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Larvae of the scleractinian coral Pocillopora damicornis are induced to settle and metamorphose by' the presence of marine bacterial biofilms. and the Iarvae of Monttpora capitata respond to a combination of filamentous and crustose coralline algae. The primary goal of this study was to better understand metamorphosis of cnidarian larvae by determining what types of' receptors and signal-transduction pathways are involved during stimulation of metamorphosis of P. damicornis and M. capttata. Evidence from studies on larvae of hydrozoans suggests that G-protein-coupled receptors (GPCRs) are good candidates. Settlement experiments were conducted in which competent larvae were exposed to neuropharmacological agents that affect GPCRs and their associated signal-transduction pathways. AC/cAMP and PI/DAG/PKC. On the basis of the results of these experiments, we conclude that GPCRs and these pathways do not mediate settlement and metamorphosis in either coral species. Two compounds that had an effect on both species, forskolin and phorbol-12-myrisiate-13-acetate (TPA), may be acting on other cellular processes not related to GPCRs. This study strengthens our understanding of the underlying physiological mechanisms that regulate metamorphosis in coral larvae., Introduction Research on the environmental cues and physiological mechanisms that induce coral larvae to settle and metarnorphose has expanded significantly within the last two decades (Ritson-Williams et al., 2009). Studies [...]

Cidaroids, one of the two major sister clades of sea urchins, first appeared during the lower Permian (ca. 270 mya) and are considered to represent the primitive form of all living echinoids. This study of Cidaris blakei, a deep-sea cidaroid urchin with planktotrophic larvae, provides a description of development from fertilization through early juvenile stages and is the first report of a deep-sea urchin reared through metamorphosis. C. blakei resembles other cidaroids in its lack of a cohesive hyaline layer, the absence of an amniotic invagination for juvenile rudiment formation, and the presence of spines with a single morphotype at metamorphosis. C. blakei differed from other cidaroids in the presence of an apical tuft, the extent of fenestration of postoral skeletal rods, the shape of juvenile spines, and an extended (14-day) lecithotrophic stage prior to development of a complete gut. The development of C. blakei, 120 days from fertilization to metamorphosis, was protracted relative to that of shallow-water cidaroids. Preliminary work on temperature tolerances suggests that C blakei larvae would be unable to survive the warmer temperatures higher in the water column and are therefore unable to vertically migrate., Introduction Extant echinoids are divided into two major sister clades, the Cidaroida and the Euechinoida. Euechinoids are diverse, comprising more than 720 living species, including both regular and irregular echinoids [...]

I used histological sections and 3D reconstructions to document development through metamorphosis of the foregut and proboscis in the conoidean neogastropod Conus lividus. A goal was to determine how highly derived features of the post-metamorphic feeding system of this gastropod predator develop without interfering with larval structures for microherbivory. A second goal was to compare foregut development in this conoidean with previous observations on foregut development in the buccinoidean neogastropod Nassarius mendicus. These two neogastropods both have a feeding larval stage, but they show major differences in post-metamorphic foregut morphology. Basic events in development of the proboscis and proboscis sheath in C. lividus and N. mendicus were similar. However, the elongate buccal tube of C. lividus forms during metamorphosis as a composite of apical epidermal tissue that grows inward and ventral foregut tissue that extends outward. The larval mouth is not carried through metamorphosis. Comparative observations on foregut development in caenogastropods, which now include data on C. lividus, suggest that the foregut incorporates dorsal and ventral modules having different ontogenetic and functional fates. This developmental modularity may have facilitated evolutionary diversification of the post-metamorphic foregut. Foregut diversification in predatory gastropods may have been further fast-tracked by developmental uncoupling of larval and post-metamorphic mouths., Introduction Predatory gastropods have complex feeding systems that are highly derived relative to those of the herbivorous grazers that populate more basal gastropod clades (Fretter and Graham, 1994; Ponder and [...]

Artificial inducers have been used to study signal-transduction pathways involved in metamorphosis of some marine invertebrates. However, the transduction mechanisms for echinoderms have been less explored. In the present study, participation of protein kinase C (PKC), G-protein-coupled receptors (GPCRs), and calcium has been investigated during metamorphosis of the sea urchin Stronglylocentrotus purpuratus. Competent larvae were induced with different drugs that activate (PKC and GP activators, [Ca.sup.2+] ionophores, and inhibitors of [Ca.sup.2+] ATPase) or inhibit (PKC, G-protein, and [Ca.sup.2+] flux inhibitors) metamorphosis. Six of the compounds were effective: the PKC activators TPA and indolactam; the G-protein inhibitors suramin and guanosine; the calcium ionophore A23187, and the calcium ATPase inhibitor thapsigargin. TPA was effective at 0.001 µM; indolactam was effective at 0.001 µM. In the presence of KCl as inducer, the G-protein inhibitor suramin was effective at 10 µM and guanosine at 0.001 µM. In the presence of a bacterial film as inducer, suramin was effective at 50 µM, and guanosine inhibited metamorphosis at 1 µM. A23187 was effective at 5 and 10 µM and thapsigargin at 50 and 100 µM. Our results indicate that GPCRs, protein kinase C, and calcium participate in the metamorphosis of S. purpuratus. These elements of the transduction pathways triggered during metamorphosis may be part of a cascade of signal transduction routes that interact from induction to the end of the morphogenetic events that shape the postlarval form. In addition, according to the results obtained with G-protein inhibitors, the GPCRs may be shared between the artificial (KCl) and natural (biofilm) inducers., Introduction In marine invertebrates, the stimuli that induce metamorphosis are specific agents, chemical or biological, that are associated with certain natural substrata such as microbial films, potential prey, or algae [...]

