a distinctive radula morphology, known as 'docoglossan', with each tooth-row of the radula containing a median tooth flanked by a small number of relatively simple lateral and marginal teeth (up to three pairs of each). This contrasts with the radulae of other gastropods, in which the teeth are more numerous and/or more specialised, but resembles the radula of other molluscan groups such as chitons. As such, patellogastropods may retain the plesiomorphic radula morphology of gastropods as a whole. Patellogastropods also have a shell microstructure that differs from that of other gastropods, and they have their gonads in a ventral position relative to the visceral mass instead of the dorsal position of other gastropods (Lindberg 2008). Some have even suggested that patellogastropods may represent a remnant lineage that never underwent the coiling that characterises other gastropods, but this seems unlikely. Patellogastropods do have some features, such as an asymmetrical position of the protoconch (larval shell) on the mature shell, that suggest coiled ancestors. Molecular studies, on the other hand, have been more equivocal in positioning the patellogastropods. Some have given results consistent with the morphological analyses, but a few have failed to place patellogastropods with the other gastropods at all (e.g. Giribet et al. 2006), while many have placed them in a more nested position deeper within the gastropods (Zapata et al. 2014).

Ringed blind limpet Cryptobranchia concentrica, copyright L. Schroeder.


At face value, the fossil record might actually appear to support a more nested position for patellogastropods. The fossil record for gastropods as a whole extends back to the Cambrian, but the oldest definite record of patellogastropods goes back no further than the late Triassic (Frýda et al. 2009). If patellogastropods were indeed the earliest surviving gastropod lineage to diverge, where were they hiding for the intervening 200 million years or so? One possibility is that they were there, but did not get preserved. Phylogenetic analysis of living limpets suggests that the immediate ancestors of the patellogastropods clung to rocks in high-energy environments, with deep-water lineages occupying more nested positions in the tree (Nakano & Sasaki 2011). Such environments are not favourable to the fossil record, as the shells of dead limpets tend to get broken up by wave action before they have the chance to be fossilised. Another, more likely, possibility is that we have failed to recognise the coiled ancestors of the Patellogastropoda for what they are. The Palaeozoic gastropod fauna included a number of groups that are not present in the modern day; it is quite possible that one of these groups gave rise to the patellogastropods. One group that has specifically been nominated as a possible relative of the patellogastropods is the euomphaloids. But for now, Clamp the Mighty Limpet is holding the secrets of his affinities well hidden, and it will not be easy prising them off him.

REFERENCES

Frýda, J., P. R. Racheboeuf, B. Frýdová, L. Ferrová, M. Mergl & S. Berkyová. 2009. Platyceratid gastropods—stem group of patellogastropods, neritimorphs or something else? Bulletin of Geosciences 84 (1): 107-120.

Giribet, G., A. Okusu, A. R. Lindgren, S. W. Huff, M. Schrödl & M. K. Nishiguchi. 2006. Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons. Proceedings of the National Academy of Sciences of the USA 103 (20): 7723-7728.

Lindberg, D. R. 2008. Patellogastropoda, Neritimorpha, and Cocculinoidea. In: Ponder, W. F., & D. R. Lindberg (eds) Phylogeny and Evolution of the Mollusca, pp. 271–296. University of California Press.

Nakano, N., & T. Sasaki. 2011. Recent advances in molecular phylogeny, systematics and evolution of patellogastropod limpets. Journal of Molluscan Studies 77 (3): 203-217.

Zapata, F., N. G. Wilson, M. Howison, S. C. S. Andrade, K. M. Jörger, M. Schrödl, F. E. Goetz, G. Giribet & C. W. Dunn. 2014. Phylogenomic analyses of deep gastropod relationships reject Orthogastropoda. Proceedings of the Royal Society of London Series B 281: 20141739.

Sybra punctatostriata

Sybra punctatostriata, from here.


Just a very brief post today, because once again I've drawn a species that I haven't been able to find too much about. Sybra punctatostriata is a member of the Cerambycidae, the longicorn beetles, a diverse but (usually) fairly distinctive group of beetles whose larvae burrow into wood and plant stems. This species was first described by H. W. Bates in 1866 as part of a collection of beetles from Taiwan. Since then, it has been recorded over a wide area stretching from Japan in the north to Hainan in the south (though mostly in Japanese and Chinese sources that I don't have access to, and probably couldn't follow if I did). However, Samuelson (1965) indicated that S. punctatostriata was one of a number of very similar species in the genus Sybra, hinting that its range may require revision. Samuelson himself only identified a single specimen of S. punctatostriata from Okinawa in collections of beetles from the Ryukyus, which he described as showing some small differences from Taiwan specimens.

