Echinoderm Classes Research Articles

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.

Regional specification in the early embryo of the brittle star Ophiopholis aculeata

Early embryogenesis has been examined experimentally in several echinoderm and hemichordate classes. Although these studies suggest that the mechanisms which underlie regional specification have been highly conserved within the echinoderm + hemichordate clade, nothing is known about these mechanisms in several other echinoderm classes, including the Ophiuroidea. In this study, early embryogenesis was examined in a very little studied animal, the ophiuroid Ophiopholis aculeata. In O. aculeata, the first two cleavage planes do not coincide with the animal–vegetal axis but rather form approximately 45° off this axis. A fate map of the early embryo was constructed using microinjected lineage tracers. Most significantly, this fate map indicates that there is a major segregation of ectodermal from endomesodermal fates at first cleavage. The distribution of developmental potential in the early embryo was also examined by isolating different regions of the early embryo and following these isolates though larval development. These analyses indicate that endomesodermal developmental potential segregates unequally at first, second, and third cleavage in O. aculeata. These results provide insight into the mechanisms of regional specification in O. aculeata and yield new material for the study of the evolution of echinoderm development.

Parablastoid holdfasts from the Lower Ordovician of South Wales

AbstractProblematical discoid fossils from the Lower Arenig (Moridunian Stage) of the Llangynog Inlier, South Wales, are interpreted as parablastoid holdfasts. They constitute the first record of holdfasts in this rare echinoderm class. The lower surface has a radial ribbed structure, in contrast to the concentric, reticulate structure of the upper surface. The substrate for attachment was not lithified and may instead have been sediment stabilized by microbial mats. An associated pluricolumnal indicates that the parablastoid column was homeomorphic. Copyright © 2005 John Wiley & Sons, Ltd.

First Report of the Enigmatic EchinodermXyloplaxfrom the North Pacific

More than 100 specimens of the rare and enigmatic echinoderm Xyloplax, a deep-sea wood-fall specialist described as the new class Concentricycloidea on the basis of nine specimens from New Zealand (1), were recently collected from a depth of 2675 m in the North Pacific. These first submersible-collected specimens of Xyloplax from the Pacific contrast with the submersible-collected specimens of the Atlantic’s X. turnerae in developmental mode. Embryos of the Pacific specimens—including those from New Zealand—undergo extended development in the ovary; members of the Atlantic species apparently release small embryos or eggs. In what may be an associated difference, females appear to dominate the North Pacific population in numbers and exceed males in individual size. In addition, the marginal spines of North Pacific specimens show a nonlinear relationship to body diameter on an ln-ln plot; relatively rapid growth of marginal spines in small individuals may enhance dispersal on near-bottom currents. In the Atlantic species, the sexes are equally represented (although sexual size dimorphism exists), and marginal spines grow at a constant rate. The increased biomass of females among North Pacific specimens may reflect their increased investment in each offspring. Animals specialized to live on deep-sea wood falls remain little known, both taxonomically and biologically. Nine specimens from five New Zealand wood falls collected at depths from 1057 to 1208 m were assigned to the genus Xyloplax, which has been interpreted as composing a new class of echinoderms, the Concentricycloidea (1) and, alternatively, as a highly derived genus of the Asteroidea (2 and references therein). Detailed anatomical study of the genus (3), and a brief report of population parameters (4) were based on more than 200 specimens collected from an experimental wood deployment at 2066 m off the Bahamas. The recent collection of more than 100 specimens of the genus from an experimental wood deployment in the North Pacific provides a contrast between this viviparous species and the Atlantic species with its shorter embryonic development. Despite distinct reproductive strategies, both species exploit the rare and unpredictable habitat provided by deep-sea wood falls. This study reports the population composition and growth patterns of specimens of the undescribed North Pacific species. The specimens probably represent the original population accurately because the deployments were recovered inside a closed box on the Deep Submergence Vehicle Alvin. C. Mah, University of Illinois at Urbana-Champaign, is describing the new species represented by these specimens.

