Effect of zinc on growth and development of larvae of the Pacific oyster Crassostrea gigas

Following the observation of periodic high concentrations of zinc in estuarine waters used in the White Fish Authority's oyster hatchery at Conway, North Wales, two beaker trials were conducted to study the effect of zinc, over the range recorded, on the young stages of larvae of Crassostrea gigas. Zinc, added to sea water both as zinc sulphate and as a natural mine-adit water, was applied for a period of 5 days, after which larvae were maintained for a further 5 days in sea water alone. Increasing concentrations over the range 125 to 500 g/l Zn resulted in decreasing growth, and increasing incidence of abnormality and larval mortality. A second trial with zinc sulphate showed 50 g/l Zn to have little effect on larval development, a progressive decrease in growth at 100 and 150 g/l, and no growth at 200 g/l. It is suggested that the deleterious effect of short-term exposure to zinc may well have contributed to the intermittent failure of larvae and irregular productivity previously recorded at the hatchery. It is also possible that zinc contamination in estuaries may affect natural oyster breeding, and may have to be considered in the future siting of hatcheries for seed production.     

Total labile carbon in the euphotic zone of the Baltic Sea as measured by BOD

Total labile organic carbon was estimated in the euphotic zone of the Baltic Sea by a method based on the determination of the biological oxygen demand (BOD); the method is extremely simple, requires little sample water, and is quite precise (±4.5 μmol O₂/l or ±40.4 μgC_lab/l). The amount found was 1,150 μgC_lab/l, or about 29% of the total organic carbon. At 1 m, a relationship between biological activity, as measured by chlorophyll content, and the amount of labile carbon was found: ln C_lab (μg/l)=ln 6.87 (μg/l) +0.272 ln chlorophyll (μg/l), with a correlation coefficient of 0.86, which is significantly greater than zero at the 0.001 level.     

Biofilm cue for larval settlement in Hydroides elegans (Polychaeta): is contact necessary?

Larvae of many sessile marine invertebrates settle in response to surface microbial communities (biofilms), but the effects of soluble compounds from biofilms in affecting larval behavior prior to settlement, attachment, and metamorphosis have been little studied. This question was addressed by videotaping the behavior of competent larvae of the serpulid, Hydroides elegans, above settlement-inducing biofilms. Adult worms were collected in Pearl Harbor, Hawaii, USA in November 2012 and spawned almost immediately. Six-day old larvae were placed in five replicated treatments in small cups: (1) with a natural biofilm; (2) with a natural biofilm on an 8-µm screen, 1 mm above the bottom of a clean cup; (3) with a natural biofilm beneath a clean screen; (4) in a clean cup; and (5) in a clean cup with a clean screen. Using the videotapes, larval swimming speeds and trajectories were quantified within 5 min of the larvae being placed in a treatment. Only larvae that touched a biofilm, i.e., in treatments (1) and (2), slowed their swimming speed and increased the amount of time spent crawling rather than swimming. This shows that under these conditions, any soluble cues emanating from a biofilm do not affect settlement behavior. Furthermore, after 24 h close to 100 % of larva in the two accessible biofilm treatments had metamorphosed and <15 % in treatments that included a biofilm under a clean screen and no biofilm at all, strongly suggesting that soluble cues for settlement were not produced by the biofilms over the longer time period.     

Effects of the insecticide sevin and its first hydrolytic product, 1-naphthol,on some early developmental stages of the bay mussel Mytilus edulis

Six developmental stages of the bay mussel Mutilus edulis, from fertilization to 32 h after fertilization, and also unfertilized eggs, were exposed for 1 h to different concentrations of the insecticide Sevin and its first hydrolytic product, 1-naphthol. After exposure, the larvae or eggs were separated from the pesticide solution and returned to clean water. At 48 h after fertilization, the numbers of normal and abnormal larvae were determined and 1-h EC₅₀ values (the effective concentrations that caused anomalous development of 50% of the test animals) were calculated. The most sensitive developmental stage was the one that occurred shortly after fertilization at the time of appearance of the first polar body. Thereafter, sensitivity decreased as age increased. The EC₅₀ of Sevin for the first polar body stage and 32-h stage were 5.3 and 24.0 mg/l of Sevin, respectively. The EC₅₀ of l-naphthol for the first polar body stage was 5.2 mg/l. Effects of the toxicants on development were characterized by disjunction of blastomeres, a reduction in the rate of development, and asynchronous and unaligned cleavages.     

