Metamorphosis In Response Research Articles

The role of phenotypic plasticity and corticosterone in coping with pond drying conditions in yellow-bellied toad (Bombina variegata, Linnaeus 1758) tadpoles.

Amphibian larvae inhabiting temporary ponds often exhibit the capacity to accelerate development and undergo metamorphosis in challenging conditions like desiccation. However, not all species exhibit this ability, the yellow-bellied toad (Bombina variegata) is one such example. The underlying mechanisms behind the inability to accelerate development under desiccation remain largely unexplored. The hypothalamic-pituitary-interrenal (HPI) axis and corticosterone (CORT), which act synergistically with thyroid hormone, are thought to facilitate metamorphosis in response to desiccation stress. In this study, we aimed to investigate whether modification in the HPI axis, particularly CORT levels, contributes to the absence of adaptive plasticity in B. variegata under desiccation stress. The study design included four treatments: high water level, high water level with exogenous CORT, low water level, and low water level with metyrapone (a CORT synthesis inhibitor). The main objective was to evaluate the effects of these treatments on whole-body corticosterone levels, life history, morphological traits, and oxidative stress parameters during the prometamorphic and metamorphic climax developmental stages. While low water level had no effect on total corticosterone levels, larval period, body condition index, and metamorphic body shape, it negatively affected metamorph size, mass, and growth rate. Our findings suggest that constant exposure to desiccation stress over generations may have led to modifications in the HPI axis activity in B. variegata, resulting in adaptation to changes in water level, evident through the absence of stress response. Consequently, CORT may not be a relevant stress indicator in desiccation conditions for this species.

Diacylglycerol, PKC and MAPK signaling initiate tubeworm metamorphosis in response to bacteria

External environmental cues can have significant impacts on the timing and outcomes of animal development. For the swimming larvae of many marine invertebrates, the presence of specific surface-bound bacteria are important cues that help larvae identify a suitable location on the sea floor for metamorphosis and adult life. While metamorphosis in response to bacteria occurs in diverse animals from across the animal tree of life, we know little about the signal transduction cascades stimulated at the onset of metamorphosis upon their interaction with bacteria. The metamorphosis of a model tubeworm, Hydroides elegans, is triggered by the bacterium Pseudoalteromonas luteoviolacea which produces a stimulatory protein called Mif1. In this work, we define three key nodes in a signaling cascade promoting Hydroides metamorphosis in response to Mif1. Using metabolomic profiling, we find that the stimulation of Hydroides larvae by P. luteoviolacea leads to an increase in diacylglycerol during the initiation of metamorphosis, and that Mif1 is necessary for this upregulation. Genomic and pharmacological examination suggests that diacylglycerol triggers a phosphotransferase signaling cascade involving Protein Kinase C (PKC) and Mitogen-Activated Protein Kinase (MAPK), to induce Hydroides metamorphosis. Additionally, Mif1 activates the expression of two nuclear hormone receptors, HeNHR1 and HeNHR2 in the cerebral ganglia of Hydroides larvae. Our results define a post-translational signal transduction pathway mediating bacteria-stimulated metamorphosis in a model invertebrate animal.

Larval metamorphosis is inhibited by methimazole and propylthiouracil that reveals possible hormonal action in the mussel Mytilus coruscus

Larval metamorphosis in bivalves is a key event for the larva-to-juvenile transformation. Previously we have identified a thyroid hormone receptor (TR) gene that is crucial for larvae to acquire “competence” for the metamorphic transition in the mussel Mytilus courscus (Mc). The mechanisms of thyroid signaling in bivalves are still largely unknown. In the present study, we molecularly characterized the full-length of two iodothyronine deiodinase genes (McDx and McDy). Phylogenetic analysis revealed that deiodinases of molluscs (McDy, CgDx and CgDy) and vertebrates (D2 and D3) shared a node representing an immediate common ancestor, which resembled vertebrates D1 and might suggest that McDy acquired specialized function from vertebrates D1. Anti-thyroid compounds, methimazole (MMI) and propylthiouracil (PTU), were used to investigate their effects on larval metamorphosis and juvenile development in M. coruscus. Both MMI and PTU significantly reduced larval metamorphosis in response to the metamorphosis inducer epinephrine. MMI led to shell growth retardation in a concentration-dependent manner in juveniles of M. coruscus after 4 weeks of exposure, whereas PTU had no effect on juvenile growth. It is hypothesized that exposure to MMI and PTU reduced the ability of pediveliger larvae for the metamorphic transition to respond to the inducer. The effect of MMI and PTU on larval metamorphosis and development is most likely through a hormonal signal in the mussel M. coruscus, with the implications for exploring the origins and evolution of metamorphosis.