Equinodermos (Echinodermata) de las aguas mexicanas del Golfo de México

Echinoderms (Echinodermata) from the Mexican waters of the Gulf of Mexico . We present a systematic list of the echinoderms from Gulf of Mexico’s Mexican waters based on specimens of the Coleccion Nacional de Equinodermos, Instituto de Ciencias del Mar y Limnologia, Universidad Nacional Autonoma de Mexico and the National Museum of Natural History, Smithsonian Institution, Washington, D.C. This list reveals an important echinoderm biodiversity present in the Gulf of Mexico, where five of the six echinoderm classes are represented. A total of 209 echinoderm species is recorded, distributed in 129 genera, 63 families and 25 orders. 31 new records for the Gulf of Mexico are presented: Asteroidea (16), Ophiuroidea (nine), Echinoidea (one) and Holothuroidea (five). Rev. Biol. Trop. 53(Suppl. 3): 53-68. Epub 2006 Jan 30.

Equinodermos (Echinodermata) del Caribe Mexicano

Echinoderms (Echinodermata) of the Mexican Caribbean. A systematic list of the echinoderms of the Mexican Caribbean based on museum specimens of the Coleccion Nacional de Equinodermos, Instituto de Ciencias del Mar y Limnologia, Universidad Nacional Autonoma de Mexico and the National Museum of Natural History, Smithsonian Institution, Washington, D.C. is presented. This list reveals an important echinoderm biodiversity in the Mexican Caribbean, where five of the six echinoderm classes are represented. A total of 178 echinoderm species is recorded, distributed in 113 genera, 51 families and 22 orders. 30 new records for the Mexican Caribbean are presents: Crinoidea (three), Asteroidea (two), Ophiuroidea (eleven), Echinoidea (one), Holothuroidea (thirteen). Rev. Biol. Trop. 53(Suppl. 3): 109-122. Epub 2006 Jan 30.

Mitochondrial DNA phylogeny and rates of larval evolution in Macrophiothrix brittlestars

Phylogenetic analysis has led to significant insights into the evolution of early life-history stages of marine invertebrates. Although echinoderms have been a major focus, developmental and phylogenetic information are relatively poor for ophiuroids, the most species-rich echinoderm class. We used DNA sequences from two mitochondrial genes to develop a phylogenetic hypothesis for 14 brittlestar species in the genus Macrophiothrix (Family Ophiotrichidae). Species are similar in adult form and ecology, but have diverse egg sizes and modes of larval development. In particular, two species have rare larval forms with characteristics that are intermediate between more common modes of feeding and non-feeding development. We use the phylogeny to address whether intermediate larval forms are rare because the evolution of a simplified morphology is rapid once food is no longer required for development. In support of this hypothesis, branch lengths for intermediate forms were short relative to those for species with highly derived non-feeding forms. The absolute rarity of such forms makes robust tests of the hypothesis difficult.

Reproduction: widespread cloning in echinoderm larvae.

Asexual reproduction by free-living invertebrate larvae is a rare and enigmatic phenomenon and, although it is known to occur in sea stars and brittle stars, it has not been detected in other echinoderms despite more than a century of intensive study. Here we describe spontaneous larval cloning in three species from two more echinoderm classes: a sea cucumber (Holothuroidea), a sand dollar and a sea urchin (Echinoidea). Larval cloning may therefore be an ancient ability of echinoderms and possibly of deutero-stomes - the group that includes echinoderms, acorn worms, sea squirts and vertebrates.

Gene expression and larval evolution: changing roles of distal-less and orthodenticle in echinoderm larvae.

