Larval development in the Antarctic nemertean Parborlasia corrugatus (Heteronemertea: Lineidae)

Embryonic and larval development were followed from fertilisation to settlement in the Antarctic heteronemertean Parborlasia corrugatus (McIntosh, 1876). The first cleavage occurred 10 to 15 h after fertilisation, and the second at 17 h. Larvae hatched at the gastrula stage, between 170 and 200 h post-fertilisation, and were 150 m in diameter. Early larval stages aggregated in dense groups near the surface of incubation vessels and were positively phototactic. Early pilidium larvae were recognisable 435 h post-fertilisation. They were 155×152 m in size, and possessed a complete apical tuft of cilia and a full marginal band of locomotory cilia. At this stage, the gust was visible through the body wall, and the mouth was open and was 40 m in diameter. Late pilidia, 222×193 m in size, were helmet-shaped. They had an apical tuft over 100 m long, and possessed a lobed marginal band of locomotory cilia. Pilidia were observed aggregating close to the bottom of incubation vessels 1200 to 1350 h (50 to 56 d) after fertilisation, and this was interpreted as settlement behaviour. At this stage, the apical tuft had been lost and they were highly contractile, being capable of compressing their bodies. However, neither developing juveniles within the larval envelope nor hatched juveniles were observed. Pilidia consumed the microalgae Tetraselmis suecica, Thalassiosira pseudonana and Isochrysis galbana. They also fed on particulate organic material < 1 m in size, as shown by the presence of material in the guts of larvae offered filtered extracts of algal cultures. There was some indication that larvae could use dissolved organic material, since pilidia held in seawater with organic material removed did not survive as long as those in filtered seawater or in filtered water with added amino acids. However, the only larvae to exhibit settlement behaviour in the feeding experiments were those offered Tetraselmis succica and Thalassiosira pseudonana, and these required a longer development time to reach this stage than pilidia in the standard cultures, where a mixed algal diet was offered.     

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.     

Early life histories in the bryozoan<Emphasis Type="Italic"> Schizobrachiella sanguinea</Emphasis>: a case study

Mediterranean populations of Schizobrachiella sanguinea seem to span a wide range of contrasting benthic habitats. We have studied adult, larval, and recruit populations of S. sanguinea from sublittoral (approximately 10 m depth) communities at Blanes (Spain, NW Mediterranean). At the study site, the species colonises semi-obscure caves and, to a lesser extent, communities of sciaphilic algae. Our first goal was to show the periods of brooding, larval release, and recruitment. Plankton was collected about twice a week between July 1998 and June 2000 over an artificial reef. A total of 303 larvae were collected in 23 of the 102 days of sampling. At the study site the species shows a single, annual larval release period, which occurs from late March to June–July every year. Recruits were found from late May to late August 1999 and in late May 2000. A recruitment peak was observed in July 1999 and again in late May 2000. The presence of brooding adults corresponded to the periods of larval supply and recruitment. Our second goal was to investigate the causes of the variable distribution of the species along several communities in the north-west Mediterranean Sea. We describe adult distribution and the dynamics of larval supply, recruitment of early (15 days from settlement) and late (4 months from settlement) recruits. Patterns of larval presence and early recruit distribution along the communities on the reef did not reflect the adult distributions, whereas this was closely matched by the distribution of late recruits. Post-recruitment mortality strongly affected recruits of the PA (photophilic algae) and SA (sciaphilic algae) communities, whereas it was very low in the communities of SOC (semi-obscure caves). Mortality of recruits was density independent and colony survivorship in the SA communities, where recruitment was the highest, appeared strongly affected by competition with brown algae. Like the majority of cheilostomate bryozoans, S. sanguinea releases a coronate larva capable of active swimming and positive reaction to light. Such swimming abilities may allow larvae to actively select the substrate on which to settle and to avoid unsuitable substrates. It seems that larval behaviour does not explain the preference of S. sanguinea for sciaphilic habitats. High post-recruitment mortality due to various factors, especially competition with fast-growing algae, seems particularly relevant in determining the adult distributions observed.Communicated by S.A. Poulet, Roscoff     

