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Larvae of many benthic marine animals settle and metamorphose in response to waterborne chemical cues. Can the behavioral responses of microscopic larvae in the water column to dissolved chemical cues affect their transport to the substratum in the turbulent, wave-driven flow characteristic of many shallow coastal habitats? We addressed this question using an individual-based model of larvae of the sea slugPhestilla sibogae, transported in the oscillatory flow above coral reefs. Larvae ofP. sibogaestop swimming and sink in response to a dissolved inducer released by their prey, the coralPorites compressa, and resume swimming when exposed to inducer-free water. The instantaneous fine-scale spatial distribution of inducer in the flow above a reef is filamentous; hence microscopic larvae swimming or sinking through the water encounter inducer above threshold concentration in on/off temporal patterns. Model results show that using a time-averaged inducer concentration gradient to calculate larval transport rates to the reef overestimates the rates by <15% (depending on the threshold concentration of inducer required to trigger larval sinking) compared with those calculated using time-varying, fine-scale inducer distributions. Aspects of larval behavior that have large effects on rates of transport to the substratum are swimming speed and direction, sinking speed, and sensitivity (threshold concentration) and responsivity (percent of encounters eliciting a response) to inducer. In contrast, lag times to start sinking after encountering inducer or to resume swimming after re-entering inducer-free water, have negligible effect.

The leaf coral Agaricia humilis occurs mainly on the undersides of surfaces in shallow water, a distribution different from the vast majority of corals at our study site in Bonaire, Netherlands Antilles. A series of hypotheses were tested for specific mechanisms that could cause the observed distributions of Agaricia humilis. We found that a suite of larval swimming and settling behaviors, in large part, drives the adult distribution of the species. These behaviors include: (1) swimming behavior that causes larvae to position themselves in shallow water, (2) orientation behavior during settlement that causes larvae to preferentially settle on the undersides of surfaces, and (3) settlement behavior where chemosensory recognition of morphogenic molecules associated with the cell walls of specific crustose red algae is required for induction of settlement and metamorphosis. The consequences of atypical larval behavior are severe and include decreased survivorship, growth, and ability to reproduce sexually.

Many benthic marine invertebrate species have a dispersive larval stage in their life histories. Larvae typically spend hours, weeks, or months developing in plankton before they become competent to settle and metamorphose. Recruitment to benthic populations depends on the numbers of competent larvae transported to sites and/or the interaction between larvae and the surface of substratum. While there is considerable evidence that on large spatial scales, the number of competent larvae transported to sites is determined primarily by hydrodynamics, success of larval settlement on small spatial scales is mediated by biotic and abiotic characteristics of substratum. Larvae of many marine polychaetes require specific cues to settle and metamorphose. Cues can originate from conspecific or congeneric individuals, microbial films, sympatric species, food items, or habitat. Larval settlement in an individual species can be controlled by a single cue or a mixture of cues. Larval settlement of multiple species can be mediated by a common cue or a mixture of cues. Although a variety of chemicals, including proteins, free fatty acids, polysaccharides, inorganic ions, and neurotransmitters, have been suggested as inducing larval settlement of marine polychaetes, few natural cues have been isolated and structurally identified.[PUBLICATION ABSTRACT]

CD4⁺ T regulatory cells (Tregs), which express the Foxp3 transcription factor, play a critical role in the maintenance of immune homeostasis. Here, we show that in mice, Tregs were most abundant in the colonic mucosa. The spore-forming component of indigenous intestinal microbiota, particularly clusters IV and XIVa of the genus Clostridium, promoted Treg cell accumulation. Colonization of mice by a defined mix of Clostridium strains provided an environment rich in transforming growth factor-β and affected Foxp3⁺ Treg number and function in the colon. Oral inoculation of Clostridium during the early life of conventionally reared mice resulted in resistance to colitis and systemic immunoglobulin E responses in adult mice, suggesting a new therapeutic approach to autoimmunity and allergy.