Another view of Sybra punctatostriata from the same site.


Sybra punctatostriata has been recorded feeding on Gossypium, the genus that includes cotton. However, other Sybra species are known to have wide host ranges. Sybra alternans, a species that has been recorded infesting bananas, has also been known to feed on fig trees, pineapple plants, passionfruit vines, beans... (Chen et al. 2001). It is possible that S. punctatostriata's host range may also be wider than recorded.

REFERENCES

Bates, H. W. 1866. On a collection of Coleoptera from Formosa, sent home by R. Swinhoe, Esq., H.B.M. Consul, Formosa. Proceedings of the Zoological Society of London 1866: 339-355.

Chen, H., A. Ota & G. E. Fonsah. 2001. Infestation of Sybra alternans (Cerambycidae: Coleoptera) in a Hawaii banana plantation. Proc. Hawaiian Entomol. Soc. 35: 119-122.

Samuelson, G. A. 1965. The Cerambycidae (Coleopt.) of the Ryukyu Archipelago II, Lamiinae. Pacific Insects 7 (1): 82-130.

Wasps in the Sand

Sand wasp Bembix oculata with prey (a bombyliid, I think), copyright Carlos Enrique Hermosilla.


The sand wasps of the tribe Bembicini are a diverse group of about 500 species of wasp that get their name because, obviously, of their habit of constructing burrows in sand-banks. Like other members of the wasp family Crabronidae, sand wasps provision these burrows with food for their larvae in the form of other insects. The adults themselves feed on nectar. Some of the Bembicini are among the larger members of the Crabronidae, and most of them are strikingly marked in black and yellow, white or red. Other characteristics of the group include an elongate labrum above the mouth, and the reduction of the ocelli, often to simple scars.

Bembicins are divided between a reasonable number of genera (Bohart & Menke, 1976, listed fifteen, but subsequent authors have recognised more) but the greater number of species are included in just one of these, the cosmopolitan Bembix with over 300 species (some older sources spell this name 'Bembex', but Bembix seems to be correct). Bembix is also the only genus found outside the Americas. Bohart & Menke (1976) suggested three main lineages within the Bembicini: one containing the relatively plesiomorphic genera Microbembex and Bicyrtes, a group of four genera including Stictiella and Glenostictia in which the ocelli are sunken into pits, and a large group containing genera related to Bembix with a raised welt at the front of the scutum on the thorax.

Sand Wasp - Bembix americana from Dick Walton on Vimeo.


While other wasps will lay their egg(s) on a paralysed insect in a brood chamber and then fly off never to return, many bembicins continue to bring fresh food items to the burrow throughout their larvae's development (take a look at the video above, by Dick Walton). The majority of bembecins provide their larvae with flies as food, which they paralyse with their sting and then carry back to the burrow between their mid-legs. Only a small number of genera regularly use other prey, though notably these genera include both Bicyrtes and Microbembex (so predation on flies is possibly ancestral for a clade excluding these two genera rather than for the tribe as a whole). Bicyrtes species stock their burrows with bugs (Heteroptera), most commonly nymphs. Microbembex species are the gourmands of the tribe, taking prey ranging from mayflies to midges. They are also somewhat unusual in that they stock their burrows with dead as well as paralysed insects (because they are providing food continuously, freshness over an extended period is less important than it is for other wasps). Females may compete for dead insects: in the words of J. Parker (as quoted by Bohart & Menke, 1976): "The struggles at the mouth of the burrow for the possession of a dead insect are frequent and furious, the contestants grappling and rolling over and over on the sand. Frequently it happens that the prey is dropped in the struggle, and while the pair of contestants are rolling on the sand a third wasp comes along and settles the quarrel by quietly carrying off the coveted treasure". Of the other bembecin genera, species of the genera Stictiella and Editha are predators of Lepidoptera. Editha species are found in southern South America, and include the largest of the bembecins. Xerostictia longilabris, a member of the Stictiella-group from southern North America that gets its own genus, has been recorded stocking its burrow with ant-lions and flatid bugs (Evans 2002). Members of other genera may also stock their burrows with prey other than flies, though in the majority of cases they do not do so exclusively (Evans 2002).

Sand wasp, possibly Microbembex monodonta, burying the entrance to her burrow. Copyright Tim Lethbridge.


