Regular Sea Urchins Research Articles

Testing Mechanisms of Vision: Sea Urchin Spine Density Does Not Correlate With Vision-Related Environmental Characteristics.

Sea urchins do not have eyes, yet they are capable of resolving simple images. One suggestion as to the mechanism of this capability is that the spines shade off-axis light from reaching the photosensitive test (skeleton). Following this hypothesis, the density of spines across the body determines the resolution (or sharpness) of vision by restricting the incidence of light on the photosensitive skin of the animal, creating receptive areas of different minimum resolvable angles. Previous studies have shown that predicted resolutions in several species closely match behaviorally-determined resolutions, ranging from 10º to 33º. Here we present a comparative morphological survey of spine density with species representatives from 22 of the 24 families of regular sea urchins (Class Echinoidea) in order to better understand the relative influences of phylogenetic history and three visually-relevant environmental variables on this trait. We estimated predicted resolutions by calculating spine densities from photographs of spineless sea urchin tests (skeletons). Analyses showed a strong phylogenetic signal in spine density differences between species. Phylogenetically-corrected Generalized Least Squares (PGLS) models incorporating all habitat parameters were the most supported, and no particular parameter was significantly correlated with spine density. Spine density is subject to multiple, overlapping selective pressures and therefore it is possible that either: 1) spine density does not mediate spatial vision in echinoids, or 2) visual resolution via spine density is a downstream consequence of sea urchin morphology rather than a driving force of adaptation in these animals.

Inter and intraspecific comparisons of the skeletal Mg/Ca ratios of high latitude Antarctic echinoderms

AbstractEchinoderms are vulnerable to ocean acidification because of their high magnesium calcite skeletons. Here, skeletal Mg/Ca ratios were examined within and between individuals of 20 Antarctic echinoderms representative of the asteroids, ophiuroids and echinoids. The highest mean Mg/Ca ratios occurred in the discs and arms (0.111 and 0.110, respectively) of brittle-stars and the lowest in the spines (0.010) of cidaroid sea urchins. Many taxa (11 of 14 species) from the collection sites showed no intraspecific differences in Mg/Ca ratios between given skeletal components. Exceptions were the spines of two regular sea urchins and the skeletal ossicles of the combined arms and disc of a brittle-star. The relationship between skeletal magnesium content and latitude was further evaluated and an inverse correlation was found between Antarctic echinoderm taxa skeletal magnesium content and latitude across 62° to 76°, indicating that the relationship occurs over relatively narrow latitudes. Upon examination of an even narrower range (70–76° latitude), a region where the mineralogy of echinoderm skeletons has not been investigated, the predicted inverse relationship between Mg/Ca ratio and latitude was still observed in sea-stars, but not in brittle-stars or sea urchins.

Evolución temporal del índice gonadal de los erizos marinos Paracentrotus lividus (Echinoida: Echinidae) y Diadema africanum (Diadematoida: Diadematidae) en las islas Canarias (España)

There are three main species of regular sea urchins in the Canary Islands. To establish the optimal fishing seasons for two of them, we studied the evolution of the gonadal index in several years and locations, of Paracentrotus lividus (April 2006 to March 2008 on Tenerife island in two locations: Las Galletas and La Jaca and from April 2006 to January 2009 in Gran Canaria island in two locations: Ojos de Garza and Gando), and Diadema africanum (January 2010 to May 2011 on Gran Canaria island in three locations: Risco Verde, Arguineguin and Agaete). In the case of P. lividus, located on the southern edge of their distribution, the presence of several annual maximum gonadal indez peak was observed, probably related to the temperature. These times of gonadal index increases corresponded to the stages of maturation. The main period of maturity coincided with the fall and early winter (August, October and December). There was sporadic emissions of gametes depending on location, in April, during the summer (June, July or August), autumn (September or October) and winter (December, January or February). This fragmented situation is possibly due to exposure to the hydrodynamic area’s and food abundance. The maximum presence of D. africanum occurred in the months of May to June, with virtually no variations throughout the year, in the southernmost locality (Arguineguin). In conclusion, the two species of sea urchins would be complementary shellfish resources, as their periods of maximum GI (Gonadal Index, capture time) do not overlap during the year.

