Effects of temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis

To assess the effects of both temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis (O.F. Müller), individuals collected December 1996 (winter experiment) and June 1997 (summer experiment) were maintained for 3 months in one of four experimental treatments: (1) 3 °C and fed ad libitum (high ration), (2) 3 °C and fed one-seventh of the maximum ration (low ration), and (3) 12 °C and fed the high ration; (4) 12 °C and fed the low ration. All individuals were fed an artificial diet and exposed to only 1 h of light every day. At the end of both experiments, mean gonad indices of sea urchins fed the high ration had increased significantly (11–24% and 6–19% in the winter and summer experiments, respectively), while the gonad indices of individuals fed the low ration did not change. At the high ration (both experiments), the increase in gonad index of sea urchins occurred primarily as the result of a significant increase in the mass of nutritive phagocytes, as revealed by histological analyses. Primary oocytes were significantly larger in individuals held at 3 °C than at 12 °C throughout the winter experiment, regardless of food ration; during the summer experiment, primary oocytes were significantly larger in individuals receiving the high ration, regardless of the temperature at which they were held. These results suggest that: (1) food availability is the most important factor regulating energy storage and the relative size of gonads throughout the year, (2) temperature affects the rate of growth and maturation of primary oocytes during the later stages of oogenesis, and (3) once gametogenesis has been initiated, mature ova will be produced, even under conditions of low food availability. Conditions of high food availability in summer and low temperature in winter would thus favor reproductive output in sea urchin populations. Received: 1 March 2000 / Accepted: 4 October 2000     

Fecundity and spawning in a deep-water cirromorph octopus

 Reproductive characteristics of cirromorph octopuses, assigned to the species Opisthoteuthis grimaldii, were sampled as a commercial fishing by-catch on the Hebrides Slope, west of Scotland. A total of 254 specimens (99 female, 155 males), retrieved from bottom trawls fished at 750 to 1500 m depth, were examined. A maximum of 2097 eggs was counted in a single female ovary (mean female body weight 1242.8 g), most of them <1 mm in length. At egg lengths over 1 mm, diminishing numbers of eggs were present in 1 mm size categories up to a maximum of ≈10 mm. At body sizes >500 g (wet wt), and in every female >750 g (max. female weight recorded in the sample was 2959 g), a succession of unattached eggs was present in the proximal oviduct and a single, unattached mature egg occupied the tip of the distal oviduct. These females were assumed to be in spawning condition and the characteristics of egg distribution in the reproductive tract to be consistent with sequential release of individual eggs and continuous spawning throughout the growth period and lifespan of the mature octopus. In pre-spawning females there was a positive relationship between estimated egg numbers and maximum egg size. After the onset of spawning there was no significant further increase in estimated potential fecundity over the body-size range 500 to 3000 g. Follicular sheaths remaining in the ovary after release of eggs into the proximal oviduct were counted and used to estimate the total number of eggs released up to the time of capture. Follicular sheaths first appeared at 500 to 650 g body weight and increased steeply in number to >1000 in females >1500 g. Two individuals were found with ovarian follicular sheaths but with no terminal egg in the distal oviduct; these were assumed to have released their egg just before capture. Summation of the number of follicular sheaths counted plus the number of eggs estimated as remaining attached in the ovisac, provided a revised estimate of total potential fecundity and raised the estimate for any individual to a maximum of 3202 eggs (mean = 1396 eggs). Received: 3 February 2000 / Accepted: 17 May 2000     

Egg production of Calanus finmarchicus : effect of temperature, food and season

The use of the egg production rate of herbivorous copepods as an important parameter for understanding population dynamics and as an index of secondary production requires knowledge of the regulatory mechanisms involved and of the response to changes in food concentrations and temperature. Furthermore, the effects of season and generation on egg production have to be studied. In this context data are presented for Calanus finmarchicus from the northern North Atlantic. Prefed and prestarved females were exposed to different concentrations of the diatom Thalassiosira antarctica over 1 to 2 wk at 0 or 5 °C, and egg deposition was controlled daily. Egg production increased with higher food concentrations, but much less when prestarved. The effect of temperatures between −1.5 and 8 °C on egg production was studied in females maintained at optimum feeding conditions. Egg production rate increased exponentially over the whole temperature range by a factor of 5.2, from 14.2 to 73.4 eggs female⁻¹ d⁻¹, and carbon-specific egg production by 4, from 2.1 to 8.5% body C d⁻¹. The response to starvation was also temperature dependent. In both the temperature and feeding experiments egg production rate was regulated mainly by changes of the spawning interval, while changes of clutch size were independent of experimental conditions. Different responses to optimum feeding conditions were observed in females collected in monthly intervals on three occasions between March and May. The March females deposited more clutches than the April and May females. In May, >50% of the females did not spawn at all. Maximum egg production rates were never >25% of the rate expected at 5 °C, indicating endogenous control of egg production in addition to food and temperature effects. Received: 4 August 1996 / Accepted: 11 September 1996     

