Echinoderm Classes Research Articles

Global diversity of brittle stars (Echinodermata: Ophiuroidea).

This review presents a comprehensive overview of the current status regarding the global diversity of the echinoderm class Ophiuroidea, focussing on taxonomy and distribution patterns, with brief introduction to their anatomy, biology, phylogeny, and palaeontological history. A glossary of terms is provided. Species names and taxonomic decisions have been extracted from the literature and compiled in The World Ophiuroidea Database, part of the World Register of Marine Species (WoRMS). Ophiuroidea, with 2064 known species, are the largest class of Echinodermata. A table presents 16 families with numbers of genera and species. The largest are Amphiuridae (467), Ophiuridae (344 species) and Ophiacanthidae (319 species). A biogeographic analysis for all world oceans and all accepted species was performed, based on published distribution records. Approximately similar numbers of species were recorded from the shelf (n = 1313) and bathyal depth strata (1297). The Indo-Pacific region had the highest species richness overall (825 species) and at all depths. Adjacent regions were also relatively species rich, including the North Pacific (398), South Pacific (355) and Indian (316) due to the presence of many Indo-Pacific species that partially extended into these regions. A secondary region of enhanced species richness was found in the West Atlantic (335). Regions of relatively low species richness include the Arctic (73 species), East Atlantic (118), South America (124) and Antarctic (126).

Abstracts of the 17th International Symposium on Bioluminescence and Chemiluminescence ‐ (ISBC 2012)

WARNING : The light-emitting molecular structures responsible for the chemiluminescence and fluorescence phenomena are not necessarily the same!

Neurosensory and neuromuscular organization in tube feet of the sea urchin Strongylocentrotus purpuratus

Several behavioral and electrophysiological studies indicate that all classes of echinoderms, including Echinoidia, the class to which sea urchins belong, are photosensitive and exhibit complex behavioral responses to light or changes in light intensity. However, no discrete photosensitive structure has been identified in sea urchins. The purpose of this study was to provide new insights into eye evolution by determining whether distinct photosensory structures are present in adult sea urchins. Recently, we showed that the Strongylocentrotus purpuratus genome contains orthologs of many mammalian retinal genes and that these genes are expressed in tube feet, suggesting the presence of photoreceptor neurons. To determine whether this is so, we identified several features of tube feet that relate to a possible invertebrate phototransduction system. We show that rhabdomeric opsin is expressed severalfold higher within the disk region of the tube feet and is the most abundant opsin. Immunostaining identified βIII-tubulin-expressing cells at the periphery of disk in the vicinity of the synaptotagmin-expressing nerve fibers. We also showed that Pax6 expression in the disk was restricted to the periphery, where small clusters of putative sensory neurons reside. Our results reveal neuromuscular organization of the tube foot neuromuscular system. They further support earlier studies suggesting the presence of a photosensory system in tube feet.

Resolving phylogenetic signal from noise when divergence is rapid: A new look at the old problem of echinoderm class relationships

Resolving evolutionary relationships in groups that underwent fast radiation in deep time is a problem for molecular phylogeny, as the scant phylogenetic signal that characterises short internal branches is generally swamped by more recent substitutions. We implement an approach, that maps how the support for rival phylogenies changes when analysing subsets of sites with either faster and more heterogeneous rates or slower and more homogeneous rates, to address a long-standing problem in deuterostome phylogeny – the interrelationships of the eleutherozoan echinoderm classes. We show that miRNA genes are phylogenetically uninformative as to the relationships of asteroids, echinoids and ophiuroids, consistent with a rapid radiation of these groups as suggested by their fossil record. Using three nuclear rRNAs and seven nuclear housekeeping genes, we map the support for the three possible phylogenetic arrangements of asteroids, ophiuroids and echinoids when moving between subsets of the data with very similar or very different rates of evolution. Only one of the three possible topologies (asteroids (ophiuroids+echinoids)) strengthens when the most rate-homogeneous subset of data are analysed. The other two possible pairings become stronger in a less reliable data subset, which includes the fastest and thus homoplasy-rich data in our alignment. Thus, while superficial analysis of our concatenated alignment identifies asteroids and ophiuroids as sister taxa, more thorough analyses suggest that ophiuroids may be more closely related to echinoids. Divergence of these echinoderm groups, using a relaxed molecular clock, is estimated to have occurred within ∼5 million years. Our results illustrate that the analytic approach of phylogenetic signal dissection can be a powerful tool to investigate rapid radiations in deep geologic time.

