Larval recruitment in response to manipulated field flows

Settlement responses to boundary-layer flow of several invertebrate taxa, including the hydroid Tubularia crocea, the bryozoans Bugula turrita and Schizoporella unicornis, and the tube-building polychaete Hydroides dianthus were studied in manipulated field flows in Great Harbor, Massachusetts, USA. During three experiments in 1989 and two in 1990, densities of newly-recruited larvae were measured on flat plates, whose flow regimes had been manipulated by altering the leading-edge configurations. Settlement responses to flow were strongly species-specific, with T. crocea preferring regions of both high turbulence and strong shear stress, and S. unicornis settling a most exclusively in regions of high shear stress. B. turrita settled most prominently in regions of reduced shear stress, exhibiting settlement patterns that closely approximated predictions from a model of passive particle contact. H. dianthus showed a moderate avoidance of regions with high shear stress. These results indicate that boundary-layer flows affect settlement of several common encrusting species, a probable consequence of larval behaviors such as substrate rejection or exploration in response to flow. These responses are likely to generate patchiness during initial colonization of natural habitats, and certainly affect colonization of settlement panels commonly used in marine ecological studies.     

Effects of duration of larval swimming period on early colony development in Bugula stolonifera (Bryozoa: Cheilostomata)

Laboratory experiments with larvae of the cheilostome bryozoan Bugula stolonifera Ryland, 1960 assessed the time to settlement in the presence of a constantly available polystryrene substrate, the development of competence for metamorphosis, and the effects of the duration of swimming period on early colony development. Sexually mature colonies of B. stolonifera were collected on 11 and 18 September 1987; 2 and 18 August, 1988; and 6, 12, 19, and 26 September 1988, from Eel Pond (Woods Hole, Massachusetts, USA) and were maintained at 20°C. In the presence of a constantly available substrate, cumulative percent settlement curves were sigmoid, with 75% of larvae settled in 3.2±0.5 h. Typically, 50% of the larvae settled in less than 3 h and 95% settled in 6.1±1.2 h. The number of settled individuals that developed feeding ancestrulae by 3 d and the number that developed first-feeding autozooids by 6 d was assessed as a function of duration of larval swimming. Individuals which were kept swimming for 8 and 10 h after hatching developed significantly more slowly to the ancestrula and autozooid stages in 13 out of 14 experiments than did larvae that swam 2 or 6 h. This is the first report for any bryozoan that prolongation of the larval free-swimming period affects the rate of colony development.     

Substrate choice and settlement preferences of planula larvae of five Scyphozoa (Cnidaria) from German Bight,North Sea

The settlement behaviour of planula larvae and their development to young polyps was investigated in laboratory experiments in five scyphozoan species [Aurelia aurita (L.), Cyanea capillata (L.), Cyanea lamarckii Péron and Leseur, Chrysaora hysoscella (L.), and Rhizostoma octopus (L.)]. The undersides of settling plates were strongly preferred for settlement. Shells, the only natural substrate type offered, were less attractive than artificial substrates (concrete, machined wood, polyethylene, and glass). The advantages of colonization of substrate undersides for survival and reproduction of polyps are discussed. It is supposed that the increase of artificial substrates in our seas, due to marine litter pollution and submarine building activities, enlarge the areas of distribution of scyphozoan polyps, in coastal as well as in off-shore regions. Subsequent increases in ephyra production by polyps are probably one reason for the increase in mass occurrences of jellyfish recognized worldwide during the last few decades. It is suggested that the early developmental stages in the cnidarian life cycle, the planula larvae, and the polyps, play the key role in the development of jellyfish outbursts.     

