Analysis of lipid components for determining the condition of anchovy larvae,Engraulis mordax

It has been presumed that intertidal spawning by Limulus polyphemus minimizes the loss of egges to subtidal predators; however, this strategy involves considerable risks. Massive beach strandings of adults accompany seasonal spawning migrations of crabs along Cape May in Delaware Bay, (USA). At least 190000 horseshoe crabs, approximating 10% of the adult population, died from beach stranding along the New Jersey shore of Delaware Bay during the 1986 (May to June) spawning season. Abnormalities of the telson (which is used in righting behavior) were significantly more common among stranded crabs than among individuals actively spawning on the intertidal beach. The number of stranded crabs per day was not correlated with tidal height or environmental variables (wind speed, wave height) which characterized the conditions at spawning. A complex suite of factors, including the size of the available spawning population, tidal and weather conditions, and beach slope, influence the number stranded during the breeding season. Horseshoe crab stranding results in a large loss of gravid females from the population, and may represent a major input of organic matter to intertidal sandy beaches in certain regions of Delaware Bay.     

Nutritional condition and growth rate of anchovy larvae (Engraulis mordax) in the California Current: two contrasting years

A large number of northern anchovy larvae, Engraulis mordax, were collected in the Southern California Bight on two CalCOFI cruises during spring 1987, and the amounts of triacylglycerol, cholesterol and polar lipids were measured in each individual larva using the latroscan TLC/FID (thin-layer chromatography/flame ionization detection) system. The 1987 larvae were compared to larvae taken in the Bight during the previous year. Canonical correlation analyses revealed a significant relationship between the larval lipid components and physical/biological data collected at each occupied station during the late 1987 spring cruise, similar to that recorded in 1986. The short-term nutritional condition of the copepod Calanus pacificus was the most important station variable in both cruises. Their triacylglycerol:cholesterol ratio indicated that few of the 1987 larvae had been starving, but their very low contents of length-specific cholesterol and polar lipid indicated that they had been growing very slowly. Otolith readings on defatted larvae confirmed that the 1987 larvae had been growing significantly more slowly than the 1986 larvae. Furthermore, the data suggested that in the 1986 population the faster growing larvae were also those with the highest lipid content. This situation was reversed for the 1987 larvae, when the faster growing larvae in the population seemed to achieve their faster growth only at the expense of a lowered lipid content. The abundance of anchovy larvae was much higher in spring 1987 than in spring 1986 but, despite this, the apparent recruitment (number of Age 0 fish in July of each year) was not higher in 1987.     

Tissue distribution and neurotoxic effects of domoic acid in a prominent vector species, the northern anchovy Engraulis mordax

The planktivorous northern anchovy is a prominent vector of the phycotoxin domoic acid (DA) to organisms at higher trophic levels, including fish-eating seabirds and mammals. Although there are abundant data reporting DA-induced excitotoxic symptoms in higher vertebrates, to date there has been no reported evidence of neurotoxic effects in lower vertebrate vectors such as fish. To explain this apparent lack of toxicity, it has been suggested that DA may not reach the brain in anchovies and/or that fish are not as sensitive neurologically to DA. In the present study, intracoelomic (IC) injection of DA, at doses ranging from 1 to 14 μg DA g⁻¹ total fish weight, resulted in severe neurotoxic symptoms such as spinning, disorientation, inability to school, and mortality, indicating that anchovies are neurologically susceptible and that DA crosses the blood–brain barrier in fish. An ED₅₀ of 3.2 μg DA g⁻¹ total body weight was determined via IC injection of DA in 83 anchovies. Comparable intraperitoneal injection studies with mice, rats, and monkeys report similar DA-induced neurotoxic symptoms at doses near 3.2 μg DA g⁻¹, suggesting a similar neurologic sensitivity and mechanism of toxicity between anchovies and mammals. DA tissue distribution measurements from freshly collected field-exposed anchovies and orally gavaged anchovies indicate that DA uptake from the gastrointestinal tract does occur. Levels as high as 1,175 μg DA g⁻¹ were measured in anchovy viscera, while muscle and brain tissue DA levels were 3 orders of magnitude lower, indicating low but measurable DA uptake. Further evidence is needed to confirm that uptake is sufficient during field events to induce symptoms in anchovies. Our work provides the first reported evidence of neurotoxic symptoms in fish and suggests that anchovies may be affected by DA during toxic diatom blooms. If sufficient uptake occurs, DA-induced neurotoxic symptoms and mortality may make fish easier prey targets, thereby selecting for the highest toxin levels transferred, as well as providing a possible pathway for the transfer of DA to benthic communities. Received: 19 May 2000 / Accepted: 29 November 2000     