Role of environmental change in rock-boring echinoid trace fossils

Abstract Well preserved groove-borings related to the boring activity of regular sea urchins are preserved on Neogene rocks associated with rocky palaeoshores in a volcanic oceanic island (Santa Maria, Azores archipelago) and a continental margin (Foz da Fonte, Central West Portugal). The new trace fossil consists of a series of relatively deep grooves, with sinuous pathway and with a fairly uniform width along its trajectory. A series of forward oriented concave markings occur frequently inside the grooves, reflecting the movement direction of the producer. The trace fossil is designated as Ericichnus igen. n. with two new ichnospecies, Ericichnus bromleyi isp. n. and Ericichnus asgaardi isp. n. Ericichnus igen. n. could have been produced by regular echinoids, similar to the extant echinometrid echinoid Echinometra lucunter ( Linnaeus, 1758 ) for E. bromleyi isp. n. and similar to the echinid Paracentrotus lividus Lamarck, 1816 for E. asgaardi isp. n. Ethologically, the compound trace fossil Ericichnus igen. n-Circolites may correspond to an agrichnion–pascichnion and/or domichnion. Our study suggests that the new ichnospecies E. bromleyi may be used as a palaeoenvironmental tool, indicating the boring activity of termophile rock-boring echinoids in the fossil record.

Calcite orientations and composition ranges within teeth across Echinoidea

Sea urchin’s teeth from four families of order Echinoida and from orders Temnopleuroida, Arbacioida and Cidaroida were studied with synchrotron X-ray diffraction. The high and very high Mg calcite phases of the teeth, i.e. the first and second stage mineral constituents, respectively, have the same crystallographic orientations. The co-orientation of first and second stage mineral, which the authors attribute to epitaxy, extends across the phylogenic width of the extant regular sea urchins and demonstrates that this is a primitive character of this group. The range of compositions Δx for the two phases of Ca1-xMgxCO3 is about 0.20 or greater and is consistent with a common biomineralization process.

Patrones de distribución y tasas de bioerosión del erizo <i>Centrostephanus coronatus</i> (Diadematoida: Diadematidae), en el arrecife de Playa Blanca, Pacífico colombiano

Regular sea-urchins are one of the main bioeroding organisms affecting coral reefs around the world. The abundance, distribution and bioerosion rate of the sea-urchin Centrostephanus coronatus, were determined in different reef zones of Playa Blanca fringing reef (Gorgona Island, Colombian pacific coast) during 1997 and 1998. The erosion rates were determined calcinating the gut content of the sea-urchins to eliminate all organic components and preserve the inorganic portion of calcium carbonate. C. coronatus showed the highest densities towards the central zones of the reef (plain-crest and front) (12.4 ind/m2; range 0-48 ind/m2). The highest mean bioerosion rate was 0.103 kgCaCO3/m2/yr in the reef plain-crest (0-0.69 kgCaCO3/m2/yr). In the other zones, (back reef and reef front) the mean bioerosion rates were 0.071 (range 0-0.39) and 0.052 (range 0-0.31) kgCaCO3/m2/yr respectively. According to the present data, it can be seen that the destruction of coralline skeletons, produced in this reef by sea-urchins is rather low, compared with the abrasion caused by these organisms in other places of the world. However, the combined action of C. coronatus and other bioeroding organisms (borers and grazers). along with some adverse environmental factors to corals, can be causing a negative balance between normal processes of reef accretion-destruction in Gorgona Island reefs.

Do regular sea urchins show preference in which part of the body they orient forward in their walk?