We describe the expression of the homeobox genes orthodenticle (Otx) and distal-less (Dlx) during the larval development of seven species representing three classes of echinoderms: Holothuroidea, Asteroidea, and Echinoidea. Several expression domains are conserved between species within a single class, including Dlx expression within the brachiolar arms of asteroid larvae and Otx expression within the ciliated bands of holothuroid larvae. Some expression domains are apparently conserved between classes, such as the expression of Dlx within the hydrocoel (left mesocoel) in all three classes. However, several substantial differences in expression domains among taxa were also evident for both genes. Some autapomorphic (unique derived) features of gene expression are phylogenetically associated with autapomorphic structures, such as Dlx expression within the invaginating rudiment of euechinoids. Other autapomorphic gene expression domains are associated with evolutionary shifts in life history from feeding to nonfeeding larval development, such as Otx expression within the ciliated bands of a nonfeeding holothuroid larva. Similar associations between evolutionary changes in morphology and life history mode with changes in regulatory gene expression have also been observed in arthropods, urochordates, and chordates. We predict that recruitment of regulatory genes to a new developmental role is commonly associated with evolutionary changes in morphology and may be particularly common in clades with complex life cycles and diversity of life history modes. Caution should be used when making generalizations about gene expression and function based on a single species, which may not accurately reflect developmental processes and life histories of the phyla to which it belongs.

Autotomy as a prelude to regeneration in echinoderms

'Autotomy' refers to the adaptive detachment of animal body parts where this serves a defensive function, is achieved by an intrinsic mechanism, and is nervously mediated. With regard to each echinoderm class, this article itemises those structures that are autotomous, evaluates the extent to which autotomy precedes regeneration in natural populations, reviews current knowledge of the morphology of autotomy planes and mechanisms that effect fracture at autotomy, and comments on autotomy-related issues arising from studies of the cellular events of regeneration. Each autotomy plane can be regarded as an assemblage of breakage zones traversing the individual anatomical components of the autotomous structure. In any one autotomy plane some breakage zones are permanent sites of weakness that are fractured by external forces and some are potential sites of weakness that undergo a loss of tensile strength only at the time of autotomy. The latter occur predominantly in mutable collagenous structures, although there are a few examples of muscles that undergo an endogenous rupturing process. Available evidence indicates that autotomy is by far the commonest proximate cause of structural loss in echinoderms. Most echinoderm regeneration is therefore necessitated by autotomy and proceeds from the retained side of a fractured autotomy plane. Due to a lack of relevant research there is as yet little evidence for or against the presence of specific regeneration-promoting adaptations at autotomy planes, although it is argued that an autotomy plane designed primarily to effect rapid detachment would by itself increase regenerative efficiency.