Early growth and mortality of the Caribbean king crab Mithrax spinosissimus reared in the laboratory

The berried females of the Caribbean king crab Mithrax spinosissimus (Lamarck) used in this study were collected from canals on Big Pine Key, Sugarloaf Key and Lower Matecumbe Key (south Florida, USA) on 9 August, 8 October and 15 November 1986. Viable spawns hatched as first zoeae and molted to second zoeae within ca. 10 to 12 h. Most of the larvae reached the megalopa stage 1 d later, and molted to first crab 4 to 8 d after hatching (water temperature: 27.2° to 28.8°C). Low water temperature and/or early lack of food had a negative effect not only on stage duration, but also on the size of the early crab stages. Successful molt to first crabs occurred, however, in the absence of food. The growth rate (carapace length) between molts in early crab stages varied between ca. 20 and 30%. When provided with good water exchange, stocking density could be very high (>22 500 individuals m^-2), with no increase in mortality. The highest mortality rate was recorded when the larvae molted to first crab, and the highest rates of survival were always recorded when feeding was not initiated until after 5 to 8 d after hatching. No cannibalism was observed among larvae, and cannibalism was low in early crab stages. The study indicates that to achieve viable hatches and high larval survival in rearing M. spinosissimus, a continuous and adequate supply of high-quality seawater is a prerequisite both in larviculture and in maintaining brooding females.Contribution No. 93, Department of Oceanography and Ocean Engineering, Florida Institute of Technology     

Yolk resorption,onset of feeding and survival potential of larvae of three tropical marine fish species reared in the hatchery

This paper provides basic early life-history information on milkfish (Chanos chanos), seabass (Lates calcarifer) and rabbitfish (Siganus guttatus) which may explain in part the observed differences in their survival performance in the hatchery. Egg size, larval size, amount of yolk and oil reserves and mouth size are all greater in milkfish than in seabass, and greater in the latter than in rabbitfish. During the first 24 h after hatching, rabbitfish larvae grow much faster than milkfish and seabass larvae at similar ambient temperatures (range 26°–30°C, mean about 28°C). The eyes become fully pigmented and the mouths open earlier in seabass and rabbitfish (32–36 h from hatching) than in milkfish (54 h). Seabass larvae learn to feed the earliest. Yolk is completely resorbed at 120 h from hatching in milkfish, and yolk plus oil at 120 h in seabass and 72 h in rabbitfish at 26° to 30°C. Milkfish and seabass larvae have more time than rabbitfish to initiate external feeding before the endogenous reserves are completely resorbed. Delayed feeding experiments showed that 50% of unfed milkfish larvae die at 78 h and all die at 150 h from hatching. Milkfish larvae fed within 54 to 78 h after hatching had improved survival times: 50% mortality occurred at 96 to 120 h, and 10 to 13% survived beyond 150 h. Unfed seabass larvae all died at 144 h, while 6 to 13% of those fed within 32 to 56 h after hatching survived beyond 144 h and well into the subsequent weeks. Unfed rabbitfish larvae all died at 88 h, while 7 to 12% of those fed within 32 to 56 h after hatching survived beyond 88 h. A delay in initial feeding of more than 24 h after eye pigmentation and opening of the mouth may be fatal for all three species.Contribution No. 167 from the SEAFDEC Aquaculture Department     