Développement larvaire de deux Tellinacea,Scrobicularia plana (Semelidae) et Donax vittatus (Donacidae)

Scrobicularia plana Da Costa and Donax vittatus L. were reared in the laboratory through settlement. Fertilizable eggs were obtained by perfusing the ovary with 5% 0.1 M ammonium hydroxyde in sea water. S. plana eggs have a thick chorion, inside which the early larval stages develop; they hatch as straight-hinge larvae more than 60 h after fertilization. This brood protection is considered to be an adaptation to osmotic pressure changes and pollution in the environment. D. vittatus eggs have a very thin chorion and are unprotected. Further development is planktotrophic and very similar for the two species. Under laboratory conditions, the pediveliger stage is attained 3 weeks after fertilization and settlement occurs 1 week thereafter. S. plana spat stop growing until a suitable substratum is available. Meanwhile they undergo a byssus drifting important for postlarval dispersion. Post larvae resume growth as soon as a small quantity of fine sand is added to the rearing jar. The exhalant siphon is developed first, when the post larvae reach a length of 600 m; the inhalant siphon is formed later, at a length of approximately 900 m. Evolution from the larval hinge to the juvenile hinge stage occurs sooner in S. plana than in D. vittatus. Comparison of laboratory larval development with field development indicates that spawning occurs in June and August for S. plana in North Wales (UK).     

Prey selection by larval weakfish (Cynoscion regalis): the effects of prey size,speed, and abundance

We examined feeding by larval weakfish, Cynoscion regalis (Bloch and Schneider), in laboratory experiments conducted during the 1991 spawning season. under natural conditions weakfish larval development is ca. 3 wk, and we ran separate experiments with larvae of five different ages (5, 8, 11, 14, and 17 d post-hatching). We used two different size classes of rotifers (Brachionus plicatilis) and brine shrimp nauplii (Artemia sp.) as prey organisms. Contrary to results of previous research, weakfish larvae did not select prey based on size alone. When prey abundance was above 100 itemsl^-1 weakfish, larvae always chose large rotifers (length = 216 m) over small rotifers (length = 160 m). At 11 d post-hatching, larvae switched their diet from large rotifers to small brine shrimp nauplii (length = 449 m); however, when fed small rotifers and small brine shrimp nauplii the change in diet occurred at 14 d post-hatching. This pattern of selectivity was maintained in each larval age class. Early-stage larvae (5 and 8 d post-hatching) did not feed selectively when prey abundance was less than 100 itemsl^-1. Late-stage larvae (17 d post-hatching) fed selectively at abundances ranging from 10 to 10000 items^-1. Lwimming speeds of prey items, which ranged from 1 to 6 mms^-1, had no consistent effect on prey selection. These results suggest that weakfish larvae are able to feed selectively, that selectivity changes as larvae age, and that selectivity is also influenced by prey abundance.     

Transmission distance of chemical cues from coral habitats: implications for marine larval settlement in context of reef degradation

The present study (Ishigaki Island, Japan) explored the distance of transmission of chemical cues emitted by live versus dead coral reefs (Exp. 1: High performance liquid chromatography (HPLC) analyses with water sampling station at 0, 1, and 2 km away from the reef) and the potential attraction of these chemical cues by larval fish, crustaceans, and cephalopods (Exp. 2: choice flume experiment conducted on 54 Chromis viridis larvae, 52 Palaemonidae sp larvae, and 16 Sepia latimanus larvae). In the experiment 1, HPLC analyses highlighted that the live coral reef (and not the dead coral reef) produced different and distinct molecules, and some of these molecules could be transported to a distance of at least 2 km from the reef with a reduction of concentration by 14–17-fold. In the experiment 2, C. viridis, Palaemonidae sp, and S. latimanus larvae were significantly attracted by chemical cues from a live coral reef (sampling station: 0 km), but not from a dead coral reef. However, only C. viridis larvae detected the chemical cues until 1 km away from the live coral reef. Overall, our study showed that chemical cues emitted by a live coral reef were transported farthest away in the ocean (at least 2 km) compared to those from a dead coral reef and that fish larvae could detect these cues until 1 km. These results support the assumption of a larval settlement ineffective in degraded coral reefs, which will assist conservationists and reef managers concerned with maintaining biodiversity on reefs that are becoming increasingly degraded.     