Inhibitory effects of red algal extracts on larval settlement of the barnacle Balanus improvisus

We examined the chemical antifouling properties of four sublittoral red algae, Chondrus crispus, Delesseria sanguinea, Osmundea ramosissima, and Polyides rotundus, which are all rarely fouled in the field. Two different approaches were used. Firstly, we tested the effects of lipophilic crude extracts on the settlement behaviour of cyprid larvae of the co-existing barnacle Balanus improvisus. Secondly, in a settlement preference experiment, we tested whether B. improvisus cyprid larvae settle on living algae when given a choice between natural algal surfaces and control surfaces. With this procedure, we were able to test both if the algae inhibit recruitment of cyprids, and if this inhibition is a result of chemistry. The settlement of B. improvisus larvae was strongly inhibited at concentrations estimated to be potentially ecologically relevant for all of the tested extracts. However, only C. crispus significantly inhibited settlement in the preference experiment, even though there was also a tendency for settlement inhibition on P. rotundus and O. ramosissima. In contrast, D. sanguinea seemed to stimulate settlement. This contradiction probably resulted from an extraction of metabolites that naturally occur only inside the alga. However, as this study shows, a combination of settlement assays with whole-cell extracts and preference tests of ecologically relevant fouling organisms on natural algal and control surfaces may be a useful procedure to avoid erroneous conclusions regarding natural antifouling roles of compounds based on settlement assays with only whole-cell extracts. Furthermore, this study also shows that production of inhibitory metabolites may explain the low degree of fouling, especially by B. improvisus, on C. crispus.Communicated by L. Hagerman, Helsingør     

Settlement of larval blacklip abalone, Haliotis rubra, in response to green and red macroalgae

Surfaces from the habitat of adult Haliotis rubra were tested as inducers of larval settlement to determine the cues that larvae may respond to in the field. Settlement was high on the green algal species Ulva australis and Ulva compressa (Chlorophyta), the articulated coralline algae Amphiroa anceps and Corallina officinalis, and encrusting coralline algae (Rhodophyta). Biofilmed abiotic surfaces such as rocks, sand and shells did not induce settlement. Ulvella lens was also included as a control. Treatment of U. australis, A. anceps and C. officinalis with antibiotics to reduce bacterial films on the surface did not reduce the settlement response of H. rubra larvae. Similarly, treatment of these species and encrusting coralline algae with germanium dioxide to reduce diatom growth did not significantly reduce larval settlement. These results suggest that macroalgae, particularly green algal species, may play an important role in the recruitment of H. rubra larvae in the field and can be used to induce larval settlement in hatchery culture.     

Larval settlement in the serpulid polychaete Hydroides elegans in response to bacterial films: an investigation of the nature of putative larval settlement cue

Larval settlement in the marine polychaete Hydroides elegans (Haswell) is induced by certain bacteria in marine biofilms. The exact nature of the settlement cue that larvae of H. elegans receive from these bacteria remains unknown. In this study, we revealed some properties of the bacterially derived larval settlement cue by investigating the larval settlement inductive activity of two bacterial strains after various treatments. These two bacterial strains, Roseobacter sp. and an α-subclass Proteobacteria, are highly inductive to larval settlement of H. elegans. The larvae responded similarly to Roseobacter and Proteobacteria in all the larval settlement bioassays, suggesting that the larval settlement-inducing substances produced by these bacteria may share common characteristics. First of all, the larvae did not settle in the seawater conditioned by the bacteria attached as a film or by the bacteria that were freely suspended in seawater. The results suggest that the putative larval settlement cue is not released into seawater and, therefore, should be associated with the surface of the bacteria. Secondly, formaldehyde treatment entirely eliminated the larval settlement induction activity of the bacterial films, and streptomycin treatment reduced the percentage of larval settlement on the bacterial films in a concentration-dependent manner. Since both treatments can kill bacteria with little damage to the surface chemistry of bacterial cells, the decline in larval settlement is suggested be due to a reduction of the viable bacterial population in the bacterial films. In fact, the reduction of larval settlement in the streptomycin treatments coincided with the decrease in viable bacterial populations in broth cultures containing respective concentrations of streptomycin. These results suggest that the viability of Roseobacter and Proteobacteria is important to their settlement induction effect. Since the larval settlement induction activity of the bacterial strains appears to correlate with their viability, we suggest that the putative larval settlement cue is derived from a metabolic pathway in the bacteria and that the cue is exported to and concentrated at the extracellular polymer matrix of the bacterial cell, at which the larvae establish contact with the bacteria. The larval settlement cue may be highly susceptible to degradation so that a metabolically active bacterial film is needed to maintain the putative cue at a concentration that surpasses the threshold for induction of larval settlement. Received: 14 October 1998 / Accepted: 5 September 2000     

