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).

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.

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.

November 2010