Deep-sea echinoderm oxygen consumption rates and an interclass comparison of metabolic rates in Asteroidea, Crinoidea, Echinoidea, Holothuroidea and Ophiuroidea

Echinoderms are important components of deep-sea communities because of their abundance and the fact that their activities contribute to carbon cycling. Estimating the echinoderm contribution to food webs and carbon cycling is important to our understanding of the functioning of the deep-sea environment and how this may alter in the future as climatic changes take place. Metabolic rate data from deep-sea echinoderm species are, however, scarce. To obtain such data from abyssal echinoderms, a novel in situ respirometer system, the benthic incubation chamber system (BICS), was deployed by remotely operated vehicle (ROV) at depths ranging from 2200 to 3600 m. Oxygen consumption rates were obtained in situ from four species of abyssal echinoderm (Ophiuroidea and Holothuroidea). The design and operation of two versions of BICS are presented here, together with the in situ respirometry measurements. These results were then incorporated into a larger echinoderm metabolic rate data set, which included the metabolic rates of 84 echinoderm species from all five classes (Asteroidea, Crinoidea, Echinoidea, Holothuroidea and Ophiuroidea). The allometric scaling relationships between metabolic rate and body mass derived in this study for each echinoderm class were found to vary. Analysis of the data set indicated no change in echinoderm metabolic rate with depth (by class or phylum). The allometric scaling relationships presented here provide updated information for mass-dependent deep-sea echinoderm metabolic rate for use in ecosystem models, which will contribute to the study of both shallow water and deep-sea ecosystem functioning and biogeochemistry.

A new echinoderm Lagerstätte from the Pliensbachian (Early Jurassic) of the French Ardennes

A succession of clays with thin shell beds from the French Ardennes, dated to the late Early Pliensbachian Davoei Zone and yielding abundant well-preserved remains of all five extant classes of echinoderms, is described here as a new Lagerstatte. The echinoderms occur in the form of isolated ossicles, semi-articulated plate aggregates and abundant intact skeletons with even the most delicate appendices preserved in place and the finest skeletal structures hardly blurred by recrystallisation. The outstandingly good preservation of the echinoderm specimens allows for a morphological assessment of the represented taxa in detail only rarely achievable by fossil material. The material described herein thus significantly contributes to a better understanding of the systematic position and phylogenetic background of many Early Jurassic echinoderm taxa. The echinoderms are considered to have been buried as an autochthonous or at least parautochthonous assemblage among clumps and beds of Modiolus bivalves. The depositional setting most likely represented a relatively shallow, near-shore soft-bottom environment in which conditions repeatedly favoured the settlement and subsequent effective burial of bivalve-echinoderm assemblages, leading to the formation of the thin highly fossiliferous shell beds intercalating the otherwise nearly sterile clays. The new Lagerstatte opens a unique window into the palaeoecology of an extinct shallow-water soft-bottom community closely comparable to recent analogues.

The coeloms in a late brachiolaria larva of the asterinid sea star Parvulastra exigua: deriving an asteroid coelomic model

Morris, V.B., Selvakumaraswamy, P., Whan, R., and Byrne, M. 2011. The coeloms in a late brachiolaria larva of the asterinid sea star Parvulastra exigua: deriving an asteroid coelomic model. —Acta Zoologica (Stockholm) 92: 266–275. The coeloms and their interconnexions in a late pre-metamorphic brachiolaria larva of a sea star are described from the series of images in the frontal, transverse and sagittal planes obtained by confocal laser scanning microscopy. A larval, brachial coelom connects with the coeloms of the adult rudiment that lie posteriorly. The connexion is through the anterior coelom, which lies over the head of the archenteron, to the right anterior coelom and then to the left posterior coelom through the ventral horn of the left posterior coelom. The right posterior coelom is a separate coelom. The hydrocoele is on the larval left side separated from other coeloms except for a connexion to the anterior coelom. On the larval right side, the anterior coelom and right anterior coelom connect with the pore canal that opens to the exterior at the hydropore. From these coeloms, we derived an asteroid coelomic model comprising the larval left and right coeloms linked over the head of the archenteron by a common anterior coelom. The asymmetry of the hydrocoele and the left posterior coelom on the left side linked through the common anterior coelom to the right side, with the external opening, translates into the oral and aboral coeloms of the adult stage. The coelomic model has application in the search for morphological homology between the echinoderm classes and the deuterostome phyla.