Effect of diet on reproductive characteristics of Ophryotrocha labronica (Polychaeta: Dorvilleidae)

Effects of food quality on fecundity (defined as the number of eggs deposited in the entire life of a female), egg size and egg energy content (measured by differential scanning calorimeter were studied in two populations of Ophryotrocha labronica (La Greca and Bacci): one taken from the natural environment immediately prior to the experiment, and the other from the same original environment, but bred for more than 5␣years in the laboratory. Results indicated that fecundity is mediated both by food quality and population origin. Specimens fed Tetramin generally spawned more frequently than those receiving the other two diets, while individuals bred in the laboratory since 1990 spawned more frequently than the wild population. Also the number of eggs per spawning and the total fecundity vary with the quality of the diet; Tetramin gives the highest fecundity values. The differences in egg size and energy content seem essentially related to the population origin, in fact, with all diets laboratory specimens produced larger eggs with higher energy contents than wild specimens. Received: 1 August 1997 / Accepted: 29 April 1998     

Physiological and behavioural responses of the intertidal scavenging gastropod Nassarius festivus to salinity changes

Physiology (oxygen consumption and ammonia excretion) and behaviour (feeding and activity) of the intertidal gastropod Nassarius festivus (Powys) at five different salinities [15, 20, 25 (control), 30 and 35‰] were studied for 4 weeks. Oxygen consumption and ammonia excretion rates were reduced immediately after salinity was either elevated or reduced. Subsequently, both rates were increased while the O:N ratio was decreased at all salinities, including the control, in the first 2 weeks and then levelled off. Such changes were probably attributed to osmotic adjustment and reproductive activity. Activity and feeding were reduced at low salinity, particularly in the first week. Reproductive output, in terms of the total number of egg capsules and the mean number of eggs per capsule, was also lowered at reduced salinities. Nevertheless, individuals at all salinities are able to maintain a positive energy balance. Results are discussed with respect to the distribution of N. festivus in Hong Kong waters. Received: 14 April 1997 / Accepted: 9 May 1997     

Seasonal reproductive biology of the egg-carrying calanoid copepod Pseudodiaptomus marinus in a eutrophic inlet of the Inland Sea of Japan

In situ egg production of the egg-carrying calanoid copepod Pseudodiaptomus marinus was investigated in Fukuyama Harbor, a eutrophic inlet of the Inland Sea of Japan, at 3- to 5-d intervals for a year. This species reproduced throughout the year, and the adults showed a large abundance peak in June/July and a small peak in September/October. Females usually outnumbered males, comprising 61.4% of the annual mean. The composition of ovigerous females varied from 7.9 to 100%, with an annual mean of 55.7%. Adult prosome length was consistently large throughout winter and spring, and decreased with increasing temperature in summer and fall. Egg diameter varied from 98 to 121 μm, and was negatively correlated to temperature. The seasonal variation in clutch size (range: 15.1 to 38.2 eggs) was bicyclical, with peaks in May and December. The egg production rate of breeding females was low in January to March (mean: 2.3 eggs female⁻¹ d⁻¹), while it was constantly high from mid-May to early October (mean: 12.1 eggs female⁻¹ d⁻¹). The specific egg production rate for the breeding females was highly correlated to temperature; it increased linearly from 0.03 d⁻¹ at 9 °C to 0.27 d⁻¹ at 26 °C. Compared to other co-occurring copepods, the reproductive rate of P. marinus was lowest, which is one of the reasons why this species never dominates in this inlet. Received: 11 November 1996 / Accepted: 7 December 1996     

Variability in larval development of the tropical reef fish Pomacentrus amboinensis (Pomacentridae): the parental legacy

