Echinoids (Echinodermata: Echinoidea) from the Gulf of Mexico

The echinoid fauna of the Gulf of Mexico collected during three research cruises (20-1260 m depth) was surveyed from samples were taken at 43 stations. A total of 190 individuals were identified (eight orders, 11 families, 15 genera and 18 species). Six species are new records for the Gulf of Mexico: Stylocidaris lineata, Phormosoma placenta placenta, Plesiodiadema antillarum, Plethotaenia spatangoides, Brissopsis atlantica and Hypselaster limicolus. This adds to the little information available on the echinoid fauna of Tamaulipas, Veracruz, Tabasco, Campeche and Yucatan states in Mexico.     

Phagostimulation of Lytechinus variegatus (Lamarck) (Echinodermata: Echinoidea)

Artificial agarose foods containing either starch, glycogen, carragheenan, alginate, casein, gelatin, albumin, or peptones at a concentration of 1% dry weight were not phagostimulatory. Artificial foods containing sugars, especially galactose, at a concentration of 0.005 M were phagostimulatory. Of the common amino acids, only L‐phenylalanine was phagostimulatory at this concentration. The initiation of feeding postures (movement of the spines, tube feet, peristome, and Aristotle's lantern) of Lytechinus variegatus was little affected by the concentration of plant extracts. Many aspects of this behavior were attributable to physical rather than chemical stimulation. Feeding rates of L. variegatus upon artificial agarose foods containing either Thalassia testudinum or Gracilaria verrucosa were directly dependent upon plant tissue concentration. All artificial agarose food‐items were initially moved to the mouth and fed upon, however the time spent feeding was directly dependent upon plant type and tissue concentration. Retention of the artificial food‐items upon the test after feeding was not influenced by plant type or tissue concentration. Differences in consumption of foods by L. variegatus are determined by the degree of persistence in feeding upon encountered items and not be selection before initiating feeding or by retention of food‐items for subsequent feedings.     

The genus Sterechinm (Echinodermata: Echinoidea) on the Weddell Sea shelf and slope (Antarctica): distribution,abundance and biomass

Summary Two species of the echinoid genus Sterechinus were documented from 92 trawl stations and 55 photographic stations in the eastern and southern Weddell Sea between 100 and 1200 m water depth. We found two species occuring along the whole shelf and slope, S. neumayeri being more abundant above 450 m water depth and S. antarcticus dominating the deeper regions. The size-frequency distributions of both species indicate differences in growth, mortality and longevity. First estimates of abundance and biomass of S. neumayeri and S. antarcticus are 0.085 ind/m² & 0.005 gAFDW/m² and 0.022 ind/m² & 0.005 gAFDW/m², respectively.AWI Publication No. 392     

Behavioral ecology of Strongylocentrotus droebachiensis (Muller) (Echinodermata: Echinoidea)

Summary Field observation indicated that extensive aggregation behavior occurs in Strongylocentrotus droebachiensis, an important echinoid herbivore in rocky subtidal zones of northern New England. The relationship of this behavior to the behavioral ecology of this species was investigated. Laboratory studies indicated that urchins are chemically sensitive to the stimuli of food and other urchins. Quantitative field experiments with tagged animals and transects showed the existence of two types of aggregations: feeding and nonfeeding, which appear to be pervasive phenomena in the biology of S. droebachiensis.It is concluded that, depending upon their hunger state, urchins shuttle between feeding aggregations in exposed locations and non-feeding aggregations located in relatively sheltered areas (where they spend most of their time). Individuals of S. droebachiensis aggregate actively through response to other urchins in the form of chemotaxis. The aggregative behavior may be maintained for its selective value in terms of defense from predation and, to lesser extents, facilitation of feeding and breeding.     

Histology of the globiferous pedicellariae of Psammechinus miliaris (Echinodermata: Echinoidea)

The general morphology, function and histology of the globiferous pedicallariae of the regular sea-urchin, Psammechinus miliaris are described. There is a ganglion under the sensory epidermis on the inner surface of each jaw. This is the first discovery of a ganglion in any echinoid.
The regeneration of globiferous pedicellariae in laboratory conditions takes 25 to 40 days.     

Phylogeny of Early Cretaceous spatangoids (Echinodermata: Echinoidea) and taxonomic implications

A phylogenetic analysis of 36 species provides a test for the taxonomy and the history of Early Cretaceous spatangoids. Most taxonomic units from genera to suborders are consistent with the proposed phylogenetic framework. We retain Hemiasterina, Micrasterina, Hemiasteridae, Schizasteridae, Hemiaster , Heteraster , Mecaster , and Periaster as original monophyletic groups. However, all of these clades originate without the classical apomorphies normally ascribed to them. We suggest a revision of their diagnoses and of the generic attributions of basal species. Some ill-defined, 'primitive', and paraphyletic taxa are recognised: Toxaster , Epiaster , Palhemiaster , and Toxasteridae. Even if they do not have phylogenetic meaning, they are retained here, pending a more complete revision.     

