Tube Feet Research Articles

Organization of the coelomic lining and a juxtaposed nerve plexus in the suckered tube feet of Parastichopus californicus (Echinodermata: Holothuroida)

The coelomic lining of the water-vascular canal in a suckered tube foot from the sea cucumber, Parastichopus californicus, is a pseudostratified myoepithelium consisting of flagellated adluminal cells and myofilament-bearing retractor cells. The bodies of adluminal cells flank the water-vascular canal and send basal processes between the underlying retractor cells to confront the podial connective tissue. Retractor cells have a contractile apparatus of unregistered thick and thin myofilaments. The contractile apparatus is confined to the medullary sarcoplasm and oriented parallel to the primary axis of a tube foot. The bodies and processes of retractor cells intermingle with the basal processes of adluminal cells at the basal lamina of the coelomic lining. A ganglionated nerve plexus in the podial connective tissue approximates the basal lamina. Neuronal connectives link the ganglia to one another and to the nerve plexus in deep sectors of the podial epidermis. External laminae enveloping the ganglia and connectives in the podial connective tissue are continuous with the basal lamina of the epidermis. The adventitial nerve plexus, since it merges with the epidermal nerve plexus, is a component of the ectoneural division of the echinoderm nervous system.

Flexibility in starfish behavior by multi-layered mechanism of self-organization

Understanding animal behavior as a product of natural selection sometimes result in an underestimation of the animal's adaptability: lower animals with poor mental capabilities are usually considered to simply exhibit innate behavioral patterns. Self-organized behavior may exhibit both stability of certain behavioral patterns and flexibility in adopting those patterns. Thus, the self-organization processes of starfish arm and tube feet movements are investigated, by observing obstacle avoidance behavior and tube feet of moving starfish. As starfish have no central nervous systems, their behaviors are the result of certain self-organization processes. Starfish have hierarchically constructed motor organs consisting of arms and tube feet. The collective behavior of the tube feet does not function only as simple fluctuations in the arms’ coordination. As a result, starfish seem to exhibit more versatile behavioral changes than expected from the original model of a self-organized behavior.

Engrailed is expressed in larval development and in the radial nervous system of Patiriella sea stars

We documented expression of the pan-metazoan neurogenic gene engrailed in larval and juvenile Patiriella sea stars to determine if this gene patterns bilateral and radial echinoderm nervous systems. Engrailed homologues, containing conserved En protein domains, were cloned from the radial nerve cord. During development, engrailed was expressed in ectodermal (nervous system) and mesodermal (coeloms) derivatives. In larvae, engrailed was expressed in cells lining the larval and future adult coeloms. Engrailed was not expressed in the larval nervous system. As adult-specific developmental programs were switched on during metamorphosis, engrailed was expressed in the central nervous system and peripheral nervous system (PNS), paralleling the pattern of neuropeptide immunolocalisation. Engrailed was first seen in the developing nerve ring and appeared to be up-regulated as the nervous system developed. Expression of engrailed in the nerve plexus of the tube feet, the lobes of the hydrocoel along the adult arm axis, is similar to the reiterated pattern of expression seen in other animals. Engrailed expression in developing nervous tissue reflects its conserved role in neurogenesis, but its broad expression in the adult nervous system of Patiriella differs from the localised expression seen in other bilaterians. The role of engrailed in patterning repeated PNS structures indicates that it may be important in patterning the fivefold organisation of the ambulacrae, a defining feature of the Echinodermata.

Biology of the basket star Gorgonocephalus caputmedusae (L.)

