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