Spatiotemporal coupling/decoupling of planktonic larvae and benthic settlement in decapods in the Scottish east coast

Settlement patterns and the relationship between meroplanktonic larvae and settlement in decapods were studied on the Scottish east coast. Artificial settlement substrates (ASS), deployed at two locations (sandy vs. rocky sea substrates), were employed to collect megalopae and newly settled juveniles. Abundance of meroplanktonic larvae was used as an indicator of larval supply. The results showed a clear seasonality in settlement rates, and in some cases, significant differences between sites were detected. Nevertheless, the interference of the ASS with the surrounding habitat limits the study of spatial variability in settlement rates. Significant cross-correlation was found between the abundance of megalopae and juveniles in the collectors and planktonic larval abundance a month earlier. For individual species, this relationship was observed only in Pisidia longicornis. Complexities caused by the great variety of pre- and post-settlement processes, alongside effects of secondary dispersals of early juveniles may have obscured the relationship between meroplanktonic larvae and juveniles in other species.     

Larval growth and settlement of the European oyster (Ostrea edulis) as a function of food quality measured as fatty acid composition

Larval growth rate and settlement of the European flat oyster Ostrea edulis were experimentally studied as a function of the composition of dietary fatty acids. Diets differing in fatty acid composition were composed by mixtures of the microalgae Isochrysis galbana, Pavlova lutheri and Chaetoceros calcitrans. Fatty acid content in the tissue of the feeding larvae, analyzed by gas chromatography and mass spectrometry, reflected the composition in the diet. Larval growth rate was significantly correlated to the three omega-3 polyunsaturated fatty acids (PUFA) C18:3, C18:4 and C22:6, with minor differences for neutral and polar lipids. No relation between growth rate and the omega-3 PUFA C20:5 was detected, a PUFA often implied as essential for bivalves. It is suggested that naturally occurring variability in fatty acid composition may constrain larval growth. In settlement experiments in both still water and flume flow little substrate selectivity was found for some contrasting substrates. It is concluded that differences in dietary fatty acids may explain as much of settlement success as the variability of substrates. Received: 12 October 1998 / Accepted: 6 April 1999     

Settlement of <Emphasis Type="Italic">Macoma balthica</Emphasis> larvae in response to benthic diatom films

The role of multi-species benthic diatom films (BDF) in the settlement of late pediveliger larvae of the bivalve Macoma balthica was investigated in still-water bioassays and multiple choice flume experiments. Axenic diatom cultures that were isolated from a tidal mudflat inhabited by M. balthica were selected to develop BDF sediment treatments characterized by a different community structure, biomass, and amount of extracellular polymeric substances (EPS). Control sediments had no added diatoms. Although all larvae settled and initiated burrowing within the first minute after their addition in still water, regardless of treatment, only 48–52% had completely penetrated the high diatom biomass treatments after 5 min, while on average 80 and 69% of the larvae had settled and burrowed into the control sediments and BDF with a low diatom biomass (<3.5 μg Chl a g⁻¹ dry sediment), respectively. The percentage of larvae settling and burrowing into the sediment was negatively correlated with the concentration of Chl a and EPS of the BDF. This suggests higher physical resistance to bivalve penetration by the BDF with higher diatom biomass and more associated sugar and protein compounds. The larval settlement rate in annular flume experiments at flow velocities of 5 and 15 cm s⁻¹ was distinctly lower compared to the still-water assays. Only 4.6–5.8% of the larvae were recovered from BDF and control sediments after 3 h. Nonetheless, a clear settlement preference was observed for BDF in the flume experiments; i.e., larvae settled significantly more in BDF compared to control sediments irrespective of flow speed. Comparison with the settlement of polystyrene mimics and freeze-killed larvae led to the conclusion that active selection, active secondary dispersal and, at low flow velocities (5 cm s⁻¹), passive adhesion to the sediment are important mechanisms determining the settlement of M. balthica larvae in estuarine biofilms.     