Influence of limited starvation periods on growth and elemental composition (C,N,H) of Carcinus maenas (Decapoda: Portunidae) larvae reared in the laboratory

Zoea-1 larvae of Carcinus maenas L. (Decapoda: Brachyura: Portunidae) were from Helgoland in March 1984 and reared in the laboratory at 18°C through ecdysis. Dry weight (DW) and elemental composition of carbon (C), nitrogen (N), and hydrogen (H) were analyzed in newly hatched zoea-1, after different initial starvation periods, and in newly moulted zoea-2. Continually starved zoea-1 lost biomass and energy steadily, and logarithmic functions show best fit of empirical and predicted data. Biomass and energy equivalents of newly moulted zoea-2 are significantly correlated with starvation periods in the zoea-1, showing lower values with longer initial starvation. After about 25 to 34% individual biomass and energy losses, larvae exceed the point-of-noreturn (PNR), and do not recover or moult to the zoea-2, even if re-fed. When starvation ceases before the PNR, larvae moult to the zoea-2, and develop with lower average growth rates (AGR) after prolonged periods of initial food deprivation. The later larvae were re-fed, the less absolute amounts of DW, C, H, and individual energy, but more DW-related energy equivalents and N accumulated during subsequent feeding towards ecdysis. It is suggested that lipid, rather than protein, is the main source of energy controlling the maintenance of larval moult cycles. After lipid reserves are depleted, zoea-1 larvae live on body protein, and lose the ability to absorb and restore sufficient lipid if re-fed later than the PNR.Contribution to research project An-145/1-1 granted by the Deutsche Forschungsgemeinschaft (DFG)     

Growth and elemental composition (C,N, H) in Inachus dorsettensis (Decapoda: Majidae) larvae reared in the laboratory

Larvae of the spider crab Inachus dorsettensis were reared in the laboratory at constant 12 °C. Development lasted 8 to 10 d in the Zoea I, 10 to 12 d in the Zoea II and 14 to 20 d in the megalopa stage. During this time, larval growth was measured in samples taken every 2 to 4 d as dry wt (W), carbon (C), nitrogen (N), and hydrogen (H); energy content (E) was calculated from C. Biomass and energy (per individual) increased in each larval stage as a parabola-shaped function of age, which could be fitted by a power equation. C, H, and E show a higher percentage gain (relative to the initial values at hatching) than W or N, suggesting that proportionally more lipid than protein is accumulated during larval development. There are cyclical changes in the relative (per unit of W) biomass and energy figures, corresponding to the larval moult cycles: immediately after each ecdysis all these values decrease, presumably due to rapid uptake of water and minerals, then they increase again due to tissue growth and remain high until the next moulting occurs. Cyclical changes in the C/N ratio suggest that proportionally more lipid than protein is accumulated during the initial (postmoult) phase of the moult cycle, followed by a period of balanced or protein-dominated gain during the intermoult and premoult phases. These patterns of growth and elemental composition observed during the complete larval development and in single moult cycles of I. dorsettensis are compared with those described in the literature for other decapod species. This comparison suggests a high degree of similarity in biochemical composition and growth characteristics of larval decapod crustaceans.     

RNA:DNA ratios and growth of herring (Clupea harengus) larvae reared in mesocosms