Ophiurid basket stars belonging to the family Gorgonocephalidae are distributed from the Arctic to the Antarctic and from the shallow subtidal to the deep sea, but their biology remains poorly known. In situ observations at the mouth of the Oslofjord by a remotely operated vehicle showed that Gorgonocephalus caputmedusae had a patchy distribution at 85 to 120 m water depth and frequently occurred in association with the gorgonian Paramuricea placomus and the coral Lophelia pertusa. Morphological and histological studies show that G. caputmedusae is well adapted to capture macroplanktonic prey. Histological examination of the arms revealed the presence of a thick layer of dermal mutable connective tissue which is probably an energy-efficient way to maintain its feeding posture against the current. This layer is connected to the nerve cord suggesting that the passive mechanical properties (stiffness) is controlled by the nervous system. In the distal parts of the arms, each segment has a pair of sticky tube feet and a sophisticated system of spines and hooks, which are connected to muscles and collagenous tendons. In combination, these features were shown, in an experimental flume study, to be used for capturing the locally abundant krill species Meganyctiphanes norvegica. This is the first documentation of G. caputmedusae of this kind.

Thermal history-dependent expression of the hsp70 gene in purple sea urchins: Biogeographic patterns and the effect of temperature acclimation

Thermal variation associated with large spatial scales is thought to significantly affect the physiology of organisms in marine systems and consequently limit their distribution. To examine this phenomenon, this study compared the heat shock protein ( hsp70) mRNA expression of purple sea urchins ( Strongylocentrotus purpuratus) collected from different geographic locations and those acclimated to different temperatures. Tube feet were removed from S. purpuratus freshly collected from Fogarty Creek, Oregon (FC-OR) and Carpinteria, California (CRP-CA) and from urchins acclimated to 5 °C and 20 °C in the laboratory. Tissue samples were then immediately frozen or incubated at various temperatures between 5 °C and 36 °C for 30 min in vitro. Real-time PCR was used to measure the hsp70 mRNA levels of each sample. The results showed evidence for differential regulation of hsp70 between S. purpuratus in both field and laboratory studies. For example, the incubation temperature that induced maximum hsp70 mRNA expression ( T max) was higher in CRP-CA urchins (26 °C) than those from FC-OR (23 °C). Additionally, although the T max of the two acclimation groups was identical (26 °C), S. purpuratus acclimated to 5 °C failed to induce hsp70 mRNA after incubation at 32 °C, while tube feet of the 20 °C-acclimated urchins remained transcriptionally active. In general, the extent of hsp70 mRNA induction was substantial, exceeding 250-fold in the 5 °C-acclimated urchin tissues after incubation at 26 °C. These data suggest that the thermal variation encountered across S. purpuratus' range is sufficient to effect the regulation of thermally sensitive genes, like hsp70.

Adhesion of echinoderm tube feet to rough surfaces

Echinoderms attach strongly and temporarily to the substratum by means of specialized organs, the podia or tube feet. The latter consist of a basal extensible cylinder, the stem, which bears an apical flattened disc. The disc repeatedly attaches to and detaches from the substratum through adhesive and de-adhesive secretions. In their activities, echinoderms have to cope with substrata of varying degrees of roughness as well as with changing hydrodynamic conditions, and therefore their tube feet must adapt their attachment strength to these environmental constraints. This study is the first attempt to evaluate the influence of substratum roughness on the temporary adhesion of echinoderm tube feet and to investigate the material properties of their contact surface. It was demonstrated that tube foot discs are very soft (E-modulus of 6.0 and 8.1 kPa for sea stars and sea urchins, respectively), have viscoelastic properties and adapt their surface to the substratum profile. They also show increased adhesion on a rough substratum in comparison to its smooth counterpart, which is due mostly to an increase in the geometrical area of contact between the disc and the surface. Tenacity (force per unit area) increases with roughness [e.g. 0.18 and 0.34 MPa on smooth polymethyl-methacrylate (PMMA), 0.21 and 0.47 MPa on rough PMMA for sea stars and sea urchins, respectively] if only the projected surface area of the adhesive footprint is considered. However, if this tenacity is corrected to take into account the actual substratum 3-D profile, surface roughness no longer influences significantly the corrected adhesion strength (e.g. 0.18 and 0.34 MPa on smooth PMMA, 0.19 and 0.42 MPa on rough PMMA for sea stars and sea urchins, respectively). It can be hypothesized that, under slow self-imposed forces, disc material behaves viscously to adapt to substratum roughness while the adhesive fills out only very small surface irregularities (in the nanometer range). It is deposited as a thin film ideal for generation of strong adhesion. Under short pulses of wave-generated forces, attached discs probably behave elastically, distributing the stress along the entire contact area, in order to avoid crack generation and thus precluding disc peeling and tube foot detachment.