Two-Round Ca2+ transient in papillae by mechanical stimulation induces metamorphosis in the ascidian Ciona intestinalis type A.

Marine invertebrate larvae are known to begin metamorphosis in response to environmentally derived cues. However, little is known about the relationships between the perception of such cues and internal signalling for metamorphosis. To elucidate the mechanism underlying the initiation of metamorphosis in the ascidian, Ciona intestinalis type A (Ciona robusta), we artificially induced ascidian metamorphosis and investigated Ca2+ dynamics from pre- to post-metamorphosis. Ca2+ transients were observed and consisted of two temporally distinct phases with different durations before tail regression which is the early event of metamorphosis. In the first phase, Phase I, the Ca2+ transient in the papillae (adhesive organ of the anterior trunk) was coupled with the Ca2+ transient in dorsally localized cells and endoderm cells just after mechanical stimulation. The Ca2+ transients in Phase I were also observed when applying only short stimulation. In the second phase, Phase II, the Ca2+ transient in papillae was observed again and lasted for approximately 5-11 min just after the Ca2+ transient in Phase I continued for a few minutes. The impaired papillae by Foxg-knockdown failed to induce the second Ca2+ transient in Phase II and tail regression. In Phase II, a wave-like Ca2+ propagation was also observed across the entire epidermis. Our results indicate that the papillae sense a mechanical cue and two-round Ca2+ transients in papillae transmits the internal metamorphic signals to different tissues, which subsequently induces tail regression. Our study will help elucidate the internal mechanism of metamorphosis in marine invertebrate larvae in response to environmental cues.

Kairomone-induced changes in mosquito life history: effects across a food gradient

The aquatic immature stages of species with complex life histories exhibit a range of defense mechanisms in response to predator released kairomones (PRK). Employing these costly mechanisms often results in delayed metamorphosis. Larvae of the house mosquito Culex pipiens (Linnaeus) show a rare exception of accelerated metamorphosis in response to kairomones originated from the mosquitofish Gambusia affinis (Baird and Girard). In a series of lab experiments we examined whether this response is context-dependent with respect to food availability (i.e. applied only when food is abundant and cost is low). We examined life history variables of C. pipiens larvae, reared at different levels of food availability, either with or without PRK. We further examined the effect of PRK on the foraging behavior of the larvae at different instars. We also examined the effect of PRK-induced behavior on larvae survival under actual predation. We showed that the response of C. pipiens larvae to PRK was independent of food availability. Larvae exposed to PRK were less active and survived longer when exposed to direct predation. Exposure to both PRK and small food amounts also resulted in reduced adult size and survival period. The effects of food and PRK were independent of one another. We argue that for organisms with short development time, such as mosquitoes, decreasing time to metamorphosis may be the main feasible refuge from increased predation risk. Hence, Culex larvae exploit their capability for rapid development rate as a main anti-predator mechanism, minimizing the time spent in high-risk environments by accelerating metamorphosis, regardless of available resources, at the expense of other life history traits.