Autumn-spawned North Sea herring larvae (Clupea harengus L.) were released in two outdoor mesocosms of 2500 m³ (A) and 4000 m³ (B). The mesocosms were monitored for temperature, salinity, oxygen, chlorophyll a, zooplankton and herring larvae abundance. The density of suitable prey for first feeding larvae (mainly copepod nauplii) was initially low in Mesocosm A (<0.11^-1) compared to in Mesocosm B (>11^-1). Half-way through the experiment the situation was reversed, with higher densities of prey in Mesocosm A (>31^-1) as compared to Mesocosm B (～11^-1). The average temperature declined steadily in both mesocosms from 18°C at release to 11–12°C by the end of the experiment 60 d later. The RNA:DNA values of individual herring larvae were related to protein growth rates and temperature adjusted according to Buckley (1984). A corresponding DNA growth index (Gdi) was given as: Gdi=0.68 TEMP+3.05 RNA:DNA-9.92. The RNA:DNA based growth indices were significantly correlated with other somatic growth estimates. The average estimated protein growth rate in the two mesocosms followed the same temporal pattern as the somatic growth rate, but with a lag of 2 d or more. Residual analysis of the regression of ln RNA versus ln DNA also showed the same temporal pattern as the RNA:DNA ratios, but the shift in condition as estimated by this method occurred more in synchrony with the other somatic growth measures. Larvae in Mesocosm A had RNA:DNA values similar to the starvation control kept in the laboratory the first days after release, confirming that larvae in Mesocosm A initially were in poor nutritional condition. On the other hand, the majority of the herring from Mesocosm B were characterised as starving or in poor nutritional condition towards the end of the experiment. The assessment of growth and nutritional condition were in accordance with independent survival estimates which suggested that the majority of the total mortality occurred during the first 15 d in Mesocosm A and there-after in Mesocosm B.     

Larval growth of anchovy,Engraulis encrasicolus,in the Western Mediterranean Sea

Engraulis encrasicolus (Linnaeus, 1758) were sampled in July/August 1985 in the Western Mediterranean Sea; they were aged by means of growth rings in the sagittal otoliths. Daily growth rings were observed and subdaily rings were visible starting with the third or fourth daily increment. The Gompertz growth equation, commonly employed in larval growth analysis, suitably describes the growth of this species in the length range sampled; however, extrapolation to greater lengths is not reliable. An instantaneous growth rate of ca. 0.9 mm d^-1 was calculated for 8 mm larvae at a temperature of 20°C. This rate is higher than those reported by other authors for the related species E. mordax from colder waters.     

Effects of non-circadian light/dark cycles on the growth and moulting of Palaemon elegans reared in the laboratory

The growth and moulting of Palaemon elegans Rathke has been compared under a circadian (12 h light: 12 h dark) and two non-circadian (8 h light:8 h dark and random light:dark) light/dark cycles. In prawns reared individually from hatching to the late juvenile phase, growth, measured as increase in total length, was significantly retarded in the non-circadian regimes, during zoeal, post-larval and early juvenile development. This effect was primarily the result of reduced increments at moulting in the non-circadian regimes. Growth of prawns reared from hatching to the post-larval phase, measured as wet and dry weights, was similarly reduced in a non-circadian regime. These effects support the hypothesis that the integrity of an animal's physiology is partially dependent on maintaining, through the action of daily environmental cycles, correct timing relationships between its oscillating sub-systems.     

RNA:DNA ratios and growth of herring (Clupea harengus) larvae reared in mesocosms

Autumn-spawned North Sea herring larvae (Clupea harengus L.) were released in two outdoor mesocosms of 2500 m³ (A) and 4000 m³ (B). The mesocosms were monitored for temperature, salinity, oxygen, chlorophyll a, zooplankton and herring larvae abundance. The density of suitable prey for first feeding larvae (mainly copepod nauplii) was initially low in Mesocosm A (<0.11^-1) compared to in Mesocosm B (>11^-1). Half-way through the experiment the situation was reversed, with higher densities of prey in Mesocosm A (>31^-1) as compared to Mesocosm B (11^-1). The average temperature declined steadily in both mesocosms from 18°C at release to 11–12°C by the end of the experiment 60 d later. The RNA:DNA values of individual herring larvae were related to protein growth rates and temperature adjusted according to Buckley (1984). A corresponding DNA growth index (Gdi) was given as: Gdi=0.68 TEMP+3.05 RNA:DNA-9.92. The RNA:DNA based growth indices were significantly correlated with other somatic growth estimates. The average estimated protein growth rate in the two mesocosms followed the same temporal pattern as the somatic growth rate, but with a lag of 2 d or more. Residual analysis of the regression of ln RNA versus ln DNA also showed the same temporal pattern as the RNA:DNA ratios, but the shift in condition as estimated by this method occurred more in synchrony with the other somatic growth measures. Larvae in Mesocosm A had RNA:DNA values similar to the starvation control kept in the laboratory the first days after release, confirming that larvae in Mesocosm A initially were in poor nutritional condition. On the other hand, the majority of the herring from Mesocosm B were characterised as starving or in poor nutritional condition towards the end of the experiment. The assessment of growth and nutritional condition were in accordance with independent survival estimates which suggested that the majority of the total mortality occurred during the first 15 d in Mesocosm A and there-after in Mesocosm B.     

