Acceptability and digestion of diets fed to larval stages ofHomarus gammarus and the role of dietary conditioning behaviour

Whilst the role of chemoreception in juvenile crustacean feeding behaviour has received considerable attention, the chemosensory responses of larval planktonic crustacean stages have been neglected. The present study investigated the acceptability of processed, microparticulate and microencapsulated diets and the possible role of chemoattractants for larval and juvenile stages ofHomarus gammarus (Linnaeus) and juvenile stages ofPalaemon elegans (Rathke). Larval and juvenile lobster were obtained from MAFF (Ministry of Agriculture, Fisheries and Food, Conwy, UK), and juvenileP. elegans were collected from the Menai Straits (Gwynedd, UK) during the summer months of 1985/1986. During the larval stages, food capture results from chance encounter, and the role of low molecular-weight chemoattractants appears to be in the determination of the edible from the inedible. Dietary conditioning behaviour in decapod larvae, which has until now not been investigated, enables the larvae to discriminate more rapidly between particles of different digestibilities. The digestibility and assimilation efficiency displayed on artificial diets in the present study suggest that lobster larvae possess limited enzymatic capabilities, although later stages are more efficient and display longer food-retention time. Growth (0.114 mm d^–1) and survival (80.0%) of lobster larvae fed natural diets were significantly better (F=14.8;P=0.001, andF=54.0,P=0.001, respectively) than of individuals fed artificial diets. Although pre-digested artificial diets (ingredients pre-treated with pancreatin) resulted in higher survival (74 and 63%) compared to microencapsulated diets (37%), there was no significant growth difference between the artificial diets tested.     

Responses of estuarine crab megalopae to pressure,salinity and light: Implications for flood-tide transport

The megalopal larval stage of many estuarine brachyuran crabs appears to return to adult habitats by undergoing rhythmic vertical migrations which result in saltatory up-estuary transport on flood tides. Larval ascent into the water column during rising tides may be cued by changing hydrologic variables. To test this hypothesis, we investigated the responses of field-caught megalopae of the blue crab Callinectes sapidus and the fiddler crab Uca spp. to constant rates of pressure and salinity change under laboratory conditions. For both genera, pressure changes resulted in increased movement (barokinesis) and upward migration in the test chamber, with C. sapidus megalopae having a lower response threshold (2.8×10^-2 mbar s^-1) than Uca spp. larvae (5×10^-2 mbar s^-1). Similarly, larvae ascended in response to increasing salinity, with C. sapidus larvae being more sensitive. Larvae were negatively phototactic and failed to respond to pressure increases at light levels above 1.0×10¹⁵ and 1.0×10¹³ photons m^-2 s^-1 for C. sapidus and Uca spp. megalopae, respectively. Such responses are thought to explain the low abundances of larvae in the water column during daytime flood tides. Nevertheless, threshold sensitivities to increasing pressure for both genera were above levels experienced during floodtide conditions in the field. Similarly, it is unlikely that increasing salinity is sufficient to induce ascent in Uca spp. postlarvae. However, rates of salinity increase during midflood tide typically reach levels necessary to induce an ascent in C. sapidus megalopae. These results are consistent with the hypothesis that fiddler crab megalopae utilize an endogenous activity rhythm for flood-tide transport, while blue crab megalopae rely upon external cues, especially salinity changes, to time their sojourns in the water column.     

Effects of the juvenile hormone mimic ZR-515 (Altosid®) on larval development of the mud-crab Rhithropanopeus harrisii in various salinities and cyclic temperatures

Effects of 0.01, 0.1 and 1.0 ppm methoprene (Altosid^®: ZR-515), a juvenile hormone (JH) mimic which shows high activity against some economically important insect pests, especially Diptera, were tested on larvae of the mud-crab Rhithropanopeus harrisii (Gould) (Brachyura: Xanthidae) from hatching to the first crab stage under optimum and stress conditions of a number of salinities and cyclic temperatures. There was a significant reduction in survival of zoeal larvae with increasing concentrations of methoprene in nearly all combinations of salinity and temperature. On the average there was 9% less survival in the 0.01 ppm concentration of methoprene than in the control, and in the 0.1 ppm concentration the survival was further reduced by another 16%. At 1.0 ppm methoprene no larvae survived beyond the first zoeal stage under optimum conditions or under stressful combinations of salinity and temperature. Except at 0.2 ppm in 27.5% S, survival of the megalopa was not significantly reduced in 0.01 or 0.1 ppm methoprene in any salinity or temperature, although the percentage of abnormal megalopa increased under stress conditions. The first zoeal stage was the most sensitive of the larval stages to methoprene as well as to salinity and temperature stress. The duration of zoeal development was significantly lengthened with an increase in concentration of methoprene under nearly all conditions of salinity and temperature. The JH mimic had, however, no significant effect on the duration of megalopa development. A significant synergism between methoprene, salinity and temperature was not observed. It can be concluded from the results that methoprene does not inhibit metamophosis of R. harrisii larvae at the 0.1 ppm level or lower. Reduction in survival of zoeal stages and increased duration of zoeal development with increasing concentrations of methoprene are presumably related to stress.     