Global contribution of echinoderms to the marine carbon cycle: CaCO3 budget and benthic compartments

The contribution of carbonate‐producing benthic organisms to the global marine carbon budget has been overlooked, the prevailing view being that calcium carbonate (CaCO3) is predominantly produced and exported by marine plankton in the “biological pump.” Here, we provide the first estimation of the global contribution of echinoderms to the marine inorganic and organic carbon cycle, based on organism‐level measurements from species of the five echinoderm classes. Echinoderms' global CaCO3 contribution amounts to ~0.861 Pg CaCO3/yr (0.102 Pg C/yr of inorganic carbon) as a production rate, and ~2.11 Pg CaCO3 (0.25 Pg C of inorganic carbon) as a standing stock from the shelves, slopes, and abyssal depths. Echinoderm inorganic carbon production (0.102 Pg C/yr) is less than the global pelagic production (0.4–1.8 Pg C/yr) and similar to the estimates for carbonate shelves globally (0.024–0.120 Pg C/yr). Echinoderm CaCO3 production per unit area is ~27.01 g CaCO3·m−2·yr−1 (3.24 g C·m−2·yr−1 as inorganic carbon) on a global scale for all areas, with a standing stock of ~63.34 g CaCO3/m2 (7.60 g C/m2 as inorganic carbon), and ~7.97 g C/m2 as organic carbon. The shelf production alone is 77.91 g CaCO3·m−2·yr−1 (9.35 g C·m−2·yr−1 as inorganic carbon) in contrast to 2.05 g CaCO3·m−2·yr−1 (0.24 g C·m−2·yr−1 as inorganic carbon) for the slope on a global scale. The biogeography of the CaCO3 standing stocks of echinoderms showed strong latitudinal variability. More than 80% of the global CaCO3 production from echinoderms occurs between 0 and 800 m, with the highest contribution attributed to the shelf and upper slope. We provide a global distribution of echinoderm populations in the context of global calcite saturation horizons, since undersaturated waters with respect to mineral phases are surfacing. This shallowing is a direct consequence of ocean acidification, and in some places it may reach the shelf and upper slope permanently, where the highest CaCO3 standing stocks from echinoderms originate. These organism‐level data contribute substantially to the assessment of global carbonate inventories, which at present are poorly estimated. Additionally, it is desirable to include these benthic compartments in coupled global biogeochemical models representing the “biological pump” and its feedbacks, since at present all efforts have focused on pelagic processes, dominated by coccolithophores. The omission of the benthic processes from modeling will only diminish the understanding of elemental fluxes at large scales and any future prediction of climate change scenarios.

Design of phylum‐specific hybrid primers for DNA barcoding: addressing the need for efficient COI amplification in the Echinodermata

Recent research has shown the usefulness of the Folmer region of the cytochrome oxidase I (COI) as a genetic barcode to assist in species delimitation of echinoderms. However, amplification of COI is often challenging in echinoderms (low success or pseudogenes). We present a method that allows the design of phylum-specific hybrid primers, and use this to develop COI primers for the Echinodermata. We aligned COI sequences from 310 echinoderm species and designed all possible primers along the consensus sequence with two methods (standard degenerate and hybrid). We found much lower degeneracy for hybrid primers (4-fold degeneracy) than for standard degenerate primers (≥48-fold degeneracy). We then designed the most conserved hybrid primers to amplify a >500-bp region within COI. These primers successfully amplified this gene region in all tested taxa (123 species across all echinoderm classes). Sequencing of 30 species among these confirmed both the quality of the sequences (>500 bp, no pseudogenes) and their utility as a DNA barcode. This method should be useful for developing primers for other mitochondrial genes and other phyla. The method will also be of interest for the development of future projects involving both community-based genetic assessments on macroorganisms and biodiversity assessment of environmental samples using high-throughput sequencing.