Ultraviolet radiation effects on the behavior and recruitment of larvae from the reef coral Porites astreoides

We tested the rarely considered hypothesis that the ultraviolet portion (UVR, 280–400 nm) of the light spectrum affects patterns of recruitment in reef-building corals. The premise for this hypothesis rests in the fact that biologically relevant intensities of UVR penetrate to considerable depths (>24 m) in the clear waters surrounding many coral reefs, and that reef organisms allocate substantial resources to prevent and repair UVR damage. The ability of larvae spawned by the brown morph of the Caribbean coral, Porites astreoides, to detect and avoid UVR was assessed in petri dishes where one-half of the dish was shielded from UVR and the other exposed. Observations made every 30 min between 10:30 and 13:30 h showed significantly higher densities of larvae swimming in regions shielded from UVR. To determine how this behavior affects settlement patterns, larvae collected from P. astreoides adults at 18 m depth were released into chambers deployed at 17 m depth where they were given a choice of three different light regions in which to settle: PAR (PAR=400–700 nm), PAR+UVAR (UVAR=320–400 nm), and PAR+UVAR+UVBR (UVBR=280–320 nm). At the end of the experiment, greater numbers of P. astreoides larvae had settled in the region of the tube where UVR was reduced than would be expected if dispersion were random. To our knowledge, this is the first demonstration in any reef-building coral species that planula larvae can detect UVR and that it affects their choice of a settlement site. These results indicate that the capacity to detect and avoid habitats with biologically damaging levels of UVR may be one factor contributing to the successful recruitment of coral larvae.     

Waterborne chemical compounds in tropical macroalgae: positive and negative cues for larval settlement

Settlement sites of marine invertebrate larvae are frequently influenced by positive or negative cues, many of which are chemical in nature. Following from the observation that many shallow-water, Hawai'ian marine macroalgae are free of fouling by sessile invertebrates, we predicted that the algae are chemically protected and dependent on either surface-bound or continuously released soluble compounds to deter settling invertebrate larvae. To address the importance of waterborne algal compounds, we experimentally determined whether larvae of two of Hawai'i's dominant hard-surface fouling organisms, the polychaete tube worm Hydroides elegans and the bryozoan Bugula neritina, would settle in the presence of waters conditioned by 12 species of common Hawai'ian macroalgae (representing the Phaeophyta, Chlorophyta, Rhodophyta and Cyanophyta). The results included a full spectrum of biological responses by each larval species to waterborne algal compounds. Larval responses to conditioned water were consistent for each algal species, but the outcomes were not predictable based on the taxonomic relationships of the algae. For example, among the species of Phaeophyta examined, different conditioned waters were: (1) toxic, (2) inhibited settlement, (3) simulated settlement, or (4) had no effect, compared to larvae in control dishes containing filtered seawater. Additionally, larval responses to aged (24 h) conditioned waters could not be predicted from the results of assays run with conditioned waters utilized immediately after preparation. Finally, settlement by larvae of one species did not predict outcomes of tests for the other species. Four of 12 shallow-reef Hawai'ian macroalgae tested released compounds into surrounding waters that immediately killed or inhibited settlement by both H. elegans and B. neritina (toxic: Dictyota sandvicensis; inhibitory: Halimeda discoidea, Sphacelaria tribuloides, Ulva reticulata); the remaining 8 algal species prevented settlement by one of these fouling organisms but for the other had no effect or, in some cases, even stimulated settlement     

Inhibition and facilitation of bryozoan and ascidian settlement by natural multi-species biofilms: effects of film age and the roles of active and passive larval attachment