Feeding behaviour and prey size selection of gilthead seabream,Sparus aurata,larvae fed on inert and live food

Prey selection shortly after the onset of feeding by laboratory-reared gilthead seabream, Sparus aurata L., larvae was studied using larvae fed on two types of microcapsule (hard- and soft-walled) having diameters ranging from 25 to 300 m. Preferences between inert food and live prey (rotifers and Artemia sp. nauplii) were also studied. Seabream larvae were able to ingest inert food from first feeding. Larvae of all size classes ingested hard microcapsules with diameters in the range 25 to 250 m. However, larvae with a total length (TL) below 4 mm preferentially selected particles 25 to 50 m in diameter, larvae of TL 4 and 5 mm preferred particles 51 to 100 m in diameter, while larvae above 5 mm TL preferred particles 101 to 150 m in diameter. With soft microcapsules, larvae always preferred particles larger than in the previous case, and above 4.5 mm TL they preferentially selected particles 201 to 250 m in diameter. In addition, the gradual increase of preferred diameters with increasing TL was more pronounced when larvae were increasing TL was more pronounced when larvae were fed on soft particles. Mean values for prey width/mouth width ratios were approximately 0.24 and 0.30 when larvae were fed on hard-walled and soft-walled microcapsules, respectively, irrespective of the absolute value of larval length. When a mixed diet of live and inert food items was offered, live prey were always preferentially selected, even if the prey width/mouth width ratio was apparently not favourable. Only a physical constraint such as excessive prey width could counter this preference for living prey vs inert microcapsules. These results contribute to our knowledge in larval feeding behaviour, especially in the presence of inert food, and represent a fundamental step in developing prepared food for marine fish larvae.     

Activity and metabolism of larval Atlantic cod (Gadus morhua) from Scotian Shelf and Newfoundland source populations

Patterns of activity and metabolism were investigated in larval Atlantic cod (Gadus morhua L.) between December 1991 and July 1992: (1) throughout larval development; (2) between two genetically discrete populations (Scotian Shelf and Newfoundland) and (3) as a function of two different culture temperatures. During the yolk-sac stage (0 to 5 d post-hatch), changes in swimming speed were not related to mass-specific metabolic rates; no portion of the mass-specific oxygen consumption could be explained by changes in activity. In the mixed feeding stage (6 to 14 d posthatch), there was a tendency for oxygen consumption to be related to changes in swimming speed. In the exogenous feeding stage (>14 d post-hatch), oxygen consumption significantly increased with swimming speed. These ontogenetic patterns of activity and metabolism were the same for larvae from the Scotian Shelf and Newfoundland populations. However, over the entire larval life and among ontogenetic stages, the metabolic cost of activity (mass-specific O₂ consumption/swimming speed) of Scotian Shelf larvae was significantly higher than that of Newfoundland larvae. When cod larvae, that had developed at 5°C, were acutely exposed to 10°C, Scotian Shelf larvae had a higher intrinsic cost of activity than Newfoundland larvae, over the entire larval life. During the exogenous feeding stage, the mean metabolic cost of activity for Newfoundland larvae raised at 10°C and tested at 10°C was significantly higher and more variable than that of larvae raised at lower temperatures. However, the metabolic cost of activity of larvae raised and tested at 10°C was not significantly different between source populations. Together these findings suggest that differences in swimming energetics reflect changing energy requirements for activity among ontogenetic stages, and reflect adaptation to regional environments among genetically discrete populations.     