The tube feet of sea urchins and sea stars contain functionally different mutable collagenous tissues

Echinoderms possess mutable collagenous tissues (MCTs), which are capable of undergoing rapid changes in their passive mechanical properties mediated by secretions from a specific cell type, the juxtaligamental cell. In this study, the possible presence of MCTs in the tube feet of the echinoid Paracentrotus lividus and the asteroid Marthasterias glacialis was investigated by measuring their extensibility, tensile strength, stiffness and toughness after different treatments known to influence the physiological state of MCTs. Calcium removal reversibly induced a significant plasticization of the tube feet of both species. When exposed to cell-disrupting solutions, the tube foot stem of sea urchins and sea stars showed a significant increase in strength, stiffness and toughness in the absence of calcium. This response, combined with the ultrastructural observation of juxtaligamental-like cells in the connective tissue, confirms that an MCT is present in both echinoid and asteroid tube feet. It was observed, however, that the tube foot stems of P. lividus and M. glacialis are affected differently by exposure to cell-disrupting solutions in the presence of calcium, indicating that their MCTs could be functionally different. In their soft state, MCTs could assist the muscles in tube foot protraction, bending and retraction; in their stiff state, they could play a role in the energy-sparing maintenance of position; for example, during strong attachment to the substratum to resist hydrodynamically generated loads.

The phylogenetic status of Paxillosida (Asteroidea) based on complete mitochondrial DNA sequences

One of the most important issues in asteroid phylogeny is the phylogenetic status of Paxillosida. This group lacks an anus and suckers on the tube feet in adults and does not develop the brachiolaria stage in early development. Two controversial hypotheses have been proposed for the phylogenetic status of Paxillosida, i.e., Paxillosida is primitive or rather specialized in asteroids. In this study, we determined the complete mitochondrial DNA nucleotide sequences from two paxillosidans ( Astropecten polyacanthus and Luidia quinaria) and one forcipulatidan ( Asterias amurensis). The mitochondrial genomes of the three asteroids were identical with respect to gene order and transcription direction, and were identical to the previously reported mitochondrial genomes of Asterina pectinifera (Valvatida) and Pisaster ochraceus (Forcipulatida) in this respect. Therefore, the comparison of genome structures was uninformative for the purposes of asteroid phylogeny. However, molecular phylogenetic analyses based on the amino acid sequences and the nucleotide sequences from the five asteroids supported the monophyly of the clade that included the two paxillosidans and Asterina. This suggests that the paxillosidan characters are secondarily derived ones.

Behavioural and metabolic responses of the Antarctic sea star Odontaster validus to food stimuli of different concentration

Experiments were carried out to study the behavioural and metabolic responses of the Antarctic sea star, Odontaster validus, to different concentration of food signals (glutamic acid, aspartic acid, glicyne, alanine, tyrosine and a mixture of 11 amino acids). Stimulus concentration increase caused a rise in both the percentage of reacting animals and in the reaction intensity. At low stimulus concentration, O. validus reaction consisted of tube foot waving, and only high concentrations elicited a complicated sequence of several types of behaviour. The metabolic rate of small O. validus was more sensitive to chemical stimuli than of large individuals, with small sea stars reacting even to low signal concentration. It can be speculated that small body size can be a factor restricting sea star capabilities and influencing its reaction to chemical signals.