Effects of the juvenile hormone mimic ZR-512 (Altozar®) on larval development of the mud-crab Rhithropanopeus harrisii at various cyclic temperatures

Effects of the juvenile hormone (JH) mimic hydroprene (Altozar^®: ZR-512), which exhibits high activity against Lepidoptera, were studied on the larval development of the mud-crab Rhithropanopeus harrisii (Gould) (Brachyura: Xanthidae). Larvae reared in 20 S at 3 cycles of temperature of 20° to 25°C, 25° to 30°C and 30° to 35°C, were exposed to 0.01, 0.1 and 0.5 ppm hydroprene from hatching to the first crab stage. Larvae were also exposed to 0.1 and 0.5 ppm hydroprene only from the megalopa stage to the first crab stage. When larvae were treated with hydroprene throughout larval life, survival was significantly reduced with increasing concentrations of the compound at all temperature cycles. Synergistic effect between hydroprene and temperature on survival of zoeal larvae was not observed. On the average there was 11% less survival in the zoeal stages at the 0.01 ppm concentration. of hydroprene than in the control, an additional reduction of 13% occurred at 0.1 ppm, and finally there was a further decrease of 46% at 0.5 ppm hydroprene. Significant decrease in survival in the megalopa stage occurred only in the 0.5 ppm concentration of hydroprene at the lowest temperature cycle when larvae were exposed to the compound from hatching. When larvae were treated with hydroprene only within the megalopa stage, a significant reduction in survival was not observed. First-stage zoeae were the most sensitive of the larval stages to hydroprene. Duration of zoeal development was significantly delayed at 0.5 ppm hydroprene at the two lower temperature cycles, whereas in the megalopa stage the delay began at the 0.1 ppm level at all 3 temperature cycles when larvae were exposed to hydroprene from hatching. A significant delay was also observed at 0.1 ppm hydroprene at the two lower cycles when larvae were exposed to hydroprene only in the megalopa stage; at 30° to 35°C a significant delay was observed only at the 0.5 ppm level. The results show that metamorphosis to the first crab stage was not inhibited at the 0.5 ppm level of hydroprene or lower. Reduction in survival and increase in duration of larval development were presumably related to stress conditions caused by hydroprene. The results also suggest an interaction between temperature and hydroprene on survival of megalopa larvae and duration of larval development.     

A dispersal model for larvae of the deep sea red crab Geryon quinquedens based upon behavioral regulation of vertical migration in the hatching stage

Behavioral responses to gravity, hydrostatic pressure, and thermoclines are described for Stage I zoeae of the deep sea red crab Geryon quinquedens Smith. Survival and rate of development as a function of temperature is presented for all larval stages. Although temperatures between 10° and 25°C have no direct effect upon survival, development time is five times longer at 10°C than at 25°C. Stage I larvae show strong negative response to gravity. Swimming rate increases with an increase in pressure up to 20 atm above ambient at 11°C, but not at 15°C. Swimming rates at 15°C are higher than those measured at 11°C at each pressure tested. Stage I larvae readily penetrate sharp thermoclines. Potential dispersal ranges of G. quinquedens larvae in the Mid-Atlantic Bight are suggested based on larval behavior, development time, and coastal hydrography. A testable recruitment model is proposed for G. quinquedens.Contribution no. 1365 of the Center for Environmental and Estuarine Studies     

Long-term effects of water-soluble fractions of Kuwait crude oil on the larval and juvenile development of the mud crab Eurypanopeus depressus