The effects of food availability, female size, and social interactions on the quality of Pomacentrus amboinensis larvae at hatching were examined using two field-based experiments. In Experiment 1, food availability and female size significantly influenced size, eye diameter and levels of yolk reserves of larvae at hatching. Small females (47 to 52 mm standard length, SL) whose diets were not supplemented, produced the longest larvae (3.0 ± 0.01 mm total length, TL) with the least yolk reserves (50.1 ± 1.04 μm²). Irrespective of female size, those that received additional food produced larvae with the largest yolk-sacs (large females: 87.60 ± 1.53 μm²; small females: 80.14 ± 1.24 μm²). In Experiment 2, interactions with conspecifics had a greater affect on the somatic development of larvae at hatching than food availability. Increased social interactions resulted in larvae that were ⋍3% longer, with 2% greater head depth, than larvae from females that spawned in isolation on the experimental reefs. Fed females produced larvae with ⋍20% more yolk than larvae from females whose diets were not supplemented. All three factors (food availability, female size, and intensity of social interactions) tested within these experiments vary spatially and temporally among reefs. There is the potential, therefore, for larvae at the onset of the planktonic stage to vary in quality, level of development, and probability of survival. Received: 12 August 1996 / Accepted: 26 August 1996     

Longevity of subitaneous and diapause eggs of Centropages hamatus (Copepoda: Calanoida) from the northern Gulf of Mexico

Copepod resting eggs are abundant in the seabed of many bays and estuaries where they provide a potential source of recruits for growth of planktonic populations. In the northeastern Gulf of Mexico the copepod Centropages hamatus (Lillejeborg) occurs in the water column only during the late fall, winter and early spring. The species produces subitaneous and diapause eggs, and both egg types have been found in the seabed. We determined the longevity of these two egg types to ascertain their potential for contributing to the growth of the planktonic population and for sustaining a persistent egg bank. Eggs were collected from females and incubated in the laboratory under temperature and oxygen conditions chosen to simulate field conditions. The diapause eggs were also exposed to sulfide. The total hatching success of subitaneous eggs in two experiments declined from highs of 78 and 97% to zero after 60 and 90 d of exposure to anoxia. The total hatching success of diapause eggs that were exposed to anoxia for 90 d however was typically greater than 80%. Some diapause eggs hatched after being incubated under anoxia for 437 d. Diapause eggs survived longer at ambient field temperatures when incubated under anoxia (437 d) compared to normoxia (118 d). Exposure to sulfide did not result in greater mortality of diapause eggs compared to anoxia alone. Diapause eggs that were incubated at ambient field temperatures did not hatch when exposed to normoxia until the temperature dropped to <20 °C. The results of this study suggest that C.␣hamatus sustain a short-term reserve of subitaneous eggs in the seabed that provides recruits for the current year's population. The greater longevity of diapause eggs suggests that they sustain the seasonal reappearance of the species year after year in the northeastern Gulf of Mexico. However, the contribution of diapause eggs of C. hamatus from the Gulf of Mexico to a persistent egg bank is questionable since hatching ceased after 437 d. Received: 30 July 1997 / Accepted: 18 January 1998     

Impact of temperature on juvenile growth and age at first egg-laying of the Pacific reef squid Sepioteuthis lessoniana reared in captivity

Cephalopod mollusks exhibit highly plastic life cycle traits influenced primarily by the interactive effects of food availability, light cycle and temperature, with the latter perhaps the most influential. Hatchlings of the tropical reef squid Sepioteuthis lessoniana were hatched from field-collected eggs in the laboratory and cultured at different temperatures to evaluate the effect of temperature on growth rates. All groups showed rapid, sustained growth rates from hatching to a size of 10–25 g. Beyond this size range, growth was slower and not clearly exponential in form. Growth rate was closely linked to temperature. Squids grown at approximately 27 °C attained a size of 10 g in as little as 45 days at sustained growth rates of 12.2% body weight day⁻¹ (%bw day⁻¹), while squids cultured at 20 °C required almost 100 days to attain the same size at rates of 5.7%bw day⁻¹. At an age of 55 days and approximately 1 g body weight, juvenile squids cultured at 20 °C were able to accelerate growth rates from 5.7%bw day⁻¹ to over 12%bw day⁻¹ when temperature was raised to 27 °C. They maintained this growth rate to a size of about 10 g and an age of at least 75 days post-hatching, indicating that body size and not age is the limiting factor for this rapid post-hatching growth. By comparison, conspecifics cultured near 27 °C from hatching had shifted out of the rapid post-hatching growth phase by day 50 at sizes between 10 and 50 g. The hatchlings from temperate to subtropical Japan had consistently higher growth rates at comparable temperatures than hatchlings from tropical Okinawa. When plotted as growth rate versus temperature, the Japanese group had a clearly higher slope to the relationship than the tropical populations, equivalent to a 2%bw day⁻¹ difference in growth rate at 25 °C. Age at first egg-laying was decreased at higher culture temperatures; however, overall life span was not. Received: 21 February 2000 / Accepted: 6 September 2000     