Zooplankton standing stock and community size structure within the epipelagic zone: a comparison between the central Red Sea and the Gulf of Aden

Standing stock and size composition of the zooplankton comunity (>100 m) were studied in four depth strata of the upper 200 m of the water column during a Meteor cruise to the central Red Sea and Gulf of Aden in spring 1987. The central Red Sea was divided into a northern area of higher salinity and a less saline southern part. Both areas exhibited significant differences in zooplankton abundance and standing stock. The latter increased by the ratio 1:2:3 from the northern central Red Sea to its southern part and further south to the Gulf of Aden. For size structure analysis samples were fractionated into three size classes (100 to 300, 300 to 500, 500 to 5000 m). In the central Red Sea the smallest size was dominant whereas in the Gulf of Aden the largest size fraction played a greater relative role than in the central Red Sea. This shift in size structure of the zooplankton community from the Red Sea to the Gulf of Aden is apparently primarily related to ecosystem difference between both areas, leading to a change in species composition. In addition, size reduction of individual species common to both seas may be of some significance in the extreme environment of the Red Sea.     

Egg laying and embryonic-larval development in the snail<Emphasis Type="Italic"> Thais (Stramonita) chocolata</Emphasis> (Duclos, 1832) with observations on its evolutionary relationships within the Muricidae

Oviposition and embryonic-larval development are described for the muricacean snail Thais (Stramonita) chocolata from the Southeast Pacific coast. As with numerous other muricacean snails, this species engages in communal egg laying, with females depositing egg capsules in clusters on subtidal rocks. Each cluster of capsules contains 100–150 pedunculate, ampulliform egg capsules, with each capsule containing an average of 2,600 small (130 m) eggs. Intracapsular development was followed using light and scanning electron microscopy to describe the successive embryonic stages of the species. Free-swimming veliger larvae of about 225 m length were released from capsules after 49 days incubation at 13.6°C. The planktotrophic larvae were cultured in seawater aquaria by feeding with pure cultures of phytoplankton, recording growth and form of the larvae. Larvae reached competence after 4 months at 22°C, at 1,450–1,740 m in size, and a few larvae were observed through metamorphosis and early definitive growth. The embryonic-larval development of T. chocolata coincides with the general characteristics of the ontogeny observed in other Thais species as well as of other genera of the Rapaninae such as Concholepas. This lent support to grouping these genera into a single clade. The lack of knowledge of the development of free larvae of Thais spp. means that we do not know whether these similarities also include an extensive larval phase as generally characteristic of other members of the clade. The mode of development may be useful in characterizing some clades of this family. Thus for example, the transference of some Thais to the genus Nucella (Subfamily Ocenebrinae) is supported by differences in the mode of embryonic development, which differentiates these subfamilies. Paleobiological data reported for Neogastropoda allow postulation of primitiveness in planktotrophic larval development compared to more recent developmental strategies such as direct development of different types, which characterize various clades of this family.Communicated by O. Kinne, Oldendorf/Luhe     

Endogenous swimming rhythms in estuarine crab megalopae: implications for flood-tide transport

Up-estuary migration of crab larvae to adult habitats is thought to be accomplished by selective tidal transport in which late-stage larvae enter the water column on flood tides and remain on or near the bottom on ebb tides. This study measured endogenous rhythms in swimming by the last larval stage (megalopa) of blue crabs Callinectes sapidus and fiddler crabs Uca spp. Previous field studies found that megalopae of both species were only abundant in the estuarine water column on nocturnal rising tides. Megalopae were collected from the Newport River Estuary, North Carolina (34°41N; 76°40W) during August–September 1992 and swimming activity was recorded for 4.5 to 7 d under constant conditions with a video system. Rhythms exhibited by both genera in the laboratory were not identical to those recorded in the field. Uca spp. displayed a circatidal rhythm, with maximum swimming occurring near the time of high tide in the field. Rhythm amplitude increased when crushed oyster shells were present, which suggested that megalopae bury or cling to the substrate during quiescent periods. In contrast, C. sapidus had a circadian rhythm in which maximum swimming coincided with the day phase in the field. In most trials, the activity of blue crab megalopae was unrelated to the expected tidal cycle. It was concluded that a tidal rhythm in swimming was the behavioral basis of flood-tide transport for fiddler crab larvae. The endogenous rhythm in blue crabs does not participate in transport, which probably results from behavioral responses to environmental cues associated with flood tide.     

