Symbiosis: Amphiura - Eulimidae (parasitism)

Amphiura joubini: host

Eulimids: endoparasite

In a Amphiura belgicae specimen, caught in South Georgia, Mortensen (1936) described a gastropod that could belong to the same species as the one from the present study. Mortensen described as a possible mechanism of parasite infestation its entrance through an orifice. He also indicated that the adult looked like a sack and had no visible shell.

In the examined specimens of Amphiura joubini from South Shetland we can  see externally one or two large whitish interradios. When we opened the animals, we saw a bag with numerous white or flesh colored shells inside the coelom, the adult being globular.

There is no observable orifice through which the gastropod entered, so there is a possibility of its regeneration. However, in one of the specimens that had two parasites in contiguous interradios, the other interradios are open ventrally, which leads us to think that this would be an “exit” mechanism for the gastropods.

This parasitism does not necessarily mean the death of the brittle star, possibly due to its great regenerative capacity. Although this is only a supposition, based on the large number of examined specimens of Amphiura joubini with badly damaged interradios due to the process of youth parasitic forms leaving their host.

There have also been numerous open specimens of Ophiolimna antarctica, without most of the aboral disk, it is hard to find a “whole” specimen in our collection. In one of the specimens a pair of juvenile gastropods were found, so it does not seem to be a specific Amphiura genus parasitism.

Although there is a reference in the literature on the presence of internal eulimidae parasites, not yet been identified at specific level (Anders Waren, pers. Comm.)

Live Amphiura jouvini

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

Symbiosis: Ophioplinthus - Ascothorax (parasitism)

Ophioplinthus brevirima: Host
Ascothorax sp: endoparasite

Inside there were several parasites (Ascothorax sp) with globose aspect; the smaller ones (probably males) with better distinguished morphology having two valve, while the larger specimens are possibly females. This difference in size, and better conservation of structures in male specimens is a common phenomenon in many parasitic crustaceans.

Ophioplinthus brevirima has short genital  slits, hence its specific epithet. In the studied specimens one of the slits, which coincides with the deformed area of the disc has a deformation making it larger, and the proximate plates look abnormal. It is likely had been caused when the endoparasites were leaving their host.

This is supposedly the first record of endoparasites for Ophioplinthus brevirima.

Materials studied from Expediton ANTARKTIS-XXIII/8 Polarstern 2006/2007 in Snow Hill and Dundee Islands. Specimens conserved in 70% ethanol, all photography were made in ethanol using Motic SMZ-168 TL  stereo microscope.
Collected by Pablo J. González-López
Identified by Rafael Martín-Ledo
July 2010

Live Ophioplinthus brevirima, covered by Iophon sponge

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

Pycnogonid on Ophioplinthus

This morning, after a delicious breakfast at Vicente’s, I was in my laboratory ready to observe specimens, when I suddenly found a beautiful composition: a pycnogonid (Austrodecus sp) grasping the spicules of the Iophon sponge which covers the disk of a Ophioplinthus gelida.

The pycnogonids, sea spiders, are marine arthropods that feed mainly on hydroids, bryozoans, anemones and other soft-bodied animals in which insert their proboscis to suck their fluids. In Antarctic waters there are about 175 species, which amounts to be 18% of the pycnogonids on a world scale.

Hardly any brittle star can be found among their prey because of having the body covered with calcareous plates which, like an armor-plate, may dissuade many predators who are not armed with appropriate dentition or stylet (pricker).

So, this picture might seem to be a portrait of a process of micropredation or even ectoparasitism, but I tend to consider it just an accident, although there can appear a supposition that what the pycnogonid might have been really looking for could have been the abundat offspring of gemmules of the Iophon sponge.

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.

March 2010

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.

November 2011

Astrotoma agassizii Lyman, 1875

One of the most emblematic animals from Antarctic waters is the euryalid Astrotoma agassizii Lyman, 1875. Gigant, long-lived, with a disk diameter of up to 70 mm and an age which could overpass 100 years; Dahm (1996) calculated, by the growth rings of the arm vertebrae, the age of 90 years for a specimen of disc less than 50 mm.

Astrotoma is a predator, who climbs up the gorgonians from Primnoella and Thouarella genus, waits patiently for the arrival of copepods which will be caught by its ambulacral tentacles Dearborn et al (1986). His rough and tough skin defends it from the nematocysts that the polyps of the Gorgon throw without success. Astrotoma grabs the gorgonia with its modified arm spines and the hooklets that cover the skin of its arms arranged in rings.

1. ambulacral pore, 2. arm spine, 3. arm ring with hooklets, 4. jaws, 5. genital slit, 6. radial shields, 7. madreporite

Astrotoma is a hermaphrodite incubator Bernasconi (1965), something common in Antarctic Brittle stars, but the thing that is not so common is that, without a planktonic development that would help to expand its geographical distribution, it however presents a wide circumpolar distribution in shelf waters, in addition to being presented in the waters of Magellan. The Antarctic Circumpolar Current is a divisor of polar and magellanic populations Hunter (2008), there is no gene flow but these are isolated populations, molecularly distinct, although it doesn’t seem to be any morphological differentiation. Smirnov (1986) suggested the possibility of phoresis by macrophytes, an interesting option that could explain their wide distribution.

Arm ring with hooklets

References

Bernasconi I (1965) Astrotoma agassizii Lyman, especie vivipara del Atlántico Sur. Physis 25 (69): 1-5

Dahm C (1996) Ökologie und populationsdynamik antarktischer ophiuroiden (Echinodermata). Ber Polarforsch 194: 1-289

Dearborn, JH, Ferrari FD,  Edwards KC (1986) Can pelagic aggregations cause benthic satiation? Feeding biology of the Antarctic brittle star Astrotoma agassizii (Echinodermata: Ophiuroidea). Ant. Res. Ser., 44 (Biology of the Antarctic Seas XVII): 1-28

Hunter RL & Halanych KM  (2008) Evaluating Connectivity in the Brooding Brittle Star Astrotoma agassizii across the Drake Passage in the Southern Ocean. J Hered 99:137-148

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

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

May 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