Settlement of the tube worm Hydroides dianthus (Polychaeta: Serpulidae): cues for gregarious settlement

The larvae of many benthic marine invertebrates settle to form conspecific aggregations and are thought to rely on chemical cues associated with adults as indicators of habitat suitability, although the identification of inductive compounds has proven difficult. Still-water laboratory assays carried out during the summers of 1992 and 1993 with larvae of the serpulid polychaete, Hydroides dianthus (Verrill, 1873), demonstrate that unidentified water-borne compound(s) were responsible for gregarious settlement of competent larvae. Unlike inductive compounds associated with other tube-dwelling polychaetes, the settlement cue was soluble in water and was not associated with the tube, but rather with the body of live adults. In assay chambers divided by a 52-m mesh barrier, a greater percentage of larvae settled on biofilmed substrata when adult worms were present on the other side of the barrier than when adults were absent. Settlement in response to conspecific adults, live worms removed from their tubes, and amputated tentacular crowns of live worms was significantly greater than settlement in response to dead worms, empty tubes, or biofilmed slides. The settlement inducer appears to emanate from the openings of occupied tubes; settlement was greatest along the anterior two-fifths of the tube of living conspecific adults. A single adult was equally capable of eliciting a gregarious response as were five or 25 conspecifics, and newly settled juveniles began to elicit gregarious settlement after approximately 96 h. Extraction of aggregations of adult worms with organic solvents removed the inductive capacity of the tissue, and activity was found in both nonpolar and polar fractions of an extraction series.     

Patterns of bioaccumulation of an essential trace element (zinc) and a pollutant metal (cadmium) in larvae of the prawn Palaemon serratus

Palaemon serratus Pennant larvae from females caught near Roscoff (France) or the Bay of Biscay in 1983, were reared in the laboratory. From hatching to the first stage after metamorphosis, the larvae were exposed to zinc and cadmium introduced separately or together into the rearing medium. At the end of experimental contamination, the concentrations of the two metals in the organisms were determined. The level of zinc in organisms was mainly independent of its concentration in the water, suggesting that the bioaccumulation of this essential trace element is controlled by some physiological process. The accumulation of cadmium in the larvae paralleled the overloads of this metal in the water. Increased zinc concentrations in water have little or no effect on the cadmium levels in organisms. The most important interaction between these two metals is an antagonism exerted by cadmium on the biological uptake of zinc.     

