Brittle stars from Southern Ocean (Echinodermata: Ophiuroidea)

The present biogeographic study on the ophiuroid fauna from the Southern Ocean contains an updated checklist, based on a compilation of all the published information provided for the Antarctic and sub-Antarctic regions as well as the information available in SCAR-MarBIN database. Faunal composition and geographical and bathymetric distribution are included. So far, 219 species have been recorded, of which 126 are endemic to the SO, 76 are exclusive to Antarctic waters, and 30 are exclusive to sub-Antarctic waters. This study corroborated the circumpolar and eurybathic character of the ophiuroid fauna of the SO, but some differences are discussed when considering shelf and deep-sea fauna in the whole SO, or in the Antarctic and sub-Antarctic regions separately. The biogeographic affinities of 17 areas considered in the SO are revised, based on a presence/absence datamatrix of the 219 species. This similarity analysis shows three main groups, two of them including sub-Antarctic areas and one for Antarctic areas. The faunal movement patterns between the main geographical connections have been based on historical site records of each species. These movements have a level of faunal exchange that exceeds that of other Antarctic benthic groups. Such movements are mainly from Antarctic and sub-Antarctic regions to the subtropical waters of South America, and from New Zealand and southern Australian waters to sub-Antarctic areas. In this context, the origin of the ophiuroid Antarctic fauna is discussed.

 

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Ophiacantha wolfarntzi: a new brooding species of brittle star (Echinodermata: Ophiuroidea) from Antarctic waters

A new brittle star attributable to the genus Ophiacantha is described from Antarctic waters, in the diffuse limits of the Antarctic Polar Front at Shag Rocks, South Georgia. The new species can be differentiated from its Southern Ocean congeners by the striations on the arm plates. Several adult individuals were observed brooding their juveniles, as the arms of the juveniles were seen emerging from the genital slits of the parent.

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A day at the Natural History Museum

One typical August morning in London (that means drizzling) Elena and me, accompanied by Chester Sands (molecular ecologist at the British Antarctic Survey BAS) found ourselves at the gates of the British Natural History Museum (NHM).

In the NHM, an architectural ensemble of unique and spectacular beauty, which blends a “cathedral” museum with modern research facilities, we were welcomed by Mr Andrew Cabrinovic, echinoderms curator of the Department of Zoology of the NHM. His kindness and attention to every single detail of our work were really praiseworthy.

The collections, magnificently preserved and neat, were at our disposal, and our “temporary” place of work was equipped with a great stereomicroscopy and a camera to take pictures of the specimens.

The research project which I participate in as ophiuroids taxonomist is about phylogeny and biogeography of Southern Ocean benthos. I’ve already identified several thousand of specimens from several expeditions made by the BAS. During my second stay in Cambridge, I identified the ophiuroids collected by Chester Sands during the expedition ANT XXVII / 3  to South Georgia, Larsen and Bouvet Island on board the German ship Polarstern, organized by the Alfred Wegener Institute.

To make the identifications reliable, it was necessary to compare our specimens with the specimens preserved in museums, either because some species are represented only by the holotype, or because of the variability or the lack of information of the original descriptions. Thus the aim of the visit was to see the specimens of the ophiuroids collected during expeditions Discovery I (1925) and Discovery II (1936) that constituted the base for one of the major monographs on the Southern Ocean ophiuroids, the work of Theodor Mortensen 1936. I also had the opportunity to see specimens from other expeditions (Challenger 1873-1876 and Quest 1921-1922).

What did that experience mean to me? I mean, the fact of spending a day working in one of the most important zoological research centers worldwide and having in my hands those specimens from the Discovery expeditions after years of rereading the work of Mortensen. Well, I suppose that, besides of a mere fact of collecting information necessary for identification, it could be comparable to the feelings of a Treki in a Star Trek convention being able to take a picture with Leonard Nimoy. Yes, just a day, but a day to remember.

© Rafael Martín-Ledo  2011

lllustrations of Southern Ocean ophiuroids: the origen

The scientific illustrations are an essential tool in the taxonomy. Drawings and photographs can be complementary and not redundant. The more precise in representing the characters of diagnosis, the more useful for identification of specimens. They should be used with the original descriptions and redescriptions which provide information on variability.

In 1843 the Germans Johannes Müller and Franz Hermann Troschel described the first species of ophiuroids in the Southern Ocean, it was Ophiolepis chilensis (species with subantarctic distribution) and was collected by Rudolph Amandus Philippi in the coastal waters of southern Chile. It was first illustrated only in 1875 by Theodore Lyman in Results of the Hassler Expedition. Quite poor drawing, if you can call it that way, that only represents one piece of a jaw and the first ventral arm segment of two arms. In the same work by Th. Lyman we can find a much more precise illustration of gorgonocephalidae Astrotoma agassizii and Astrophyton pourtalesii (Antarctic and sub-Antarctic species). Both illustrations were the first ones of the ophiuroids in the Southern Ocean.

