Zooplankton growth rates: the influence of size in nauplii of tropical marine copepods

Growth rates and development times were determined for nauplii of the genera: Acartia, Centropages, Corycaeus, Oithona, Paracalanus, Parvocalanus and Temora in nearshore waters of Jamaica from in situ microcosm incubations. At these high local temperatures (∼28 °C), total naupliar development time was short: 3 to 4 d inshore and 4 to 5 d offshore. Mean instantaneous growth rates (g) ranged from as high as 0.90 d⁻¹ for Parvocalanuscrassirostris to as low as 0.41 d⁻¹ for Corycaeus spp. In general, nauplii of cyclopoid copepods appeared to grow more slowly than those of calanoids of the same size. Naupliar growth rates were significantly related to body size (r ²= 0.43 to 0.50), but were unrelated to chlorophyll concentration in any measured size-fraction. This suggests that nauplii are generally not limited by resources, but are growing at their maximum temperature and size-dependent rates. Received: 30 May 1997 / Accepted: 13 May 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     

Growth and development rates of Calanus finmarchicus nauplii during a diatom spring bloom

Growth and development rates were determined for nauplii of Calanus finmarchicus (Gunnerus) in the near-shore waters of a western Norwegian fjord from in situ mesocosm incubations. The major food source for the nauplii was diatoms, but Phaeocystis sp., dinoflagellates and ciliates were also part of the diet. At local temperatures ranging from 4.8 to 5.2 °C the cumulative median development time from hatching to Nauplius VI was 19 d. The time taken to molt to the next naupliar stage was approximately constant (3 d) from Stages IV to VI, but Stage III needed the longest development time (5 d). The instantaneous growth rate in terms of body carbon was negative from hatching to Nauplius Stage II, but as high as 0.25 to 0.30 d⁻¹ from Stage III to V. Enhancement of food resources by nutrient addition led to no significant change in specific growth rates. Additionally, the cohorts from different nutrient regimes showed almost equal development time, size and body carbon within stages. Length–weight relationships of nauplii from the two different food resources were: W _{low resources} = 4.17 × 10⁻⁶ × L ^2.03 (r ² = 0.84) and W _{high resources} = 4.29 × 10⁻⁶ × L ^2.05 (r ² = 0.92), where weight (W) is in micrograms of C and body length (L) in micrometers. The natural body morphology of naupliar stages I to VI is illustrated with digital images, including the final molt from Nauplius VI to Copepodid Stage I. In general, development of the nauplii was faster than that of the copepodids of C. finmarchicus, and structural growth was exponential from naupliar stages III to VI. This study validates our earlier results that nauplii of C. finmarchicus can obtain high growth and nearly maximal developmental rates at relatively low food levels (∼50 μg C l⁻¹), suggesting that nauplii exhibit far less dependence on food supply than copepodids. Received: 30 July 1999 / Accepted: 7 March 2000     

Zooplankton growth rates: the influence of female size and resources on egg production of tropical marine copepods

Egg production was measured in 17 species of copepods from the genera Acartia, Calanopia, Centropages, Clausocalanus, Corycaeus, Eucheata, Euterpina, Oithona, Oncaea, Paracalanus, Parvocalanus, Temora and Undinula in Jamaican waters. At the high local temperatures (∼28 °C), mean egg production ranged from 3.2 to 88 eggs female^–1 d^–1, and instantaneous female growth (g, as egg production) ranged from 0.04 to 0.87 d^–1. Female growth was positively related to ambient chlorophyll concentration (r ² = 0.44) and negatively to female body size (r ² = 0.29). Together these two variables explained 60% of the variation in growth. When quadratic terms for chlorophyll and a term for interaction of body size and chlorophyll were introduced, 82% of the variance in growth rate was explained. Egg production rates represent an extension of the resource and size-dependent relationship established for copepodites. In smaller species (<3.5 μg), egg production was comparable to prior copepodite somatic growth; in larger species (>3.5 μg), egg production is compromised at lower resource concentrations than copepodite somatic growth. Thus, it appears that egg production in tropical copepods may be frequently limited by resources in a size-dependent manner. Under conditions where growth is resource limited, we caution against the application of egg production rates for the calculation of total copepod production. Received: 30 May 1997 / Accepted: 13 May 1998     