Experimental studies on the effects of copper on a marine food chain

Effects of copper sulphate on a marine food chain were investigated in large tanks at Loch Ewe on the west coast of Scotland. The food chain consisted of phytoplankton, the bivalve Tellina tenuis da Costa, and O-group plaice (Pleuronectes platessa L). During initial months after settlement, T. tenuis siphons are an important food for juvenile plaice, which feed by grazing the siphon tips; these subsequently regenerate. Copper dose rates of 10, 30 and 100 μg Cu/1 were investigated; the distribution of added copper was monitored and its metabolic effects were determined. Copper levels in water, sand, in algae on tank walls, in T. tenuis shell and flesh, and in plaice muscle and viscera were measured by atomic absorption spectrophotometry. Copper accumulated in sand, T. tenuis flesh and shell, and plaice viscera. Accumulations were dose-dependent and in no case was a plateau concentration reached. Most of the added dose was taken up by the sand; accumulations in T. tenuis shell were small — less than twice background in all cases. High levels were found in T. tenuis flesh, where concentrations after 100 days were 270, 470 and 1100 μg Cu/g dry flesh for dose concentrations of 10, 30 and 100 μg Cu/1, respectively. The mean control concentration was less than 50 μg Cu/g dry flesh. There was no accumulation of copper in plaice muscle. Visceral accumulations after 100 days were 71, 147 and 567 μg Cu/g dry flesh for dose concentrations of 10, 30 and 100 μg Cu/1, respectively. The mean control level was 30 μg Cu/g dry flesh. The effect of copper on phytoplankton metabolism was investigated by measuring plant pigment levels and rates of primary photosynthetic fixation of C¹⁴-labelled bicarbonate. All dose concentrations reduced both the standing crop and the rate of photosynthesis per unit of chlorophyll a. The effects of copper on growth and condition of T. tenuis and P. platessa were investigated. All dose concentrations adversely affected T. tenuis condition (dry flesh weight for standard individual). The effect was most marked during deposition of winter reserves. In the absence of plaice predation, there was a decrease in the mean siphon weight for T. tenuis exposed to 30 and 100 μg Cu/1. For plaice, all dose concentrations of copper resulted in reduced growth, but there was no significant change in condition or biochemical composition. The ash weight of fish exposed to copper was significantly higher than for controls.     

Toxizität und anreicherung von blei bei der miesmuschel Mytilus edulis im laborexperiment

In a laboratory experiment, conducted over 130 days, three batches of a total of 100 common mussels, Mytilus edulis, were maintained in media containing different lead concentrations. Two vessels served as controls. There was no acute injury to the mussels; but, over an extended period of time, a marked increase in mortality occurred which was related to the lead concentration in the medium. The median lethal time (LT₅₀) was computed to be 218 days for the control, 150 days with 0.5 mg Pb/l in the medium, 129 days with 1 mg/l, and 105 days with 5 mg/l. Quantitative analyses of the soft parts of M. edulis by flameless atomic-absorption-spectrophotometry revealed a distinct accumulation of lead. From a natural lead content of 8.4 μg Pb/g dry weight, the lead concentration increased to 12,840 μg/g at 0.5 mg/l, to 20,770 μg/g at 1 mg/l and to 39,830 μg/g at 5 mg/l. The ratio of the concentration of lead in the soft parts of M. edulis to the concentration in the medium remained in the same order of magnitude both under laboratory conditions using high lead concentrations and under in situ lead levels. Over a period of 130 days, lead uptake expressed as percentage of lead offered was 10.9% at 0.5 mg/l, 9.5% at 1 mg/l, and 3.4% at 5 mg/l, respectively.     

Die Bedeutung in Meerwasser gelöster Glucose für die Ernährung von Anemonia sulcata (Coelenterata: Anthozoa)

Anemonia sulcata Pennant absorbs and accumulates tritiated D-glucose dissolved in sea water in natural concentrations (80 μg/l). The rate of decrease of glucose concentration in the sea water can best be described by an exponential function, when the concentration of added glucose exceeds 20 to 25 μg/l. The glucose, absorbed through the ectoderm, is immediately metabolized. This is determinable experimentally by tritiated water which is emitted into the sea water. The rate of uptake estimated experimentally was in the range 3 to 4 μg glucose/g wet weight/h, whilst the rate calculated by V _max (maximum absorption rate) was 20 μg/g wet weight/h. The concentration of free glucose in the tentacle tissue is greater by a factor of 10⁵ than that of the medium in which absorption takes place. Absorption is temperature-dependent; the Q₁₀ between 15° and 25°C exceeds 2; translocation is, therefore, energy-dependent. Absorption was diminished by inhibitors, especially by phlorizin. The actinians increase their energy reserves by absorption of glucose (and other dissolved organic substances). The energetic net profit by taking up glucose comprises up to 50% of the energy amount which is equivalent to the oxygen consumption.     