Th. Studer described several species in 1876 (Gazelle expedition 1874-1876), but the illustrations made by him appeared in his later works only in 1880.

Th. Lyman made what is considered the most important work within the ophiurology: Report on the Ophiuroidea dredged by H.M.S. Challenger during the years 1873-76 which was published in 1882. In this major work he described numerous species present in the Southern Ocean. The illustrations are really beautiful and precise, but the descriptions of new species were published in two previous studies in 1878 and 1879 in the Bulletin of the Museum of Comparative Zoology of Harvard, with accurate illustrations but lacking the beauty of the anterior work.

In 1876 Edgar A. Smith from British Museum described the species Ophioglypha hexactis in the Magazine of Natural History. But only in 1879, in Echinodermata of Kerguelen Island of the Philosophical Transactions of the Royal Society he illustrates with great accuracy the species, as well as Ophiacantha vivipara ( species described by Ljungman in 1870 which had not been illustrated up to the date).

As far as the photographs of Southern Ocean species are concerned, the first of them which was photographed, was the aforementioned Ophiolepis chilensis, by Hubert Lyman Clark in Echinoderms of Peru in 1910. The photo of the brittle star is oral position, though you can hardly see any characters needed for correct identification.

The first photos of great descriptive quality were published in the monograph of Ophiuroidea of the Australian Antarctic Expedition (1911 – 1914) by Rene Koehler in 1922, it’s not known whether the pictures were taken by himself or by James Francis Hurley, the official photographer to the Australian Antarctic Expedition. There’s no doubt these pictures are the great example of good taxonomy work.

Another story is the case when the esthetic component was as important as the scientist one. Then, the purpose was to surprise an audience which from the last decade of the  nineteenth century were being marveled at the risky scientific voyages of exploration as well as at the organisms collected. The organisms which, in the case of the Southern Ocean could survive in such hostile conditions for a human beeing. Thus we can enjoy the illustration dated on 1882 (Challenger expedition) of Ophiacantha vivipara carrying over its offspring as a good caring mother with her youngsters in a glacial ocean.

In conclusion, some pictures which still amaze me. Koehler in his monograph Ophiures et Echinides dated on 1902  on Belgica expedition (1897-1899) illustrated the specimens with an idealized touch, but the most surprising thing about it is the degree of precision of the drawings, they could perfectly be confused with some 3D computer graphics illustrations.

 

 

 

 

 

 

© Rafael Martín-Ledo  2011

Can not be reproduced without author’s permission

Roofs and eyeglasses in Antarctic ophiuroids

In 1961 Fell, in his monograph on the Ross Sea Ophiuroidea, drew attention to the development of skeletal excrecence in Antarctic ophiuroids from Euvondrea and Ophiosteira genera. Fell found no explaination for these outgrowths.

These expansions, especially in the dorsal arm plates, can also be seen in other Antarctic species such as Ophiura (Ophiuroglypha) carinifera, Anophiura banzarei, Ophiomages cristatus, Ophiomastus bispinosus and some species of Ophioplinthus genus.

Ignoring the mere evolutionary whim, there could be various interpretations. For example, passive defense: the protrusive elements could dissuade the predators which would prefer something easier to chew and swallow. Or, acting as peaked roofs somewhere in Siberia or Pirinei Mountains –  little flat surface on aboral part protects them from heavy “snowfalls” and, so, doesn’t let them be buried in funds with a high degree of sedimentation. Or, the explanation could be the possible presence of amplifier lenses for photoreception (see symmetrical structures as honeycomb cells of Ophiosteira).

At any rate, nature manifests its spectacularity through the tiniest details of organisms which did not evolve to amaze us, nonetheless, we can’t help being marvelled at them.

© Rafael Martín-Ledo  2011

Can not be reproduced without author’s permission

Ophiocymbium antarcticus Martynov, 2010 sp. nov.

The study of the biodiversity of Antarctic waters has increased since 1993 so far, from 4000 to 8200 known species, many of which are brand new for the science. But it seemed that everything “was over” for the ophiuroids, since the last species of Antarctic waters ophiuroids was Ophiomastus trispinosus described by Bernasconi & D’Agostino in 1977, i.e. for the last 33 years the study of the biodiversity Antarctic ophiuroid fauna has not increased at all.
But everything has changed thanks to the great monograph of the Russian scientist Alexander Martynov (Zoological Museum, Moscow State University), called “Reassessment of the classification of the Ophiuroidea (Echinodermata), based on morphological characters. I. General character delineation and evaluation of the Families and Ophiacanthidae Ophiomyxidae“, and published in Zootaxa. This is the first one of a series that, when finished, will undoubtedly be one of the most important works on systematic of the ophiuroids in a global scale.
Based on a specimen deposited at the Smithsonian, collected in 1975 by the ship Orkney Islands (Expedition USAP) at a depth of about 2750 m in the waters of the Scotia Sea (South Sandwich Island) and identified due to a number of common characters as Ophiodaces cf. inanis Koehler, 1922, Martinov described a new species of Ophiocymbium.