Barnacle larvae actively select flow environments supporting post-settlement growth and survival

Many marine dispersive propagules select specific settlement sites based on a range of environmental cues. However, the link between larval choice and post-settlement growth and survival is still poorly understood. Here we show that cypris larvae of the barnacle Balanus improvisus actively reject surfaces exposed to local flow speeds exceeding 5-10 cm/s. Field experiments show that post-settlement growth and survival decline in freestream flows above 15 cm/s. Moreover, studies in flume flow at local speeds exceeding 10 cm/s reveal that early juveniles show reduced feeding rates caused by deformation of the cirral fan, reduced retention efficiency, and a decrease in time spent feeding. We conclude that cypris larvae actively reject flow environments that will be suboptimal for suspension feeding in the early post-settlement phase. Our study suggests that larval choice can be adaptively connected to a specific part of the life cycle, in this case the very sensitive time after metamorphosis.     

Rearing Pandalus borealis larvae in the laboratory

Larvae of the northern shrimp Pandalus borealis (Krøyer) are pelagic. In the Estuary and Gulf of St. Lawrence, Canada, the early stages are found in the upper 25-m of the water column in spring and early summer and are expected to experience a range of water temperatures from as low as 0°C to as high at 6°C. Little is known of the impact of water temperature on metabolic requirements of northern shrimp larvae. In this study, routine respiration (VO₂), maximum respiration (electron transport system activity, ETSA) and metabolic scope for growth (MS, ETSA–VO₂) of northern shrimp larvae were measured as a function of temperature (3, 5 and 8°C), developmental stage (I–V at 3°C, I–VII at 5°C and 8°C) and growth rate in dry mass. After logarithmic transformation, all three metabolic variables were linearly related to dry mass. The increase in VO₂ with body mass was faster at 5°C than at 3 or 8°C, whereas with ETSA this increase was slower. As a result, MS increased more slowly with dry mass at 5°C than at 3 and 8°C. However, MS did not limit growth in this study, since it explained only 39% of the variability in growth. All three metabolic variables as well as growth varied together as a function of temperature and ontogeny. Q₁₀ of all three metabolic variables ranged from 1.6 and 2.2 for stages I–V larvae, except for VO₂ at stage I (3.9) and stage III (2.9).     

Effects of temporary starvation on the survival,and on subsequent feeding and growth,of oyster (Crassostrea gigas) larvae

Larvae of oysters, Crassostrea gigas, were maintained without food for 1 to 8 d after fertilization, and fed daily thereafter. There was little difference in survival and growth between controls and larvae kept without food for 2 or 3 d. Survival and growth rates were depressed in larvae starved for 4 or 5 d. For larvae starved for 6 to 8 d, survival was negligible or nil; even those larvae which survived the starvation period died later in the presence of food, apparently because of impaired digestion. Therefore, food availability in the first few days after spawning appears to be of paramount importance to the successful recruitment of Pacific oysters.     