Sessile marine invertebrate larvae can recognize suitable settlement substrata by using various environmental cues, including organic/microbial biofilms. In laboratory choice assays, the effect of biofilms of varying ages on the settlement behaviour of two fouling organisms was assessed. The species included the arborescent cheilostome bryozoan Bugula flabellata (Thompson) and the solitary ascidian Ciona intestinalis (L.), both of which are characteristic of temperate sublittoral hard substratum assemblages in northwest Europe. Experiments were carried out using polystyrene petri dish substrata preconditioned with multispecies biofilms from natural laboratory-aquarium seawater for 1, 3, 6 or 12 d. Unfilmed (new, initially sterile) dishes were used as control substrata. Whereas the coronate larvae of B. flabellata generally were inhibited by biofilming, irrespective of film age, the settlement of tadpole larvae of C. intestinalis was facilitated on biofilmed substrata, and numbers of settled larvae generally increased with biofilm age: the highest mean numbers were counted on 12 d-old biofilms. In C. intestinalis, settlement and metamorphosis are processes which can be temporally separate and are possibly induced by different environmental cues. This study therefore distinguished between C. intestinalis larvae which were attached to the biofilm surface by the anterior, and those larvae entrapped by the biofilm but not settled in the conventional meaning of the term. As reported in previous studies, we did observe that such entrapped larvae could subsequently attach and develop successfully into sessile juveniles. Both the numbers of “attached” and “trapped” tadpoles increased with biofilm age. Assuming that “settlement” is essentially a process involving the active behavioural response of larvae to environmental cues, it seems that the facilitated attachment of C. intestinalis onto biofilmed substrata is due to the combined effect of active habitat selection and passive deposition/“entrapment” of larvae onto the “sticky” substratum. Received: 21 August 1996 / Accepted: 21 November 1996     

Life history characteristics of a coral reef sponge

Study of the life history characteristics of the common Red Sea sponge Mycale fistulifera (Poecilosclerida: Demospongiae) reveals several traits which may categorize it as an opportunistic species: (1) at least part of the population is reproductively active throughout the year, providing a continuous supply of larvae for settlement; (2) sponges may produce and release a large number (152±39 larvae d^-1) of brooded larvae; (3) released larvae are capable of fast settlement and metamorphosis (minutes to 30 hours post-release); (4) the turnover of the population is high and population size varies with time. From 48 initial sponges plus 94 recruits, only 25 remained alive after 14 months of observation; (5) most members of the population (>70%) have a small body size (<30 cm²); (6) sexual maturity may occur at an early age. These traits facilitate continuous establishment of M. fistulifera in new spaces on the reef; (7) M. fistulifera, preferred substrate is another opportunistic species, the coral Stylophora pistillata which it overgrows.     

Experimental assessment of sensory modalities of coral-reef fish larvae in the recognition of their settlement habitat

One of the great mysteries of coral-reef fish ecology is how larvae locate the relatively rare patches of coral-reef habitat on which they settle. The present study aimed to estimate, by experiments in aquaria, the sensory modalities of coral-reef fish larvae for senses used in searching for their species settlement habitat. Larval recognition of settlement habitat can be based on the detection of conspecifics and/or of characteristics of coral habitat using visual, chemical and mechanical cues. For this study, larvae were captured with crest nets and were then introduced into experimental tanks that allowed testing of each type of cue separately (visual, chemical or mechanical cues). Among the 18 species studied, 13 chose their settlement habitat due to the presence of conspecifics and not based on the characteristics of coral habitat, and 5 species did not move toward their settlement habitat (e.g. Scorpaenodes parvipinnis, Apogon novemfasciatus). Among the different sensory cues tested, two species used the three types of cues (Parupeneus barberinus and Ctenochaetus striatus: visual, chemical and mechanical cues), six used two types (e.g. Myripristis pralinia: visual and chemical cues; Naso unicornis: visual and mechanical cues), and five used one type (e.g. Chrysiptera leucopoma: visual cues; Pomacentrus pavo: chemical cues). These results demonstrate that many coral-reef fish larvae could in practice use sensory cues for effective habitat selection at settlement, and have the ability to discriminate species-specific sensory cues.Communicated by J. Krause     

Seasonal variation in settlement and spiculation of sponge larvae

A settlement study, based in the Hauraki Gulf, Auckland, New Zealand, has revealed 3 different seasonal reproductive patterns among Demospongiae. Species which settled on our substrates can be divided into summer settlers, winter settlers, or all-year-round settlers. The composition of these groups is given, and the affinities and habitats of the constituent species are commented on. Variation in aspects of the spicule complement have been observed and discussed for three species: Mycale macilenta, Microciona coccinea and Ectyomyxilla kerguelensis.     