Comparative histological and immunohistochemical study of sea star tube feet (Echinodermata, Asteroidea)

Adhesion in sea stars is the function of specialized structures, the tube feet or podia, which are the external appendages of the water-vascular system. Adhesive secretions allow asteroid tube feet to perform multiple functions. Indeed, according to the sea star species considered, the tube feet may be involved in locomotion, fixation, or burrowing. Different tube foot shapes usually correspond to this variety of function. In this study, we investigated the variability of the morphology of sea star tube feet as well as the variability of the composition of their adhesive secretions. This second aspect was addressed by a comparative immunohistochemical study using antibodies raised against the adhesive material of the forcipulatid Asterias rubens. The tube feet from 14 sea star species representing five orders and 10 families of the Class Asteroidea were examined. The histological study revealed three main tube foot morphotypes, i.e., knob-ending, simple disc-ending, and reinforced disc-ending. Analysis of the results suggests that tube foot morphology is influenced by species habitat, but within limits imposed by the evolutionary lineage. In immunohistochemistry, on the other hand, the results were very homogeneous. In every species investigated there was a very strong immunolabeling of the adhesive cells, independently of the taxon considered, of the tube foot morphotype or function, or of the species habitat. This indicates that the adhesives in all the species considered are closely related, probably sharing many identical molecules or, at least, many identical epitopes on their constituents.

Evaluation of the attachment strength of individuals of Asterina gibbosa (Asteroidea, Echinodermata) during the perimetamorphic period

A turbulent channel flow apparatus was used to determine the adhesion strength of the three perimetamorphic stages of the asteroid Asterina gibbosa, i.e. the brachiolaria larvae, the metamorphic individuals and the juveniles. The mean critical wall shear stresses (wall shear stress required to dislodge 50% of the attached individuals) necessary to detach larvae attached by the brachiolar arms (1.2 Pa) and juveniles attached by the tube feet (7.1 Pa) were one order of magnitude lower than the stress required to dislodge metamorphic individuals attached by the adhesive disc (41 Pa). This variability in adhesion strength reflects differences in the functioning of the adhesive organs for these different life stages of sea stars. Brachiolar arms and tube feet function as temporary adhesion organs, allowing repetitive cycles of attachment to and detachment from the substratum, whereas the adhesive disc is used only once, at the onset of metamorphosis, and is responsible for the strong attachment of the metamorphic individual, which can be described as permanent adhesion. The results confirm that the turbulent water channel apparatus is a powerful tool to investigate the adhesion mechanisms of minute organisms.

Morphometry and mechanical design of tube foot stems in sea urchins: a comparative study

To withstand hydrodynamic forces, sea urchins rely on their adoral tube feet, which are specialized for attachment. Although it has been often suggested that the degree of development of these tube feet is intimately related to the maximum environmental energy a species can withstand, it has never been demonstrated by mechanical testing. To address this subject, we studied the mechanical properties of the stem of adoral tube feet from three species of sea urchins, Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis, which have distinct taxonomic, ecological and morphological characteristics. The tube feet of the three species have a very similar morphology. When a tensile force is applied to the tube foot stem, the connective tissue is the only tissue layer bearing the load. The mechanical properties of this tissue give the tube feet an ideal balance of extensibility (139–166%), strength (23–29 MPa) and stiffness (152–328 MPa), which together produce a material with adequate toughness (2.5–2.9 MJ/m 3) to absorb the impact of waves and currents, and thus to resist the environmental challenges of the habitats in which sea urchins live. Extended stems of P. lividus were significantly stiffer (328 MPa) than those of the other two species (152 and 183 MPa, for A. lixula and S. granularis, respectively). No interspecific difference was found in terms of extensibility, strength, initial stiffness and toughness between the tube feet from the three species. The difference in local distribution between the species investigated is therefore not only explained by the mechanical properties of their tube feet, but may involve other factors such as tube foot number and arrangement, tube foot disc tenacity or sea urchin size.