Many bembecins (most notably in the genera Microbembex and Bembix) nest gregariously, and may form sizable colonies. Species of Bembix will maintain the same colony from year to year. In some species, such as B. pallidipicta, the females may dig accessory burrows near the main burrow without laying eggs in them; these have been presumed to act as decoys to discourage parasitoids or kleptoparasites. Males perform prolonged 'sun dances' above the colony in which they fly in circles or figures-of-eight, looking out for attractive females (O'Neill 2001). Males of many species have a serrated mid-femur that they use to hold down the female's wings while mating; in species without these leg serrations, mating may involve more of a struggle (Bohart & Menke 1976). The male produces a loud chirping during mating, presumably just to add atmosphere.

REFERENCES

Bohart, R. M., & A. S. Menke. 1976. Sphecid Wasps of the World: a generic revision. University of California Press.

Evans, H. E. 2002. A review of prey choice in bembicine sand wasps (Hymenoptera: Sphecidae). Neotropical Entomology 31 (1): 1-11.

O'Neill, K. M. 2001. Solitary Wasps: behavior and natural history. Cornell University Press.

Continued Adventures with Rake-legged Mites

Dorsal view of Neocaeculus kinnearae; photo modified for Taylor (2014).


It's been a noteworthy couple of weeks here at chez CoO. My contract at the university has reached its end, and I've become a Free Agent ('free' as in 'I don't get paid for any of this stuff'). That bit in the sidebar where I describe myself as "an entomologist working on the identification of terrestrial invertebrates" is, for the nonce, more of an aspiration. Or, to put another way, a lie. And my lunch breaks have gotten a lot more generous. Time will tell how long this state of affairs will continue, but in the meantime, there's still research to be done and papers to produce. Which segue's nicely into the subject of today's post: my newest paper, "Two further Neocaeculus species (Acari: Prostigmata: Caeculidae) from Barrow Island, Western Australia".

Back in December, I commented on the publication by myself and my colleagues of the rake-legged mite Neocaeculus imperfectus. In the comments for that post, I indicated that N. imperfectus was not the only new rake-legged mite species that I had on hand*. As it turns out, there was a total of five different species of caeculid in our collection (including N. imperfectus). Two of these were species that had already been described by Coineau & Enns (1969), but two more were new. The first of these I have dubbed Neocaeculus kinnearae, after my colleague Adrianne Kinnear, to whom I owe all of my understanding of mites. Adrianne first identified many of the Barrow Island mites, and trained me in mite identification prior to her recent retirement. Neocaeculus kinnearae is very similar to a species originally described from the Kimberley region of northern Western Australia, N. knoepffleri (some of you not familiar with Australian geography may still know the Kimberley as one of the few regions that diamonds come from). The two primarily differ in size (N. kinnearae is distinctly smaller) and while the large leg-spines in N. knoepffleri end in sharp points, those of N. kinnearae are blunter. Interestingly, N. knoepffleri is also present on Barrow Island, which did lead me to wonder if the specimens I ended up assigning to N. kinnearae might be just smaller individuals of N. knoepffleri. But I have seen several specimens of both from Barrow by now, and I'm yet to see any overlap between the two, so I do think that they are both good species.

*You may wonder why, if I knew that there was more than one species present, I didn't just put them all in the one paper. The reason was that, because I had never prepared a mite taxonomic paper before, I wanted to just do the one species at first as a test run, and then do the others once I felt a bit more confident that I knew what I was doing.

Neocaeculus knoepffleri also has a particular claim on my affections in that I've seen it alive. This is a bigger deal than it sounds: caeculids are cryptic and slow-moving, so observing them in the field is notoriously difficult. I already explained in my earlier post how I've never seen Neocaeculus imperfectus out and about, despite it turning up in samples in numbers that suggest absolute plagues of the things. But on my last trip to Barrow back in March, we were out collecting at night near the shore when I spotted a small grey point on a grey rock move slightly. Closer inspection revealed a caeculid sitting on the rock with spiny front legs outstretched, in the classic caeculid ambush pose. If I moved my forceps close to it, the mite would turn towards them as if warding them off. Having found one, I looked a bit further, and found several more, all perfectly camouflaged against the rock.

Dorsal view of Neocaeculus nudonates; original version of photo used in Taylor (2014).


The second new species was the smallest caeculid I had seen from Barrow so far, but was none the less remarkable. For a start, it didn't have the slender spines on its front legs of other rake-legged mites; instead, the spines were modified into rounded paddles. There are a few other caeculids known to have this feature (one of them, Neocaeculus bornemisszai, is another Kimberley species now also known from Barrow). Where their habits are known, it seems to be an adaptation for digging in sand, something that I can assure you Barrow Island has no shortage of. The other interesting feature of the new species is that the dorsal plates that usually cover the rear half of the body in other caeculids are unusually small. It was the appearance given by these small plates that inspired the name I gave this species: nudonates, from the Latin nudus, naked, and nates, buttocks. This is, literally, the bare-arsed mite.

