Rafael Microfauna Marina: Ophiacantha

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Symbiosis: Cancerilla - Ophioika (parasitism)

Ophiuroidea – Copepoda

First the ectoparasites copepods of Antarctic ofiuroids were cited by Mortensen (1936), but without identifying the species, it was Heegaard (1951) who identified them. In the taxonomic literature they have also been cited by Madsen (1967).

Ophiacantha antarctica: host

Cancerilla sp: ectoparasite

The copepods are adhered to the dorsal surface of the disc. Females are carrying their bags of eggs like balls, males have not been found.

Ophiacantha antarctica: host

Ophioika sp: endoparasite

Copepods with modified morphology occupy nearly an entire interradius of the disk. In this species very small males are situated inside of females brood-pouch.

In his review of Ophiuroidea reproduction, Hendler (1991), cites a work of Bartsch (1975) in which he described a castration as a consequence of an ectoparasite copepod (Cancerilla oblonga) in Amphiura capensis. This is an issue to investigate whether in Antarctic ofiuroids there are similar consequences.

Specimens conserved in 70% ethanol, all photographs were made in ethanol.

Collected by Pablo J. González-López. Cruiser ANTARKTIS XXIII/8 Polarstern.

Identified by Rafael Martín-Ledo.
The study was made using Motic SMZ-168 TL stereo microscope.

References

Heegaard, P (1951) Antarctic parasitic copepods and an ascothoracid cirriped from brittle-stars. Vidensk. Medd. Dan. Naturhist. Foren. 113, 171-190

Hendler, G (1991) Echinodermata: Ophiuroidea. In: Giese, A.C.,

Pearse, J.S. & Pearse, V.B. (Eds.), Reproduction of marine invertebrates: echinoderms and lophophorates. The Boxwood Press, Pacific Grove, CA, pp. 355–511

Madsen FJ (1967) Ophiuroidea. B.A.N.Z. Antarctic Research Expedition (1929–1931) under the Command of Sir Douglas Mawson Rep Ser B 9:123–145

Mortensen T (1936) Echinoidea and Ophiuroidea. Discovery Reports, National Institute of Oceanography Cambridge 12:199–348

July 2010

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

Soy Rafael Martín

Profesor de Biología y Geología en el I.E.S. Santa Bárbara de Málaga. Licenciado en Ciencias Biológicas con especialidad en Zoología y Master en Investigación Científica.
Estudio organismos marinos de bentos; con especialización en la taxonomía y biogeografía de ofiuroideos (equinodermos) antárticos.

Con un estereomicroscopio (Zeiss Discovery V8) en el Natural History Museum de Londres; viendo especímenes de la expedición Challenger (1872-1876) .

Así es como llegan los especímenes de las expediciones, conservados en formaldehído o etanol, a la espera de una ardua labor de identificación.

BAS (British Antarctic Survey -Cambridge-).

He realizado estancias de investigación en la British Antarctic Survey (Cambridge; Inglaterra), para el estudio de diversas colecciones de ofiuroideos pertenecientes a expediciones británicas y alemanas al Océano Sur alrededor Antártida.

En el laboratorio de BAS con Chester Sand, ecólogo molecular y Huw Griffths, biogeógrafo marino.

En el laboratorio de BAS con los botes de muestras de ofiuroideos de una expedición británica en aguas del Arco de Scotia en la Antártida; el estereomicroscopio es un Zeiss Stemi SV 6.

En el laboratorio de BAS con estereomicroscopio Leica M65

Ophiosteira bullivanti, fotografía tomada con cámara CCD en un estereomicroscopio Leica M65.

Microscopio electrónico (Quanta 3D FEG/FEI SEM) en la Universidad de Extremadura.

Fotografías SEM (microscopía electrónica de barrido) de Ophiacanta wolftarntzi. Una nueva especie descrita, por el que esto escribe, colectada por el buque de investigación alemán Polarstern en aguas antárticas.