Spawning,larval development and settlement behaviour of Gibbula cineraria (Gastropoda: Prosobranchia) with a reappraisal of torsion in gastropods

Cultures of developing larvae of Gibbula cineraria (L.) were obtained from adults spawning in the laboratory, and these cultures were reared to settlement of the larvae at 9 days. Dispersal of the outer jelly coat of the egg appeared to be delayed in the presence of spermatozoa. Early cleavage followed the typical spiral pattern, and gastrulation was by epiboly. The trochophore larvae hatched at about 28 h, before shell-formation began. The first 90° of torsion was completed between 48 and 56 h. The second part of torsion was completed within 4 days, and preliminary attempts to retract into the shell were made at 76h. After 96 h, although torsion was complete, the larvae were still unable to retract fully into the shell. The larvae remained in a stage of swimming-attempted creeping until the 4th or 5th day after torsion was complete, but were unable to pull the shell upright until the end of this time. Throughout this stage, the velum was reduced and was finally shed. The animals all died after metamorphosis, which occurred at about 9 days, when the larval shell comprised 1 1/2 whorls, with a breadth of 250 to 300m. Settlement appeared to be passive, i.e., governed by wave action and tides. In the light of these observations, theories concerning torsion are reappraised. The advantage to the larva of torsion is not clear, as the larvae are not able to retract into the shell until after metamorphosis, yet it is clear from previous authors' observations that the first part of torsion in most gastropods involves only larval structures. This makes it difficult to suggest that torsion is entirely of advantage to the adult snails. It is thus proposed that the two components of torsion may have evolved independently. The first, relatively rapid component of torsion, resulting in 90° of displacement of the mantle cavity in relation to the visceral mass, may be of advantage to the swimming larva. The final, slower component of torsion may be of advantage to the newly-metamorphosed benthic snail, and is the only component of torsion found in those gastropods which have no free-swimming larva.     

Early recognition of sediment during settlement of<Emphasis Type="Italic"> Eupolymnia nebulosa</Emphasis> (Polychaeta: Terebellidae) larvae

Analysis of the displacements and activity of larval stages of the terebellid polychaete Eupolymnia nebulosa were carried out on different sediment types collected in the Bay of Banyuls-sur-Mer (western Mediterranean). Results indicated sediment recognition by the larvae. Time devoted to exploration and settlement trials were significantly longer when sediments of similar granulometry had been previously reworked by adults. There were increased responses of settling larvae if the tentacles of adults of the same species had previously been in contact with the tested sediment. The early settlement activity was stronger at the sediment boundaries and increased in the coarse sediment. Our results suggest that larvae are able to recognize biologically bioturbed sediments and modify their behaviour accordingly. There is a larval settlement before the competent stage. Possible consequences of early settlement and the limitation of dispersal in Eupolymnia nebulosa larvae are discussed together with the results of automated measurements of early larval swimming.Communicated by S.A. Poulet, Roscoff     

Ecological investigations of the late-stage phyllosoma and puerulus larvae of the western rock lobster Panulirus longipes cygnus