Regular sea urchins, which have pentaradial symmetry, have been believed to show no preference in which part of the body forward they proceed with. Through use of circular statistics, we show that the regular sea urchin Hemicentrotus pulcherrimus had no preference with respect to what body part, as determined by Loven’s system, should be anterior in locomotion. The sea urchins, however, preferably proceeded with the body part, which had contacted with the aquarium walls at rest, forward. When the contact part was artificially altered, the body part facing forward in the following proceeding changed accordingly: the animals walked with the part that had contacted last forward. The biological significance of this behavior was discussed in relation to the aggregation formation and homing behavior.

Bilaterality in the regular sea urchin Anthocidaris crassispina is related to efficient defense not to efficient locomotion

Regular sea urchins show pentaradial symmetry: they have round bodies made of five almost identical sectors. There is no preference in which sector should be anterior in their locomotion. Anthocidaris crassispina is exceptional among regular sea urchins. Morphometry showed that their tests had radial symmetry but the overall body shape showed bilateral symmetry because of the uneven distribution of spine length. This bilateral axis did not correspond to the ambulacrum III—interambulacrum 5 axis, which is the bilateral axis of irregular sea urchins. Their behavior in relation to this bilaterality was observed in the aquarium. At rest, most of them stayed in the upper corner of the aquarium keeping the sector with the shortest spine length (the shortest sector) toward the water surface. At walking, most of them moved on the vertical wall just under the water surface keeping the shortest sector directing to the water surface. When they walked in the open space, they moved in a random direction with respect to the bilaterality of the spines. The bilaterality and the behavior of this sea urchin were interpreted as the adaptation to defense not to efficient locomotion.

Evolutionary modification of T-brain ( tbr) expression patterns in sand dollar

The sand dollars are a group of irregular echinoids that diverged from other regular sea urchins approximately 200 million years ago. We isolated two orthologs of T-brain ( tbr), Smtbr and Pjtbr, from the indirect developing sand dollar Scaphechinus mirabilis and the direct developing sand dollar Peronella japonica, respectively. The expression patterns of Smtbr and Pjtbr during early development were examined by whole mount in situ hybridization. The expression of Smtbr was first detected in micromere descendants in early blastula stage, similar to tbr expression in regular sea urchins. However, unlike in regular sea urchin, Smtbr expression in middle blastula stage was detected in micromere-descendent cells and a subset of macromere-descendant cells. At gastrula stage, expression of Smtbr was detected in part of the archenteron as well as primary mesenchyme cells. A similar pattern of tbr expression was observed in early Peronella embryos. A comparison of tbr expression patterns between sand dollars and other echinoderm species suggested that broader expression in the endomesoderm is an ancestral character of echinoderms. In addition to the endomesoderm, Pjtbr expression was detected in the apical organ, the animal-most part of the ectoderm.

Revision of new species of Eocene sea urchins from Istria, described in the 19th and 20th centuries

In paper are presented drawings and pictures of sea urchins from original works of authors who determined and documented new species from Istria. TARAMELLI (1874), BITTNER (1880), OPPENHEIM (1901), TONIOLO (1909) and DEGLI INNOCENTI (1924 a, b) established in Paleogene beds of Istria 18 new species of Eocene sea urchins. Determined were four species of regular sea urchins of genera Stereocidaris, Phyllacanthus, "Eucidaris" and Arachniopleurus, and fourteen species of irregular sea urchins of nine genera - Conoclypus, Echinolampas, Rhyncholampas, Gitolampas, Ditremaster, Pericosmus, Prenaster, Brissopsis and Cyclaster. The highest species diversity was recorded within genus Echinolampas. New sea urchins species originate prevailingly in the Middle Eocene - Lutetian beds of localities in the narrower and broader surroundings of Koper, Buzet, Roč, Grdoselo, Pazin, Pićan, Čepić and Labin.

Bilateral symmetry and locomotion: do elliptical regular sea urchins proceed along their longer body axis?