Role of bacteria in larval settlement and metamorphosis of the polychaete Hydroides elegans

The serpulid polychaete Hydroides elegans Haswell, 1883 is an early colonist of new substrata in Pearl Harbor, Oahu, Hawaii. When metamorphically competent, larvae of H. elegans will settle rapidly upon an acceptably biofilmed surface, but not on a clean surface. In this study we found the ability of larvae to respond selectively to inductive surfaces to be retained for at least 3 wk. Of a series of bacterial strains isolated from Hawaiian marine biofilms, 13 induced larval settlement, 11 gave moderate or mixed results, and 10 others did not stimulate the settlement of H. elegans. The amount of settlement induced by monospecific strains was rarely as great as with natural, multispecies films. Most of the isolated bacteria were motile Gram-negative rods, but Gram-positive strains were also present, and diverse metabolic types were represented in the study. Biofilms killed by treatment with heat, ultraviolet radiation or chemical fixatives were no longer inductive. Soluble, dialyzable, heat-stable bacterial products induced settlement and metamorphosis more slowly. The range of bacteria producing an inductive signal suggests either that there are multiple cues, or that the cue is common to many bacteria. Chemical signals characteristic of early microbial biofilms may indicate freshly available substrata with optimal potential for the growth and survival of H. elegans. Received: 30 January 1998 / Accepted: 12 September 1998     

Ontogenetic changes in the vertical distribution of giant scallop larvae,Placopecten magellanicus,in 9-m deep mesocosms as a function of light,food, and temperature stratification

To understand how thermal stratification and food abundance affects the vertical distribution of giant scallop larvae Placopecten magellanicus (Gmelin), a mesocosm study was conducted in January and February 1992. The position of larvae was followed over 55 d in replicated 9-m deep tanks in relation to a sharp thermocline and the presence or absence of phytoplankton. Growth and vertical position of larvae were monitored in separate treatments which included phytoplankton added above the thermocline, below the thermocline, throughout the mesocosm, or absent from the mesocosm. Changes in the vertical position of larvae over time were quantified with a new, profiling, video-optical instrument capable of semi-automatically identifying, counting and sizing larvae. The strong diurnal migration of scallop larvae resulted in aggregations at two interfaces: the air/water interface during the night, and at the thermocline during the day. At times, the concentration of larvae within cm of the surface was > 100 times that in the remaining water column. The formation of bioconvective cells of swimming larvae at the air/water interface allowed larval aggregations to persist throughout the period of darkness. Regardless of the distribution of food, larvae remained above the thermocline during most of the experiment. Therefore, only in those treatments where food was also present above the thermocline was larval growth relatively high. Larger larvae penetrated the thermocline only after reaching a shell length of about 200 m; thus larval size, rather than chronological age, was more important in describing their vertical distribution. The rapid increase in kinematic viscosity with decreasing water temperature at the thermocline may retard the movement of larvae and contribute to aggregation at this interface. The influence of larval size on their vertical distribution, and the resulting potential for horizontal transport to settlement sites, points to the importance of persistent hydrographic features as critical factors contributing to settlement variance in scallops.     

Östrogenartige wirkungen von bisphenol a auf vorderkiemenschnecken (Mollusca: Gastropoda: Prosobranchia)