The distribution and abundance of the late-stage phyllosoma larvae of Panulirus longipes cygnus George and the distribution and densities of the final larval stage, the puerulus, both in the plankton and at settlement along the coast, were investigated. A total of 3,617 late-stage phyllosoma (Stages VI to IX) and 301 puerulus larvae were caught at 187 plankton stations during the July to November periods 1974, 1975 and 1976 off the west coast of Australia between 29°00 to 32°30S and 113°30 to 115°00E. The depth range sampled was 0 to 35 m on the continental shelf and 0 to 90 m off the shelf. During onshore/offshore cruises with similar sampling effort on and off the shelf, 1,169 late-stage phyllosoma larvae were taken, of which only 9 were caught on the shelf, and these near the outer edge. A series of cruises sampling two areas beyond the shelf near 29°30 and 32°00S yielded 2448 late-stage phyllosoma, with greater densities of larvae in the northern location. The settlement of puerulus-stage larvae along the coast in the same geographical range was also greater in the north than in the south. The data from the onshore/offshore cruises showed a definite effect of moon phase on numbers of puerulus larvae caught on the shelf, with higher catches near new moon. The low numbers of puerulus larvae (usually 0, 1 or 2 individuals) caught at all stations showed that the puerulus stage is sparsely distributed in the plankton. Fewer puerulus larvae were present at the surface than at lower depths, but it was not possible to determine a depth preference for the puerulus between 10 m and the lowest depths sampled because of the low catch numbers. No relationships were found between puerulus larvae density and surface-water temperature, salinity, or plankton biomass at each station. Data on the larval distributions indicate that, near the end of their planktonic existence, the majority of the late-stage phyllosoma larvae of P. longipes cygnus are not carried onto the shelf, where mixing of oceanic and continental shelf waters occurs only on the outer third, but are transported southward by oceanic circulation beyond the shelf. The puerulus moults from the last phyllosoma stage beyond the shelf and completes the larval cycle by swimming across the shelf and settling in the shallow reef areas.     

Translocation of fixed carbon from symbiotic bacteria to host tissues in the gutless bivalve Solemya reidi

Naturally-occurring lipophilic inducers of larval settlement and metamorphosis wer isolated and identified for Phragmatopoma californica, a gregarious tube worm from southern California. Organic solvent extraction of the sand/organic cement matrix of tubes diminished the inducing capacity of the tube matrix. The inducing capacity was restricted to a single, highly active, HPLC-purified fraction of the organic solvent extract. Chemical analysis of this fraction revealed a mixture of free fatty acids (FFAs), dominated by eicosapentaenoic acid (20:5, 20%), palmitic acid (16:0, 14%) and palmitoleic acid (16:1, 12%). In assays of the nine FFAs that each contributed 3% or more to the active fraction, only 16:1, 18:2, 20:4 and 20:5 induced larval settlement and metamorphosis, while the others were ineffective. The larval response was contact-dependent, highly specific, and concentration-dependent, with a significant response to 16:1 and 20:4 at as low as 10 g FFA spread onto 1 g of sand (surface area 36 cm²). Active FFAs were extracted at approximately 14 g g^-1 sand from the tube matrix, although the levels encountered by larvae in nature are believed to be higher.     

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     

Effects of copper and cadmium on growth,swimming and predator avoidance in Eurytemora affinis (Copepoda)

In this study we demonstrate the sensitivity of swimming behavior and predator-escape responses of nauplii of the estuarine copepod Eurytemora affinis to sublethal doses of Cu and Cd. Behavior was generally altered at metal doses below those affecting growth rates or survival of the copepods. Swimming velocities of Cu-dosed nauplii were different from controls at all concentrations of Cu tested (10–50 g l^-1 total Cu) after 24- to 48-h exposure, whereas development rate of nauplii was significantly reduced only after 96 h at 25 g l^-1. The 96 h LC50 for Cu was approximately 30 g l^-1 Cu. Naupliar swimming velocity was also affected by Cd. Swimming speeds were reduced after 24 h at 130 g l^-1, and development was slowed after 48 h at 116 g Cd l^-1. The 96-h LC50 was >120 g l^-1. Little is known of the adaptive role of specific motile behaviors in the success of larval copepods. We investigated the relationship of swimming speed to predator — prey interactions of the nauplii using both real and simulated predators. Nauplii exposed to Cu for 24 h were observed to be generally hyperactive, a condition which could increase their encounter frequency with predators. Reduced numbers of escape responses of nauplii to a simulated predator, another indication of increased vulnerability to predation, were observed only after 48-h exposure to Cu. Nevertheless, feeding rates of non-dosed larval striped bass on dosed nauplii (24 h at 25 g Cu l^-1) were significantly higher than on control nauplii. Feeding rates of larval mysid shrimp, however, were not higher on similarly dosed nauplii; 24 h exposure of nauplii to >30 g Cu l^-1 did result in increased predation by mysids.Contribution No. 272 of the US EPA Environmental Research Laboratory, Narragansett, Rhode Island 02882, USA     