Chemical cues are important in the exogenous and endogenous control of metamorphosis in many marine invertebrate larvae. In the queen conch, Strombus gigas Linnaeus, larval metamorphosis is induced by low molecular weight compounds associated with dominant species of red algae found in conch nursery grounds; these species include the foliose rhodophyte Laurencia poitei (Lamouroux). The responses of conch larvae to the algal-associated cues are dependent on concentration and length of exposure, with the initial events of metamorphosis occurring within 10 min of treatment with an aqueous extract of L. poitei. The free amino acids valine and isoleucine mimic the effects of the natural inducer, and they may bind to and be recognized by the same sites on the larvae as the algal cues. Hydrogen peroxide, vanadate, and [Gamma]-aminobutyric acid (GABA), as well as elevated [K.sup.+] concentrations (i.e., above ambient seawater levels), also induce larval metamorphosis. Acetylsalicylic acid decreases the responses of conch larvae to the algal-associated cues and to the free amino acids, but it has no effect on the induction triggered by hydrogen peroxide. The chemical induction of metamorphosis in conch larvae shares many general features with chemoreception in aquatic invertebrates. The natural inducers of metamorphosis, like the cues involved in olfactory responses in other marine organisms, are of low molecular weight and water soluble. In addition, the results of the experiments with hydrogen peroxide, vanadate, and GABA suggest that second messenger pathways are involved in conch metamorphosis., Introduction Queen conch, Strombus gigas, are marine benthic gastropods found in seagrass beds and sand flats throughout the tropical Atlantic (Randall, 1964; Brownell and Stevely, 1981). As juveniles, conch occur [...]

The free-swimming parenchymella larvae of Haliclona permollis have a surface of flagellated cells that function in locomotion. These flagellated cells disappear from the larval surface soon after larval settlement, but the debate about their fate during metamorphosis has not been resolved. An improved method for electron microscopy enabled us to visualize minute ellipsoid granules characteristic of the larval flagellated cells of this sponge and to use these granules as natural markers to follow the fate of flagellated cells in metamorphs. On metamorphosis, the axonemes of the flagellated cells are withdrawn into the cell body and persist for some time, thus serving as a second natural marker for flagellated cells in postlarval forms. Within 12 h after settlement, the metamorphs have both markers in amoeboid cells derived from flagellated cells. The minute ellipsoid granules are found in the amoeboid cells of the metamorphs 24 h after settlement, and in the choanocytes of the juveniles 36 h after settlement. Therefore, it is proposed that the choanocytes derive from the larval flagellated cells by way of an amoeboid cell stage. These results suggest that the flagellated cells of parenchymella larvae participate in the formation of juveniles., Introduction Most sponges, which are sessile as adults, have free-swimming larvae in their life cycles. The larvae use a surface layer of flagellated cells to swim around for a while, [...]

Larvae of Haminaea callidegenita (Mollusca: Cephalaspidea) were induced to metamorphose by a compound found in the gelatinous matrix composing most of the egg mass. A functionally similar compound isolated from adult tissue also induced metamorphosis in H. callidegenita larvae. Opisthobranchs are frequently induced to metamorphose by a specific prey item or a substrate characteristic of the adult habitat, but this is the first known instance of metamorphosis occurring in response to a compound produced by adult conspecifics. The inducer was purified from egg mass jelly (EMJ) by high pressure liquid chromatography (HPLC) and was found to be smaller than 1000, Da, polar, non-proteinaceous, and very stable. We isolated a compound of identical activity from egg masses produced by four other opisthobranch species, suggesting that the same or chemically similar compounds are intrinsic to opisthobranch egg masses. However, only H. callidegenita larvae metamorphosed in response to EMJ. Competent larvae of five other mollusc species did not respond to the partially purified EMJ inducer but did respond to a specific substrate associated with each species. The presence of the inducer within the egg mass causes an unusual developmental pattern in H. callidegenita, a poecilogonous species that produces both swimming veliger and crawling juvenile offspring., Introduction Larvae of most benthic marine invertebrate species are induced to metamorphose by a cue, either chemical or physical, that identifies a specific habitat as suitable for juvenile or adult [...]