Larvae of the mud crab Eurypanopeus depressus (Smith) were reared in various concentrations of the water-soluble fraction of Kuwait crude oil. The 48-h TL_m (median tolerance limit) for Zoea Stage I was approximately 10 ppm total dissolved hydrocarbons and that for Zoea Stage II approximately 17 ppm. Chronic toxicity of more dilute solutions (4.3 and 8.7 ppm) was assessed independently for each larval stage and for subsequent developmental stages through Crab Stage 5. In the group continuously exposed to oil from hatching, there was differential mortality relative to controls in every larval stage and increased duration of intermolt periods was observed at every stage through Crab Stage 5. Mortality in groups not exposed until larvae had reached Zoea Stages III or IV was not greater than controls, suggesting that toxicity to advanced larval stages may be related to accumulation of toxic compounds by the larvae. Neither concentration (4.3 or 8.7 ppm) of crude oil caused increased mortality among juvenile crab stages regardless of the time of initial exposure. Increased occurrence of an extra and morphologically abnormal megalopa stage was associated with exposure to the crude oil. This has not been reported before.Communicated by I. Morris, West Boothbay Harbor     

Biological effects of oil on early development of the Baltic herring Clupea harengus membras

The effects of petroleum hydrocarbons from two crude oils and one fuel oil (No. 1) were studied on the ontogenic development of the Baltic herring Clupea harengus membras L. Herring eggs exposed to water-soluble fractions of the oils at the time of fertilization showed no decrease in numbers of fertilized eggs compared to eggs exposed 6 or 72 h after fertilization. During embryongenesis, treatment with 3.1 to 8.9 ppm or 3.3 to 11.9 ppm total oil hydrocarbons from light fuel oil and the two crude oils respectively, gave rise to alterations in embryonic activity, decreased heart rate, and premature or delayed hatching. Although many larvae hatched from eggs exposed to contaminated water (3.1 to 11.9 ppm total oil hydrocarbons), the majority of the (70 to 100%) were malformed or dead 1 day after hatching. Exposure of eggs to 5.4–5.8 ppm total oil hydrocarbons resulted in significantly (P<0.001) decreased lengths of the larvae. Increased temperature (from 9° to 14°C) aggravated the effects of the oils. The results are discussed in relation to the potential effects of oil spills and chronic oil pollution on fish eggs and larvae in the Baltic Sea.     

Vertical distribution of early and competent larvae of Concholepas concholepas in two systems of Chilean inland seas

The transport of marine invertebrate larvae is strongly influenced by their distribution in the water column, which could be affected by the biological features of the larvae and environmental variables. Larvae can modify their swimming behavior throughout their planktonic cycle, thereby changing the observed distributional patterns. This ability, coupled with oceanographic features, could induce landward or seaward transport. We studied the vertical distribution of C. concholepas larval stages in two differently stratified systems in Chilean inland seas; Refugio Channel (a strongly stratified channel, where previously has been described as a frontal system) and Guaitecas (a gently stratified system). Combinations of 12–24 h larval collection experiments were done simultaneously with fixed temperature and salinity profiles; meteorological data were also obtained. The results suggest that both salinity and day period influence the C. concholepas distributional patterns in the water column. Early veliger and competent larvae are concentrated in different parts of the water column, probably related to their transport capacities. The upper layer of the water column at the Refugio site, unlike the Guaitecas site, showed a stratified regime, which could affect larval density and larval length between the two sites. Finally, our results suggest that Refugio may be a sink habitat for C. concholepas.     

Development and growth of eggs and larvae of Mytilus edulis exposed to a linear dodecylbenzenesulphonate,LAS

The fertilization and growth of eggs and larvae of Mytilus edulis L. were studied under the influence of an important ingredient in washing powders, linear dodecylbenzenesulphonate (LAS). Observable effects occurred in concentrations as low as 0.05 ppm. The length attained by the larvae was affected even by concentrations of 0.1 ppm. The possible ecological consequences of these effects are discussed.     