Interactive and sex‐specific life‐history responses ofCulex pipiensmosquito larvae to multiple environmental factors

AbstractSpatio‐temporal variation in aquatic habitat characteristics can have important implications for the population and community dynamics of organisms utilizing these habitats. We studied the life‐history responses ofCulex pipienslarvae (Diptera: Culicidae) to multiple environmental factors associated with habitat drying by increasing larval density (directly affecting resource availability), temperature fluctuations (influencing physiological processes), and solute concentration due to evaporation (inducing an environmental stress), using a full‐factorial design. We found that high density led to lower larval survival and to shorter development time. Larvae delayed their metamorphosis in response to fluctuating temperature. We detected inter‐sexual differences in the effects of these two factors on adult body sizes, with high larval density leading to the emergence of smaller females, and fluctuating temperature translating to larger adult males. The two sexes also differed concerning the two‐way interactive effects of different environmental factors on development time and on adult size. Our findings indicate thatC. pipienslarvae respond to multiple environmental factors via phenotypic plasticity, rather than through bet‐hedging. Future studies should account for additional inter‐stage effects, rather than body size, such as adult fecundity, longevity, and dispersal.

Brief exposure to intense turbulence induces a sustained life-history shift in echinoids.

In coastal ecosystems, attributes of fluid motion can prompt animal larvae to rise or sink in the water column and to select microhabitats within which they attach and commit to a benthic existence. In echinoid (sea urchin and sand dollar) larvae living along wave-exposed shorelines, intense turbulence characteristic of surf zones can cause individuals to undergo an abrupt life-history shift characterized by precocious entry into competence - the stage at which larvae will settle and complete metamorphosis in response to local cues. However, the mechanistic details of this turbulence-triggered onset of competence remain poorly defined. Here, we evaluate in a series of laboratory experiments the time course of this turbulence effect, both the rapidity with which it initiates and whether it perdures. We found that larvae become competent with turbulence exposures as brief as 30 s, with longer exposures inducing a greater proportion of larvae to become competent. Intriguingly, larvae can remember such exposures for a protracted period (at least 24 h), a pattern reminiscent of long-term potentiation. Turbulence also induces short-term behavioral responses that last less than 30 min, including cessation of swimming, that facilitate sinking and thus contact of echinoid larvae with the substratum. Together, these results yield a novel perspective on how larvae find their way to suitable adult habitat at the critical settlement transition, and also open new experimental opportunities to elucidate the mechanisms by which planktonic animals respond to fluid motion.

The 'male escape hypothesis': sex-biased metamorphosis in response to climatic drivers in a facultatively paedomorphic amphibian.

Paedomorphosis is a major evolutionary process that bypasses metamorphosis and allows reproduction in larvae. In newts and salamanders, it can be facultative with paedomorphs retaining gills and metamorphs dispersing. The evolution of these developmental processes is thought to have been driven by the costs and benefits of inhabiting aquatic versus terrestrial habitats. In this context, we aimed at testing the hypothesis that climatic drivers affect phenotypic transition and the difference across sexes because sex-ratio is biased in natural populations. Through a replicated laboratory experiment, we showed that paedomorphic palmate newts (Lissotriton helveticus) metamorphosed at a higher frequency when water availability decreased and metamorphosed earlier when temperature increased in these conditions. All responses were sex-biased, and males were more prone to change phenotype than females. Our work shows how climatic variables can affect facultative paedomorphosis and support theoretical models predicting life on land instead of in water. Moreover, because males metamorphose and leave water more often and earlier than females, these results, for the first time, give an experimental explanation for the rarity of male paedomorphosis (the 'male escape hypothesis') and suggest the importance of sex in the evolution of paedomorphosis versus metamorphosis.

Early metamorphosis is costly and avoided by young, but physiologically competent, marine larvae