The cost of host egg damage caused by a brood parasite: experiments on great spotted cuckoos (Clamator glandarius) and magpies (Pica pica)

Adult great spotted cuckoos, Clamator glandarius, frequently damage one or more eggs of their magpie host, Pica pica, without removing or eating them. The presence of damaged host eggs could signal parasitism thereby increasing the probability that the parasitic egg is ejected. This hypothesis was tested by experimentally introducing a model cuckoo egg with or without damaged host eggs. Magpie responses to experimental parasitism did not differ significantly between treatments implying that damaged host eggs are not used by magpies to assess parasitism. We followed the fate of magpie eggs naturally damaged by the great spotted cuckoo or experimentally damaged by us. Host response was very similar for naturally or experimentally damaged host eggs, but varied significantly according to the type of egg damage, eggs being removed more frequently when pecked than crushed, while cracked eggs were never removed. However, the egg damage that most readily causes egg removal is albumen leakage. Received: 30 November 1998 / Received in revised form: 7 June 1999 / Accepted: 12 June 1999     

Growth, carbon, nitrogen and caloric content of Solea senegalensis (Pisces: Soleidae) from egg fertilization to metamorphosis

Eggs and larvae of the Senegal sole, Solea senegalensis Kaup, were reared from fertilization until the end of metamorphosis, which occurs by Day 17 after hatching at 19.5 °C. Changes in energy content and biomass quality were studied in terms of dry weight and of carbon, nitrogen and energy content. S. senegalensis spawned eggs of about 1 mm diameter which hatched 38 h after fertilization. Average dry weight of individual eggs was 46 μg, the chorion accounting for about 18% of total dry weight. Gross energy of recently fertilized sole eggs was approximately 1 J egg⁻¹. From fertilization to hatching, eggs lost 8% of their total energy (chorion not included). After hatching, larvae lost 14% of their initial energy until the start of feeding which occurred about 48 h afterwards. The principal components catabolized during embryogenesis were carbon-rich compounds that decreased by 26%, while nitrogen-rich compounds decreased by only 10% and were practically unaltered from hatching to the start of feeding. Feeding larvae displayed constant growth during the period studied (specific growth rate on a dry weight basis was 0.26 d⁻¹). The relative proportion of carbon and nitrogen content revealed an accumulation of high energy compounds in the days before metamorphosis. By Day 14, the energy content reached values similar to those of recently hatched embryos, but decreased again during metamorphosis. Received: 10 June 1998 / Accepted: 28 January 1999     

Diet and egg production of the copepod Acartia tonsa in Florida Bay. II. Role of the nutritional environment

 As part of an ongoing study of changes in the trophic pathways of Florida Bay's pelagic ecosystem, the nutritional environment (seston protein, lipid and carbohydrate levels), diet (taxon-specific microplankton ingestion rates) and egg production rate of the important planktonic copepod Acartia tonsa were measured off Rankin and Duck Keys in July and September 1997 and in January, March and May 1998. Rankin Key has been the site of extensive sea grass mortality and persistent ultraplankton blooms since 1987. Duck Key has experienced neither of these perturbations. Protist (auto-plus heterotroph) biomass was approximately twice as high off Rankin as off Duck Key. Diatoms, dinoflagellates and heterotrophic protists dominated the food environment off Rankin Key, while cells <5 μm diam often predominated off Duck Key. Protein and carbohydrate concentrations were higher off Rankin Key than Duck Key, while average lipid levels were usually low at both stations. Ingestion rates at both stations frequently approached temperature- and food-dependent maxima for the species, exceeding 100% of estimated body C d⁻¹ on 3 of 5 occasions off Rankin Key. Egg production rates, however, were consistently low (Rankin: 3 to 16 eggs copepod⁻¹ d⁻¹; Duck: 1 to 12 eggs copepod⁻¹ d⁻¹), and gross egg production efficiencies (100% × egg production C/ingested C) averaged <10%. At Duck Key, egg production rate varied with temperature and food concentration, while off Rankin Key, egg production was strongly correlated with seston protein content. The efficiency with which lipids (which were scarce in the seston) were transferred from the diet to the eggs increased exponentially with decreasing seston lipid content. Egg production efficiencies based on protein, however, were independent of seston protein content and never exceeded 10%. Received: 23 December 1998 / Accepted: 23 March 2000     