So Barrow Island is now officially home to five different species of caeculid mite (and shortly after submitting this paper, I came across specimens of a sixth). While only one of these species has actually been observed alive, we can still infer some things about the likely habits of the others. Two, Neocaeculus bornemisszai and N. nudonates, are probably diggers of some sort. They may prefer different substrates: while N. nudonates has the afore-mentioned reduced plates, N. bornemisszai is more heavily armoured than usual. There may be a difference in preferred substrate between N. knoepffleri and N. kinnearae as well, to explain the blunter leg spines of the latter. In the paper, I suggested that N. imperfectus was probably a climber on vegetation, to explain it mostly being found in suction samples while the other species all came from pitfall traps. Since the paper was submitted, I have seen a few N. kinnearae specimens in suction samples, but I still think that it is most likely not a climber because these have been few and far between.

The other main point to be made is that there are probably a lot more caeculids out there than we realise. Though only eight species of caeculid have been recorded from Australia so far, Barrow Island is home to at least six (one of which may or may not be a further undescribed species). Caeculids are generally regarded as associated with warmer, drier habitats, and Australia is almost entirely warmer, drier habitat. I would not be surprised if, once we looked further, we would find a lot more undescribed caeculids out there.

Stunning Central American Millipedes

Blue cloud forest millipede Pararhachistes potosinus, copyright Luis Stevens.


For my semi-random selection of taxon to write about this week, I drew the millipede family Rhachodesmidae. Rhachodesmids are members of the millipede group called the Polydesmida, characterised by the presence of lateral keels on each segment of the body. The presence of these keels had lead to platydesmidans sometimes being referred to as 'flat-backed millipedes' though depending on how strong the keels are, not all species are necessarily 'flat-backed'.

In an earlier post on millipedes, I stressed the importance of genitalia in characterising millipedes, and the Rhachodesmidae are no exception. In polydesmidans, it is the front pair of legs on the seventh segment that is modified into the gonopods in males (with one notable exception that I may refer to later). Gonopods of rhachodesmids lack the solenite or coxal spur found in many other polydesmidans, and the inner side of the gonopod has a distinct elongate or oval concavity that is densely setose. Other noteworthy features of rhachodesmids are that they are often relatively large, with a conical terminal segment and more or less thickened rims to the lateral keels (Loomis 1964).

An unidentified rhachodesmid, copyright Sergio Niebla.


Beyond that, rhachodesmids become a little more difficult to characterise. Though they are not a widespread group, being restricted to Mexico and Central America, they are very diverse in appearance. Loomis & Hoffman (1962) commented that, "Rhachodesmoids collectively are members of a group notable for great variability and the development of bizarre features. Among their ranks we find millipeds which are bright blue, green, orange, and even pure white as adults; here the gonopod structure ranges from the normal polydesmoid appearance down to monoarticular fused remnants. Body form varies from a slender juliform shape to broad, flat, limaciform contour. Within the limits of this so-called single family occurs more variation than in all of the remaining polydesmoids." They also noted that the group was in need of review, something that apparently remains undone to this day (though there is someone working on it). If the photographs I've commandeered in this post are any indication, this is definitely a group that deserves more love.

Paratype of Tridontomus procerus, from Loomis & Hoffman (1962).


Loomis & Hoffman (1962) made their comments in comparing the Rhachodesmidae to another Central American polydesmidan family they were then describing as new, the Tridontomidae, and if I'm referring to the rhachodesmids then I should probably give a shout-out to these remarkable beasts as well. So far as I've found, this family is still only known from two species, Tridontomus procerus and Aenigmopus alatus, from Guatemala. Not only is the appearance of tridontomids striking, with long spinose processes on either side of the body, but the genital morphology of one species, A. alatus, is especially bizarre: it doesn't have any where it should. Where males of other polydesmidans have the legs of the seventh segment modified into gonopods, those of A. alatus have a perfectly ordinary pair of walking legs. In normal polydesmidans, the gonopods are used to transfer sperm from seminal processes on the coxae of the second pair of legs to the female's genital opening (more details are available here), but obviously Aenigmopus must do things differently. The seminal processes are still present, and the second legs themselves are thickened compared to other millipedes; it is possible that they are somehow used to transfer sperm directly from process to female without the use of gonopods. However it does it, there is no question that Aenigmopus is unique in the world of polydesmidans.

REFERENCES

Loomis, H. F. 1964. The millipeds of Panama (Diplopoda). Fieldiana: Zoology 47 (1): 1-136.