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 559:117-129 (2016) - DOI: https://doi.org/10.3354/meps11914 Early metamorphosis is costly and avoided by young, but physiologically competent, marine larvae Benjamin Mos*, Symon A. Dworjanyn National Marine Science Centre, Southern Cross University, PO Box 4321, Coffs Harbour, NSW 2450, Australia *Corresponding author: benjamin.mos@scu.edu.au ABSTRACT: Dispersing organisms often cannot assess habitat quality directly, so they employ proxies (cues) to choose habitats that maximise fitness. Theory suggests organisms should settle as soon as they find appropriate cues in order to reduce physiological costs and mortality risk incurred whilst searching. We propose that for planktotrophic marine larvae, when resources are plentiful, development of adult structures during an extended larval phase provide post-metamorphosis benefits that offset the costs of remaining in the plankton. To test this, we measured fitness consequences of metamorphosis in response to habitat cues at a range of developmental maturities in 2 sea urchin larvae, Tripneustes gratilla and Centrostephanus rodgersii. We found larvae that were capable of responding to cues and settling accrued significant benefits if they extended their pelagic development. Compared to more developed larvae, larvae without adult structures took longer to metamorphose, and after metamorphosis were 11 to 25% smaller, 0.1 to 6 times more likely to lack defensive structures and 3 to 13 times more likely to have abnormal morphology. Most early settlers died within 1 wk compared to >40% survival for more developed larvae. We found larvae avoid the costs of early metamorphosis by only responding to low concentrations of cues in the water column once they have adult structures. Our results contrast with models of habitat selection that suggest organisms should settle in habitat quickly to minimise search costs. Incorporating the trade-off between the benefits of larval development and search costs into current models of habitat selection may provide new insights into how fitness consequences affect habitat selection. KEY WORDS: Delayed metamorphosis · Development · Fitness · Habitat selection · Search costs Full text in pdf format Supplementary material PreviousNextCite this article as: Mos B, Dworjanyn SA (2016) Early metamorphosis is costly and avoided by young, but physiologically competent, marine larvae. Mar Ecol Prog Ser 559:117-129. https://doi.org/10.3354/meps11914 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 559. Online publication date: November 09, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Effects of fish cues on mosquito larvae development

We investigated the effects of predator-released kairomones on life history traits of larval Culex pipiens (Linnaeus). We compared the development time and survival of sibling larvae, reared in either water conditioned by the presence of Gambusia affinis (Baird and Girard) or fishless control-water. Our results indicate that larvae developing in fish-conditioned water (FCW) pupated faster than larvae in fishless-control water. The effect of FCW on larval survival was evident only in females. Surprisingly, FCW increased female survival. In both development-time and survival, boiling the water eliminated the FCW effect, supporting our hypothesis that fish conditioning is based on kairomones. Accelerated metamorphosis in response to predator released kairomones, evident in our results, is a rarely described phenomenon. Intuitively, when exposed to predator associated signals, aquatic larvae should metamorphose earlier to escape the higher risk of predation. However, theoretical models predict this outcome only under specific conditions. Indeed, longer – rather than shorter – time to metamorphosis is usually observed in response to predation risk. We argue that the response of larval mosquitoes to predation risk is context-dependent. Shortening larval development time may not be an exceptional response, but rather represents a part of a response spectrum that depends on the level of predation risk and resource abundance.