Effects of contrasting modes of larval development on the genetic structures of populations of three species of prosobranch gastropods

 In south-eastern Australia, the prosobranch gastropods Morula marginalba (Blainville), Cominella lineolata (Lamarck) and Bedeva hanleyi (Angas) have similar fine-scale distributions, but appear to possess very different dispersal capabilities due to contrasting modes of larval development. M.marginalba produce planktonic larvae, whereas C. lineolata and B. hanleyi undergo direct development in benthic egg capsules and emerge as crawling juveniles. To test for possible effects of contrasting life histories on levels of genetic variation within and among populations, a survey was conducted of allozyme variation at six polymorphic loci in 8 to 9 local populations of each species. Collections of snails were made between June 1992 and November 1993. Sampling ranges spanned between 162 and 180 km of coast. Regardless of larval type, proportions of single-locus genotypes in each collection were consistent with the recruitment of offspring which had been generated through random mating. However, genotypic diversity was lower in those species that undergo direct development. Loci surveyed in C. lineolata and B. hanleyi were polymorphic (i.e. frequency of most common allele <95%) in fewer populations than those examined for M.␣marginalba (P <0.001) and, where polymorphisms occurred, also possessed significantly fewer alleles (P <0.001). Consequently, average levels of expected heterozygosity were greater in populations of M. marginalba than in those of either of the other species (P <0.001). Genetic variation among populations, expressed as the standardised variance in allele frequencies (F _ST ), was inversely related to expected larval dispersal capability. The nine collections of M. marginalba showed little overall differentiation (F _ST  = 0.017; P <0.001), reflecting the ability of planktonic larvae to interconnect local populations, and so limit divergence due to drift and natural selection. In contrast, there were high levels of allelic heterogeneity among the nine collections of C. lineolata (F _ST  = 0.523; P <0.001) and eight collections of B. hanleyi (F _ST  = 0.140; P <0.001). These data imply that for species which undergo direct development, local populations are effectively closed and evolve largely independent of one another. Received: 3 May 1996 / Accepted: 12 July 1996     

Effect of Pb,Cd and Cu on survival and development of Culex quinquefasciatus (Diptera: Culicidae)

ABSTRACT

The present research aimed to determine the lowest levels of three heavy metals (Pb, Cd and Cu) to which the larvae of Southern House Mosquito, Culex quinquefasciatus are susceptible in water. The study also aimed to investigate the effects of these heavy metals on the development of Cx. quinquefasciatus at concentrations set by Pakistan Environmental Protection Agency (Pak-EPA) as permissible levels for liquid industrial effluents. The 2nd instar larvae of Cx. quinquefasciatus were exposed to different concentrations of Pb, Cd and Cu and their effects on oviposition preference, egg hatching rate and larval development were studied. The LC₅₀ values of Pb, Cd and Cu were 12.6, 6.3 and 2.6?ppm, respectively. Gravid female mosquito adults deposited a significantly lower number of egg rafts in containers containing 0.50?ppm Pb or 1.0?ppm Cu in water. Each of the heavy metals in water resulted in significantly (p?<?0.05) lower egg hatching rate, prolonged time to pupation, lower pupation rate, prolonged time to adult emergence, lower adult emergence rate and higher female to male ratio. It is concluded that the 2nd instar larvae of Cx. quinquefasciatus are susceptible to Pak-EPA permissible levels of Pb, Cd and Cu in municipal and liquid industrial effluents.     

Effect of light intensity on activity and food-searching of larval herring,Clupea harengus: a laboratory study

Larvae of Clupea harengus were reared from spawning herring caught in March 1982 and 1983 in the Firth of Clyde, Scotland. An infra0red observation technique was used to record the behaviour of larval herring both in shallow dishes using a top view and in a tank 2 m deep using a side view. The amount of time larvae spent swimming, which was minimum in complete darkness, increased with increasing light intensity and as the larvae grew. Maximum swimming speeds of feeding larvae were recorded at light intensities between 10 and 100 lux. The presence of food organisms (Artemia sp., Brazilian strain) at light intensities below the feeding threshold (0.1 lux) caused an increase in the proportion of time spent active, but light intensities above the threshold had different effects, depending on developmental stage: larvae of 12 mm increased swimming speed, but 21 mm larvae decreased speed. In the 2 m deep tank in darkness, larvae displayed inactive periods wherein they sank head first, interspersed with periods of upward swimming. As light intensity increased, vertical swimming was replaced by horizontal swimming. These results are discussed with reference to food searching and vertical migration of larval herring in the sea.     