Loomis, H. F., & R. L. Hoffman. 1962. A remarkable new family of spined polydesmoid Diplopoda, including a species lacking gonopods in the male sex. Proceedings of the Biological Society of Washington 75: 145-158.

Antipatharia: The Black Corals

The black coral Antipathes, copyright Jez Tryner.


One piece of trivia I've learnt while looking stuff up for this post: the genus name Antipathes, from which the whole group of the Antipatharia derives its name, was coined to refer to the supposed ability of black coral to cure illnesses and protect against evil. It almost goes without saying that I found no indications that this evaluation was warranted.

The black corals of the Antipatharia are a group of colonial, sessile cnidarians that are found in marine waters around the world. They are predominantly deep-water animals, found mostly below the level of light penetrance. Those individuals that are found in shallower waters still keep to secluded habitats out of the light. Some of the shallowest communities are found in New Zealand at depths of only 4 m in the fiords of the South Island, where a rich concentration of tannins in the top layer of the water prevents light from reaching even that far down (Wagner et al. 2012). Black corals have been harvested in many parts of the world for jewellery (and also for their supposed curative properties referred to above), but they are very slow-growing animals. At least one colony subjected to radiocarbon dating was estimated to be over 4000 years old (Roark et al. 2009).

Wire coral Cirrhipathes, copyright Frédéric Ducarme.


Colonies of antipatharians may be highly branched, or they may form an unbranched whip (the latter forms are sometimes referred to as wire corals or whip corals). They may be only a few centimetres tall, or they may reach a length of several metres in the case of some wire corals (Wagner et al. 2012). The core of the colony is a stalk composed of chitin that varies in colour from jet black in the main stem to golden yellow at branch tips. The stalk is lined with spines that may be simple cones, or may be covered with denticles, or may even be branched and antler-like. In life, the stalk is encased in living tissue, so black corals are not actually black. Unlike other skeletonised cnidarians in which the polyps are recessed within the skeleton, those of antipatharians are entirely external to it. As a result, black corals are rarely found in locations where there is a lot of moving sediment in the water, as they lack the ability to entirely retract the polyps to protect them from abrasion. The individual polyps are usually only a few milimetres wide and up to a few centimetres tall when extended. All antipatharian polyps have six tentacles and six primary mesenteries; depending on the species, there may also be four or six secondary mesenteries, though members of the family Cladopathidae lack secondary mesenteries altogether.

The most recent classification of the Antipatharia divides it between seven families, some of which have been recognised only very recently. Because their deep-water habitat makes the study of live colonies difficult, and many features of the minute polyps become obscured in preserved material, earlier classifications focused heavily on features such as the branching arrangement of the colony, or the morphology of the spines on the skeletal axis. However, these features may be influenced by environmental factors, and their significance may have been overestimated. For instance, a molecular phylogenetic analysis by Brugler et al. (2013) found that the unbranched wire coral genus Cirrhipathes was polyphyletic and not separated from the branched genus Antipathes. Nevertheless, Brugler et al. did find that the higher-level relationships within the Antipatharia were mostly concordant with morphology, including the distinction of the seven families. These relationships included a divergent position for Leiopathes, the only genus with six secondary mesenteries; a clade including the bathyal families Schizopathidae and Cladopathidae, in which the polyps are transversely elongated; a close relationship between the families Myriopathidae and Stylopathidae, with polyps that are not elongated and have relatively short, subequal tentacles; and an association of the families Antipathidae and Aphanipathidae, in which the sagittal tentacles tend to be quite elongate relative to the lateral tentacles. There was still, of course, room for investigation: one notable anomaly is that the type species of Antipathes, A. dichotoma, was identified as a member of 'Aphanipathidae' rather than 'Antipathidae'. If correct, this would mean that aphanipathids should be called antipathids, while antipathids would be... something else.

REFERENCES

Brugler, M. R., D. M. Opresko & S. C. France. 2013. The evolutionary history of the order Antipatharia (Cnidaria: Anthozoa: Hexacorallia) as inferred from mitochondrial and nuclear DNA: implications for black coral taxonomy and systematics. Zoological Journal of the Linnean Society 169: 312-361.

Roark, E. B., T. P. Guilderson, R. B. Dunbar, S. J. Fallon & D. A. Mucciarone. 2009. Extreme longevity in proteinaceous deep-sea corals. Proceedings of the National Academy of Sciences of the USA 106 (13): 5204-5208.

Wagner, D., D. J. Luck & R. J. Toonen. 2012. The biology and ecology of black corals (Cnidaria: Anthozoa: Hexacorallia: Antipatharia). Advances in Marine Biology 63: 67-132.