Within a Federal Environmental Agency research project to develop a biological test for hormone-mimetic compounds using the freshwater snail Marisa cornuarietis, the effects of the suspected xenoestrogenic substance bisphenol A (BPA), not only on freshwater but also on marine prosobranch snails, were investigated. For the laboratory experiments the ramshorn snail Marisa cornuarietis and the ovoviviparous snail Potamopyrgus antipodarum were considered as freshwater species and two marine prosobranchs, the netted whelk Nassarius reticulatus and the dog whelk Nucella lapillus, were additionally employed. N. reticulatus, as a typical sediment-living species, was exposed via artificial sediments, while the three other prosobranchs were exposed via water. The test series with Marisa cornuarietis covered a nominal concentration range between 1 and 100 μg BPA/l in a 5 month experiment with adult snails and a complete life cycle test for 12 months. Additionally, a third test in the nominal range between 0.05 and 1 μg BPA/l (measured: 0.0079–0.404 μg/l) was performed with adults snails for 6 months. In these experiments, BPA induced a complex syndrome of physiological and morphological alterations in female Marisa referred to as the induction of ‘superfemales’. Affected specimens were characterised by the formation of additional female organs, an enlargement of the accessory pallial sex glands, gross malformations of the pallial oviduct section resulting in an increased female mortality, and a massive stimulation of oocyte and spawning mass production. For these tests, an LOEC of 48.3 ng/l, an NOEC of 7.9 ng/l and an EC₁₀ of 13.9 ng/l were calculated. Superfemales occurred also in the BPA exposure experiment with the other snail species, but comparable oviduct malformations as in Marisa were not found, probably due to species differences in the gross anatomical structure of the pallial oviduct. During the 9 week test with Potamopyrgus antipodarum in the nominal concentration range between 1 and 100 μg/l, BPA induced an enhancement of embryo production even in the sexual repose phase of the reproductive cycle. A characteristic inverted U-type concentration response relationship was found. Nassarius reticulatus was exposed via BPA-spiked artificial sediments (nominal concentration range: 10–1000 μg/kg dry wt.) for 3 months. BPA exhibited a significant and concentration dependent uterotrophic effect which could be detected not only by an enlargement but also by a weight increase of the accessory pallial gland complex in the pallial oviduct section. Adult Nucella lapillus were tested for three months in the laboratory in a nominal concentration range between 1 and 100 μg BPA/l. Superfemales in the dog whelk were also characterised by enlarged accessory pallial sex glands and an enhancement of egg production, but the test compound also affected the males in this species. A lower percentage of exposed specimens had ripe sperm stored in their vesicula seminalis and male Nucella exhibited a reduced length of penis and prostate gland when compared to the control. Because statistically significant effects were observed already at the lowest nominal test concentration (1 μg/l), it can be assumed that even lower concentrations may have a negative impact on the snails. The results show that prosobranch snails are affected by BPA at lower concentrations compared to other systematic taxa in the animal kingdom. Consequently, it has to be claimed that the results of these experiment have to be considered for the current EU risk assessment for BPA in order to achieve a sufficient protection of wildlife in aquatic ecosystems.     

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     

The indirect effects of eutrophication on habitat choice and survival of fish larvae in the Baltic Sea

The structure of the habitat is usually crucial for growth and survival of young life stages. Presently, some nursery areas of fish larvae are changing due to eutrophication, e.g. due to enhanced growth of ephemeral filamentous algae at the expense of perennial species. We studied the influence of two habitats, one with filamentous algae (Cladophora glomerata) and the other with bladder wrack (Fucus vesiculosus), on habitat choice of pike larvae (Esox lucius) in the absence/presence of a predator or a competitor. We further tested whether the habitat choice is adaptive in increasing survival under predation threat. In contrast to expectations, pike larvae preferred the habitat with ephemeral filamentous algae to the bladder wrack, thriving in clean waters, independent of the presence/absence of both predator/competitor. In addition, the survival of the larvae was higher in the filamentous algae in the presence of predators, which suggested that the habitat preference of the larvae was adaptive. The structure of the bladderwrack habitat was probably too open for newly hatched larvae, which implies that F. vesiculosus and other large brown algae are not as important refuges for young larvae as previously thought.     

Metal interaction during accumulation by the mussel Mytilus edulis planulatus

Mytilus edulis planulatus (Lamarck) were collected from Howden, South-east Tasmania in autumn 1981. Interaction effects of cadmium, copper and zinc during accumulation by mussels exposed for ten days to all three metals simultaneously were examined in a series of experiments in which each metal was tested at three concentrations. In general, interaction effects were most evident at the highest concentrations tested (20 g l^-1 Cd; 20 g l^-1 Cu; 200 g l^-1 Zn) and led to a reduction in the accumulation of cadmium and an increase in that of copper and zinc. More specifically, high levels of zinc caused a decrease in cadmium uptake and an increase in copper accumulation. The presence of copper resulted in depressed cadmium accumulation while zinc accumulation increased. Cadmium tended to enhance zinc accumulation, but copper accumulation was only affected to any great extent when zinc was also present.     