Goethe’s Phenomenological Way of Thinking and the Urphänomen

Goethe’s Phenomenological Way of Thinking and the Urphänomen Iris Hennigfeld Das Höchste wäre, zu begreifen, daß alles Factische schon Theorie ist. Die Bläue des Himmels offenbart uns das Grundgesetz der Chromatik. Man suche nur nichts hinter den Phänomenen; sie selbst sind die Lehre. (FA 1.13:49) [The ultimate goal would be: to grasp that everything in the realm of fact is already theory. The blue of the sky reveals to us the basic law of chromatics. Let us not seek for something behind the phenomena—they themselves are the theory.1] Der Mensch kennt nur sich selbst, insofern er die Welt kennt, die er nur in sich und sich nur in ihr gewahr wird. Jeder neue Gegenstand, wohl beschaut, schließt ein neues Organ in uns auf. (FA 1.24:596) [The human being knows himself only insofar as he knows the world; he perceives the world only in himself, and himself only in the world. Every new object, well contemplated and clearly seen, opens up a new organ within us. (39)] These two maxims suggest that Goethe’s thinking about and intuition of the world are phenomenologically grounded. They can be read as programmatic statements for his specific kind of phenomenological thinking, evident in his poetry and his natural science and philosophical writings. I suggest reading the first aphorism in the light of the phenomenological shibboleth “Back to the things themselves”2 (Zurück zu den Sachen selbst), pronounced by Edmund Husserl, the founder of the phenomenological movement. Similarly, the second aphorism reflects the phenomenological intertwinement of subject and object; it shows how each new experience might widen the faculties of perception, cognition, and intuition and thus suggests that the observer undergoes perpetual metamorphosis in response to each new object of perception and cognition. As I will show, reading Goethe’s method—his way of perceiving, experiencing, and reflecting—as grounded in and guided by what Husserl calls the “principle of all principles,” that is, pure and “original intuition,”3 allows new insights into Goethe’s works. Hence, I propose a phenomenological reading of Goethe, especially of his philosophical and scientific writings.4 Husserl’s philosophical language and phenomenological method, oriented toward [End Page 143] clarity and truth, help to illuminate Goethe’s phenomenological thinking and to transform implicit into explicit philosophical concepts. The relation between Goethe’s and Husserl’s phenomenology can be viewed from two perspectives: from the historical point of view, Goethe can be seen as a forerunner of phenomenological insights; from the philosophical angle, Goethe and Husserl share similar philosophical ideas. I want to show how Goethe’s thinking sheds new light on central questions in phenomenological philosophy, in particular regarding Husserl and his followers. If we regard Goethe as a spiritual predecessor of the phenomenological movement in the twentieth and twenty-first centuries, his analyses can provide new insights into the problems that also drove Husserl’s work. In the following, I examine some key ideas in Goethe’s thinking, including his notion of the Urphänomen from a phenomenological perspective. My aim is to show that the Urphänomen can be understood as an example of a phenomenological science of nature that explores the limits of intuition and cognition. Furthermore, Goethe’s phenomenological way of thinking, exemplified by his notion of the Urphänomen, invites radical openness especially toward phenomena that, like the Urphänomen, elude the standards of modern science and its mathematization of nature and exceed the limits of mere objectification and representation. This article is divided into five parts. Section 1 focuses on the phenomenological method in its relation to different kinds of givenness in the phenomenal world. Section 2 analyzes the relation between art and science in Goethe’s thinking and explains, from a phenomenological point of view, why Goethe used imagination as a tool of cognition. Section 3 explains how Goethe’s phenomenological method—particularly, his enhanced notion of experience and his showing (Aufweisen or Ausweisen) or illumination of the manifold conditions of appearances—led him to the “discovery” of the Urphänomen. Section 4 parses the relation between world and consciousness, phenomena and thinking, in phenomenological philosophy. I link...

Anuran developmental plasticity loss: the cost of constant salinity stress

In animals with a complex life cycle, changes in biotic and abiotic conditions during development can alter growth and maturation rates, causing carry-over effects in postmetamorphic phenotypes. In anurans, this developmental plasticity can result in a trade-off between length of larval period and body size at metamorphosis in stressful environments. Secondary salinisation has been identified as a substantial stressor to amphibians; however, little is known about how salinity-induced developmental plasticity differs between anuran populations. We examined differences in survival, time to metamorphosis, size at metamorphosis (mass and snout–vent length) and body condition at metamorphosis in response to elevated salinity in three populations of the brown tree frog (Litoria ewingii). Significant differences in size at metamorphosis between salinity treatments were observed in tadpoles sourced from freshwater wetlands and ephemeral wetlands, with tadpoles showing a reduced mass and snout–vent length at metamorphosis in the higher-salinity treatment. There were no significant differences in metamorphic traits between salinity treatments in tadpoles sourced from a consistently brackish wetland, suggesting either an erosion of developmental plasticity in response to elevated salinity, or the magnitude of salinity required to alter developmental traits is higher in this population. Our results indicate that environmental conditions of source populations need to be considered when studying life-history adaptations in response to environmental change.