These Stars Have Eyes

The ability to detect and react to light can be critically important for an animal’s self preservation, and many varieties of eyes have been developed in the animal kingdom. Recently, a team of researchers from Bell Laboratories, Lucent Technologies, the Weizmann Institute, and the Natural History Museum of Los Angeles have found evidence for a novel photoreceptor system in some species of brittle stars. 1 1. J. Aizenberg, A. Tkachenko, S. Weiner, L. Addadi, G. Hendler, Nature 412, 819 (2001) https://doi.org/10.1038/35090573. Spiny brittle stars (top figure) are a class of echinoderms, relatives of starfish and sea urchins. With their wiggly arms, brittle stars are capable of very fast motion. These marine invertebrates lack anything immediately recognizable as eyes, yet some species, such as Ophiocoma wendtii, show a remarkable light sensitivity: As shown in the top figure, they change color from day (upper) to night (lower), and they can detect and dart into shadows or crevices when predators approach.This photosensitivity has been attributed to diffuse dermal receptors. Now, Joanna Aizenberg (Bell Labs) and colleagues have found that the skeletal elements (middle figure) that protect the upper (dorsal) part of the brittle star arms may form what’s essentially an extended compound eye in the light-sensitive species. As in all echinoderms, each of these skeletal elements is composed entirely of single-crystal calcite (calcium carbonate). Whereas the dorsal arm plates of light-insensitive brittle stars have an irregular porous structure, those of O. wendtii and other light-sensitive species have on their outer surface a regular array of nearly uniform nodules 20–50 µm in diameter (bottom figure).These protuberances have many properties of an engineered microlens array. Calcite is birefringent, but in these structures the optical axis is normal to the plates and parallel to the lens axis; this optimal orientation prevents the formation of double images. In addition, the researchers found that each protuberance has a nearly ideal double-lens shape that closely resembles the shapes proposed in the 17th century by Descartes and Huygens to minimize spherical aberrations. Such shapes maximize a lens’s light-gathering ability and sensitivity. The only other known calcitic lens structures like this in the animal kingdom are from the extinct trilobites, which thrived about 200–550 million years ago. 2 2. E. N. K. Clarkson, R. Levi-Setti, Nature 254, 663 (1975) https://doi.org/10.1038/254663a0. Aizenberg and company studied the optical properties of the microlens arrays from O. wendtii using photolithography. Similar to someone moving a magnifying glass up and down until focused sunlight ignites a piece of paper, the researchers exposed photoresist at various distances below the lens layer. From the sizes of the exposed spots in the resist, the researchers not only confirmed the focusing ability of the calcitic structures but also determined the focal length and the spot size in the focal plane. The possible role of these microlenses in the brittle star’s photoresponse system is supported by the presence of bundles of nerve fibers located just beneath the lens layer at the calculated focal points of the lenses. The sizes of the bundles also agree well with the experimentally determined focus spot size (about 3 µm). Furthermore, the color change in O. wendtii is due to mobile pigment-containing cells that act somewhat as irises, regulating the amount of light that reaches the lenses. Such a response explains why O. wendtii is sensitive to lower levels of illumination at night than it is during the day. Some questions remain, though, such as whether it is the nerve bundles themselves or associated tissue that is photoreceptive, and how the brittle star’s nervous system processes the signals received from the microlenses. With their regularity and precise curved shape and orientation, the brittle star microlens arrays should inspire new ideas of what may be possible in the emerging field of biomimetics, or “biologically inspired materials synthesis.”G. HENDLERPPT|High resolutionJ. AIZENBERGPPT|High resolutionJ. AIZENBERGPPT|High resolutionREFERENCESSection:ChooseTop of pageREFERENCES <<1. J. Aizenberg, A. Tkachenko, S. Weiner, L. Addadi, G. Hendler, Nature 412, 819 (2001) https://doi.org/10.1038/35090573. Google ScholarCrossref, CAS2. E. N. K. Clarkson, R. Levi-Setti, Nature 254, 663 (1975) https://doi.org/10.1038/254663a0. Google ScholarCrossref, CAS© 2001 American Institute of Physics.

Evolution of the echinoderm Hox gene cluster

Extant echinoderms are members of an ancient and highly derived deuterostome phylum. The composition and arrangement of their Hox gene clusters are consequently of interest not only from the perspective of evolution of development, but also in terms of metazoan phylogeny and body plan evolution. Over the last decade numerous workers have reported partial Hox gene sequences from a variety of echinoderms. In this paper we used a combined methods approach to analyze phylogenetic relationships between 68 echinoderm Hox homeodomain fragments, from species of five extant classes--two asteroids, one crinoid, one ophiuroid, one holothuroid, and three echinoids. This analysis strengthens Mito and Endo's (2000) proposition that the ancestral echinoderm's Hox gene cluster contained at least eleven genes, including at least four posterior paralogous group genes. However, representatives of all paralogous groups are not known from all echinoderm classes. In particular, these data suggest that echinoids may have lost a posterior group Hox gene subsequent to the divergence of the echinoderm classes. Evolution of the highly derived echinoderm body plan may have been accompanied by class-specific duplication, diversification and loss of Hox genes.

Phylogenetic relationships of extant echinoderm classes

A well-supported phylogeny of echinoderm classes has eluded morphological, embryological, molecular, and combined analyses. From this body of work it is apparent that (i) echinoids (sea urchins) and holothuroids (sea cucumbers) are related, and (ii) crinoids (sea lilies) are the sister taxon to extant eleutherozoan classes (asteroids, ophiuroids, echinoids, and holothuroids). However, the relationships of asteroids and ophiuroids to other echinoderm classes have been difficult to recover. To address relationships between the asteroids and ophiuroids and other echinoderms, I have sequenced additional nuclear loci and taxa and used novel computational approaches for co-optimizing morphological with molecular evidence at the level of sequence alignment. Support for the monophyly of each class is strong. Support for a monophyletic Asteroidea + Xyloplax is as strong as for Asteroidea. Support for Asterozoa (Asteroidea + Ophiuroidea) is apparent, albeit not as strong as for other clades (e.g., Echinozoa, Eleutherozoa, and Echinodermata). I also present detailed sensitivity analyses to provide (i) a test of the monophyly of groups under a variety of evolutionary models and (ii) a statement of the evidential value of various character systems.