Sniffing Out New Data and Hypotheses on the Form, Function, and Evolution of the Echinopluteus Post-Oral Vibratile Lobe

The performance requirements of ciliary band feeding explain the convoluted forms of many marine invertebrate larvae. Convolutions increase surface area and therefore feeding rates per unit body volume. We review recent advances in morphology, neural development, and behavior at settlement of the echinoid Lytechinus pictus and provide new ultrastructural and expression data on larvae of its congener, L. variegatus. Larvae of the echinometrid Colobocentrotus atratus contain neurons identified by their expression of nitric oxide synthase (NOS), indicating that this character is not unique to Lytechinus. We hypothesize that in some echinoids the convoluted shape of the post-oral vibratile lobe (POVL) covaries with the distribution of identified sensory neurons to enable olfaction during settlement. An analysis of variation in structural elaboration of the post-oral transverse ciliary band (PTB) within Echinoida and in feeding larvae of other echinoderm classes indicates that only echinoids, but not all echinoids, possess this novel character; larvae that do are distributed heterogeneously within the class. In recognition of this specialized function for the POVL and surrounding ectoderm, and because it is lobate and grows toward the mouth, we propose naming this structure the adoral lobe.

Quinoid pigments from the sea urchin Toxopneustes pileolus

Many studies of the sea urchin Toxopneustes pileolus (Lamarck) are dedicated to toxic compounds released from various organs, especially the pedicellariae [1]. However, its pigments have not yet been studied. Quinoid pigments of the spinochrome are a distinguishing feature of this class of echinoderms and impart color to the testae and spines as the Ca and Mg salts [2]. The set of spinochromes for a certain species of sea urchin is not random but is determined by the maternal side [3]. Nevertheless, the color is variable and can differ not only in different populations [4] but also over several days in an individual [5] or from the effect of various factors [6]. These facts have not yet been fully explained. Although these compounds exhibit antimicrobial [7] and anti-oxidant [8] properties, their functions are apparently not limited to these. We investigated pigments of T. pileolus collected near Nya Chang Bay of the South China Sea during the 30th expedition of the SRS Academic Oparin. The color of the sea urchins varied from pale-blue to brownish-green although the pedicellariae of all were pale-rose. We selected one representative from each of three differently colored groups of sea urchins, pale-blue, green, and brownish-green. The remaining specimens (20) were combined to obtain a total pigment extract by collecting from them beforehand air-dried pedicellariae (∼3 g). Before pigments were extracted the testae with spines were first washed with EtOH and n-BuOH. Spinochromes were extracted exhaustively with n-BuOH, decomposing the material with dilute HCl. The resulting extract was washed with water to remove HCl, evaporated, and chromatographed several times over KSK silica gel with elution by benzene with a gradient of increasing acetone content. Pure compounds were purified further over Sephadex LH-20 using CHCl3:EtOH (4:1) to afford 1 (12 mg), 2 (2 mg), and 3 (52 mg). These were identified by comparing their UV, mass, and NMR spectra with the literature [2] and with those of known spinochrome samples that were isolated by us earlier from Strongylocentrotus nudus [9] as spinochromes A, C, and B, respectively. Only spinochrome A was observed in pedicellariae of this species of sea urchin. The composition and ratio of spinochromes in representatives of the various color groups were determined in extracts obtained as described above except for the use of Et2O as the extractant. Spinochromes were separated from the resulting extracts by saturated NaHCO3 solution and were again transferred to an Et2O solution by acidifying the extract with HCl. The extract was washed with H2O to remove HCl, dried over Na2SO4, evaporated, and analyzed by TLC and HPLC using spinochromes isolated during this work as references. Differently colored testae and spines contained all three spinochromes observed in the total extract from this species. Spinochrome B dominated (>97 and 86.4%) in green and brownish-green specimens, respectively. Spinochromes A and C made up 0.5% for the green specimens. The brownish tone was imparted to the animals by higher contents of these compounds, 6.7 and 4.2%, respectively. The main component in extracts from pale-blue specimens was spinochrome A. Spinochromes B and C were present in trace quantities. These results and those from a single study in the literature [4] indicate that the pigment compositions of testae and spines of sea urchins are characteristic of a species. However, color variability in individuals is determined by the quantitative ratio of spinochromes.