Patterns of growth and triacylglycerol content in snow crab Chionoecetes opilio (Brachyura: Majidae) zoeal stages reared in the laboratory

Snow carb Chionoecetes opilio zoea I and zoea II larvae, hatched from females in a controlled mating experiment, were reared in the laboratory at 10.1 °C and 28.0 salinity, to resolve the patterns of growth (dry weight [DW]) and change in energy reserves (triacylglycerols [TAG]) within a given moult cycle. The patterns of growth and change in TAG reserves were similar in each zoeal stage. Following hatching or a moult, the zoeae entered a phase of rapid size increase, i.e. high daily growth rates (5.5 to 12.8% DWd^-1), for 1/3 to 2/5 of the duration of the moult cycle. During the same period, the zoeae accumulated TAG reserves until a maximum (TAG DW^-1) was reached at the end of the phase of rapid growth. The period of high growth rates and of TAG accumulation is interpreted as the required time for the zoeae to reach a point in development [i.e. point of reserves saturation (PRS); Anger and Dawirs (1981)] where sufficient growth and energy reserves allow moulting to the next stage. Following the phase of rapid growth and TAG accumulation, the zoeae entered a phase of low daily growth rates (0 to 1% DWd^-1) during which the TAG reserves decreased to a minimum at the end of the phase. Prior to, and during the moult to zoea II, a phase of negative growth was observed in the zoea I larvae. We conclude that measurement of zoeal size and TAG content, along with morphometric criteria (e.g. epidermal retraction), can be used to assess growth and nutritional condition of C. opilio zoeal stages from the sea.     

Growth and reproduction ofCaprella danilevskii (Crustacea: Amphipoda) reared in the laboratory

Caprella danilevskii Czerniavski, an epifaunal amphipod crustacean, was successfully reared for two generations under laboratory conditions. Amphipods were maintained in pairs or fours in a Petri dish. Temperature and photoperiod were maintained at 20°C, and 14 h light: 10 h dark, respectively. Colonies of the diatomCylindrotheca closterium (Ehrenberg) Reumann and Lewin were provided as food source. The red algaGelidium amansii (Lamouroux) Lamouroux was used to provide an attachment substrate for the caprellids. Juveniles that emerged from the brood pouch were described as instar I. Subsequent instars were identified by molts. Juveniles molted successively at 2.5 to 6.5 d intervals. The body length of males at each instar increased exponentially, while that of females followed a sigmoid curve. A different pattern of flagellar segment addition in the antenna I of males and females was revealed. At the age of 20.8 d, females reached instar VII, and produced their first embryos. After 5.1 d, juveniles emerged from the brood pouch. On average, females produced 5.4 clutches successively at 5.0 d intervals. The number of offspring per spawning increased from 6.3 at instar VII to 22.0 at instar XII. The total number of offspring was 69.0 for a female throughout her life span.     

Feeding,growth and bioluminescence of the heterotrophic dinoflagellate Protoperidinium huberi

Feeding, growth and bioluminescence of the thecate heterotrophic dinoflagellate Protoperidinium huberi were measured as a function of food concentration for laboratory cultures grown on the diatom Ditylum brightwellii. Ingestion of food increased with food concentration. Maximum ingestion rates were measured at food concentrations of 600 g C l^-1 and were 0.7 g C individual^-1 h^-1 (1.8 D. brightwelli cells individual^-1 h^-1). Clearance rates decreased asymptotically with increasing food concentration. Maximum clearance rates at low food concentration were ca. 23 l ind^-1 h^-1, which corresponds to a volume-specific clearance rate of 5.9x10⁵ h^-1. Cell size of P huberi was highly variable, with a mean diameter of 42 m, but no clear relationship between cell size and food concentration was evident. Specific growth rates increased with food concentration until maximum growth rates of 0.7 d^-1 were reached at a food concentration of 400 g C l^-1 (1000 cells ml^-1). Food concentrations as low as 10 g C l^-1 of D. brightwellii (25 cells ml^-1) were able to support growth of P. huberi. The bioluminescence of P. huberi varied with its nutritional condition and growth rate. Cells held without food lost their bioluminescence capacity in a matter of days. P. huberi raised at different food concentrations showed increased bioluminescence capacity, up to food concentration that supported maximum growth rates. The bioluminescence of P. huberi varied over a diel cycle, and these rhythmic changes persisted during 48 h of continuous darkness, indicating that the rhythm was under endogenous control.     