Effects of municipal piers on the growth of juvenile fishes in the Hudson River estuary: a study across a pier edge

The growth rates of two fish species, the winter flounder Pseudopleuronectes americanus (Walbaum) (19.3 to 42.6 mm total length, TL) and the tautog Tautogaonitis (Linnaeus) (23.9 to 55.9 mm TL), were used to evaluate habitat quality under and around municipal piers in the Hudson River estuary, USA. Growth rates were measured in a series of 10 d field caging-experiments conducted at two large piers in the summers of 1996 and 1997. Cages (0.64 m²) were deployed along␣transects that stretched from underneath the piers to beyond them, encompassing the pier edge (the transitional zone between the pier interior and the outside). Growth in weight (G _w ) was determined at five locations along the transect, 40 m beneath the pier, 20 m beneath the pier, at the pier edge, 20 m beyond the pier edge, and 40 m beyond. Under piers, mean growth rates of winter flounder and tautogs were negative (xˉG _W = −0.02 d⁻¹), and rates were comparable to laboratory-starved control fishes (xˉG _W = −0.02 d⁻¹). In contrast, mean growth rates at pier edges and in open waters beyond piers were generally positive (xˉG _W ranged from −0.001 to +0.05 d⁻¹), with growth at pier edges often being more variable and less rapid than at open-water sites. Analyses of stomach contents upon retrieval of caged fishes revealed that dry weights of food were generally higher among fishes caged at open-water stations (xˉ range = 0.02 to 0.72 mg dry wt) than at pier-edge (xˉ range = 0.01 to 0.54 mg) or under-pier (xˉ range = 0.03 to 0.11 mg) stations, although it was apparent that benthic prey were available at all stations on the transect. Our results indicate poor feeding conditions among fishes caged under piers, and suboptimal foraging among fishes caged at pier edges. Inadequate growth rates can lead to higher rates of mortality, and, based on these and other earlier experiments, we conclude that under-pier environments are poor-quality habitats for some species of juvenile fishes. Received: 12 March 1998 / Accepted: 9 November 1998     

Effects of inorganic and organic phosphates on feeding, feeding absorption, nutrient allocation, growth and righting responses of the sea urchin Lytechinus variegatus

The sea urchin Lytechinus variegatus is capable of surviving chronic exposure to sodium phosphate (inorganic phosphate) concentrations as high as 3.2 mg l⁻¹, and triethyl phosphate (organic phosphate) concentrations of 1,000 mg l⁻¹. However, chronic exposure to low (0.8 mg l⁻¹ inorganic and 10 mg l⁻¹ organic phosphate), medium (1.6 mg l⁻¹ inorganic and 100 mg l⁻¹ organic phosphate) or high (3.2 mg l⁻¹ inorganic and 1,000 mg l⁻¹ organic phosphate) sublethal concentrations of these phosphates inhibits feeding, fecal production, nutrient absorption and allocation, growth and righting behavior. Food consumption and fecal production declined significantly in individuals exposed to medium and high concentrations of inorganic phosphates and all levels of organic phosphates. Feeding absorption efficiencies for total organics and carbohydrates decreased significantly in individuals held in the highest concentration of organic phosphate. Feeding absorption efficiencies for lipids were significantly reduced in the highest inorganic phosphate concentration only, while they decreased significantly for protein with increasing phosphate exposure. Carbohydrate and lipid levels in gonad and gut tissues decreased significantly with exposure to increasing phosphate concentrations, potentially impairing both gametogenesis and nutrient storage in the gut. Moreover, gonad indices significantly decreased in individuals exposed to the highest concentrations of either phosphate. Growth rates decreased significantly under the influence of all phosphate concentrations, while increasing in seawater alone. Individuals exposed to increasing phosphate concentrations showed reduced righting responses (a measure of stress) and no acclimation in righting times during chronic exposure to phosphates over a 4 week period. These findings indicate that shallow-water populations of L. variegatus subjected to inorganic and organic phosphate pollutants will exhibit stress and be inhibited in their growth and performance due to reductions in feeding, nutrient absorption and allocation of nutrients to key somatic and reproductive tissues. Received: 10 April 2000 / Accepted: 2 October 2000     