Egg survival,embryonic development,and larval characteristics of northern shrimp (<Emphasis Type="Italic">Pandalus borealis</Emphasis>) females subject to different temperature and feeding conditions

Laboratory experiments on ovigerous females of northern shrimp (Pandalus borealis) were used to assess the effects of temperature and food ration on female condition during incubation and examine how combined effects of temperature and female condition influenced egg survival, embryonic development, and larval characteristics. Ovigerous females were maintained at 2°C, 5°C, and 8°C and fed on a low (three times/week; 2–2.7% W/W) or high ration (five times/week at satiation). The increase in temperature accelerated the developmental time of the eggs but their survival at 8°C was reduced. Conversion efficiency of yolk reserves in developing embryos was significantly reduced at elevated temperatures and larvae hatching at 2°C and 5°C were significantly larger and heavier than those hatching at 8°C. The experimental design did not result in any effect of food ration on the energetic condition of females or on egg characteristics and their biochemical composition. However, lower energy reserves were observed for females held at 8°C.     

Population dynamics and production of the planktonic copepods in a eutrophic inlet of the Inland Sea of Japan. IV. Pseudodiaptomus marinus, the egg-carrying calanoid

Population dynamics and production of the egg-carrying calanoid copepod Pseudodiaptomus marinus were studied for a year in Fukuyama Harbor, a eutrophic inlet of the Inland Sea of Japan. This species was perennial, with a large numerical peak in June and small peaks in September/October and November/December. During the study period, at least 11 generations could be detected. For each generation, the stage-specific survival from egg to Copepodite Stage (C) V was determined; it was very high during early life stages (egg to NIII), and gradually decreased beyond. On average, 94% of eggs recruited into NIII, which is strongly contrasted with very high (>ca. 90%) mortality during the corresponding stages for free-spawning copepods, i.e. Acartia omorii, Centropages abdominalis and Paracalans sp. This demonstrates that the egg-carrying strategy has a great advantage to reduce mortality in egg stage. The biomass of this species showed marked seasonal variations largely in parallel with numerical abundance. The instantaneous somatic growth rate increased linearly with temperature. The population production rate was estimated as the sum of somatic growth of larval stages and egg production of adult females; the annual integration was 51.0 mg C m⁻³ yr⁻¹ or 0.38 g C m⁻² yr⁻¹. Received: 11 November 1996 / Accepted: 7 December 1996     

Feeding and assimilation kinetics of Artemia franciscana fed Isochrysis galbana (clone T. Iso)

Artemia franciscana was grown on Isochrysis galbana Green (clone T. Iso) at saturated food concentrations (13 to 20 mg C l⁻¹) for 11 d at 26 to 28 °C, and 34 ppt salinity. Three groups of brine shrimp were used in the feeding experiments: metanauplius III and IV (Group 1), post-metanauplius II and III (Group 2) and post-metanauplius VIII (Group 3), corresponding to 4-, 7- and 11-d-old animals, respectively. The ingestion rate, clearance rate and carbon balance were estimated for these stages at different concentrations of ¹⁴C-labeled I. galbana ranging from 0.05 to 30 mg C l⁻¹. The handling time of algae was determined for all three groups. The ingestion rate (I, ng C ind⁻¹ h⁻¹) increased as a function of animal size and food concentration. In all three groups, the ingestion rate increased to a maximum level (I _max) and remained constant at food concentrations ≥10 mg C l⁻¹ (saturated food concentrations). The clearance rate (CR, μl ind⁻¹ h⁻¹) increased with increasing food concentration up to a maximum rate (CR _max), after which it decreased for even higher food concentrations. The functional response of A. franciscana was most consistent with Holling's Type 3 functional response curve (sigmoidal model), which for the two oldest groups (Group 2 and 3) differed significantly from a Type 2 response (p < 0.05). The gut passage time for the three groups of A. franciscana, feeding on saturated food concentration (20 mg C l⁻¹), varied between 24 and 29 min. As the nauplii developed to pre-adult stage the handling time of the algae increased as a function of animal size. The assimilation rate (ng C ind⁻¹ h⁻¹) in the youngest stages (Group 1 and 2) increased with increasing food concentrations, reaching a maximum level close to 10 mg C l⁻¹. At higher food concentrations the assimilation rate decreased, and the proportions of defecated carbon increased, reaching 60 to 68% in the post-metanauplius stages (Group 3). The assimilation efficiency (%) was high at the lowest food concentrations in all three groups (89 to 64%). At higher concentrations, the assimilation efficiency decreased, reaching 56 to 38% at the highest concentrations. Received: 2 February 2000 / Accepted: 25 March 2000     