Effects of ultraviolet-B enhancement on marine trophic levels in a stratified coastal system

We examined the effects of enhanced UV-B radiation (relative to ambient) on marine trophic levels inhabiting a stratified coastal ecosystem, using living models (13 000 liter marine enclosures) of a temperate estuarine water column. The experiment was carried out in June and July 1994 on a plankton community drawn from lower Narragansett Bay, Rhode Island, USA. The effects of altered UV-B radiation (elevated 50% over ambient, tenfold DNA-weighted) on three trophic strata: the primary producers (photosynthetic algae), primary herbivores (copepods), and fish eggs and larvae (Anchoa mitchilli Cuvier and Valenciennes) were examined. The goal was to determine if UV-B–induced alterations at the base of the food chain had impacts on other elements of the trophic web. Phytoplankton abundance (P=0.02) and biomass (P=0.007) were significantly reduced in UV-B–enhanced treatments, above but not below the thermocline (2.25 m), during the second week of the study. Copepod nauplii were significantly less abundant in UV-B–enhanced mesocosms than in control treatments during the third and fourth weeks of the study (P=0.01). A portion of the impact on nauplii may be a result of alterations at the base of the food web. The greater mortality of nauplii in UV-B–enhanced systems did not translate to reduced abundance of copepodite (P=0.83) or adult (P=0.29) copepods. No significant effects were observed for microzooplankton (P=0.15). Neither the mortality rates nor the growth rates of larval anchovy were significantly affected by the experimental increase in UV-B (P>0.05). Despite the tenfold increase in biologically damaging UV-B, effects were not seen at higher trophic levels, most likely because of the rapid extinction of UV-B in the highly colored coastal water. Received: 13 November 1996 / Accepted: 18 July 1997     

Effect of copper and cadmium on carbon assimilation and uptake of metals by algae

Two species of blue green algae Spirulina platensis and Anacystis nidulans grown in artificial aqueous media were treated with Cu and Cd in concentrations of 0.01, 0.1, 1.0 and 10 ppm to study carbon assimilation and Chlorophyll (Chl) A content. The species were treated with concentrations of 0.1, 0.5, 1.0, 5.0 and 10.0 ppm to study the uptake of metals with exposure time. Carbon assimilation and Chl A content showed responses proportional to the concentration in the general form y = K + n ln C, where C is the concentration of metal in ppm, while in case of uptake the relation was y = KC”; (where C is the molar concentration x 10^‐6 of the metal). The n values in case of uptake was found to be < 1 indicating a non‐Langmuir type of sorption. The concentration factors of metals decreased with metal concentration in the medium.     

Effects of algal and larval densities on development and survival of asteroid larvae

Effects of larval and algal culture density and diet composition on development and survival of temperate asteroid larvae were studied in the laboratory at Santa Cruz, California, USA, during summer and fall of 1990. Larvae of Asterina miniata were reared at two densities, 0.5 or 1.0 ml^-1, and fed one or two species of cultured phytoflagellates — Dunaliella tertiolecta alone or mixed with Rhodomonas sp. — at three concentrations of 5x10², 5x10³, and 5x10⁴ total cells ml^-1. Algal concentration strongly influenced larval development; however, larval density also had a marked effect. Development progressed further with increasing algal concentration. Larval growth and differentiation were sometimes uncoupled; i.e., growth measures were directly related to food level, while differentiation indicators were less so. At the lowest food level, growth was negative and differentiation was arrested at early precompetent stages; these larvae never formed juvenile rudiments or brachiolar attachment structures. Development times of larvae given more food ranged from 26 to 50 d and depended directly on food availability. Development time to metamorphosis at the highest food concentration was similar for siblings fed D. tertiolecta alone or mixed with Rhodomonas sp. In contrast, when food level was an order of magnitude lower, larvae fed the algal mixture metamorphosed significantly earlier than larvae fed the unialgal diet. This suggests interactive effects of food quantity and food quality. Survival was little affected by larval or food density, except at the lowest ration. Feeding experiments in well-controlled laboratory conditions are useful to predict and compare the physiological or developmental scope of response of larvae to defined environmental factors; however, results from such studies should not be extrapolated to predict rates and processes of larval development in nature.     

Effects of fertilization and elevated CO<Subscript>2</Subscript> on larval food and butterfly nectar amino acid preference in<Emphasis Type="Italic"> Coenonympha pamphilus</Emphasis> L.