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     

Differential increment-deposition rate in embryonic statoliths of the loliginid squid Loligo vulgaris

 Apart from one study that reported growth of less than one increment per day in statoliths of the squid Alloteuthis subulata, most studies so far have presumed that one increment was laid down per day in the statoliths of the squid species they examined. The present study provides evidence of differential daily growth rates in embryonic statoliths of the squid Loligo vulgaris Lamarck, 1798, thus confirming a previous report for A. subulata. Incremental growth rates of L. vulgaris statoliths differ as a function of temperature. Squid embryos were incubated in the laboratory at three temperatures (12.0, 15.5 and 21.1 °C), and tetracycline staining was used to follow statolith growth. This growth slowed in squid exposed to the lowest temperature, but recovered when the squid were returned to warm conditions, indicating statolith adaptation. Statolith growth rate after incubation at 12 °C was 1.3% d⁻¹ and reached 6.1% d⁻¹ for squids exposed to 21.1 °C. Statoliths from embryos incubated at 15.5 °C yielded a rate of 1 increment d⁻¹ and a mean daily growth of 2.2 μm in the dorsal dome area of the statolith. In contrast, the slow growth of statoliths incubated at 12 °C yielded a mean daily growth of only 0.9 μm in the dorsal dome and the readings resulted in a less-than-daily increment-deposition rate. Received: 9 October 1999 / Accepted: 30 March 2000     

Asymptotic growth trajectories of larval sardine (Sardinops melanostictus ) in the coastal waters off western Japan

Growth trajectories of individual larvae of Japanese sardine, Sardinops melanostictus, caught in the coastal waters off western Japan were back-calculated from the first feeding stage up to date of capture (approximate size of 20 to 35 mm total length; TL) based on individually determined allometric relationships between otolith daily ring radii and fish total lengths. The larvae in January-, February-, and March-hatched cohorts in the coastal waters grew faster and more uniformly than those in the oceanic waters offshore of the Kuroshio current. Growth trajectories of the three hatch-month cohorts were similar and could be expressed by the Gompertz model. The inflection points of the growth curves were reached at 9 to 11 d after hatching, when larvae were 10.8 to 11.8 mm TL. Maximum growth rates at these points were 0.80 to 0.85 mm d⁻¹. Growth rates gradually declined after the inflection points, and larval TLs converged into the infinite length of 29 to 32 mm, the sizes at which metamorphosis from larvae to juveniles is initiated. This asymptotic growth pattern in the larval stage resulted in the narrow ranges in TLs in spite of the wide range of ages of the larvae caught by boat seiners in the coastal waters. Slow growth and therefore long duration of the metamorphosing stage could be influential in determining the cumulative total mortality in the early life stages of the Japanese sardine. Received: 14 July 1996 / Accepted: 20 August 1996     

Effect of temperature on statolith growth of the European squid Loligo vulgaris during early life

Over the past decade, statolith interpretation has resulted in a major advance in our knowledge of squid population-dynamics, but the way in which environmental conditions affect the statolith increment-deposition ratio remains virtually unknown. The object of the present study was to determine the effect of temperature on this process, using tetracycline marks to validate statolith growth in Loligo vulgaris Lamarck, 1798 under rearing conditions equivalent to severe winter (11 °C) and summer (19 °C) temperature regimes. Tetracycline marking was performed every 10 d (at 10, 20, 30, 40, 50 and 60 d of age). The newly hatched squid paralarvae were slightly smaller in summer than those hatched in winter. Survival rates were similar in both cultures, but growth rates (wet mass) of summer squids were double those in winter. At hatching, statoliths were already longer in the summer squids, and growth rates were 2% d⁻¹ as opposed to 0.9% d⁻¹ for winter statoliths. For the dorsal dome area of the statolith, where more increment counts were made, statolith growth was of 3.25 μm d⁻¹ in summer, and daily increment deposition was confirmed in 87% of the statoliths. The slow growth of statoliths at winter temperatures yielded a mean growth of 1.1 μm d⁻¹– insufficient to discern the increments using light microscopy. Subsequent SEM observation enabled only 21% of the winter statoliths to be read; these also indicated a deposition rate of one increment d⁻¹. Since the life span of L. vulgaris is ≃1 yr, squids will experience at least one winter during their life cycle, and this might be visible on the statolith. Received: 28 June 1999 / Accepted: 20 December 1999     