In the monograph the author also makes (with the help of electron microscopy) an exhaustive morphological study of the taxonomical valuable characters of numerous species and, in some cases, making important systematic changes. 

Antarctic species that have been studied in the monograph are Ophionotus victoriae, Ophiosparte gigas, Ophiolimna antarctica and Ophiolycus nutrix (formerly Ophioscolex), as well as other species of wider distribution and also present in sub-Antarctic waters Ophiocymbium cavernosum, Ophiomyxa brevirima, Ophiernus vallincola and Amphipholis squamata.
Thirty-three years’ drought is over!

Gigantism and dwarfism

Gigantism is well known among Antarctic benthic organisms, although the fact is that size abnormalities occur in both directions (gigantism and dwarfism). Is it possible that it would be a case of evolution with a disruptive selection in which the organisms with extreme phenotypes would be the dominant forms?  That means, being small size organisms (ophiuroids with disc less than 5mm) and large size ones (ophiuroids with disk of about 30mm) more abundant in species compared to the middle size organisms (ophiuroids with disk of approximately 10 mm).

 A giant (Ophiosparte gigas) and a dwarf (Ophiosteira bullivanti)

The origin of dwarfism seems to be found in the difficulty to precipitate calcium carbonate at low temperatures, which is supposed to be a limiting factor for organisms with calcareous skeletons, such as calcareous foraminifera, prosobranch gastropods, bivalves and brachiopods, among which we can find Antarctic species of very small size. Antarctic echinoderms with a skeleton composed of calcareous dermal ossicles are “doomed” to higher energy consumption in order to incorporate calcium carbonate, compared to echinoderms from other geographical areas, for this reason, it seems logical to consider this enviroment to be a hostile one for this zoological group development, although it does not look to be like that, as  the echinoderms are among the benthic groups with most diversity and biomass in the Antarctic benthos.
Whatever the case and considering dwarf ophiuroid species those that do not reach 5 mm disk diameter , we have: Ophiacantha paramedea, Ophiomitrella ingrata, Amphiophiura antarctica, Ophiocten banzarei, Ophiocten bisquamatum, Ophiomastus conveniens, Ophiomastus ludwigi, Ophiomastus perforatus, Ophiomastus primula, Ophiomastus trispinosus, Ophiosteira bullivanti, Ophiopyrgus australis, Amphiura lymani,  Amphiura algida, Amphiura microplax, Amphiura monorima and Ophiozonella antarctica.

 Antarctic animals with siliceous skeletons or those with no mineralized skeleton, can multiply by up to ten the size of the individuals of the species compared to ones of the same groups in other geographical areas, as it happens to nemerteans, pycnogonids and giant isopods, like if meeting the Bergmann’s rule. The waters with low temperatures and seasonal food shortages (in winter) reduce basal metabolism which may cause a delay in sexual maturity and increased longevity, the two factors that, along with  an environment with little amount of predators can favor the development of a giant size.
Among the ophiuroids, even despite its calcareous skeleton, appear giant Antarctic species, considering giant the species with disk diameter over 30 mm, we have: Gorgonocephalus chilensis  (though the gorgonocephalids in general have large size) Astrotoma agassizii, Ophiosparte gigas, Ophionotus victoriae, Ophionotus hexactis, Ophiura flexibilis, Ophiura lenticularis, Ophiocamax gigas and Ophiocamax drygalskii.

 Taking the data of 117 species of ophiuroids present in Antarctic waters (I’ve excluded those with high distribution outside Antarctic waters) and represented according to disk sizes in ranges, we obtain the following graphic:

 

It shows a classic Gaussian bell curve, which clearly indicates an evolutionary “stabilizing type” tendency, i.e. where corporal phenotypes of middle size have been selected predominantly
When compared with species from other latitudes as in the case of brittle stars found in British waters (excluding cosmopolitan species), we have the following graphic as a result:

where we can see presented proportionally even more anomalous cases respecting size.
Therefore it doesn’t seem to be that the dwarfism or gigantism have been selective factors at group level in the Antarctic ophiuroids, but it is more about cases that have to be treated in a particular way.

Ophiosteira bullivanti: collected by Chester Sands (Britihs Antarctic Survey)

Ophiosparte gigas : collected by Pablo J. González-López (University of Seville)

Identified by Rafael Martín-Ledo

References

ARNAUD, P.M. 1974. Contribution a la bionomie marine benthique des regions antarctiques et subantartiques. Téthys, 6, 467–653.

SOUTHWARD, E.C. & CAMPBELL, A.C. 2006. Echinoderms. Synopses of the British Fauna. Edited by Crothers, J.H. & Hayward, P.J. The Linnean Society of London and Estuarine and Coastal Sciences Association.

TORTONESE, E. 1965. Echinodermata. Fauna D’Italia. Edizioni Calderini, Bologna.