The effects of larval diet on the nutritional preferences of butterflies has rarely been examined. This study investigates whether alterations in the larval diet result in changes in adult preferences for nectar amino acids. Larvae of Coenonympha pamphilus were raised on fertilized or unfertilized Festuca rubra, grown under ambient (350 ppm) or elevated (750 ppm) atmospheric CO ₂environments. Fertilization led to marked increases in leaf nitrogen concentration. In plants grown under elevated CO ₂conditions, leaf water and nitrogen concentrations were significantly lower, and the C/N-ratio increased significantly. Fertilization of the host plant shortened the development time of C. pamphilus larvae, and pupal weight increased. In contrast, larvae of C. pamphilus developed significantly slower on F. rubra grown under elevated CO ₂, but adult emergence weight was not affected by CO ₂treatment of the plant. C. pamphilus females showed a clear preference for nectar mimics containing amino acids, whereas males, regardless of treatment, either preferred the nectar mimic void of amino acids or showed no preference for the different solutions. Female butterflies raised on fertilized plants showed a significant decline in their preference for nectar mimics containing amino acids. A slight, but not significant, trend towards increased nectar amino acid preference was found in females raised on plants grown under elevated CO ₂. We clearly demonstrate that alterations in larval host quality led to changes in butterfly nectar preferences. The ability of the butterfly to either rely less on nectar uptake or compensate for poor larval conditions represents a trade-off between larval and adult butterfly feeding.     

Behavioral responses of a larval crustacean to hydrostatic pressure: Rhithropanopeus harrisii (Brachyura: Xanthidae)

Responses of the four zoeal stages of the crab Rhithropanopeus harrisii (Gould) to step and continuous changes in hydrostatic pressure were analyzed with a video system. Crabs were collected from the Neuse River estuary (North Carolina, USA) from June to August, 1987. The lower thresholds for step increases and decreases in pressure were 3 and 8 to 10 mbar, respectively. There was little change in sensitivity with zoeal development. Tests of larval responses in a light field that simulated the underwater angular light distribution indicated positive phototaxis does not occur upon pressure changes. In darkness, rates of pressure increase at and above 0.175 mbar s^-1 induced high barokinesis and negative geotaxis in all but Stage IV zoeae, which had a threshold of 1.19 mbar s^-1. Since larval sinking and descent swimming speeds exceed these threshold rates, larvae can move rapidly enough to produce suprathreshold changes in pressure which evoke behavioral responses. Slow rates of pressure decrease induced passive sinking while rapid rates caused an active ascent. This ascent response upon a pressure decrease is unreported among crustaceans, and is hypothesized to function for avoidance of feeding and respiratory currents of benthic invertebrates. The descent response occurs in all zoeal stages, except IV, at rates of pressure decrease (0.4 to 0.71 mbar s^-1) that are within the range of ascent swimming speeds. These results support Sulkin's negative feedback depth regulation model. The absolute distances moved before corrective vertical responses to threshold rates of pressure change are initated delimit the depth regulatory window. In darkness, the asymmetry of the window would lead to an ascent. It is hypothesized that light is an additional component in depth regulation, and that the limits and symmetry of the depth regulatory window may be controlled by the level of light adaptation.     

A long-term study of the effects of water-soluble fractions of No. 2 fuel oil on the survival,development rate,and growth of the mud crab Rhithropanopeus harrisii

Mud crabs, Rhithropanopeus harrisii (Gould), were exposed continuously for 6 months after hatching to water-soluble fractions (WSF) of No. 2 fuel oil. Survival, growth and development rate were monitored during this time. The zoeal stages were the most sensitive to fuel oil. A 20% WSF (0.36 ppm total naphthalenes, 1.26 ppm total hydrocarbons) was acutely toxic to these stages. Of the zoeal stages, the first stage appeared to be the most sensitive. The combined duration of the 4 zoeal stages was significantly increased by increasing WSF exposure concentrations. The megalopa and crab stages were not particularly sensitive to continued petroleum hydrocarbon exposure, particularly when compared to zoeal stages. However, mean duration of the megalopa and first crab stages was significantly affected by oil exposure. Individuals which survived the highest exposure concentrations as larvae appeared to grow larger during the crab stages, so that at the end of 6 months comparably staged crabs were equal to or larger than both control crabs and those exposed to low WSF concentrations. Stage distributions at the end of 6 months showed no differences due to WSF exposure. Sex ratios, which could be determined at the end of 6 months, were approximately 1, indicating no sex-related differential sensitivity to WSF exposure, at least as larvae or juveniles. The data indicate that these crabs possess considerable ability to recover from the effects of chronic sublethal exposure to petroleum hydrocarbons. The most deleterious effects of oil pollution on this species may be due to its impact on larval recruitment into the adult population.     