Growth and grazing responses of tintinnid ciliates feeding on the toxic dinoflagellate Heterocapsacircularisquama

Growth and feeding rates of two tintinnid species, Favellaazorica and Favellataraikaensis, were determined under various concentrations of the dinoflagellate Heterocapsacircularisquama which has been reported as highly toxic to shellfish. Mean growth rates of F. azorica and F. taraikaensis on a diet of H.circularisquama (ca. 10² cells ml⁻¹) were 2.15 and 1.97 doublings d⁻¹, respectively. These values are similar to those on a diet of Heterocapsatriquetra which is suitable food for various zooplankton. However, growth rates of both tintinnid species decrease with increasing concentrations of >10³ cells ml⁻¹ of H. circularisquama. In particular, H. circularisquama under conditions of >10³ cells ml⁻¹ caused mortality in F.taraikaensis, probably due to toxins. Clearance and ingestion rates of F. azorica on H. circularisquama were 4.1 to 27.5 μl ind⁻¹ h⁻¹ and 1.5 to 28.7 cells ind⁻¹ h⁻¹, respectively, at concentrations of <10⁴ cells ml⁻¹ and those of F. taraikaensis were 0.9 to 22.1 μl ind⁻¹ h⁻¹ and 0.1 to 13.0 cells ind⁻¹ h⁻¹, respectively, at concentrations of <10³ cells ml⁻¹. Both clearance and ingestion rates on H.circularisquama were higher for replicates fed on H.triquetra. Daily grazing impact of the two species of Favella on the initial stage of a bloom of H.circularisquama were estimated to reach 6 to 50% of H. circularisquama at a concentration of 540 cells ml⁻¹, indicating that grazing by tintinnids such as Favella spp. may significantly regulate the initial stages of blooms of H. circularisquama. Received: 3 January 1997 / Accepted: 27 January 1997     

Effect of nutrient enrichment in the field on the biomass, growth and calcification of the giant clam Tridacna maxima

Nutrients were added separately and combined to an initial concentration of 10 μM (ammonium) and/or 2 μM (phosphate) in a series of experiments carried out with the giant clam Tridacna maxima at 12 microatolls in One Tree Island lagoon, Great Barrier Reef, Australia (ENCORE Project). These nutrient concentrations remained for 2 to 3 h before returning to natural levels. The additions were made every low tide (twice per day) over 13 and 12 mo periods for the first and second phase of the experiment, respectively. The nutrients did not change the wet tissue weight of the clams, host C:N ratio, protein content of the mantle, calcification rates or growth rates. However, ammonium (N) enrichment alone significantly increased the total population density of the algal symbiont (Symbiodinium sp.: C = 3.6 · 10⁸ cell clam⁻¹, N = 6.6 · 10⁸ cell clam⁻¹, P = 5.7 · 10⁸ cell clam⁻¹, N + P = 5.7 · 10⁸ cell clam⁻¹; and C = 4.1 · 10⁸ cell clam⁻¹, N = 5.1 · 10⁸ cell clam⁻¹, P = 4.7 · 10⁸ cell clam⁻¹, N + P = 4.5 · 10⁸ cell clam⁻¹, at the end of the first and second phases of the experiment, respectively), although no differences in the mitotic index of these populations were detected. The total chlorophyll a (chl a) content per clam but not chlorophyll a per cell also increased with ammonium addition (C = 7.0 mg chl a clam⁻¹, N = 13.1 mg chl a clam⁻¹, P = 12.9 mg chl a clam⁻¹, N + P = 11.8 mg chl a clam⁻¹; and C = 8.8 mg chl a clam⁻¹, N = 12.8 mg chl a clam⁻¹; P = 11.2 mg chl a clam⁻¹, N + P = 11.3 mg chl a clam⁻¹, at the end of the first and second phases of the experiment, respectively). The response of clams to nutrient enrichment was quantitatively small, but indicated that small changes in inorganic nutrient levels affect the clam–zooxanthellae association. Received: 2 June 1997 / Accepted: 9 June 1997     