The pharmacology of gamma-aminobutyric acid and acetylcholine receptors at the echinoderm neuromuscular junction.

This review describes the various subtypes of gamma-aminobutyric acid (GABA) receptors found at the echinoderm neuromuscular junction (NMJ), based on pharmacological and physiological studies. The review focuses mainly on holothurian GABA receptors at the NMJ located between the radial nerve and longitudinal muscle of the body wall (LMBW) and compares them to GABA receptors described at other echinoderm NMJs. Since a primary action of GABA on the holothurian LMBW is to modulate contractile responses to the excitatory neurotransmitter, acetylcholine (ACh), the pharmacology of echinoderm nicotinic ACh receptors (nAChRs) and muscarinic ACh receptors (mAChRs) is also addressed. GABA responses have been described in the asteroids, echinoids and holothuroids but not in the other echinoderm classes. Some actions of GABA on echinoderm muscle include regulation of basal tone and spontaneous rhythmic contractions and modulation of cholinergic responses. Both GABA A and B receptor subtypes are present at the echinoderm NMJ, a feature also common to the arthropods, molluscs and chordates. Echinoderm GABA A receptors may mediate the excitatory responses to GABA. The GABA A receptor antagonist bicuculline has a paradoxical effect on contractility, stimulating large protracted contractions of the LMBW. The GABA A agonist muscimol potentiates cholinergic contractions of the holothurian LMBW. Another population of GABA receptors is inhibitory and is sensitive to the GABA B agonist baclofen and GABA B antagonists phaclofen and 2-OH-saclofen. The pre- and/or postsynaptic location of the GABA A and B receptors is not currently known. The folded GABA analogue 4-cis-aminocrotonic acid has no effect on the contractility of the holothurian LMBW so GABA C receptors are probably lacking in this preparation. Pharmacological studies have shown that distinct nAChRs and mAChRs are colocalized in numerous echinoderm muscle preparations. Most recently, nAChR agonists were used to characterize pharmacologically receptors at the holothurian LMBW that bind ACh. Nicotinic AChRs with unique pharmacological profiles are localized both pre- and postsynaptically at this NMJ, where their physiological action is to enhance muscle tone. Muscarinic agonists also have excitatory actions on the LMBW but their action is to stimulate phasic, rhythmic contractions of the muscle. The location of mAChRs at the echinoderm NMJ, however, is unknown. Since most of the studies described in the present review have used whole-mount preparations consisting largely of a combination of muscle fibers, neurons and connective tissue, it is extremely difficult to determine pharmacologically the exact location of the various receptor subtypes. Additional electrophysiological studies on isolated neurons and muscle fibers are therefore required to clearly define extra-, pre- and/or postsynaptic sites for the receptor subtypes at the echinoderm NMJ.

Stratocladistic analysis of blastoid phylogeny

Stratocladistics combines morphological and stratigraphic data in a parsimony-based analysis of evolutionary relationships. We use stratocladistics here to provide an overview of the phylogeny of the extinct echinoderm class Blastoidea. Both cladistic and stratocladistic methods evaluate alternative phylogenies by comparing the number of ad hoc hypotheses needed to reconcile each alternative to observed data. Minimization of ad hoc hypotheses selects the phylogeny best supported by data and enables phylogenetic analyses to incorporate data from different sources. Cladistics treats ad hoc hypotheses of homoplasy, whereas stratocladistics additionally considers ad hoc hypotheses of differential preservation probability of lineages in the stratigraphic record.The blastoid phylogeny derived using stratocladistics is more resolved than hypotheses selected by cladistics. Although the morphological characters are relatively homoplasious, in this instance the stratigraphic ordering of fossils provides both structure and altered polarity for the stratocladistic hypothesis. The stratocladistic phylogeny supports previous paleontological conclusions of convergence among blastoid lineages and facilitates evaluation of specific hypotheses of character transformation that are integral to recent systematic revisions. Additionally, consideration of temporal data makes some hypotheses of ancestor-descendant relationships more parsimonious than hypotheses of derivation from a common ancestor. The ability to recognize sequential members within single lineages allows more accurate estimation of faunal diversities and more specific reconstruction of evolutionary histories. Chief among possible confounding factors in stratocladistics are instances where preservation potential shows significant geographic variation, although problems of preservation are more tractable than the difficulties homoplasy presents for cladistic analysis.