Electrophoretic survey of genetic variation in Pecten maximus,Chlamys opercularis,C. varia and C. distorta from the Irish Sea

Food selection by young larvae of the gulf menhaden (Brevoortia patronus) was studied in the laboratory at Beaufort, North Carolina (USA) in 1982 and 1983; this species is especially interesting, since the larvae began feeding on phytoplankton as well as microzooplankton. When dinoflagellates (Prorocentrum micans), tintinnids (Favella sp.), and N1 nauplii of a copepod (Acartia tonsa) were presented to laboratory-reared, larval menhaden (3.9 to 4.2 mm notochord length), the fish larvae ate dinoflagellates and tintinnids, but not copepod nauplii. Larvae showed significant (P<0.001) selection for the tintinnids. Given the same mixture of food items, larger larvae (6.4 mm notochord length) ate copepod nauplii as well as the other food organisms. These feeding responses are consistent with larval feeding in the northern Gulf of Mexico, where gulf menhaden larvae between 3 and 5 mm in notochord length frequently ate large numbers of dinoflagellates (mostly P. micans and P. compressum) and tintinnids (mostly Favella sp.), but did not eat copepod nauplii. As larvae grew, copepod nauplii and other food organisms became important, while dinoflagellates and tintinnids became relatively less important in the diet. Since the tintinnids and nauplii used in the laboratory feeding experiments were similar in size as well as carbon and nitrogen contents, the feeding selectivity and dietary ontogeny that we observed were likely due to a combination of prey capturability and larval fish maturation and learning.Contribution No. 5575 of the Woods Hole Oceanographic Institution     

Effect of Ca2+ on survival and development of metamorphosing bonefish (Albula sp.) leptocephali

Bonefish (Albula sp.) larvae (leptocephali) from the Gulf of California complete metamorphosis in ˜10 d in natural seawater (35‰S; Ca²⁺ conc = 10.5 mM). The increase in ossification that occurs near the end of the non-feeding metamorphic period, in addition to the ability of larvae to complete metamorphosis in dilute seawater (8‰ S) prompted the present study, where the effects of varying the external calcium ion concentration, [Ca²⁺]_e, of artificial seawater (ASW) on the survival, development and internal (whole-body) calcium ion content, (Ca²⁺)_i, of unfed metamorphosing larvae were investigated. Early-metamorphosing larvae placed in␣ASW, where [Ca²⁺]_e = 10.1 mM, survived for up to 10 d and developed normally without exogenous nutrients. In shorter-term experiments (4 to 5 d), no differences in survival were found for larvae in ASW with [Ca²⁺]_e rang-ing from 1.5 to 10.1 mM. However, in Ca²⁺-free ASW, most larvae died within 27 h and no larvae survived more than 42 h; the median lethal time (LT₅₀), and its 95% confidence limits, were 14.5 (10.0 to 20.9) h. High mortality (81% after 20 h) also occurred in 1.0 mM Ca²⁺ ASW, but 2 of 16 larvae tested survived for 96 h. The 96 h median tolerance limit (TL_M), corrected for control mortality, was 1.2 mM Ca²⁺. In natural seawater, larval (Ca²⁺)_i remained relatively constant ( = 0.419 mg larva⁻¹)␣in early- and intermediate-metamorphosing larvae, and then increased to a mean value of 0.739 mg larva⁻¹ in advanced larvae, indicating that Ca²⁺ was␣taken up from the medium at this stage; the increase in (Ca²⁺)_i corresponded to the period of ossification of the vertebral column. Internal (whole-body) magnesium ion content (Mg²⁺)_i showed no significant change during metamorphosis ( = 0.089 mg larva⁻¹). No significant differences in (Ca²⁺)_i were found in advanced larvae in natural seawater and those in ASW, with [Ca²⁺]_e ranging from 2.0 to 10.1 mM. However, clearing and staining revealed that ossification of the vertebral column had not yet occurred in advanced larvae from 2.0 to 10.1 mM Ca²⁺ ASW. Also, low [Ca²⁺]_e (1.0 to 2.0 mM) usually produced deformed larvae that swam erratically, at times showing “whirling” behavior. Received: 21 August 1996 / Accepted: 26 August 1996