Production of tropical copepods in Kingston Harbour, Jamaica: the importance of small species

The copepod community observed during an 18-month period at the mouth of eutrophic Kingston Harbour, Jamaica, was dominated by small species of Parvocalanus, Temora, Oithona, and Corycaeus. Mean copepod biomass was 22.1 mg AFDW m⁻³ (331 mg m⁻²). Annual production was 1679 kJ m⁻², partitioned as 174 kJ m⁻² naupliar, 936 kJ m⁻² copepodite, 475 kJ m⁻² egg and 93 kJ m⁻² exuvial production. All nauplii, most copepodites and many adults, equivalent to half of the biomass and production, were missed by a standard 200-μm plankton net, emphasizing the importance of nauplii and small species in secondary production estimates. The evidence suggests that growth rates and production are generally not food limited, and we speculate that size-selective predation shapes the structure of the harbour community. Biomass and production are higher than previous estimates for tropical coastal waters, but comparable to other eutrophic tropical embayments and many productive temperate ecosystems. Far from being regions of low productivity, tropical zooplankton communities may have significant production and deserve greater research attention than they currently receive. Received: 19 September 1997 / Accepted: 21 October 1997     

Chemical composition and growth indices in leptocephalus larvae

 Leptocephali grow at extremely high rates (>1 mm d⁻¹), but, unlike most fish larvae, leptocephali may remain in the plankton as larvae for several months before metamorphosing into the juvenile form. During their planktonic phase, leptocephali accumulate energy reserves in the form of glycosaminoglycans which are then expended along with lipid reserves to fuel metamorphosis. Otolith growth rates were determined using scanning electron microscopy for four species of leptocephali common in the Gulf of Mexico, Paraconger caudilimbatus (Poey, 1867), Ariosoma balearicum (Delaroche, 1809), Gymnothorax saxicola (Jordan and Davis, 1891), and Ophichthus gomesii (Castelnou, 1855). Proximate composition, RNA:DNA ratios and protein growth rates were examined with respect to mass, length and age. The leptocephalus growth strategy was strongly reflected in the growth indices. Mass (Y) in all four species increased with increasing age (X) according to the equation Y = aX ^b , where a is a species-specific constant and 1.05 < b < 2.40. The accumulation of acellular mass was evident in protein growth rates and RNA:DNA ratios, and was observed as a shift in increasing size from rapid growth in length to a greater increase in mass with age. These results suggest that the proportion of actively metabolizing tissue declines with size and is replaced by the metabolically inert energy depot: the glycosaminoglycans. Leptocephali can thus grow to large size very rapidly with minimal metabolic penalty, an unusual and successful developmental strategy. Received: 27 December 1999 / Accepted: 8 June 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     

Light and carbon balance in the seagrass Thalassia testudinum : evaluation of current production models

The production dynamics and carbon balance of Thalassia testudinum in the lower Laguna Madre, Texas, USA, were examined during the 1995 summer period based on in situ photosynthesis vs irradiance (PI) measurements and continuous measurements of underwater photon-flux density (PFD). The validity of applying the H _sat model, used to calculate production for Zostera marina as the product of the maximum rate of photosynthesis (P _max) and daily hours of saturating irradiance (H _sat) was assessed for T. testudinum by comparison with integrated production estimates derived through numerical integration. Gross integrated production values were combined with dark-respiration measurements of photosynthetic (PS) and non-photosynthetic (NPS) tissues and areal biomass to generate daily whole-plant carbon balance. Production and whole-plant carbon balance are discussed in relation to surface and underwater PFD measurements, biomass and other physical and chemical parameters collected during a 1 yr period from January to December 1995. The H _sat model significantly underestimated production during all summer months, averaging 70% of integrated production over the entire study period. Gross integrated production ranged between 11.5 mg C g⁻¹ leaf dry wt d⁻¹ in June (during a period of unseasonably low PFDs caused by a drift-alga mat covering the seagrass bed) to 26.7 mg C g⁻¹ leaf dry wt d⁻¹ in July. Modeled net carbon gain was highest in July at 454 mg C m⁻² d⁻¹ (1.4 g dry wt m⁻² d⁻¹), sufficient to account for measured rates of leaf production in the study area and representative of T. testudinum populations of low productivity. During part of the summer period, however, the population was in negative carbon balance. The relatively low productivity of this population and the periods of negative carbon balance are attributed to low net photosynthesis:dark respiration (P _net:R _d) ratios, sporadic low-light periods, the small fraction of PS tissue relative to whole-plant biomass (5 to 13%) and nutrient limitation. Production models are sensitive to both light availability and the proportion of PS tissue supporting NPS biomass as reflected in whole-plant P _net:R _d ratios. Received: 13 August 1997 / Accepted: 6 March 1998     