Immunocytochemical investigations of the eyestalk of Carcinus maenas zoeal larval stages, using an antiserum directed agaist putative Carsinus molt-inhibiting hormone (MIH), revealed immunopositive neuronal structures. These structures included perikarya associated with the medula terminalis X-organ, parts of the sinus gland tract, and the neurohemal organ -- the sinus gland. Apart from an increase in volume of the sinus gland between zoeal stage I and II, no striking changes in the topography or morphology of the MIH neurosecretory system were observed. Immunopositive structures were found in similar locations to those seen in adult crabs. Our results suggest that the control of molting by MIH in crustacean larvae may be similar to the currently accepted model of molt control in adult decapod crustaceans.

Most benthic invertebrates have complex life cycles with planktonic larvae that return to the substratum to settle and metamorphose into a benthic stage. Although naturally produced chemical cues have long been thought to be important for the settlement or metamorphosis of invertebrate larvae, few ecologically relevant chemical cues have been clearly identified. The marine echinoid Holopneustes purpurascens has a complex life cycle, with a planktonic, nonfeeding dispersive larva that metamorphoses into a benthic stage that lives in the canopy of subtidal benthic algae such as the red alga Delisea pulchra and the kelp Ecklonia radiata. Recently recruited juveniles are found primarily on D. pulchra, and we hypothesized that this was in response to a chemical cue produced by this alga. Competent larvae metamorphosed in the presence of D. pulchra, or seawater surrounding this alga, but not in response to the presence of E. radiata or its extracts. A cue for metamorphosis was isolated and characterized from D. pulchra and found to be a water-soluble complex of the sugar floridoside and isethionic acid in a 1:1 molar ratio. The floridosideisethionic acid complex also triggered settlement in H. purpurascens; however, this response was less specific than metamorphosis and was reversible. Larvae of H. purpurascens also metamorphosed in the presence of several other species of red, but not brown or green, algae from their habitat. Floridoside is found only in red algae, suggesting that the floridoside-isethionic acid complex may be acting as a cue for metamorphosis in other red algae as well as in D. pulchra., Introduction A fundamental problem in marine biology and ecology is understanding the demography of organisms with complex life histories in which planktonic and benthic stages occur in spatially distinct habitats [...]

Larvae of the nudibranch mollusc Phestilla sibogae metamorphosed when exposed to 10 of 14 organic solvents. The active solvents included five alcohols and ethanolamine, acetonitrile, acetone, dichloromethane, and toluene. Inactive solvents were ethylene glycol, DMSO, benzene, and hexane. These compounds span a wide range of polarities and contain a number of functional groups. Ethanol induced metamorphosis after 1-5 days of exposure at 0.5-0.001 M, and maximally induced about 65% of larvae to metamorphose in 3-5 days at 0.1 M. Ethanol was lethal to larvae above 0.75 M (ca. 4%). Methanol was lethal only above 1.75 M (ca. 7%), but produced less metamorphosis than ethanol at most concentrations. The natural inducer of metamorphosis in P. sibogae produced higher percentages of metamorphosis more rapidly than did any of the solvents. The mechanism of metamorphic induction by the solvents is not known, but they probably interfere with a wide range of neuronal activities and trigger an existing metamorphic pathway. Precompetent (young) larvae did not metamorphose in response to ethanol or methanol, but juveniles produced by exposure of competent (mature) larvae to ethanol or methanol survived to reproduce. Larvae of one other mollusc species also metamorphosed in response to ethanol, suggesting that larvae of other invertebrates may also be induced to metamorphose by organic solvents. Larval biologists should be aware of this possibility.

Larvae of most marine invertebrates delay metamorphosis if they fail to contact an appropriate environmental stimulus. We conducted seven experiments with the slipper shell snail, Crepidula fornicata, to determine if delaying metamorphosis decreases juvenile fitness. Larvae were reared in the laboratory at 25 [degrees] C on the unicellular alga Isochrysis sp. (clone T-ISO, 18 X [10.sup.4] cells [ml.sup.-1], and were induced to metamorphose after long, medium, or short periods of delayed metamorphosis. Long delay larvae were reared until they metamorphosed spontaneously in acid-cleaned glassware. Medium and short delay larvae were induced to metamorphose with adult-conditioned seawater or 20 mM elevations of KC1. Juveniles were subsequently reared for about one to two weeks at 25 [degrees] C in the laboratory on a diet of T-ISO. Delaying metamorphosis generally did not lower juvenile weight-specific feeding rates, increase juvenile weight-specific respiration rates, or lower juvenile shell or tissue growth rates, any of which effects would suggest that delaying metamorphosis reduces juvenile fitness. Moreover, there was no indication that delaying metamorphosis reduced juvenile tolerance of temperature-salinity stress. Delaying metamorphosis of C. fornicata does not appear detrimental, at least for the first few weeks of juvenile life.

Hermissenda crassicornis (Eschscholtz, 1831), an opistho-branch mollusc used as a model in neurobiological studies, is being cultured in our laboratory. A major aim in opisthobranch mariculture is to increase the [...]