Effects of gamma-aminobutyric acid on the settlement of larvae of the black chiton Katharina tunicata

Laboratory-cultured larvae of the black chiton Katharina tunicata (Wood), collected from central California in May and June of 1979 and 1980, settled preferentially on the encrusting coralline alga Lithothamnium sp. Metamorphosis, which involves the loss of the prototrochal ciliary band, occurred within a period of 2.5 h after larvae began crawling upon the alga. In response to gamma-aminobutyric acid (GABA) at a concentration of 10^-6 M, the lecithotrophic trochophore larvae ceased swimming and settled rapidly. Developmental metamorphosis did not follow settlement in the absence of encrusting coralline alga. However, when both GABA and Lithothamnium sp. were present, larval metamorphosis occurred within 2 h of settlement. Significantly different rates of settlement exist for sibling larvae of different ages; 12 d-old larvae responded more rapily and in greater numbers to treatment with GABA than 10 d-old larvae. Differences in settling rates are attributed to the onset of metamorphic competence.     

Larvae of a colonial ascidian use a non-contact mode of substratum selection on a coral reef

The rate at which larvae successfully recruit into communities of marine benthic invertebrates is partially dependent upon how well larvae avoid benthic predators and settle on appropriate substrata. Therefore, to be able to predict recruitment success, information is needed on how larvae search for settlement sites, whether larvae preferentially settle on certain substrata, and the extent to which there are adequate cues for larvae to find these substrata. This article describes how larvae of the colonial ascidian Diplosoma similis find settlement sites on a coral reef. Direct field observations of larval settlement were made on a fringing reef in Kaneohe Bay, Oahu, Hawaii, between September 1985 and April 1986. A comparison of the substrata that larvae contacted prior to settlement relative to the percentage cover of these substrata on the study reef suggests that larvae are using a non-contact mode of substratum identification to locate suitable settlement sites. This mode of substratum identification allowed 74% of larvae to evade predation by benthic organisms who would otherwise have eaten larvae if they had been contacted. Of those larvae that evaded predation, 88% subsequently settled on the same two substrata upon which most adults are found (dead coral or the green alga Dictyosphaeria cavernosa). This pattern of settlement was probably a result of active selection, since the two substrata cover only 14.4% of the reef's surface and currents had little effect on the direction in which larvae swam. An important contributing factor to the high success rate of larval settlement on suitable substrata was the lack of any temporal decay in substratum preference. It is concluded that for Diplosoma similis larval supply is a sufficient predictor of larval settlement rate. However, for marine invertebrates whose larvae are passively dispersed and exhibit a greater temporal decay in substratum preference, larval settlement should generally have a greater dependency on spatial variation in the abundance of benthic predators and suitable substrata.     

In situ swimming and settlement behaviour of larvae of an Indo-Pacific coral-reef fish, the coral trout Plectropomus leopardus (Pisces: Serranidae)