Development of the five primary podia from the coeloms of a sea star larva: homology with the echinoid echinoderms and other deuterostomes.

Confocal laser scanning microscopy of larvae of the asteroid Parvulastra exigua was used to investigate the development of the five primary podia from the coeloms in the echinoderm phylum in an approach to the problem of morphological homology in the deuterostome phyla. The development is shown from an early brachiolaria larval stage to a pre-settlement late brachiolaria larval stage. In the early brachiolaria larva, a single enterocoele connected to the archenteron has formed into two lateral coeloms and an anterior coelom. The primary podia form from the coelomic regions on the left side of the brachiolaria larva, while on the right the coelomic regions connect with the exterior through the pore canal and hydropore. The anterior coelom forms the coelom of the brachia. Homology between the primary podia of the asteroid and the echinoid classes of echinoderms is described and extended to coeloms of other deuterostome phyla.

Development and Growth of the Feather Star Oxycomanthus japonicus to Sexual Maturity

Crinoids, including feather stars, are the most basal group among extant echinoderm classes and share a basic body plan. In spite of their importance for evolutionary developmental study, information on the development of crinoids has been limited, because there are not many species whose spawning season is known, and artificial spawning is impossible. Therefore, it is not easy to obtain fertilized eggs of crinoids. We have observed the spawning and development of the feather star Oxycomanthus japonicus for 7 years. We have established a cultivation system that has enabled us to culture large numbers of O. japonicus from eggs through to sexually mature adults. In the present study, we show that (1) individuals take 2 years to reach sexual maturity; (2) the skeleton of the theca of a stalked juvenile consists of five orals, five basals, five radials, five infrabasals, and an anal plate; and (3) the onset of spawning has shifted by about two weeks since 60 years ago. Our cultivation system can provide enough embryos, larvae, juveniles, and adults for further experiments, extending the possibilities for crinoid research.

Echinodermata da Praia do Cabo Branco, João Pessoa, Paraíba, Brasil

Este estudo retrata a fauna de equinodermos encontrada na Praia do Cabo Branco, João Pessoa, Paraíba, Brasil. Esta praia é formada por um terraço de abrasão marinha do tipo arenito-ferruginoso e está localizada no ponto mais oriental das Américas. Foram identificadas 32 espécies de equinodermos pertencentes a quatro classes (Asteroidea, Ophiuroidea, Echinoidea e Holothuroidea), distribuídas em 18 famílias e 24 gêneros. O número de táxons identificados corresponde a 9,7% das espécies registradas para o litoral Brasileiro e mostra a significativa diversidade das espécies de Echinodermata encontrada nesta área. A classe Ophiuroidea foi a mais diversa com 13 espécies, seguida por Holothuroidea (10 spp.), Asteroidea (5 spp.) e Echinoidea (4 spp). Do total de espécies registradas, as seguintes são novas ocorrências para o litoral do Estado da Paraíba: Ophiocnida scabriuscula, Ophiomyxa flaccida, Ophiactis lymani, Ophionereis reticulata, Ophioderma januarii, Ophiophragmus brachyatis, Thyone cognita, T. suspecta, Ocnus suspectus, Phyllophorus occidentalis, P. communis, Lissothuria sp. Holothuria grisea, H. arenicola, Chiridota rotifera, Synaptula hydriformis, Echinaster nudus, Linckia guildingii, Lytechinus variegatus, Mellita quinquiesperforata e Leodia sexiesperforata.