Effects of salinity and adult extract on settlement of the oligohaline barnacle Balanus subalbidus

Balanus subalbidus (Henry) has the most oligohaline distribution of three congeneric barnacles in Chesapeake Bay and tolerates prolonged exposure to fresh water. We studied larval settlement (i.e., permanent attachment and metamorphosis) of B. subaldius in the laboratory, over a 3 yr period, May 1989 to March 1992, under the following conditions: (1) across an array of salinities at 25°C in the presence and absence of settlement factor consisting of adult B. subalbidus extract; (2) in the presence of conspecific or congeneric settlement factors; and (3) cyprids which were, and were not, induced to delay metamorphosis were compared in their capacities to settle in a range of salinities. Discrepancies between salinity profiles of larval settlement in the laboratory and adult oligohaline distribution in the estuary were striking, and there was a significant interaction between salinity and settlement factor. Averaging results of four different batches of larvae, although peak settlement (87±9%) of B. subalbidus occurred at 2 ppt salinity in the presence of adult cue, substantial settlement also occurred at higher salinities: >70% at 5, 10 and 15 ppt; and 47% at 20 and 25 ppt. In addition, settlement in the absence of settlement factor was relatively high (>50%) and peaked at mid-salinity ranges (e.g. 56±10% at 15 ppt). Variation observed in settlement among larval batches reflected detailed differences in settlement between adjacent test salinities. No difference in settlement occurred between replicate aliquots of cyprids within a batch. Cyprids of B. subalbidus settled most abundantly in the presence of settlement factor extracted from conspecifics, followed in decreasing order by settlement factor extracted from B. improvisus and B. eburneus. Delay of metamorphosis produced by keeping B. subalbidus cyprids for 8 d at 5°C resulted in a decreased level of settlement, but settlement frequency patterns of delayed and non-delayed cyprids were indistinguishable relative to salinity. These results indicate that the oligohaline distribution of adult B. subalbidus is probably not determined by larval behavior at settlement. We suggest that pre-settlement behavior, resulting in larval retention in low saline waters, could be an important factor in determining distribution of this species.     

Local and systemic induced responses to cabbage root fly larvae (Delia radicum) in Brassica nigra and B. oleracea

Summary. Feeding by belowground herbivores may induce systemic changes in shoot defence levels that affect the performance of above ground herbivores and higher trophic levels. In this paper two wild Brassica species, B. nigra and B. oleracea were experimentally infested with 10 larvae of the cabbage root fly, Delia radicum. Plant dry masses and glucosinolate levels in shoots, main roots, and fine roots were determined at 3, 7, 12 and 14 days after infestation and compared to those of control plants. The systemic response in the leaves differed between plant species. In B. nigra shoot glucosinolate levels in D. radicum infested plants steadily increased with time until they were almost twice those of controls 14 days after infestation. B. oleracea plants infested with D. radicum did not show significant changes in shoot glucosinolate levels within 14 days, which may be due to the unexpected poorer performance of D. radicum on this species. Both plant species showed a local increase in indole glucosinolates in the main roots, which are the preferred feeding site of D. radicum larvae. B. oleracea plants however showed a stronger (1.9 – 4.7 times) increase in indole glucosinolate levels than B. nigra (1.5 – 2.6 times). The increase in indole glucosinolates in B. nigra main roots, was counterbalanced by a significant decrease in aromatic glucosinolate levels. These differences in local responses to D. radicum feeding between the two species may have contributed to the slower growth rates of the larvae on B. oleracea. D. radicum feeding did not result in altered glucosinolate levels in the fine roots in either plant species. The differences in glucosinolate induction patterns between the summer annual B. nigra and the perennial B. oleracea are discussed in the light of their different life histories.     