In Haliotis, NO means YES

Settlement and metamorphosis of marine invertebrate larvae is an ecological process of profound importance to the structure of benthic marine communities. Yet, metamorphosis is also a developmental process and our understanding of the mechanisms that regulate it remains relatively poor. Over the last 15 years, a nitric oxide (NO) based signaling system has emerged as the most phylogenetically widespread regulator of settlement and metamorphosis. Among a sea urchin, three sea squirts, three molluscs, a crustacean, and an annelid, NO (and often its downstream effector cGMP) regulates metamorphosis (Froggett and Leise, 1999; Bishop and Brandhorst, 2001; Bishop et al., 2001, 2006, 2008; Comes et al., 2007; Pechenik et al., 2007; Biggers et al., 2012; Zhang et al., 2012; Romero et al., 2013). Two important aspects to this pattern of NO function during metamorphosis are that (i) NO always acts as an inhibitory signal and that (ii) the functionality of NO signaling varies according to the settlement ecology of the larva. Ueda and Degnan’s recent work has challenged the first pattern and supported the second. First, why should NO signaling be inhibitory in these phylogenetically disparate, morphologically distinct (and, in some cases, CDB would argue), independently evolved larval forms? Based on a model in which the directionality of NO signaling during metamorphosis is stochastic over evolutionary time periods, there is a small probability, given current taxonomic sampling, that all larvae so far investigated would employ NO in an inhibitory manner. So either NO signaling during metamorphosis is conserved because larvae are homologous or because the use of NO in this manner during metamorphosis has been selected repeatedly. In the foundational study of NO during metamorphosis of the mud snail Illyanassa obsoleta Froggett and Leise (1999) hypothesized that the function of an inhibitory signaling system is to delay the onset of metamorphosis until chemical inducers or “cues” are detected, signaling habitats that are favorable for growth and development of the animals (Hadfield and Paul, 2001). Thus, Froggett and Leise provided a way to think about the directionality of a signal (i.e., stimulatory or inhibitory) in the context of a larval behavior that is presumably ecologically relevant and adaptive (Pechenik, 1990). It seemed as if widespread inhibitory NO function could be placed in the context of an equally widespread ecological reality. Nobuo Ueda and Sandie Degnan at the University of Queensland in Brisbane, Australia, have shown that NO positively regulates metamorphosis in the ascidian Herdmania momus (Ueda and Degnan, 2013) and, in the present study, positively modulates (see below for the difference between a regulator and a modulator) in the abalone Haliotis asinina. Therefore, the inhibitory function of NO in metamorphosis does not appear to be a fixed character. In this current work, NO is demonstrated to serve as a necessary inductive facilitator for metamorphosis in response to a natural inducer that is needed to transduce the metamorphic chemical cue from red coralline algae Amphiroa. Although in neither study did the authors test whether NO generators could antagonize NOS inhibitors (or other “epistasis”-type experiments) and did not use a non-enzymatic method of reducing NO activity, all of their pharmacological evidence does point to a novel directionality of NO signaling and so is highly notable in this regard. Interestingly, pharmacological manipulation of NO levels alone was not sufficient to induce larval settlement and metamorphosis; NO signaling could only modulate larval responses to algal-derived settlement cues. This takes us to the second emerging aspect of studies on marine invertebrate metamorphic signaling: the relationship between the function of identified signaling pathways and ecological specialization of larval settlement. One particular aspect of settlement ecology concerns differences in the behavior of competent larvae in the absence of settlement cues, depending upon whether larvae are settling into specialized niches or not (Elkin and Marshall, 2007; Toonen and Tyre, 2007). The “desperate larva” hypothesis, coined by Toonen and Pawlik (1994), but informed by earlier seminal experiments (Knight-Jones, 1953; Wilson, 1953) states that larvae should become less choosy about where they settle as a function of time spent in a settlementcompetent state. As a mechanism that

Regulation of Polyp-to-Jellyfish Transition in Aurelia aurita

The life cycle of scyphozoan cnidarians alternates between sessile asexual polyps and pelagic medusa. Transition from one life form to another is triggered by environmental signals, but the molecular cascades involved in the drastic morphological and physiological changes remain unknown. We show in the moon jelly Aurelia aurita that the molecular machinery controlling transition of the sessile polyp into a free-swimming jellyfish consists of two parts. One is conserved and relies on retinoic acid signaling. The second, novel part is based on secreted proteins that are strongly upregulated prior to metamorphosis in response to the seasonal temperature changes. One of these proteins functions as a temperature-sensitive "timer" and encodes the precursor of the strobilation hormone of Aurelia. Our findings uncover the molecule framework controlling the polyp-to-jellyfish transition in a basal metazoan and provide insights into the evolution of complex life cycles in the animal kingdom.