Siphons and siphonal grooves in the digestive systems of the Echinoidea (Echinodermata)

Here we use histological sections and, to a limited extent, scanning electron microscopy to ascertain whether siphons or siphonal grooves parallel the stomach of five sea urchin species representing five higher taxa in the Echinoidea. We find a siphonal groove in Centrostephanus coronatus of the Diadematidae and in Caenopedina diomedeae of the Pedinidae; we find a siphon in Tromikosoma panamense of the Echinothurioida and in Strongylocentrotus purpuratus of the Camarodonta; and we find a hemisiphon in Aspidodiadema hawaiiense of the Aspidodiadematidae (it is currently unsettled whether this last group belongs in the Pedinoida or the Diadematoida). Several recent accounts of echinoid gut anatomy have claimed that species in the Echinothurioida have a siphonal groove instead of a siphon and that species in the Diadematidae and Pedinidae have siphons instead of siphonal grooves. The present work shows that these claims are mistakes and confirms that Holland and Ghiselin (1970) correctly described the distribution of siphons and siphonal grooves in the Echinoidea.     

Origin and evolutionary plasticity of the gastric caecum in sea urchins (Echinodermata: Echinoidea)

Background  

The digestive tract of many metazoan invertebrates is characterized by the presence of caeca or diverticula that serve secretory and/or absorptive functions. With the development of various feeding habits, distinctive digestive organs may be present in certain taxa. This also holds true for sea urchins (Echinodermata: Echinoidea), in which a highly specialized gastric caecum can be found in members of a derived subgroup, the Irregularia (cake urchins, sea biscuits, sand dollars, heart urchins, and related forms). As such a specialized caecum has not been reported from "regular" sea urchin taxa, the aim of this study was to elucidate its evolutionary origin.     

Evolution of a novel muscle design in sea urchins (Echinodermata: Echinoidea)

The sea urchin (Echinodermata: Echinoidea) masticatory apparatus, or Aristotle's lantern, is a complex structure composed of numerous hard and soft components. The lantern is powered by various paired and unpaired muscle groups. We describe how one set of these muscles, the lantern protractor muscles, has evolved a specialized morphology. This morphology is characterized by the formation of adaxially-facing lobes perpendicular to the main orientation of the muscle, giving the protractor a frilled aspect in horizontal section. Histological and ultrastructural analyses show that the microstructure of frilled muscles is largely identical to that of conventional, flat muscles. Measurements of muscle dimensions in equally-sized specimens demonstrate that the frilled muscle design, in comparison to that of the flat muscle type, considerably increases muscle volume as well as the muscle's surface directed towards the interradial cavity, a compartment of the peripharyngeal coelom. Scanning electron microscopical observations reveal that the insertions of frilled and flat protractor muscles result in characteristic muscle scars on the stereom, reflecting the shapes of individual muscles. Our comparative study of 49 derived "regular" echinoid species using magnetic resonance imaging (MRI) shows that frilled protractor muscles are found only in taxa belonging to the families Toxopneustidae, Echinometridae, and Strongylocentrotidae. The onset of lobe formation during ontogenesis varies between species of these three families. Because frilled protractor muscles are best observed in situ, the application of a non-invasive imaging technique was crucial for the unequivocal identification of this morphological character on a large scale. Although it is currently possible only to speculate on the functional advantages which the frilled muscle morphology might confer, our study forms the anatomical and evolutionary framework for future analyses of this unusual muscle design among sea urchins.     

Phylogeny of Cidaroida (Echinodermata: Echinoidea) based on mitochondrial and nuclear markers

We present the first molecular phylogeny of Cidaroida, one of the most problematic groups within the echinoids. Two genes??the nuclear ribosomal gene 28?S rRNA and the mitochondrial protein-encoding gene COI??were obtained from 21 specimens representing 17 genera and 20 species, among which 13 species belong to Cidaroida. Phylogenetic analyses of the combined molecular data using parsimony and maximum likelihood optimality criteria resulted in a well-resolved phylogeny. Our results are broadly compatible (with the notable exception of Cidaris cidaris) with previous results obtained from morphological data. We find that Cidaroida represent a monophyletic group sister to the non-cidaroid Echinoidea. The family Cidaridae sensu Mortensen (1928) and Fell (1966) is paraphyletic because of the placement of Psychocidaris ohshimai as sister-group to Histocidaris elegans. Inside the Stylocidarina, we show that the two Atlantic species Stylocidaris affinis and Stylocidaris lineata constitute a well-supported clade. However, these two taxa could also represent two morphotypes within a single species showing high morphological variation.     

Hettangian Asteriidae (Echinodermata: Asteroidea) from southern Germany: taxonomy,phylogeny and life habits

Germanasterias amplipapularia andHystrixasterias hettangiurnus are new genera and species of Asteriidae (Asteroidea: Forcipulatida) described from the Hettangian (lowest Jurassic) of southern Germany. They are among the oldest known relatively complete asteroids assignable to surviving families, yet they are remarkably similar to certain living species. Modern asteriids are efficient predators of sessile and slow-moving stoutly-armored mullusks, however much asteriid morphology is suggested to have originated with suspension-feeding habits. The lineage leading to the modern Forcipulatacea might have separated from that of the remainder of the modern asteroids as early as the Carboniferous.     

