Grazing in juvenile stages of some estuarine calanoid copepods

The grazing of juvenile Eurytemora affinis, Acartia tonsa and A. clausi from the Chesapeake Bay (USA) was investigated using natural particle distributions and freshly caught copepods, live-sorted into stages. Data were analyzed in 110 size channels using an electronic particle counter, and filtering rates (FR) were estimated based on total particle removal (mean FR), and that for each size channel (giving maximum FR). Mean and maximum filtering rates increased from NVI (Nauplius Stage VI) through CVI (Copepodid Stage VI). Both rates plotted against weight satisfied a log fit best for A. tonsa, and a linear fit best for E. affinis. Results for A. tonsa were quite variable, apparently due to differences in temperature between experiments. Particle selection was investigated from the shape of the filtering rate curve over particle size. We define selective feeding by a FR curve which is higher in some size categories, and non-selective feeding by a flat FR curve. The general pattern was one of selective feeding in all copepodid stages of the three calanoid copepods investigated. E. affinis tended to track biomass peaks while Acartia spp.'s feeding was more variable, including feeding in size ranges of greatest particle concentration, on larger particles, and in other size categories as well. Experiments with nauplii tended to yield flat FR curves, and it may be that selective grazing appears with, or is greatly accentuated by, metamorphosis from NVI to CI (Copepodid Stage I).University of Maryland, Center for Environmental and Estuarine Studies Contribution No. 762.     

Modification of the feeding behavior of marine copepods by sub-lethal concentrations of water-accommodated fuel oil

The feeding behaviors of Acartia clausi and A. tonsa were measured in samples of water containing low levels of a water-accommodated fraction of No. 2 fuel oil. The copepods fed normally at a hydrocarbon concentration of 70 g l^-1, but their feeding behavior was altered both quantitatively and qualitatively at a concentration of 250 g l^-1. Three types of response to the higher oil level were found. The first was total suppression of feeding. Both other types involved suppression of feeding on particles between 7 and 15 m diameter, but one showed no change in the ingestion of larger particles, whereas the other displayed increased feeding on particles larger than 15 m diameter. These results suggest that the species of Acartia studied use three different modes of feeding, each on a different size range of particulate material. Low-level hydrocarbon pollution affects each feeding mode differently.Contribution No. 973, Center for Environmental and Estuarine Studies of the University of Maryland     

Zooplankton of the Bristol Channel and Severn Estuary. The distribution of four copepods in relation to salinity

The seasonal variations in distribution and abundance of the common zooplankton species in the Bristol Channel and Severn Estuary were related to the salinity regimes observed over the period November 1973 to February 1975. The dominant constituents in all regions were the calanoid copepods, which reached maximum densities in July: approximately 100 times their winter levels. Four zooplankton assemblages were recognised using an objective classification program which computed similarity coefficients and used group-average sorting. The assemblages existed along the salinity gradient observed from the Severn Estuary to the Celtic Sea. The assemblages were classified as true estuarine, estuarine and marine, euryhaline marine and stenohaline marine and were characterized by the copepods Eurytemora affinis (Poppe) (<30S), Acartia bifilosa var. inermis (rose) (27 to 33.5S), Centropages hamatus (Lilljeborg) (31 to 35S) and Calanus helgolandicus (Claus) (>33S), respectively.     

Dietary constraints on reproductive periodicity in two sympatric deep-sea astropectinid seastars

Examination of the diet of two sympatric species of seastar, Bathybiaster vexillifer and Plutonaster bifrons from 2200 m depth in the Rockall Trough, NE Atlantic Ocean (57°18'N; 10°28'W), suggested that diet may determine the different reproductive patterns found between these two species. In the non-seasonally breeding B. vexillifier, the diet showed a high Shannon-Wiener prey diversity index, the dominant prey being the irregular echinoid Hemiaster expergitus together with a variety of prosobranch gastropods and protobranch bivalves. By contrast, the prey diversity in the seasonally breeding P. bifrons was significantly lower than that of B. vexillifer. In addition, organic carbon content in the sediment residue in the stomachs of P. bifrons displayed a seasonal cycle, while no such seasonality was detected in B. vexillifer. The stomachs of P. bifrons also contained a higher proportion of scavenged material, including the seasonally available remains of mesopelagic blue whiting. These data, together with Bathysnap (time-lapse camera) observations of feeding behaviour in both species, suggest that B. vexillifer is a predator feeding deep in the sediment, whereas P. bifrons feeds close to the sediment surface where it is affected by the seasonal availability of phytodetritus and fish carcasses.     

Dependence of consumers on macroalgal (Laminaria solidungula) carbon in an arctic kelp community: δ13C evidence

Stable carbon isotope measurements (¹³C) were used to assess the importance of kelp carbon (-13.6 to-16.5) versus phytoplankton carbon (-25.5 to-26.5) to resident fauna of an isolated kelp bed community on Alaska's north arctic coast from 1979 to 1983. The predominant kelp, Laminaria solidungula, showed some seasonal variation in ¹³C which was correlated with changes in the carbon content of the tissue. Animals that showed the greatest assimilation of kelp carbon (>=50%) included macroalgal herbivores (gastropods and chitons,-16.9 to-18.2), a nonselective suspension feeder (an ascidian,-19.0) and a predatory gastropod (-17.6). Animals that showed the least incorporation of kelp carbon into body tissues (<=7%) included selective suspension-feeders (hydroids, soft corals and bryozoans,-22.8 to-25.1). Sponges, and polychaete, gastropod and crustacean omnivores exhibited an intermediate dependence on kelp carbon (15 to 40%). Within some taxonomic groups, species exhibited a broad range in isotopic composition which was related to differences in feeding strategies. In the polychaete group alone, ¹³C values identified four major feeding habits: deposit-feeders (-18.0), omnivores (-20.4), predators (-22.2) and microalgal herbivores (-23.0). Distinct seasonal changes in the ¹³C values of several animals indicated an increased dependence on kelp carbon during the dark winter period when phytoplankton were absent. Up to 50% of the body carbon of mysid crustaceans, which are key prey species for birds, fishes and marine mammals, was composed of carbon derived from kelp detritus during the ice-covered period.     