STRATOCLADISTIC ANALYSIS OF BLASTOID PHYLOGENY

Stratocladistics combines morphological and stratigraphic data in a parsimony-based analysis of evolutionary relationships. We use stratocladistics here to provide an overview of the phylogeny of the extinct echinoderm class Blastoidea. Both cladistic and stratocladistic methods evaluate alternative phylogenies by comparing the number of ad hoc hypotheses needed to reconcile each alternative to observed data. Minimization of ad hoc hypotheses selects the phylogeny best supported by data and enables phylogenetic analyses to incorporate data from different sources. Cladistics treats ad hoc hypotheses of homoplasy, whereas stratocladistics additionally considers ad hoc hypotheses of differential preservation probability of lineages in the stratigraphic record.The blastoid phylogeny derived using stratocladistics is more resolved than hypotheses selected by cladistics. Although the morphological characters are relatively homoplasious, in this instance the stratigraphic ordering of fossils provides both structure and altered polarity for the stratocladistic hypothesis. The stratocladistic phylogeny supports previous paleontological conclusions of convergence among blastoid lineages and facilitates evaluation of specific hypotheses of character transformation that are integral to recent systematic revisions. Additionally, consideration of temporal data makes some hypotheses of ancestor-descendant relationships more parsimonious than hypotheses of derivation from a common ancestor. The ability to recognize sequential members within single lineages allows more accurate estimation of faunal diversities and more specific reconstruction of evolutionary histories. Chief among possible confounding factors in stratocladistics are instances where preservation potential shows significant geographic variation, although problems of preservation are more tractable than the difficulties homoplasy presents for cladistic analysis.

Diversity patterns and biostratigraphy of Cenozoic echinoderms from Florida

Fossil echinoderms are an important faunal component in Cenozoic rocks of Florida and the southeastern Coastal Plain of the USA, that provide useful information regarding diversity patterns and paleoecology for the formations in which they were deposited. The research presented in this paper significantly improves what is known about the stratigraphic distribution and diversity patterns of echinoids, comatulid crinoids, asteroids, and ophiuroids from the Middle Eocene through Pleistocene strata of Florida. Biostratigraphic distribution of echinoderms, specifically the echinoids, has not been synthesized on a large scale for many years, and the first compilation of these taxa from the state is provided in this study. Echinoderms now are known from 25 formations that are exposed throughout the state. The echinoids are better documented and described than the other classes of echinoderms, yet even the echinoids were poorly known from some epochs, most notably the Miocene. Previously published information showed echinoid diversity decreasing significantly from the Eocene into the Oligocene, and continuing to decline into the Miocene. Our research shows the echinoid diversity does not drop from the Oligocene into the Miocene, but instead increases by at least five species. This marked change in alpha diversity during the Miocene is a result of careful examination of smaller size fractions of sediments, which contained fragmented components of echinoids such as test plates and spines, and the diligent collecting of moldic samples, which produced additional taxonomic and stratigraphic records. The record of other echinoderms is still poor, but fine-fraction sediments also have produced disarticulated ossicles of crinoids, asteroids, and ophiuroids from several formations that had no previous record of such echinoderms. The diversity pattern of echinoderms from Florida is a reflection of both intrinsic and extrinsic factors that affect (in varying degrees) the echinoderm-bearing formations from the Paleogene through the Neogene. One of the most influential appears to be collector bias against recovering incomplete echinoderms. Many of the new species or new records of taxa from stratigraphic units in the state are based on fossils that are poorly preserved or incomplete. Other biases include over-splitting of taxonomic groups, stratigraphic unit revisions, taphonomy, and preservation potential variation in carbonate versus siliciclastic environments. Significant refinement of the echinoderm biostratigraphy and diversity patterns has occurred due to detailed examination of field samples, museum collections, and literature reviews, as well as accounting for biases inherent in the fossil record of Florida.