Ancient behaviors of larval amphibians in response to an emerging fungal pathogen, <Emphasis Type="Italic">Batrachochytrium dendrobatidis</Emphasis>

Behaviors have evolved in response to various selection pressures over evolutionary time. However, not all behaviors are adaptive. Some presumably “ancient” behaviors, persistent for millions of years, may be detrimental in the face of novel selection pressures in modern times. These pressures include a multitude of emerging infectious diseases which may be stimulated by environmental changes. We examined how a globally emerging amphibian pathogen, Batrachochytrium dendrobatidis (BD), affected two key evolutionarily persistent behaviors displayed by amphibian larvae: aggregation and thermoregulation. Larval aggregation behavior is often essential for foraging, thermoregulation, and antipredator defense, but varies among species. Thermoregulatory behavior speeds larval development in ephemeral habitats. Specifically, we examined whether aggregation and thermoregulatory behaviors changed when exposed to the BD pathogen in two species (Bufo boreas and Rana cascadae) whose larvae aggregate in nature. In laboratory choice tests, larvae of neither species avoided infected conspecifics. BD-exposed B. boreas larvae aggregated, while unexposed R. cascadae larvae associated more frequently with BD-exposed conspecifics. There was no evidence of behavioral fever or altered thermoregulation in larvae of four species we examined (Pseudacris regilla, Rana aurora, B. boreas, R. cascadae). The absence of behavioral fever may suggest an inability of the larvae of some host species to mediate infection risk by this pathogen. Thermoregulatory behaviors may exhibit a high degree of evolutionary inertia in amphibian hosts because they are linked with host physiology and developmental rates, while altered aggregation behaviors could potentially elevate pathogen transmission rates, leading to increased infection risk in social amphibian species.     

Settlement patterns and presettlement behavior of the naked goby,Gobiosoma bosci,a temperate oyster reef fish

Dispersion, distribution, development and feeding incidence of larvae of the naked goby,Gobiosoma bosci (Lacepéde), were examined for linkages between larval behavior while near the reef surface and later patterns of settlement and recruitment. Field sampling and experiments were conducted during the summers of 1988 and 1989 in the Flag Pond oyster reef along the western shore of the Chesapeake Bay near Camp Conoy, Maryland, USA. Results indicated that prior to settlement most demersal larvae aggregate in shoals and exhibit distinct microhabitat preferences on the reef. In a field experiment, larvae settled both during the day and at night. Dispersion at settlement was aggregated, suggesting that demersal shoaling influences settlement patterns in this species. The distribution of demersal larvae also indicated that larval swimming behavior is sufficiently strong to permit active control of position on reefs. Large demersal larvae settled rapidly when brought to the laboratory, but small larvae in demersal shoals appeared to require additional growth and morphological development prior to settlement. Development of the pelvic fins, used by juveniles and adults for perching on the substrate, may be a good indicator of competence to settle in this species. The adaptive significance of demersal shoaling by small larvae of the naked goby, and the fate of these larvae, remains perplexing because the low feeding rates found for larvae shoaling near the reef surface should slow or prevent the growth and development required prior to settlement. Observations made by other authors indicate that demersal shoaling and the use of water directly overlying reefs may be common behaviors of temperate and tropical reef fishes.     

Settlement behavior and metamorphosis of oyster larvae (Crassostrea gigas) in response to bacterial supernatants

Veliger larvae of the oysterCrassostrea gigas (Thunberg) responded to unknown dissolved chemical inducers found in supernatants of cultures of the bacteriaAlteromonas colwelliana andVibrio cholerae. The response, which was similar to that seen when larvae were exposed to the neurotransmitter precursor L-3,4-dihydroxyphenylalanine (L-DOPA), consisted of an initial settlement phase of swimming with the foot extended and crawling on the substrate. Subsequently larvae attached to the substrate and metamorphosed. The percentage of veligers metamorphosing following inducation of settlement behavior was higher in a group of older larvae, a response similar to that seen with L-DOPA, suggesting that competence to respond to bacterial supernatants is divided into two phases: behavioral competence followed by morphogenetic competence. Following size exclusion chromatography, the molecular weight of the peak containing the activity which induced settlement behavior was determined to be 300 daltons. Autoclaved Marine Broth, which induced low levels of settlement behavior also contained this low molecular weight active peak, suggesting that an oyster settlement inducer is also present in this medium.Contribution # 137 from the Center of Biotechnology, Marine Biotechnology Institute, University of Maryland, USA