Reducing the time to settlement and metamorphosis of abalone larvae is critical for ensuring that abalone larvae settle within the seeding site for ocean ranching or to increase production in hatcheries. This study investigated the effect of biological (planktonic Nitzschia sp.) and chemical (potassium chloride) cues in inducing settlement and metamorphosis of abalone larvae Haliotis midae on diatom‐coated plastic sheets. Larvae were exposed to different concentrations of KCl (10–20 mM), with settlement being highest at 10 mM in the first 20 h. Settlement of larvae exposed to a combined KCl and Nitzschia treatment for 24 h was highest, followed by larvae exposed to KCl for 12 h, while larvae exposed to KCl for 24 h, and both controls (12 and 24 h) had the lowest settlement. However, in both experiments, larval settlement in all treatments declined after 24 h of exposure, while that of the controls (no added settlement cues) increased and surpassed the other treatments after 24 h. Finally, the settlement was very low on uncoated sheets, compared to diatom‐coated sheets, regardless of exposure to different combinations of KCl and Nitzschia. The exposure period's results should be interpreted with caution when drawing biological conclusions for field studies. This is due to the dramatic decrease in mean settlement post‐exposure to the cue. Therefore, we hypothesize that exposure of H. midae larvae to 10 mM KCl and Nitzschia sp. will not enhance settlement in the ocean, as the inducers are primarily only effective at a KCl concentration level equal to 10 mM for 12 h. However, long‐term exposure to KCl and Nitzschia over 24 h could be used in hatcheries to improve the settlement of H. midae larvae.

IntroductionCoral restoration efforts increasingly focus on enhancing larval settlement and post-settlement survival. However, the species-specific efficacy of different settlement inducers remains inadequately understood, limiting optimization of restoration protocols.MethodsThis study systematically assessed the effectiveness of three settlement inducers—Crustose coralline algae (CCA), Chemical (CaCl2), and Microbial (Metabacillus sp. cB07)—across seven coral species, including both brooders and broadcast spawners. Larvae were exposed to gradient concentrations of each inducer to determine optimal concentrations and treatment durations. Effects on larval survivorship, metamorphosis, and settlement were measured. Post-settlement recruits treated with optimized procedures were further evaluated for metabolic rates, morphology, survival, and growth.ResultsOptimal inducer concentrations and treatment durations varied significantly among coral species, with CaCl2 (10–60 mmol/L) and cB07 (3 × 106–3 × 107 cfu/mL) showing broad-spectrum activity comparable to CCA. CCA induced the highest settlement rates (43.3%–93.3%) within 1–2 days, CaCl2 showed moderate induction (23.3%–60.3%) within 0.5–4 days, and cB07 exhibited similar efficacy (26.7%–60.0%) within 2–4 days. Biological effects differed: CaCl2 accelerated metamorphosis but lowered survival in sensitive species, while cB07 delayed metamorphosis and suppressed respiratory rates, indicating higher toxicity. Post-settlement, recruits induced by CCA and CaCl2 had higher survival and calcification rates than those induced by cB07.DiscussionThese findings underscore the necessity of tailoring settlement inducer protocols to the species-specific life histories and physiological responses of corals. Integrating metabolic and ecological insights offers practical guidelines to enhance coral restoration success amid growing environmental pressures.

The sustainability of mollusc aquaculture relies, in part, on overcoming the challenges of spat production in captivity, particularly during the metamorphosis and settlement stages. The optimization of rearing technologies at these stages would ensure possible solutions for sustainably producing mollusc spat while simultaneously improving stock performance. The current work represents a large-scale trial examining the effect of biological and chemical inducers on larval settlement in Mytilus galloprovincialis. For this purpose, one batch of pediveliger larvae was directly transferred to settlement on microalgae-based biofilm (mature cylinders), while another batch was pretreated with gamma-aminobutyric acid GABA (10−4 M, 10−5 M and 10−6 M) and potassium chloride KCl (20 mM and 30 mM) according to two different exposure times (6 h and 24 h), before being transferred for settlement (immature cylinders). The impact of different treatments on larval performance was evaluated in terms of larval settlement rate (Sr), post-larval growth rate (Gr), and spat production rate (Pr). The biofilm treatment had the highest settlement rate and spat production (Sr = 65% and Pr = 46.4 spat/cm2) compared to chemical treatments. The highest settlement rate among chemical treatments occurred under short exposure times (6 h) to low GABA concentrations, i.e., Sr 40% and 45% at GABA 10−5 M and 10−6 M, respectively). GABA and KCl treatments ensured a faster post-larval growth rate than the biofilm, i.e., 15.54 ± 7.67 µm/day, 18.26 ± 9.39 µm/day, and 11.35 ± 6.73 µm/day, respectively, while control trials showed the lowest growth rate (6.80 ± 4.39 µm/day). These findings reveal a key trade-off: biofilm is the most effective measure for promoting spat production, while a targeted use of GABA and KCl at short exposure times (6 h) appears to significantly enhance post-larvae growth.