Poecilogony in an estuarine opisthobranch: planktotrophy, lecithotrophy, and mixed clutches in a population of the ascoglossan Alderia modesta

A San Diego population of the opisthobranch mollusc Alderia modesta (Lovén, 1844) exhibits poecilogony, the presence of two development modes within a single species. In spring, half of the adults spawned masses containing ∼300 eggs with a mean diameter of 68 μm. After 3 d, these egg masses hatched planktotrophic veligers with a maximum shell dimension of 116 μm. The remaining adults spawned masses containing ∼30 eggs with a mean diameter of 105 μm. These egg masses hatched after 5 to 6 d, releasing lecithotrophic larvae with a maximum shell dimension of 186 μm. About 1% of field-collected adults produced mixed clutches containing a continuum of larval sizes, spanning the size extremes of planktotrophy and lecithotrophy and hatching larvae with a mean maximum shell dimension of 152 μm. Adults producing planktotrophic and lecithotrophic larvae were interfertile, and no hybrid breakdown was observed through the F3 generation. When starved, adults which previously produced only lecithotrophic larvae switched to producing planktotrophic larvae or mixed clutches with both planktotrophic and lecithotrophic larvae. Sequence-polymorphisms from a portion of the mitochondrial cytochrome c oxidase I gene support the conclusion that the two reproductive morphs represent a single species. Most of the lecithotrophic larvae and a small percentage of the larvae from mixed clutches were metamorphically competent within 3 d of hatching. A. modesta is the only molluscan species as yet known to have both planktotrophic and pelagic lecithotrophic development within a single natural population. Received: 14 August 1997 / Accepted: 11 April 1998     

Importance of food quantity to structural growth rate and neutral lipid reserves accumulated in Calanus finmarchicus

 Growth and developmental rates were determined for copepodids of Calanus finmarchicus (Gunnerus) from experimental seawater mesocosms in a western Norwegian fjord. The instantaneous growth rates (g) from copepodid stage I (CI) to adult ranged from 0.08 to 0.10 d⁻¹. Daily per capita mortality rate of the cohorts was as low as 0.012 d⁻¹ (1.2% d⁻¹). At local increasing temperatures (5.1 to 8.3 °C), development was equiproportional, and the cumulative median development time from egg to CV was approximately 65 d. CV moulted to males and females, and egg production was initiated. Enhancement of food resources by nutrient addition caused a 23.4% increase in growth rates from CI to adult. Additionally, copepodid stages showed a generally larger body size, carbon and nitrogen content and total storage lipid content (wax esters + triacylglycerols) in response to enhanced resources. Our data support an elsewhere proposed exponential-growth hypothesis; growth of the structural compartments and store lipids (mostly wax esters) was exponential during the copepodid stages. However, a sigmoidal pattern of growth best described growth of adult stages if reared at high resources, and depot lipid accumulation in late CVs and adults at high resources. Body nitrogen growth increased exponentially, however, no significant changes in nitrogen specific growth rates were found between individuals from low and high resources. CV and adults seem to have reached near-maximal weights at high resources, whereas structural weight continued to increase at low resources. Despite the differences in structural growth dynamics, cohort development was similar until the end of CV. During the onset of sexual differentiation, the male:female ratio and the adult:CV ratio were highest at high food resources, suggesting that the time used for the final moult depends on the feeding history of the copepods in relation to food quality and quantity. It appears that relatively small changes in food availability strongly influence the biochemical composition of C. finmarchicus copepodids. Received: 28 May 1999 / Accepted: 10 February 2000     