Vagile animals usually have bilaterally symmetrical bodies and proceed with their mouth-end first. Regular sea urchins have, however, radially symmetrical bodies with their mouth facing the substratum and show no preference in which side of the body should be anterior in their locomotion. The elliptical sea urchins in the subfamily Echinometrinae are exceptional among regular sea urchins in having elongated bilateral bodies. We studied whether they showed the preference in the direction of locomotion using Echinometra sp. type A. Directional preference was not observed in the proceedings in an open space. However, they proceeded preferentially with their long axis coinciding with the direction of locomotion when they moved along the water surface or along the wall of the aquarium. The speed of locomotion was the same irrespective of the direction of proceedings and of whether sea urchins moved freely or moved along the water surface or along the walls. We suggest that the bilateral body form and the habit of long-axis lead of this sea urchin have adaptive significance to increase the protected body surfaces, not to facilitate the efficiency in locomotion.

Asociación a sustratos de los erizos regulares (Echinodermata: Echinoidea) en la laguna arrecifal de Isla Verde, Veracruz, México

Regular sea urchins substrate association (Echinodermata: Echinoidea) on Isla Verde lagoon reef, Veracruz, Mexico. The diversity, abundance, distribution and substrate association of the regular sea urchins found at the South part of Isla Verde lagoon reef, Veracruz, Mexico is presented. Four field sampling trips where made between October, 2000 and October, 2002. One sampling quadrant (23 716 m2) the more representative, where selected in the southwest zone of the lagoon reef, but other sampling sites where choose in order to cover the south part of the reef lagoon. The species found were: Eucidaris tribuloides tribuloides, Diadema antillarum, Centrostephanus longispinus rubicingulus, Echinometra lucunter lucunter, Echinometra viridis, Lytechinus variegatus and Tripneustes ventricosus. The relation analysis between the density of the echinoids species found in the study area and the type of substrate was made using the Canonical Correspondence Analysis (CCA). The substrates types considerate in the analysis where: coral-rocks, rocks, rocks-sand, and sand and Thalassia testudinum. The sand substrate with pieces of shell fragments was omitted from the analysis, since D. antillarum was the only species present in this habitat. During the two sampling climatic seasons the distribution of E. tribuloides tribuloides and C. longispinus rubicingulus was investigated to determine their substrate association on coral-rock and rock. The distribution of D. antillarum, E. lucunter lucunter and E. viridis was mainly associated to coral-rock, rock and sand substrates, while which the distribution of L. variegatus and T. ventricosus was more associated with presence of sea grass. The distribution of sea urchins is associated with the substrate type and their feeding habits which are also intimately linked. The seven species reached their biggest abundance during the raining season. E. lucunter lucunter and D. antillarum are considered in this study as bioindicators of the ecological quality of the coral reef. Rev. Biol. Trop. 56 (Suppl. 3): 281-295. Epub 2009 January 05.

Enlarged lantern size in similar-sized, sympatric, sibling species of Strongylocentrotid sea urchins: from phenotypic accommodation to functional adaptation for durophagy

Strongylocentrotus droebachiensis is a well known omnivorous sea urchin with an unrivalled capacity to destroy North Atlantic kelp forests. S. pallidus is a lesser known, morphologically similar, and closely related species with no record of destructive grazing, despite its larger lantern size. I quantify the lantern size of both species using bivariate allometric analysis, and test the hypothesis that enlarged lantern size facilitates durophagy, the consumption of hard prey, by measuring the feeding capacity of urchins with different lantern sizes when offered a hard-shelled prey, the blue mussel, Mytilus edulis. The results suggest that S. droebachiensis has a limited capacity for durophagous feeding irrespective of lantern size, whereas in S. pallidus the ability to exploit hard shelled prey is positively related to lantern size. This is apparently the first evidence of a relationship between trophic morphology and diet in regular sea urchins. The hypothesis of systematic latitudinal variation in the lantern size of S. pallidus is reappraised and rejected. S. droebachiensis had larger gonads than S. pallidus in field samples, confirming that its small lantern is not impeding nutrient acquisition in shallow habitats.

