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

Some peculiarities of the Antarctic ophiuroids

Ophiozonella sp. and Ophioceres sp. in ventral view

Mixture of Paleozoic and deep sea fauna

The benthos of shallow-waters in Antarctica is a mixture of:

–       Faunas that are common to Australia and New Zealand,  South America, South Africa, but also with a large number of endemics.

–       Similarities to paleozoic faunas.

–       Invasion of the organisms proceeding from the adjacent deep-sea communities.

The absence of crabs and sharks, as well as scarce biodiversity and biomass of teleosts implies a low predation on exoskeleton zoological groups, which causes an abundance in biodiversity and biomass of echinoderms (Aroson & Blake 2001).

Viviparism

Among the Antarctic ophiuroids there is a higher proportion of viviparous species than in other marine areas. Such a thing does not seem to be an adaptation to low temperatures, but to the geographic type of insular habitats of shallow waters and to the insulation by the circumpolar current, the adaptation that prevents the dispersal of larvae Ostergren (1912).

There are viviparous species that are multibrachial (like Ophiacantha vivipara and Ophionotus hexactis), and there are species which are proximate to the aforementioned ones that, being pentamers, are not viviparous (like Ophiacantha pentactis and Ophionotus victoriae). Such a mechanism permits to accommodate more embryos by having more bursae Smirnov (1984).

Ophionotus hexactis with an embryo

Incubation period

The viviparous species have long incubation period (in comparison with the tropical species or the species from temperate waters) and very slow rhythm of growth, measured with Von Bertalanffy growth constant, the fact that indicates that large size individuals are very long-lived, like, for example, in the case of Astrotoma agasiizii, which is one of the largest ophiuroids and whose age calculated from vertebral growth rings can reach 91 years Dahm (1996).

Gigantism

The cases of gigantism among some representatives of the Antarctic fauna can also be found among the ophiuroids; so, while the average disk size of ophiuroids, generally measured in millimeters, is about 10-15 mm, there are some Antarctic ophiuroids with disk size of about some centimeters like, for example, Ophionotus victoriae with disk diameter up to 43 mm, Ophiosparte gigas, up to 60 mm, or Astrotoma agasiizii, up to 60 mm.

Vertebra with growth rings, Ophionothus victoriae

A,distal surface; B,proximal surface; 1,canal for radial nerve; 2,fossa for lower intervertebral muscle; 3,fossa for upper intervertebral muscles; 4, central projection; 5, central depression

Vertebra with growth rings, Astrotoma agassizii

A,distal surface; B,proximal surface  1, hour-glass projection; 2, canal for radial nerve

Can they adapt?

Another peculiarity that suggest  the investigations of Peck et al (2009) may be the limited adaptability to environmental variations. His research in aquarium with increasing temperature give for Ophionotus victoriae a scarce survival time. The ophiuroids begin to die from the 24th day at 2 º C, and the 19th at 3 ° C. It seems to suggest that the current global climate change could cause the loss of biodiversity in the Antarctic benthos because of a small temperature increase.

References

Aronson RB, Blake D (2001) Global Climate Change and the Origin of Modern Benthic Communities in Antarctica AMER. ZOOL., 41:27–39

Dahm (1996) Ecology and Population Dynamics of Antarctic Ophiuroids (Echinodermata). Ber. Polarforsch. 194

Peck LL S, Massey A, Thorne M A S, Clark M S (2009) Lack of acclimation in Ophionotus victoriae: brittle stars are not fish. Polar Biol 32: 399-402

Peck LL S,  Antartctic marine stenotherms, temperature seasonality and change. Powerpoint presentation.

Östergren, H (1912) Über dieBrutpfl ege der Echinodermen in den Südpolaren Küstengenbieten. Zeitschrift Wissenschaften Zoologie, 101:325– 341

Smirnov IS (1984)  Fauna of antarctic and subantarctic brittle-stars. Ph.D . Leningrad. 1984

January 2010

Ophiosparte gigas, the monster

Among the specimens that were captured during Australian expedition in 1922, there were three examples of what Koehler considered at that time, apart from the unpleasant appearance due to its thick fleshy skin , as one of the largest ophiuroids ever seen till then, the biggest one of those three had a disk of 48 mm. He assigned it a new genus, Ophiosparte and, due to its size, called it gigas, perhaps he intuited that in future expeditions there would be caught larger specimens, though I doubt he thought that there would have been samples of up to 70 mm of the disk.

Ophiosparte gigas, is one of the most fearsome predators of the Antarctic benthos. Where there is no sharks, crabs and few teleosts, the echinoderms are one of the dominant groups and amog them the brittle stars that, forming dense swarms, cover with their arms large extentions of the bottom. Ophiosparte is not the most abundant species, that role is left to Ophionotus victoriae, something like half fox and half wolf, but once Ophiosparte – the bear – appears, that fox-wolf will not hesitate to run away, crestfallen . Wandering through the soft-bottom, using spatulate brachial spines as oars and helping itself with the chemoreceptors of its brachial tentacles, it tracks its prey and, if starving, it will not be averse to a corpse.

Males and females, probably bad-tempered, will have to meet and be close one to another for spawning to give rise to large larvae, which in Ross and Weddell seas and around the Antarctic Peninsula will be the future of the species, which will remain a mystery for taxonomists who come across it and challenge the traditional systematic doubting whether it is a Ophiacanthidae or Ophiomyxidae, reminding us that those are just the rails of our mind which want to pigeonhole such a marine monster.

The photos was made using Motic SMZ-168 TL stereo microscope.

References

Bernasconi I, D’Agostino MM (1974) Equinodermos antárticos. III. Ofiuroideos. 1. Ofiuroideos del extremo norte de la Peninsula Antartica. Revista Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’. Hidrobiologia 4(2):80-133

Dearborn JH, Hendler G, Edwards KC (1996) The diet of Ophiosparte gigas (Echinodermata: Ophiuroidea) along the Antarctic Peninsula, with comments on its taxonomic status. Polar Biol 16: 309-320

Fell HB (1961) The Fauna of the Ross Sea: Ophiuroidea. Memoirs of the New Zealand Oceanographic Institute, 18, 1–79.

Koehler R (1922) Echinodermata Ophiuroidea. Scientific Reports Australasian Antarctic Expedition 1911–1914, 8, 1–98

February 2010

Ophiacanthidae or Ophiomyxidae?

Sometimes we have it just in front of our eyes, but only with the help of an open mind and a new form of seeing things it makes apparent.

Alexander Martynov from Zoological Museum of Moscow State University, carrying out a revolutionary systematic work in accordance with the structures of the arm-spine articulation ridges, establishes a surprising and convincing change: Ophiosparte gigas is an archaic member of Ophiuridae family.

Reference

Martynov AV (2010) Structure of the arm spine articulation ridges as a basis for taxonomy of Ophiuroidea (a preliminary report). Echinoderms: Durham. Proceedings of the 12th International Echinoderm Conference. 233-239

April 2010