Inhibitors of nitric oxide synthase induce larval settlement and metamorphosis of the polychaete annelid Capitella teleta

The neurotransmitter nitric oxide (NO) has been implicated in the inhibitory control of metamorphosis of some marine gastropods, echinoderms, and ascidians. We have explored whether or not metamorphosis of metatrochophore larvae of the polychaete annelid Capitella teleta is also regulated by NO. Immunohistochemical analysis revealed a bilateral group of three large nitric oxide synthase (NOS) immunoreactive cells that lie dorsal to the pharynx and extend ventral processes toward the pharynx in the region of the dorsal pharyngeal pad. Smaller NOS-immunoreactive cells were distributed widely throughout the body but concentrated in the prostomium and pygidium. Histological analysis for NO, using the NO detector diaminofluorescein-FM, showed that NO concentration was high in the larval midgut, although diffuse amounts of NO were detected throughout the body. Inhibitors of NOS, including s-methylisothiourea sulfate, aminoguanidine hemisulfate, 7-nitroindazole, and N-methyl-L-arginine all induced settlement and metamorphosis of the Capitella larvae in a concentration-dependent manner. The NO donor nitroprusside prevented the induction of settlement and metamorphosis induced by the NOS inhibitor N-methyl-L-arginine, but did not prevent settlement and metamorphosis induced by a marine sediment extract, exogenous serotonin, or by the serotonin reuptake inhibitor fluoxetine. Pre-incubating larvae with the serotonin receptor antagonist ketanserin also inhibited settlement and metamorphosis in response to NOS inhibitors. These results suggest that endogenous production of NO maintains the larval state in C. teleta, and that endogenous serotonin stimulates metamorphosis in a way similar to that described previously for larvae from other major invertebrate phyla.

Evolutionary reduction of developmental plasticity in desert spadefoot toads

Organisms vary their rates of growth and development in response to environmental inputs. Such developmental plasticity may be adaptive and positively correlate with environmental heterogeneity. However, the evolution of developmental plasticity among closely related taxa is not well understood. To determine the evolutionary pattern of plasticity, we compared plasticity in time to and size at metamorphosis in response to water desiccation in tadpoles among spadefoot species that differ in breeding pond and larval period durations. Like most tadpoles, spadefoot tadpoles possess the remarkable ability to accelerate development in response to pond drying to avoid desiccation. Here, we hypothesize that desert spadefoot tadpoles have evolved reduced plasticity to avoid desiccation in ephemeral desert pools compared to their nondesert relatives that breed in long-duration ponds. We recorded time to and size at metamorphosis following experimental manipulation of water levels and found that desert-adapted species had much less plasticity in larval period and size at metamorphosis than nondesert species, which retain the hypothetical ancestral state of plasticity. Furthermore, we observed a correlation between degree of plasticity and fat body content that may provide mechanistic insights into the evolution of developmental plasticity in amphibians.

Aboral localization of responsiveness to a metamorphic neuropeptide in the planula larva of Acropora tenuis

Planula larvae of the coral genus Acropora ini-tiate metamorphosis in response to microorganisms on the undersea substrate. Since planulae move forward in the direction of the aboral side, it is supposed that initial metamorphic signaling localizes on this side of planula. We dissected planulae of Acropora tenuis into fragments perpendicularly to the oral-aboral axis, and tested their response to a metamorphosis-inducing neuropeptide. Aboral 1/3 fragments metamorphosed in response to the neu-ropeptide with high efficiency while most oral 2/3 frag-ments did not. The results suggest that the tissue in the aboral 1/3 region receives the metamorphic neuropeptide and releases downstream signals to complete metamor-phosis of the oral side in A. tenuis larvae. In addition, oral fragments gained a metamorphic ability in several days, revealing regeneration of the aboral tissue.