Fitting fertilisation kinetics models for free-spawning marine invertebrates

 To determine how fertilisation varied with sperm concentration for two species of scallop, Chlamys (Equichlamys) bifrons (Lamarck) and C. asperrima (Lamarck), we performed a simple series of sperm dilution experiments, and measured egg size and sperm swimming speeds. C. bifrons eggs were much larger (average diam=116.5 μm), and sperm swimming speeds faster (209.8 μm s⁻¹), than C. asperrima (71.2 μm, 166.0 μm s⁻¹). In both species, maximum fertilisation occurred at an ambient sperm concentration of around 100 sperm μl⁻¹; the maximum proportion of eggs fertilised was less than 0.70 in the C. bifrons experiments, but nearer 1.0 with C. asperrima. At high sperm concentrations (>100 sperm μl⁻¹), fertilisation decreased (presumably due to polyspermy) with increasing sperm concentration, but decreased more rapidly in C. bifrons than C. asperrima. A polyspermy-adjusted fertilisation kinetics model could be fitted to the experimental data, but unique parameter estimates could not be determined. Received: 7 October 1999 / Accepted: 8 July 2000     

The influence of ration level on growth and statolith increment width of the tropical squid Sepioteuthis lessoniana (Cephalopoda: Loliginidae): an experimental approach

Juvenile squids were grown in individual 2.6-l floating enclosures and were fed either a high- or a low-ration diet of fish and the crustacean Acetes. Squids were maintained for a maximum of 44 days in two experiments. The high-ration individuals reached a significantly larger size in both experiments (27, 25.5 mm mean mantle length, ML) compared to their low-ration siblings (19 mm mean ML) in both experiments. The statolith increment widths prior to the start of the experiment were significantly wider (between 3 and 4 μm) compared to the increment widths after the start of the experiment (between 2 and 3 μm) both for the low- and the high-ration squids. High-ration squids also had significantly wider increments and larger statoliths than their low-ration siblings. Even though we detected consistent trends in daily statolith increment widths for the different feeding regimes, we could not detect variation in increment widths at a daily level of resolution (i.e. as a result of differences in day-to-day food intake at an individual level). This was probably due to the relatively consistent diet experienced by each individual. These experiments revealed that ration level influences squid growth rate, statolith size and daily statolith increment width. Received: 30 March 2000 / Accepted: 30 October 2000     

Effect of temperature on viability and axial muscle development in embryos and yolk sac larvae of the Northeast Arctic cod (Gadus morhua)

Cod (Gadus morhua L.) eggs may develop and hatch within temperatures of −1.5 to 12 °C, but little is known about the effects of very low temperatures on larval characteristics. Eggs of the Northeast Arctic cod (Gadus morhua) were incubated at 1, 5 or 8 °C from Day 1 after fertilisation until hatching, and transferred to 5 °C after hatching. Histological samples of the axial musculature were taken at hatching and 5 d after hatching, and the data on muscle cellularity from these samples were related to survival and hatching, size, developmental data and viability of the yolk sac larvae. All larvae hatched at the same developmental stage. Incubation of eggs at 1 °C produced shorter larvae with a larger yolk sac and more, small deep fibres at hatching than larvae from eggs incubated at 5 or 8 °C. The larval size difference was still present 5 d after hatching, a time at which the larvae from 1 °C-incubated eggs were less developed and less resistant to an acute viability stress test (65 ppt salinity). Although there were no differences between temperature groups in number and size of muscle fibres 5 d after hatching, the deep fibres of the 1 °C-group contained less myofibrils than the two other groups. The phenotype of the larvae at hatching was thus affected within these incubation temperatures. Although all groups were transferred to the same temperature after hatching, the lowest egg incubation temperature (1 °C) still had a negative effect 5 d after hatching, as these larvae were both smaller, less resistant to stress and had less functional muscles at the time of first feeding. Our conclusion is therefore that 1 °C is close to, or below, the lower thermal tolerance limit for normal functional development of Northeast Arctic cod. The results are discussed in relation to larval viability and recruitment of this species in the wild. Received: 4 February 1998 / Accepted: 10 July 1998