Reproduction, dispersal and settlement of the bathyal ophiuroid Ophiocten gracilis in the NE Atlantic Ocean

  Ophiocten gracilis is an ophiuroid found at bathyal depths in the North Atlantic Ocean. The adults show strong seasonal reproduction, with an ophiopluteus in the surface plankton. Settling postlarvae were collected in sediment traps moored at 1000 and 1400 m depth in the NE Atlantic during Julian Days 142 to 212 (May to July) in 1996. During this period, growth of postlarvae in the traps was linear and the diet consisted of phytodetritus and foraminifera. Experiments suggest that postlarvae sink at rates of up to 500 m d⁻¹, although this may well be slower in the natural environment. The high fecundity, seasonality and high population density resulted in high fertilization success, and many of the offspring were advected outside the normal adult range, where they were able to settle but did not survive to adulthood. Received: 8 October 1999 / Accepted: 8 May 2000     

Resting spore formation of the marine planktonic diatom Chaetoceros anastomosans induced by high salinity and nitrogen depletion

The marine planktonic diatom Chaetoceros anastomosans, which was isolated from Sagami Bay, was used for a study of resting spore formation mechanisms in batch culture experiments. Vegetative cells could grow at salinities ranging from 20.7 to 45.5‰, and resting spore formation was enhanced significantly in nitrate-depleted, high salinity media (40.0 to 45.5‰). The rate of resting spore formation (1.9 d⁻¹) was comparable to the specific growth rate (1.8 d⁻¹) of vegetative cells in the exponential growth phase in normal salinity medium. The size of resting spores formed under high salinity conditions was smaller than that of spores formed in normal salinity media. Unlike vegetative cells, resting spores seemed to possess some mechanisms to survive over a wider range of salinities by resisting bacterial attacks on their cell walls. Received: 4 August 1996 / Accepted: 27 August 1996     

Feeding biology of a predatory and a facultatively predatory nematode (Enoploides longispiculosus and Adoncholaimus fuscus)

This paper reports on the feeding biology of a predatory and of a facultatively predatory nematode, Enoploides longispiculosus and Adoncholaimus fuscus, respectively. Both species represent genera which are common and abundant in the littoral of the North Sea and in adjacent estuaries. Observations on the foraging behaviour of both species are given, and for the former species, a range of prey from its natural habitat is identified. Respiration was determined using a polarographic oxygen electrode technique and compared to consumption determined as predation rates on the monhysterid nematode Diplolaimelloides meyli. The daily C-loss due to respiration accounted for 15% of the measured C-consumption in E. longispiculosus and for 111% in A. fuscus, proving the observed feeding rates in the latter species to have been inadequate for the maintenance of its aerobic metabolism. Daily respiration rates at an average environmental temperature were 219 ng C ind⁻¹d⁻¹ for adults of A. fuscus and 21.9 ng C ind⁻¹ d⁻¹ for adults of E. longispiculosus. Using radiotracer techniques, no uptake of bacterial cells or of organic matter in the dissolved phase was demonstrated for E. longispiculosus. In A. fuscus, however, a significant drinking of label in the dissolved or volatile fraction occurred; bacterial cells were taken up at a level insignificant to the nematode's daily C-ration. It is concluded that E. longispiculosus has a fairly strict predatory feeding strategy, while A. fuscus gains a majority of C from additional foraging strategies, among which the uptake of dissolved material and scavenging on macrofauna carcasses (as reported in the literature) may be of particular importance. Received: 28 August 1998 / Accepted: 8 March 1999     

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