Echinoderm collagen fibrils grow by surface-nucleation-and-propagation from both centers and ends

Collagen fibrils from sea cucumber (class Holothuroidea) dermis were previously found to grow by coordinated monomer addition at both centers and ends. This analysis of sea urchin (class Echinoidea) collagen fibrils was undertaken to compare the growth characteristics of fibrils from two classes of echinoderms, and to determine whether a single growth model could account for the main features of fibrils from these two taxa. Native collagen fibrils (37–431 μm long) from the spine ligaments of the sea urchin Eucidaris tribuloides were studied by scanning transmission electron microscopy and image analysis. The analyses revealed the mass per unit length, and hence the number of molecules in cross-section, along the entire length of each fibril. The fibrils were symmetrically spindle shaped. The maximum mass per unit length occurred in the center of each fibril, where the fibril contains anti-parallel molecules in equal numbers. The two pointed tips of each fibril showed similar linear axial mass distributions, indicating that the two tips retain shape and size similarity throughout growth. The linear axial mass distributions showed that the tips were paraboloidal, similar to those of vertebrate and sea cucumber fibrils. The computed maximum diameters of the fibrils increased linearly with fibril length. The overall shapes of the fibrils showed that they retain geometric similarity throughout growth. Computer modeling showed that the simplest self-assembly mechanism that can account for the features of these fibrils, and of the sea cucumber fibrils that have been described, is one in which the fibril tips produce independent axial growth, while lateral growth takes place through a surface nucleation and propagation mechanism. This mechanism produces coordinated growth in length and diameter as well as geometric similarity, characteristic features of echinoderm collagen fibrils.

Identification of Hox Gene Sequences in the Sea Cucumber Holothuria glaberrima Selenka (Holothuroidea: Echinodermata).

The Echinodermata is a unique animal group forming an early branch in the deuterostomes phylogenetic tree. In echinoids and asteroids a single Hox cluster with nine cognates of the vertebrate Hox paralogous groups has been reported, but no data are available from other echinoderm classes. We report here nine Hox-type sequences from the sea cucumber Holothuria glaberrima, a member of the class Holothuroidea. Partial homeodomain sequences were amplified by polymerase chain reaction from genomic DNA and from a regenerating gastrointestinal tract complementary DNA library. Sequence analyses suggest that the holothuroid cluster has at least three genes of the anterior, one of the medial, and five of the posterior groups. This is the first evidence of five posterior sequences in echinoderms.

PCR Survey of Hox Genes in the Crinoid and Ophiuroid: Evidence for Anterior Conservation and Posterior Expansion in the Echinoderm Hox Gene Cluster

To help elucidate the cluster organization of Hox genes in echinoderms, we amplified a homeobox region by polymerase chain reaction (PCR) and cloned and sequenced the PCR products for the comatulid crinoid Oxycomanthus japonicus and the ophiuroid Stegophiura sladeni. The crinoid had at least three anterior, four medial, and four posterior genes, and the ophiuroid had at least one anterior, three medial, and six (one of which being a possible trans-paralog) posterior genes. The survey of the crinoid detected all three anterior complements (PG1–3 genes). It was inferred that the Hox genes of each species are organized into a single cluster and that a novel cognate group of echinoderm posterior genes (tentatively termed HboxP9) exists among echinoderms in addition to the known posterior genes Hbox4, Hbox7, and Hbox10. The results, combined with the data of other echinoderm classes, strongly suggest that the presence of a single Hox gene cluster is a common feature among echinoderms and that the cluster has the general features of the deuterostome Hox gene cluster, i.e., the anterior conservation and posterior expansion. The results of the ophiuroid imply that the posterior genes in this class diversified after the phylum Echinodermata had been established.