Reproductive biology of the sand dollar,Peronella lesueuri (L. Agassiz, 1841) (Echinoidea: Clypeasteroida: Laganidae), in south-western Australia

The reproductive biology of the sand dollar Peronella lesueuri was studied between 2009 and 2011 in Cockburn Sound, a large coastal embayment in south-western Australia. Individuals of P. lesueuri did not display sexual size-dimorphism, and the population was found to have a sex ratio of 1:1. Maturity occurred over the range of 80–115 mm with all sand dollars larger than 115 mm having distinct gonads. Monthly histological analysis of gonads and changes in oocyte proportions over time indicated that P. lesueuri has an annual reproductive cycle; gametogenesis occurs in spring and spawning in summer. Differences in the rate of gametogenesis between 2009/2010 and 2010/2011 may have been influenced by higher temperatures experienced in 2010/2011. P. lesueuri have large ova (mean?=?210 μm), which suggests the species has lecithotrophic larval development.     

Functional morphology of the stone canal in the sea urchin Eucidaris (Echinodermata: Echinoidea)

The lack of mesocoelomic pores and the existence of a stone canal connecting proto-and mesocoel are characteristic peculiarities of echinoderms in contrast with the situation in other trimeric archicoelomates. The ultrastructure of the stone canal has been studied in order to understand its function in the strategically important position between two coelomic spaces with different functions. The epithelium of the Eucidaris (Echinoidea) stone canal is composed of three cell types: (1) ciliated cells, (2) cells with long basal processes containing myofilaments, and (3) granulated cells, which may represent secretory neurons. Nerve fibres of two types are common in the epithelium. We consider the stone canal to be a structure controlling fluid transport; its wall may exert peristaltic movements or tonic contractions and dilations which are under control of the nervous system. The ciliated cells additionally may have phagocytotic capacities. Similarities with the fine structure of the wall of the mesocoelomic pores in the pterobranch Cephalodiscus are discussed.     

Apluteal development of the sea urchin Holopneustes purpurescens Agassiz (Echinodermata: Echinoidea: Euechinoidea)

The external features of a shortened, apluteal development (lacking a pluteus larva) are described. Some features are unusual for echinoids. The large egg is distinctively marked by dark and pale coloured yolk. The sperm entry point is marked by a dark yolk spot and the first cleavage plane in most embryos is through the meridian on which the sperm entry point lies. Dark yolk in the animal hemisphere segregates largely to one blastomere in the two-cell embryo and pale yolk segregates to the other as a result of yolk movements during the first cell cycle. Progeny of the pale-yolk blastomere form adult oral structures and progeny of the dark-yolk blastomere form adult aboral structures. There is no feeding planktonic pluteus larva. The gastrula develops into a demersal vestibula larva with bilateral symmetry. The plane of symmetry is coincident with the Carpenter axis that defines a plane of symmetry through the madreporite in adult echinoderms. The coincidence shows that the anterior ambulacrum is vegetal with respect to egg polarity and the interradius originating at the madreporite is animal. The bilateral symmetry of the vestibula offers insight into the origin of radial symmetry in echinoderms and the body plan of an echinoderm ancestor.     

Testing the molecular clock: molecular and paleontological estimates of divergence times in the Echinoidea (Echinodermata)

The phylogenetic relationships of 46 echinoids, with representatives from 13 of the 14 ordinal-level clades and about 70% of extant families commonly recognized, have been established from 3 genes (3,226 alignable bases) and 119 morphological characters. Morphological and molecular estimates are similar enough to be considered suboptimal estimates of one another, and the combined data provide a tree that, when calibrated against the fossil record, provides paleontological estimates of divergence times and completeness of their fossil record. The order of branching on the cladogram largely agrees with the stratigraphic order of first occurrences and implies that their fossil record is more than 85% complete at family level and at a resolution of 5-Myr time intervals. Molecular estimates of divergence times derived from applying both molecular clock and relaxed molecular clock models are concordant with estimates based on the fossil record in up to 70% of cases, with most concordant results obtained using Sanderson's semiparametric penalized likelihood method and a logarithmic-penalty function. There are 3 regions of the tree where molecular and fossil estimates of divergence time consistently disagree. Comparison with results obtained when molecular divergence dates are estimated from the combined (morphology + gene) tree suggests that errors in phylogenetic reconstruction explain only one of these. In another region the error most likely lies with the paleontological estimates because taxa in this region are demonstrated to have a very poor fossil record. In the third case, morphological and paleontological evidence is much stronger, and the topology for this part of the molecular tree differs from that derived from the combined data. Here the cause of the mismatch is unclear but could be methodological, arising from marked inequality of molecular rates. Overall, the level of agreement reached between these different data and methodological approaches leads us to believe that careful application of likelihood and Bayesian methods to molecular data provides realistic divergence time estimates in the majority of cases (almost 80% in this specific example), thus providing a remarkably well-calibrated phylogeny of a character-rich clade of ubiquitous marine benthic invertebrates.