Echinothurioid phylogeny and the phylogenetic significance of Kamptosoma (Echinoidea: Echinodermata)

The Echinothurioida is an unusual group of regular sea urchins that are characterized by soft, flexible tests and in some cases, hoof-shaped spines used for locomotion across soft, deep-sea sediments. As far as is known, all species are armed with venom-bearing spines that have been known to cause serious injury in humans. There are 50 species of echinothurioids arranged in 11 extant genera. Their fossil record is very poor, being limited to two additional fossil taxa (one of which is only tentatively considered an echinothurioid), two assigned to extant taxa, and three of more dubious affinity. With very few exceptions, only disjointed plates are preserved—and those very rarely. Today, echinothurioids are found around the globe from as far north as the Arctic Circle and as far south as Antarctica, with a bathymetric distribution ranging from inshore on coral reefs to depths of almost 5000 m. The majority of species are characteristic of the deep sea, and consequently little is known about these urchins. Many are known only from type and associated material. Their fragile tests, deep benthic habitat, and rarity make it difficult to develop a complete picture of their morphology, and as we demonstrate, breakage of the test can lead to misinterpretations of plate architecture in some taxa. The sole Antarctic species, Kamptosoma asterias, is usually considered an echinothurioid, but its unusual morphology has made its position difficult to ascertain. In addition, previous genus-level phylogenies do not test the monophyly of the genera, and some studies even suggest that the echinothurioids themselves do not constitute a monophyletic group. This study focuses on finding a species level phylogeny of the echinothurioids in order to perform these tests, and to place the enigmatic Kamptosoma in a phylogenetic context that determines whether it is indeed an echinothurioid, and if so, to which clade it is most closely related. The present analysis surveys ambulacral morphology of all echinothurioids, seeking homologies that will enhance our understanding of the diversity of plate architecture in the clade. We confirm that echinothurioids are monophyletic and basal to most other extant echinoid groups, and that Kamptosoma is a relatively crownward echinothurioid, albeit a strange one. The largest genus, Araeosoma, is monophyletic, but at least one prominent genus (Sperosoma) is not monophyletic. The resulting phylogeny also helps address questions concerning biogeographic and bathymetric distributions.

Reproduction in cold water: paradigm changes in the 20th century and a role for cidaroid sea urchins

At the beginning of the 20th century, powerful ideas about the biology of marine animals in cold waters (polar and deep sea) were in early stages of development. (1) General similarities between some Arctic and Antarctic species suggested a past or present continuity between the poles, possibly through tropical submergence. (2) The discovery of subantarctic species brooding their offspring suggested that supposedly harsh polar conditions select against species with pelagic, dispersive larvae. (3) The linkage between seasonal temperature changes and seasonal reproduction suggested that where temperatures were constant, as in polar and deep seas, reproduction would be aseasonal or continuous throughout the year. (4) Recognition of the phenomenon of metabolic temperature adaptation suggested that animals living in cold water should exhibit rates of physiological processes similar to rates in warmer environments. Observations and experiments throughout the first half of the 20th century generally supported and reinforced these ideas. During the second half of the 20th century, however, the generality of these paradigms broke down. Detailed analyses of fuller data indicated that Arctic, Antarctic, and deep-sea faunas are not the same and probably have different phylogenies reflecting different vicariant histories. Moreover, many species in these habitats have pelagic larvae, they generally spawn seasonally, and their physiological processes (respiration, gametogenesis, development, feeding, growth) are slow, showing little evidence of the expected temperature adaptation. Nevertheless, entering the 21st century, we are challenged by important exceptions that do support many of the earlier ideas. Bipolarity in some groups does indicate relict distributions, and other groups show equatorial submergence. Moreover, major species-rich clades in the Antarctic (unlike the Arctic) do brood their young. Most species with pelagic larvae produce non-feeding (lecithotrophic) larvae rather than feeding (planktotrophic) larvae, and these, along with brooding species, generally do reproduce throughout the year. Yet brooding is almost certainly not an adaptation to low temperatures and low larval food supply, as supposed earlier; instead, the species-rich clades of brooders probably reflect enhanced speciation under unique conditions in the Antarctic. In addition, despite recent evidence of constraints on metabolic flexibility, low metabolic rates may themselves be adaptations to Antarctic conditions. Just as a growing body of “exceptions” to the early 20th century paradigms led to their breakdown, these persisting exceptions to current ideas demand that our existing paradigms be re-examined for further insight into the biology of marine animals in cold water. Cidaroid echinoids, in particular, appear to support some of the earlier ideas. The group consists of one or possibly two clades that make up over 80% of the species of regular sea urchins in the Antarctic; several species extend into the deep sea. One putative species is nearly bipolar, extending from the Antarctic shelf into the deep sea and then north in the eastern Pacific to Alaska. All the species appear to brood embryos, but development in the brood might be facultative. Reproduction seems to be aseasonal, albeit infrequent. Growth rates are probably very slow and longevity very long. Current work begun with ANDEEP is directed at resolving the phylogenetic history of this group to better understand unusual reproductive and other features in cold-water marine animals.