Larval settlement preferences and post-settlement survival of the threatened Caribbean corals Acropora palmata and A. cervicornis

The settlement specificity of two threatened Caribbean corals, Acropora palmata and A. cervicornis, was tested by measuring their rates of larval metamorphosis in response to crustose coralline algae (CCA) and other substrata. In the no-choice experiments, the coral larvae were placed in six treatments: filtered seawater (FSW), a fragment of biofilmed dead skeleton of A. palmata, or a fragment of one of four species of CCA (Hydrolithon boergesenii, Porolithon pachydermum, Paragoniolithon solubile, and Titanoderma prototypum). Within each CCA treatment, there were three different substrata on which to settle and metamorphose: (1) the CCA surface, (2) the rock under the CCA, or (3) the plastic dish. The 5-day-old larvae of both A. palmata and A. cervicornis had similar rates of total metamorphosis (all substrata combined) in every treatment (excluding FSW) even in the absence of CCA. However, there were differences in larval behavior among the CCA species since the larvae settled and metamorphosed on different substrata in the presence of different CCA species. In the no-choice experiments the larvae of both corals had higher rates of metamorphosis on the top surfaces of H. boergesenii and/or T. prototypum than on P. pachydermum. In the choice experiments, the coral larvae were offered two species of CCA in the same dish. When given a choice, both species of coral larvae had more settlement and metamorphosis on the surface of H. boergesenii or T. prototypum or clean rock than onto the surface of P. solubile. After 6 weeks in the field, transplanted A. palmata recruits had approximately 15% survival on both T. prototypum and H. boergesenii, but A. cervicornis recruits only survived on T. prototypum (13%). Some, but not all, CCA species facilitated the larval settlement and post-settlement survival of these two threatened corals, highlighting the importance of benthic community composition for successful coral recruitment.

Effects of Salinity on the Survival, Growth, Development, and Metamorphosis ofFejervarya limnocharisTadpoles Living in Brackish Water

We studied salinity tolerance and the effects of salinity on growth, development, and metamorphosis in Fejervarya limnocharis tadpoles living in brackish water. Specifically, we examined whether tadpoles exhibit adaptive plasticity in development when exposed to different salinities. Tadpoles collected on Green and Orchid Islands off Southeastern Taiwan were assigned to salinities of 0, 3, 5, 7, 9, 11, and 13 parts per thousand (ppt). The daily survival, weekly growth, and development of tadpoles were recorded until metamorphosis. More than 50% of tadpoles survived in 9 ppt for over a month, and a few individuals survived in 11 ppt for 20 days, suggesting that F. limnocharis tadpoles tolerate salinity better than the tadpoles of most species studied to date. Tadpoles at 9 ppt had lower survivorship, and retarded growth and development (from Gosner stage 26 to 35) compared to the other treatments. Tadpoles metamorphosed early at a smaller size as salinity increased, suggesting the existence of adaptive developmental plasticity in F. limnocharis in response to osmotic stress. Phenotypic plasticity in the age and size at metamorphosis in response to salinity may provide a means for tadpoles to adapt to the unpredictable salinity variation in coastal rock pools.

Larval metamorphosis of Phestilla spp. in response to waterborne cues from corals

Many marine invertebrates depend on their larvae for dispersal and to settle and metamorphose in the appropriate habitat for adult survival, yet the mechanisms of habitat selection remain poorly understood. In Hawaii, the nudibranch Phestilla sibogae only feeds on Porites compressa and requires a water-soluble cue from this coral for metamorphosis. On Guam, we tested three different species of Phestilla to determine if their larvae require water-soluble compounds from corals to induce metamorphosis. Larvae of P. sibogae metamorphosed at high rates to waterborne cues from multiple species of corals in the genus Porites. Larvae of Phestilla minor could distinguish among waterborne compounds from different species of Porites, but also had high rates of metamorphosis in filtered seawater and in response to corals that adults did not eat. Larvae of Phestilla sp. 2 could distinguish among water-soluble cues from different species of Goniopora and consistently had the highest rates of metamorphosis in response to waterborne cues released from Goniopora fruticosa. P. minor was the only species studied that did not require waterborne cues for larval metamorphosis.