Feeding rates of the jellyfish Aurelia aurita on fish larvae

We quantified feeding rates of field caught Aurelia aurita feeding on yolk sac cod (Gadus morhua) larvae in a series of incubation experiments. A short-time (~1 h) functional response experiment with a wide range of prey concentrations (0.5–16 prey l⁻¹, initial concentration) revealed that ingestion rates increased linearly over this range, such that clearance rates were similar between the different prey concentrations. This suggests that A. aurita is capable of efficiently utilizing dense prey patches. This indication was further supported by a linear increase of prey captured by A. aurita during 2.5 h of feeding at extremely high prey concentration (>200 prey l⁻¹). Clearance rate in darkness scaled with jellyfish diameter to a power of ~1.7 for jellyfish 3.9–9.5 cm in diameter. The jellyfish did not alter their umbrella pulse frequency in response to presence of fish larvae. There were no significant differences between A. aurita feeding rates in light and darkness for yolk sac prey ages 0–7 days (at 7.5°C). Although prey vision and escape abilities of fish may develop rapidly during early larval ontogeny, these factors apparently have little impact on interactions with predators such as A. aurita during the yolk sac stage.     

Growth and metamorphosis of Aplysia oculifera larvae in laboratory culture

Eggs of Aplysia oculifera (Adams and Reeve, 1850) were incubated in the laboratory. They hatched 8 to 9 d after spawning. Shell length (SL) of the hatched larvae was 102±2 m. Larvae were fed on the unicellular algae Isochrysis galbana in a concentration of 10⁴ cell ml^-1, and after 45 to 60 d grew to a maximum SL of 385±11 m. Larvae survived up to 330 d. A total of 12 species of algae from the natural habitat of A. oculifera were examined as metamorphosis inducers. Red algae Dasia sp., Jania sp., Hypnea sp. and Liagora sp. induced metamorphosis in 66.7±21.2, 28.3±17.7, 26.0±18.5 and 4.0±8.0% of the larvae, respectively. Green algae Enteromorpha intestinalis and Ulva sp. induced metamorphosis in 37.0±11.0 and 9.0±10.4% of the larvae, respectively. Cladophora sp. and Codium dichotomum, and the brown algae Padina pavonia, Colpomenia sinuosa, Hydroclathrus clathratus and Cystoseira sp. did not induce metamorphosis. There was no significant difference in the rate of metamorphosis between young (2 to 4 mo) and old (6 to 8 mo) larvae. Postmetamorphic juveniles grew and developed only when fed with E. intestinalis. They grew to a body length of>8 mm in 50 d. Postmetamorphic juveniles did not survive on other algae. The longevity of the planktonic A. oculifera larvae supports the hypothesis that the larvae can exist in the plankton and survive for several months until the next recruitment. The advantage of non-specificity in metamorphosis induction is discussed.     

A method for rearing Pecten maximus larvae in the laboratory

Development of the scallop Pecten maximus (L.) from egg to metamorphosis takes 33 to 38 days at 16°C. The shelled veliger first appears 2 days after fertilisation, and crawling pediveligers at around 2 days before metamorphosis. The pediveliger can attach temporarily to filamentous objects by means of a byssus thread and, even after metamorphosis, periods of attachment alternate with periods of crawling. The larvae are reared in polythene bins containing 30 l of full salinity sea water (pre-filtered through 0.2 filters), which is changed every 2 days. Mixtures of algal foods (Isochrysis galbana, Chaetoceros calcitrans, Pyramimonas obovata and Tetraselmis suecica), concentrated by centrifugation, are added at each change from the third day onwards to give a concentration of 50 cells/l. Larval density is maintained below 10/ml. Antibiotics are added at each change, but very careful sterilisation of all equipment is necessary to guard against the introduction of diseases.