Echinoderms in an anchialine cave in Mexico

Five surveys in an anchialine cave from Cozumel Island, Mexico, were made in order to determine the diversity of invertebrates, especially of the phylum Echinodermata. Aerolito sinkhole (cenote), with an open coastal waters connection, was explored and the organisms were sampled by hand. The abiotic parameters (conductivity, salinity, oxygen, temperature, light and pH) were also measured. We identified three main classes of echinoderms from this cave. The first class is Asteroidea, located below the halocline in marine waters (37 ppt) at 256 m from the entrance and at 45 m from the entrance in marine waters. The second class is Ophiuroidea, located below the halocline in marine waters at 40 and 336 m from the entrance, and the third class, Echinoidea, is located in marine waters 60 m from the entrance. Only the asteroid species shows a depigmentated body; it is the unique species not found outside the cave. To date, only few species of Holothuridae have been reported from caves. We discuss the colonization and adaptations to cave life these animals show.

Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms

Stylophorans (cornutes, mitrates) represent one of the most diverse classes of Cambro-Ordovician echinoderms. They were free-living, benthic, non-radiate forms, closely related to asterozoans and crinoids. Taphonomic, sedimentological, and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology. Such a combined approach suggests that the rarity of stylophorans in proximal environments (above storm-wave base) was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test. Conversely, stylophorans were relatively abundant in deeper settings (below storm-wave base), where they frequently occur in low-diversity dense beds (several tens to hundreds of individuals per square metre). Such assemblages were either largely dominated by stylophorans only (Middle Cambrian–Darriwilian), or by the association of both stylophorans and ophiuroids (latest Darriwilian–Silurian). Comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well-oxygenated, oligotrophic waters with low predatory pressure, and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans. Seven main stylophoran biofacies are identified and described. The associated environments and tropical distribution of primitive, heavily-plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings (earliest Middle Cambrian–Late Cambrian). However, from this primitive stock, stylophorans radiated several times towards deeper and/or colder environments, possibly because of the lower predation pressure in such settings (“cothurnocystid” cornutes in the Middle Cambrian, mitrates in the Late Cambrian, hanusiids in the Tremadocian). As a consequence, the fact that most Cambro-Ordovician cornutes and mitrates were “psychrospheric” (i.e. cold-water adapted) probably explains their relatively wide palaeobiogeographic distribution. In high palaeolatitudes, Cambro-Ordovician stylophorans were abundant and diverse in shallow to deep environments, but they were generally restricted to deep settings in low palaeolatitudes. The great Caradocian transgression favoured the extension of stylophorans from circum-polar regions (e.g. western margin of Gondwana) towards external settings at lower palaeolatitudes (Avalonia, Baltica, and eastern margin of Laurentia), where one group of mitrates underwent a major radiation (anomalocystitids), and extended towards shallower (and warmer) palaeoenvironments. In contrast, all Silurian stylophoran assemblages known so far are rare, poorly diversified, and restricted to shallow, warm to temperate environments in tropical regions. This distribution partly results from (1) the probable eradication of psychrospheric forms (e.g. cornutes) because of the prolonged anoxia of deep environments in the late Hirnantian–early Silurian of West Gondwana, (2) the radiation of some mitrates (anomalocystitids) in shallow tropical settings in the late Ordovician, and (3) of strong taphonomic and/or sampling biases (numerous ghost lineages).

Evolutionary palaeoecology of early epifaunal echinoderms: Response to increasing bioturbation levels during the Cambrian radiation

Increasing bioturbation depth and intensity during the Cambrian radiation created a transition in dominant subtidal soft substrates from firm Proterozoic-style soft substrates without a well-developed mixed layer to soupier Phanerozoic-style soft substrates with a well-developed mixed layer. Microbial mats were also relegated to settings with inhibited metazoan activity but made resurgences during later ecological crises. Previous work suggests that this substrate change strongly influenced the evolution of Cambrian benthic suspension-feeding echinoderms. In order to test this previous work and reveal longer-term evolutionary patterns, the substrate adaptations of 93 genera in the three classes of benthic suspension-feeding echinoderms with definitive Cambrian origins (edrioasteroids, eocrinoids, and helicoplacoids) were examined throughout their fossil records. This examination reveals that genera adapted to typical Proterozoic-style soft substrates were taxonomically dominant during, and were almost entirely restricted to, the Early and Middle Cambrian. Thereafter, from the Late Cambrian through the Carboniferous, genera adapted to typical Phanerozoic-style soft substrates were taxonomically dominant. This trend supports previous studies and defines a fundamental evolutionary shift in echinoderm substrate interactions in response to increased mixed layer development during the Cambrian radiation. A notable exception to this trend is the Late Devonian edrioasteroid Cooperidiscus, which is morphologically convergent with the Early and Middle Cambrian sediment-attaching edrioasteroid Stromatocystites. This convergence is likely the result of Cooperidiscus adapting as a sediment attacher to typical Proterozoic-style soft substrates that developed during or shortly after the Late Devonian mass extinction. This study indicates for the first time that adaptation to non-actualistic substrates during ecological crises can result in morphological convergence with forms adapted to similar substrates common during the Cambrian radiation.