We investigated several aspects of the larval biology of the anemone Anthopleura elegantissima, which harbors algal symbionts from two different taxa, and the non-symbiotic A. artemisia. From a 7-year study, we report variable spawning and fertilization success of A. elegantissima in the laboratory. We examined the dynamics of symbiosis onset in larvae of A. elegantissima. Zoochlorellae, freshly isolated from an adult host, were taken up and retained during the larval feeding process, as has been described previously for zooxanthellae. In addition, larvae infected with zooxanthellae remained more highly infected in high-light conditions, compared to larvae with zoochlorellae, which remained more highly infected in low-light conditions. These results parallel the differential distribution of the algal types observed in adult anemones in the field and their differential tolerances to light and temperature. We report on numerous failed attempts to induce settlement and metamorphosis of larvae of A. elegantissima, using a variety of substrates and chemical inducers. We also describe a novel change in morphology of some older planulae, in which large bulges, resembling tentacles, develop around the mouth. Finally, we provide the first description of planulae of A. artemisia and report on attempts to infect this non-symbiotic species with zooxanthellae and zoochlorellae.

Planula larvae of the scyphozoan Cassiopea xamachana settle and metamorphose on degrading mangrove leaves of Rhizophoia mangle that lie submerged in shallow water mangrove ecosystems. Our prior study (Fleck & Fitt 1999; J Exp Mar Biol Ecol 234:83–94) indicated that marine bacteria are involved in the release of at least 1 peptidic compound from such leaves. The goal of our present study was to isolate and purify at least 1 natural peptidic cue originating from deteriorating leaves by means of ultrafiltration, gel filtration and reversed phase HPLC and subsequently obtain characteristic data of this cue. The ultrafiltrate (≤10 kD) of the homogenate of decaying mangrove leaves was subjected to gel filtration on a Sephadex G 25 column, resulting in 3 fractions which were tested for their capacity to induce metamorphosis of planula larvae in bioassays performed in the laboratory. Fraction I (≥5 kD) was most effective in inducing metamorphosis of 75% of planulae at 1 mg freeze-dried material ml–1 seawater within 24 h. Fractions II and III (both ≤5 kD) resulted in metamorphosis of only 1% of larvae or less within 72 h when applied at 5 mg ml–1. Isochratic HPLC separation of Fraction I with 24% methanol yielded 2 biologically active fractions. One fraction (A/B), which induced 47% of the larvae to metamorphose at 0.9 mg lyophilized material ml–1 seawater within 24 h, consisted of a mixture of at least 2 subfractions and was not further analyzed. The other fraction (C) effected metamorphosis of 85% of larvae at a concentration of 0.5 mg ml–1 within 24 h. Matrix-assisted laser desorption ionization (MALDI) mass spectrometry of this fraction revealed a molecular weight of approximately 5.8 kD. Automated amino acid analysis showed that Fraction C was rich in proline (ca 44%) and glycine residues (ca 16%), corresponding to characteristic proline-rich cell wall proteins of plants. Automated sequencing of the natural inducer failed due to a blocked amino terminus. The results of our present study suggest that metamorphic inducers for C. xamachana may emerge nonspecifically as a byproduct of bacterial degradation of deteriorating, proteinaceous plant tissue in their habitat.