Sublethal effects of zinc and municipal effluents on larvae of the red abalone Haliotis rufescens

Experiments were conducted to develop a sensitive sublethal toxicity test protocol to determine the toxicity of municipal wastewater effluents to larvae of the red abalone Haliotis rufescens. In multiple tests, fertilized abalone embryos were exposed for 48 h to dilutions of a reference toxicant, zinc sulfate, and to dilutions of primary-and secondary-treated effluents. The resulting veliger larvae were examined microscopically for larval shell abnormalities. In a longer flowthrough experiment, abalone were exposed for the entire larval phase, from the two-cell stage through metamorphosis, to compare zinc effects on metamorphosis with zinc effects on short-term larval shell development. Dissolved oxygen, pH, salinity and temperature were measured daily in test solutions, and zinc concentrations were verified by chemical analysis. No observed effect concentrations (NOECs) for zinc were 39±2.1 g l^-1 in three 48 h exposures, and 19 g l^-1 for the 9 d exposure through metamorphosis. Median effect concentrations (EC₅₀s) were 68±6.9 g l^-1 in 48 h tests and 50 g l^-1 in the 9 d test. Abalone larvae were affected at lower concentrations of primary than of secondary effluent.     

Effect of hatching time on larval mortality and survival of the prawn Macrobrachium idae

The prawn Macrobrachium idae Heller carries 40 to 160 eggs depending upon the body size of the mother animal. After incubation, 63, 35 and 2% eggs are hatched on the first, second and third hatching nights, respectively. Mean dry weight of a single larva relased on the first night is 420 g (equivalent to 2.86 cal). Larvae released on the second or third hatching nights weighed 380 g (=2.56 cal) or 308 g (=2.02 cal). The total reserve yolk-energy available in a larva hatched on the first night is 1 cal. There is a definite shift from protein to fat metabolism as hatching is delayed. Oxygen consumption of developing eggs awaiting release on the first or second hatching night is 1.9 l/mg dry weight/h. In comparison to larvae released on the first night, those released on the second night exhibit 2% increase in total body-length and 2.4 or 14.3% decrease in the lengths of the 6th abdominal segment or the orbit. Larvae released on the first, second or third hatching nights, on exposure to starvation stress, survive for 3.8, 2.3 or 1.5 days. The swimming speed of larvae released on the first or second night is 1.0 or 0.6 cm/sec. Larvae released on the subsequent hatching nights committed nearly 2 times greater number of mistakes per cm distance in the horizontal plane. Since hatching of all developing eggs simultaneously on the first hatching night is possible by means of artificial technique(s), it is possible to improve survival rate of decapod larvae.     

Formation of initial daily increments in sagittal otoliths of reared and wild Sardina pilchardus yolk-sac larvae

Daily deposition of growth increments in sagittal otoliths of reared and wild Sardina pilchardus (Walbaum, 1792) larvae from hatching to complete yolk-sac absorption is reported. Increments laid down prior to hatching were observed. A distinct growth increment, 1 m wide and located 5 to 7 m from the focus is laid down on the day of hatching. Subsequently, narrower increments (<1 m) are laid down daily throughout the yolk-sac stage. Measurements were made on the maximum radius of the sagittal otoliths of two groups of larvae: 83 larvae reared from fertilized eggs sampled from Ría de Muros (NW Spain), and 239 wild individuals sampled along the northern coasts of the Iberian peninsula. Counting of daily ring increments can provide a direct and valid estimation of larval age without the necessity for later correction.