Late larvae of the serranid coral trout Plectropomus leopardus (Lacepède), captured in light traps, were released during the day both in open water and adjacent to two reefs, and their behaviour was observed by divers at Lizard Island, northern Great Barrier Reef. Coral trout larvae (n = 110) were present in light-trap catches from 18 November to 3 December 1997, including new moon (30 November). The swimming speed of larvae in open water or when swimming away from reefs was significantly greater (mean 17.9 cm s⁻¹) than the speed of larvae swimming towards or over reefs (mean 7.2 cm s⁻¹). Near reefs, larvae swam at average depths of 2.7 to 4.2 m, avoiding 0 to 2 m. In open water, swimming depth varied with location: larvae >1 km east of Lizard Island swam steeply downward to >20 m in 2 to 4 min; larvae >1 km west oscillated between 2.6 and 13 m; larvae 100 to 200 m east of Lizard Island oscillated between 0.8 and 15 m. Nearly all larvae swam directionally in open water and near reefs. In open water, the average swimming direction of all larvae was towards the island, and 80% (4 of 5) swam directionally (p < 0.05, Rayleigh's test). Larvae swam directionally over the reef while looking for settlement sites. The frequency of behaviours by larvae differed between two reefs of different exposure and morphology. Depending on site, 26 to 32% of larvae released adjacent to reefs swam to open water: of these, some initially swam towards or over the reef before swimming offshore. In some cases, offshore-swimming seemed to be due to the presence of predators, but usually no obvious cause was observed. Depending on the reef, 49 to 64% of the larvae settled. Non-predatory reef residents aggressively approached 19% of settlers. Between 5 and 17% of the larvae were eaten while approaching the reef or attempting to settle, primarily by lizardfishes but also by wrasses, groupers and snappers. A higher percentage of larvae settled in the second week of our study than in the first. Average time to settlement was short (138 s ± 33 SE), but some larvae took up to 15 min to settle. Average settlement depth was 7.5 to 9.9 m, and differed between locations. No settlement took place on reef flats or at depths <4.2 m. Larvae did not appear to be selective about settlement substrate, but settled most frequently on live and dead hard coral. Late-stage larvae of coral trout are capable swimmers with considerable control over speed, depth and direction. Habitat selection, avoidance of predators and settlement seem to rely on vision. Received: 7 July 1998 / Accepted: 26 January 1999     

Recruitment in the intertidal limpet Lottia digitalis (Patellogastropoda: Lottiidae) may be driven by settlement cues associated with adult habitat

The ribbed limpet, Lottia digitalis, is found high in rocky intertidal habitat throughout its geographic range. In order to identify likely natural settlement locations for larvae of this species, laboratory-reared larvae were settled onto substrata collected from within and near an adult L. digitalis habitat. Of larvae exposed to rock chiseled from within high-intertidal adult habitat, 31.0% and 23.3% underwent metamorphosis during two separate experiments. Similarly, an unidentified filamentous green alga that was isolated from this rock induced metamorphosis in 26.6% and 8.7% of larvae during additional experiments. In contrast, larvae did not metamorphose upon bare rocks or upon rocks encrusted with a crustose corraline alga (CCA) that were collected from lower intertidal zones, nor did they metamorphose upon the macroalgae Ulva sp., Enteromorpha contorta, Alaria marginata, or Polysiphonia sp. The presence of mucus from adult conspecifics during these experiments did not enhance metamorphosis onto rock taken from adult habitat, but it did induce metamorphosis in an average of 13.5% and 7.0% of larvae introduced to the mid-intertidal bare rock and CCA substrata, respectively. Finally, 38.0% and 34.4% of larvae from two experiments underwent metamorphosis when exposed to the high-intertidal barnacle Pollicipes polymerus. These results suggest that larval settlement, rather than differential post-settlement mortality and/or migration, drives recruitment of L. digitalis in high-intertidal habitats.     

Competence and delay of metamorphosis in the Pacific oysterCrassostrea gigas

The temporal relationship between the onset of behavioral and morphogenetic competence was determined inCrassostrea gigas (Thunberg) larvae by exposure to appropriate chemical inducers during development. Larvae exhibited settlement behavior in response to L-3,4-dihydroxyphenylalanine (L-DOPA) before becoming competent to metamorphose in response to epinephrine. For larvae past the onset of competence, the apparent stimulus-threshold decreased and they were increasingly likely to metamorphose subsequent to induced settlement behavior. In the absence of chemical stimulation, cultured oyster larvae were able to delay metamorphosisand maintain competence for at least 30 d. Competence was correlated with, but not dependent upon, larval size and eyespot development. A mechanistic model of oyster settlement and metamorphosis is proposed which incorporates these new data.Contribution # 136 from the Center of Biotechnology, Marine Biotechnology Institute, University of Maryland