The complete mitochondrial genomes of the sea lily Gymnocrinus richeri and the feather star Phanogenia gracilis: Signature nucleotide bias and unique nad4L gene rearrangement within crinoids

Complete DNA sequences have been determined for the mitochondrial genomes of the crinoids Phanogenia gracilis (15892 bp) and Gymnocrinus richeri (15966 bp). The mitochondrial genetic map of the stalkless feather star P. gracilis is identical to that of the comatulid feather star Florometra serratissima (Scouras, A., Smith, M.J., 2001. Mol. Biol. Evol. 18, 61–73). The mitochondrial gene order of the stalked crinoid G. richeri differs from that of F. serratissima and P. gracilis by the transposition of the nad4L protein gene. The G. richeri nad4L mitochondrial map position is unique among metazoa and is likely a derived feature in this stalked crinoid. Nucleotide compositional analyses of protein genes encoded on the major sense strand confirm earlier conclusions regarding a crinoid-distinctive T over C bias. All three crinoids exhibit high T levels in third codon positions, whereas other echinoderm classes favor A or C in the third codon position. The nucleotide bias is reflected in the relative synonymous codon usage patterns of crinoids versus other echinoderms. We suggest that the nucleotide bias of crinoids, in comparison to other echinoderms, indicates that a physical inversion of the origin of replication has occurred in the crinoid lineage. Evolutionary rate tests support the use of the cytochrome b ( cob) gene in molecular phylogenetic analyses of echinoderms. A consensus echinoderm tree was generated based on cytochrome b nucleotide alignments that placed the asteroids as a sister group to a clade containing the ophiuroids and the (echinoids + holothuroids) with the crinoids basal to the rest of the echinoderm classes: [Crinoid,(Asteroid,(Ophiuroid,(Echinoid,Holothuroid)))].

Echinoderms

Echinoderms

Developmental Plasticity in Macrophiothrix Brittlestars: Are Morphologically Convergent Larvae Also Convergently Plastic?

The pluteus larval forms of sea urchins (echinoids) and brittlestars (ophiuroids) use an internal skeleton to project arms that bear a long ciliated band used in swimming and feeding. The length of this ciliated band influences rates of maximum food clearance for larvae of both echinoderm classes and affects rates of growth and development in the plankton. Phylogenetic and morphological evidence, however, tend to support the view that the pluteus morphologies of the two classes are independently derived. Studies with echinoplutei have shown that investment in skeletal growth and ciliated band length changes in response to food conditions, with poorly fed larvae investing more in growth of the larval skeleton and arms either absolutely or in relation to other larval or developing postlarval structures. We present evidence for similar plasticity of skeletal growth in ophioplutei. We examined four species in the brittlestar genus Macrophiothrix that spanned a 3.8-fold range in egg size. Sibling larvae in 14 male-female crosses were reared with high (H) or low (L) food rations, and measurements were recorded for five skeletal arm rods and three non-arm body dimensions. The expression of adaptive plasticity (significantly longer arms in L versus H cultures on a given day) was apparent for most crosses in M. koehleri, the species with the smallest egg size. In the single cross for M. longipeda, larvae from L cultures had longer arms for their body length or stomach width than did larvae from H cultures. In these cases, plasticity was similar in timing, persistence, and magnitude to previously published results from echinoplutei. If internal skeletons are independently derived in the two classes, then plasticity in the expression of this homoplastic trait may itself be homoplastic.