Feeding ofPseudechinus magellanicus(Philippi, 1857) (Echinoidea: Temnopleuridae) in the SW Atlantic Coast (Argentina)

Gut contents of Pseudechinus magellanicus collected from different habitats from Buenos Aires to Ushuaia were analyzed. In mussel beds off the Province of Buenos Aires at 50 m depth, the most important food items were Mytilus edulis platensis spat. In communities without Mytilus, food preferences were barnacles, foraminiferans, polychaetes and algae. In beds of the mytilid Aulacomya atra at a depth of 5-10m, main food items were oyster shell fragments and different algae: Rodophyta (leafy algae), Ectocarpus sp., and other algae. In Ushuaia the sea urchins preyed on ostracods, foraminiferans and algae. Multidimensional scaling ordination analyses showed that the diet of P. magellanicus depends mostly on the kind of inhabited substrate. Our data indicate that P. magellanicus from Argentina is omnivorous, consuming any available food resource: they capture drift algae with the aboral tube feet, graze on sessile algae, and prey on other animals. The results suggest that the species play an important role in the community structure of the circa-littoral beds of Mytilus edulis platensis, mainly through an intense predation on bivalve spat.

DEMOGRAPHIC CONSEQUENCES OF AN ONTOGENETIC SHIFT BY A SEA URCHIN IN RESPONSE TO HOST PLANT CHEMISTRY

Habitat selection that maximizes both larval settlement success and subse- quent juvenile and adult performance is a fundamental challenge for marine benthic or- ganisms with complex life histories (e.g., planktonic larvae). For benthic herbivores, chem- ical cues from macroalgae can strongly influence both larval settlement/metamorphosis and subsequent performance. We compared the effects of chemical cues from host algae on different life history stages of the ''arboreal'' sea urchin Holopneustes purpurascens .I n sublittoral habitats near Sydney, Australia, H. purpurascensoccurred primarily on two algal hosts: red alga (Delisea pulchra) and kelp (Ecklonia radiata). Urchins on E. radiata were significantly larger than those on D. pulchra, but no recruits occurred on E. radiata .I n experiments, larvae rapidly metamorphosed in the presence of D. pulchra, but metamor- phosis was delayed or nonexistent in the presence of E. radiata. D. pulchra produces a polar chemical inducer of metamorphosis not found in E. radiata. In contrast to larval metamorphosis, feeding and performance of juvenile and adult urchins were considerably worse on D. pulchra than on E. radiata. Feeding experiments confirmed that nonpolar metabolites (halogenated furanones) in D. pulchra deter feeding. Adding natural concen- trations of the most abundant furanone to otherwise palatable diets completely inhibited feeding. Urchins fed D. pulchra had reduced survival, growth, and reproduction compared to those fed E. radiata. H. purpurascens did not thrive on the alga on which they prefer- entially settled; therefore, postrecruitment movement from D. pulchra to E. radiata was crucial to fitness. Urchins in D. pulchra beds moved between plants at night and preferred E. radiata in habitat preference experiments. Movement was constrained by diurnal pred- ators, particularly damselfish Parma microlepis. When H. purpurascenswas removed from their algal hosts during the day (but not at night), P. microlepis ''clipped'' their spines and tube feet. Removal of 10% of an urchin's spines and tube feet was fatal. H. purpurascens recruits onto a host that does not support persistence of the benthic phase and then shifts host plants, incurring a predation risk. Though chemically mediated, ontogenetic shifts in host plant use are known for insect herbivores, this is the first known example in a marine herbivore.

Preservation of tube feet in an ophiuroid (Echinodermata) from the Lower Devonian Hunsrück Slate of Germany and a redescription ofBundenbachia beneckei andPalaeophiomyxa grandis

The preservation of non-mineralized tissues in the fossil record is extremely rare. The Lower Devonian Hunsruck Slate of Germany has long been known for the preservation of non-mineralized tissues in pyrite but whether or not these remnants represent true soft tissues has been questioned. This is especially true for struetures visible only on radiographs that are too delicate for excavation by traditional methods. Here we report the discovery of well-preserved pyritized tube feet in six fully prepared specimens of the protasterid brittle starBundenbachia beneckei from the Hunsruck Slate. This discovery represents the first report of fossilized ophiuroid tube feet in the fossil record. The successful excavation of the delicate tube feet was made possible by improved airbrasive techniques developed by German fossil collectors.