High-Antarctic regular sea urchins – the role of depth and feeding in niche separation

Regular sea urchins of the families Cidaridae and Echinidae are widespread and sympatrically occurring epibenthic species in Antarctic waters. Food preference and water depth distribution of the five most abundant species (Ctenocidaris gigantea, C. spinosa, Notocidaris mortenseni, Sterechinus antarcticus, S. neumayeri) were analysed based on trawl and photograph samples. Both diet and water depth contribute to niche separation among these species. All sea urchins consume bryozoans and sediment, but echinids feed predominantly on diatoms in the fluff, when available. Cidarids do not consume diatoms, most likely owing to morphological constraints; their typical food consists of sponges and hydroids. C. spinosa and S. neumayeri prefer shallow water depths, whereas N. mortenseni and S. antarcticus prefer deeper regions. C. gigantea is the most variable species regarding food composition and living depth.

A spicule-reinforced contractile mesentery: organisation and mechanical behaviour of the exterior coelomic septum of Stylocidaris affinis (Echinodermata, Echinoida)

The exterior coelomic septum (ECS) is a mesentery-like structure that encloses the lantern of regular sea-urchins and connects it to the inner surface of the test. This paper describes the ultrastructure and microarchitecture of the ECS in Stylocidaris affinis (Cidaridae, Echinoida) and provides information on its contractile and passive mechanical properties. The ECS forms five interambulacral pouches each of which has adthecal (test-facing) and adambulacral (ambulacrum-facing) walls. The ECS wall comprises two coelothelia separated by a layer of connective tissue. The outer coelothelium is a single layer of monociliated cuboidal peritoneocytes and basally located axon-like processes. The inner coelothelium is a single layer of squamous peritoneocytes overlying axon-like processes and, in the adthecal regions only, parallel arrays of elongated myocytes orientated obliquely or horizontally. The intraseptal connective tissue consists mainly of collagen fibrils with sparsely distributed spherule cells and cells containing heterogeneous vesicles. In the adambulacral regions of the ECS hollow beaded microfibrils 20–23 nm in diameter form fibre-like aggregations. This layer also contains calcite spicules of variable size, shape, abundance and orientation. Isolated preparations of the ECS show concentration-dependent contractile responses to K+ ions and acetylcholine. The magnitude of the contractile force varies with the vertical position of the lantern (which determines the starting length of the ECS) in an unusual pattern. Cyclical loading-unloading tests indicate that, as the lantern is raised, the ECS shows low stiffness until the lantern reaches its normal resting position. It is concluded that the adthecal regions of the ECS help to set a limit to lantern retraction and that their contractility assists the protractor muscles in exerting a downward pull on the lantern.

Organization and mechanical behaviour of myocyte-ligament composites in a sea-urchin lantern: the compass depressors of Stylocidaris affinis (Echinodermata, Echinoida)

Certain components of the jaw apparatus, or lantern, of regular sea-urchins form a ’compass system’ the function of which has still to be established. This system includes ten compass depressors (CDs) which connect the compass ossicles to the inner edge of the test. Previous studies focused on the CDs of euechinoid sea-urchins. This paper provides the first detailed account of the organization and mechanical behaviour of the CDs of a cidaroid sea-urchin, Stylocidaris affinis. The outermost layer of its CDs is a coelothelium comprising apical peritoneocytes, sparse subapical myocytes and two types of granule-containing cells. The central core of the CD is dominated by longitudinally orientated collagen fibrils arranged in bundles surrounded by networks of beaded microfibrils. Myocytes are scattered throughout the core and are always surrounded by a basal lamina which usually encloses the same two types of granule-containing cells observed in the coelothelium. Each CD is attached to the outer coelomic septum by a mesentery, both the connective tissue and coelothelia of which lack myocytes and granule-containing cells. From data on the relationship between the vertical position of the lantern and the passive and active forces developed by intact CD sets, it appears that the CDs resist elevation of the lantern above its ’resting position’ and that they develop a maximal contractile force when their length corresponds to that at the resting position. The connective tissue of the CDs has the capacity to undergo nervously mediated changes in stretch resistance, although this is expressed more weakly than in the euechinoid CDs. These results are related to current views on the physiological role of the compass system.

Semi-quantitative study of macrobenthic fauna in the region of the South Shetland Islands and the Antarctic Peninsula

During the BENTART 95 Expedition, 24 Agassiz trawls for macrozoobenthos sampling were carried out at depths of 40–850 m, from north of Livingston Island to the Antarctic Peninsula. The samples were analysed using a semi-quantitative method, and with the resulting numerical data, transformed into a six-point scale, we constructed a Bray-Curtis similarity matrix. A total of 74,624 specimens, belonging to 38 taxonomic groups, were collected. The most abundant group was Polychaeta Sedentaria, with 36% of the total, whereas the highest relative masses were from Ascidiacea (23%), Echinoidea Regularia (18%) and Ophiuroidea (16%). The maximum number of specimens recorded was 15,600 ind./50 l. Cluster analysis separated stations located in Foster Bay (Deception Island), characterised by low taxonomic richness and high relative mass (average: 26.7 kg). A zonation of ascidians, regular sea urchins and ophiuroids was observed at Deception Island, clearly related to depth and substratum type. The remaining stations were separated into two groups. The first one comprised the shallowest stations (40–130 m), dominated by sessile active filter-feeders, belonging to Ascidiacea, Demospongia and Bryozoa, and probably related to high primary production zones. The second group comprised deeper stations and was dominated by classes exhibiting a diversity of trophic strategies: Ophiuroidea and Asteroidea, to 400 m, and Polychaeta Sedentaria at greater depths.

SYMMETRY, LOCOMOTION, AND THE EVOLUTION OF AN ANTERIOR END: A LESSON FROM SEA URCHINS.

Bilaterally symmetrical, "regular" sea urchins in the Family Echinometridae (Class Echinoidea; Phylum Echinodermata) were found to lack a locomotor anterior. Heterocentrotus mammillatus and Echinometra mathaei were observed while locomoting. Members of both ellipsoidal species were found to proceed with their short or long axis foremost with statistically equivalent frequencies. This finding demonstrates that the evolution of bilateral symmetry is not always accompanied by the evolution of a locomotor "anterior" end. The elliptical echinometrid sea urchins provide a particularly appropriate study group for investigating the relationship between the evolution of body form and locomotor behavior. Although the radially symmetrical regular sea urchins, from which the echinometrids sprang, lack a locomotor anterior, all "irregular" echinoids, which are also derived from a regular ancestor but are bilaterally symmetrical, possess an "obligate" locomotor anterior. The symmetry and behavior exhibited by the elliptical echinometrid sea urchins therefore demonstrates that the first irregular echinoids (which exhibit bilateral symmetry by definition) need not have possessed a locomotor anterior as they do today.