Surface sequestration of chemical feeding deterrents in the Antarctic sponge Latrunculia apicalis as an optimal defense against sea star spongivory

Latrunculia apicalis is a spherically shaped demosponge that previous investigations have shown is rarely preyed upon by sea stars which are the dominant spongivores in antarctic benthic communities. Prior studies have also demonstrated that L. apicalis produces organic compounds that elicit a tube foot retraction response in the keystone spongivorous sea star Perknaster fuscus that can be used as a reliable assay for feeding deterrence. L. apicalis is known to contain discorhabdin alkaloids which serve, among other roles, as the source of its green coloration. To assess the defensive nature of the discorhabdin alkaloids toward P. fuscus, we have determined discorhabdin G concentrations in discrete sponge layers and evaluated those concentrations in the P. fuscus bioassay. In discorhabdin G-bearing sponges, we found a gradient of discorhabdin G that falls off rapidly toward the center of the sponge. On average, 52% of total discorhabdin G in a given sponge was found within 2 mm of the sponge surface. Tube foot retraction responses to extracts from the surface tissues (0–2 mm depth) of L. apicalis were compared to those of an inner layer (8–10 mm depth) and to a sample comprised of the same inner layer spiked with discorhabdin G at a concentration equivalent to that of the surface tissues. Tube foot retraction response times to extracts of the surface layers and the spiked inner layers were not statistically different, but were significantly greater than responses to the unaltered inner layer and controls. These results support the predictions of the optimal defense theory as L. apicalis sequesters its defensive chemistry (discorhabdin G) in its most vulnerable surface tissues, where the likelihood of predation from sea stars is highest. As antarctic sponges are generally preyed upon by extraoral feeding sea stars rather than deeper biting predators such as fish, surface sequestration may be uniquely adaptive in sessile macroinvertebrates occurring in antarctic marine benthic environments.

DEVELOPMENTAL BREAKDOWN DURING THE EARLY EVOLUTION OF THE CODIACRINIDAE: PARAZOPHOCRINUS CALLOSUS STRIMPLE, 1963 (CLASS CRINOIDEA)

Parazophocrinus callosus Strimple, 1963, is redescribed as a cladid crinoid belonging to the Codiacrinidae. Parazophocrinus is a reasonably large cladid crinoid that is composed primarily of basal plates, radial plates, and oral plates; arms are absent; and the anal opening is along the side of the aboral cup. The radial plates are hypertrophied laterally, producing a disc-shaped aboral cup, and this very unusual crinoid is considered to be neotenic. Presumably, feeding in Parazophocrinus occurred only with tube feet projected through opened oral plates or from epidermal nutrient uptake. Typical codiacrinids may lack arms, may have the anal opening from the side of the aboral cup, and commonly have prominent oral plates; but in contrast to Parazophocrinus, they are small to very small in size. This small size and arrested development is considered progenetic.Parazophocrinus is the second oldest genus in the Codiacrinidae. Apparently, when developmental controls broke down during the early evolution of this family, both neotenic and progenetic crinoids evolved. However, ultimately, the Parazophocrinus morphology, a large crinoid lacking arms, was not successful through time. Thus, although more than one heterochronic pathway was possible, functional and ecologic constraints determined which developmental pathway would have long-term success.

Developmental breakdown during the early evolution of the Codiacrinidae:Parazophocrinus callosusStrimple, 1963 (class Crinoidea)

Parazophocrinus callosusStrimple, 1963, is redescribed as a cladid crinoid belonging to the Codiacrinidae.Parazophocrinusis a reasonably large cladid crinoid that is composed primarily of basal plates, radial plates, and oral plates; arms are absent; and the anal opening is along the side of the aboral cup. The radial plates are hypertrophied laterally, producing a disc-shaped aboral cup, and this very unusual crinoid is considered to be neotenic. Presumably, feeding inParazophocrinusoccurred only with tube feet projected through opened oral plates or from epidermal nutrient uptake. Typical codiacrinids may lack arms, may have the anal opening from the side of the aboral cup, and commonly have prominent oral plates; but in contrast toParazophocrinus, they are small to very small in size. This small size and arrested development is considered progenetic.Parazophocrinusis the second oldest genus in the Codiacrinidae. Apparently, when developmental controls broke down during the early evolution of this family, both neotenic and progenetic crinoids evolved. However, ultimately, theParazophocrinusmorphology, a large crinoid lacking arms, was not successful through time. Thus, although more than one heterochronic pathway was possible, functional and ecologic constraints determined which developmental pathway would have long-term success.

Locomotor performance in the sea urchin Paracentrotus lividus

The locomotor performance of the Mediterranean sea urchin Paracentrotus lividus was investigated under laboratory conditions. Individuals were placed singly in the centre of a glass surface positioned either horizontally or vertically in tanks with seawater, and their locomotor activity was recorded. For locomotion on a horizontal surface, speed increased with both sea urchin diameter and their straightness of path. Speeds on a vertical surface were size-independent and not related to the straightness of path, although they were affected by vertical path orientation, with the highest speeds occurring for downward movements and the slowest speeds for the upward movements. Taken together, these results suggest that the scaling of sea urchin locomotion may follow similar laws to those of legged animals, for which locomotor performance increases with size on horizontal surface, while their relative cost of locomotion increases with body size on inclined surfaces. It is suggested that differences in horizontal vs vertical locomotion may also be related to differences in the underlying locomotor mechanisms, i.e. using adhesive appendices (tube feet) or levers (spines). In a second experiment, the sea urchin speed obtained during a negative phototactic response to a direct light stimulus was recorded. The results show that speed during light stimulation is higher than that during spontaneous locomotion in sea urchins of intermediate size (2·5–4 cm), suggesting that, in addition to the direction of locomotion as shown by previous studies, light can also have an effect on speed.

Evolutionary convergence in Otx expression in the pentameral adult rudiment in direct-developing sea urchins.

Convergence is a significant evolutionary phenomenon. Arrival at similar morphologies from different starting points indicates a strong role for natural selection in shaping morphological phenotypes. There is no evidence yet of convergence in the developmental mechanisms that underlie the evolution of convergent developmental phenotypes. Here we report the expression domains in sea urchins of two important developmental regulatory genes ( Orthodenticle and Runt), and show evidence of molecular convergence in the evolution of direct-developing sea urchins. Indirect development is ancestral in sea urchins. Evolutionary loss of the feeding pluteus stage and precocious formation of the radially symmetric juvenile has evolved independently in numerous sea urchin lineages, thus direct development is an evolutionary convergence. Indirect-developing species do not express Otx during the formation of their five primordial tube feet, the ancestral condition. However, each direct-developing urchin examined does express Otx in the tube feet. Otx expression in the radial arms of direct-developing sea urchins is thus convergent, and may indicate a specific need for Otx use in direct development, a constraint that would make direct development less able to evolve than if there were multiple molecular means for it to evolve. In contrast, Runt is expressed in tube feet in both direct- and indirect-developing species. Because echinoderms are closely related to chordates and postdate the protostome/deuterostome divergence, they must have evolved from bilaterally symmetrical ancestors. Arthropods and chordates use Otx in patterning their anterior axis, and Runt has multiple roles including embryonic patterning in arthropods, and blood and bone cell differentiation in vertebrates. Runt has apparently been co-opted in echinoderms for patterning of pentamery, and Otx in pentameral patterning among direct-developing echinoids. The surprisingly dynamic nature of Otx evolution reinvigorates debate on the role of natural selection vs shared ancestry in the evolution of novel features.