Food selection capabilities of the estuarine copepod Acartia clausi

Existing viewpoints and theories of selective grazing by copepods are briefly reviewed in order to formulate explicit hypotheses to be tested experimentally. Based on these hypotheses, a series of grazing experiments was run to determine (1) the extent of the selective ingestion capabilities of Acartia clausi and (2) how these capabilities were affected by previous feeding histories. Groups of copepods were separately preconditioned on a small diatom (Thalassiosira pseudonana), a large diatom (T. fluviatilis), or a plastic sphere. The ingestive behavior was then examined on various combinations of spheres and food particles. Spheres offered alone were not ingested. In mixtures of diatoms and spheres, the copepods avoided ingesting spheres intermediate in size between the sizes of the diatoms. The copepods either ingested particles on either side of the spheres, or ignored all particles less than the size of the largest spheres. The pattern observed depended upon the size of the preconditioning food. However, if the spheres were larger than the largest food particles, the copepods still selectively ingested the food particles. The above results demonstrate that A. clausi has a complex grazing behavior consisting of (1) more efficient grazing on larger particles within its particle-size ingestion range; (2) the ability to alter effective setal spacing to optimize feeding behavior (i.e., the ability to increase efficiency of capture of food particles, and to avoid non-food particles); and (3) the ability for post-capture rejection of non-food particles when they interfere with the ingestion of food particles on which the copepod has been preconditioned. The behavioral patterns observed depend heavily on the food preconditioning and the presence or absence of non-food particles. These results clearly indicate that a simple mechanistic explanation of selective grazing is insufficient.     

Concentrations of Mn,Fe, Cu,Zn and Cd in the mesopelagic decapod Systellaspis debilis from the East Atlantic Ocean

Specimens of the oceanic decapod Systellaspis debilis were collected from six sites in the East Atlantic Ocean between 1970 and 1984, and were analysed for Mn, Fe, Cu, Zn and Cd. The data confirm that there are small but significant differences in mean metal concentrations from some sites which showed no obvious pattern in relation to geographic location of the samples. As a result, ranges of site means are quoted as baseline levels for each metal (g g^-1 dry wt): 2.3 to 2.9 g Mn g^-1, 31.2 to 77.8 g Fe g^-1, 25.9 to 83.4 g Cu g^-1, 41.9 to 92.9 gZn g^-1, 11.1 to 31.8 g Cd g^-1. The concentration of cadmium in S. debilis from all sites was raised relative to cadmium concentrations reported for coastal decapods, perhaps as a result of dietary enrichment. Metal accumulation may provide useful information for understanding the complex feeding behaviour of many oceanic animals.     

Vertical distribution and nocturnal migration of Nyctiphanes couchi (Crustacea: Euphausiacea) in relation to the summer thermocline in the Celtic Sea

Vertical distributions and nocturnal migrations of the developmental stages of Nyctiphanes couchi (Bell) in relation to the summer thermocline in the Celtic Sea, 25 to 26 August 1982, have been investigated using the Longhurst-Hardy Plankton Recorder (LHPR). The vertical distributions of the metanauplii and adult females suggest that N. couchi liberates its young within the euphotic zone as mature metanauplii which, in a matter of hours, moult into the first feeding stage (Calyptopis I). The ascent migration by adult females took a maximum of 3 h (17.10 to 20.05 hrs) and had an amplitude of 50m (54 to 4 m) from below to above the thermocline. A 7C° thermocline occurred between 20 to 30 m in these profiles. The nocturnal migrations by the females were for the purpose of breeding as well as feeding within the euphotic zone and were not influenced by the presence of the thermocline. The majority of the calyptopes and furciliae remained above the thermocline over the sampling period. The post-larval males and females migrated; their vertical distributions showed a pattern similar to those of the adult females. The larger the developmental stage, the deeper was the mode of its vertical distribution. The zooplankton dry weight in the profiles ranged from 3.74 to 6.91 g per haul (=1.85 to 3.45 g C m^-2, 0 to 100 m). The euphausiids represented 35% of total zooplankton dry weight and their migrations removed a large percentage of the total zooplankton biomass from the euphotic zone for 18 h d^-1. Such a large displacement of biomass would have a major impact on the biological interactions within the ecosystem.     

The zooplankton of the inshore waters of Dar es Salaam (Tanzania,S. E. Africa) with observations on reactions to artificial light

From 18th December, 1968 to 5th January, 1970, zooplankton samples were taken in darkness and using artificial light in selected areas of the Dar es Salaam coast (Tanzania, S. E. Africa). Surface sea temperature was measured on most occasions, salinity for the first 7 months only. The neritic waters of Dar es Salaam experience a warm period during January to March and a cool period during July to September. The annual salinity cycle is not known. Thirtysix dark and 17 light zooplankton samples were analysed; where possible, organisms were identified to species, others to generic or higher taxonomic level. The principal taxa and their mean percentage proportions (figures in parentheses) in the dark samples were: Calanoida (49. 1), Larvacea (11. 9), Corycaeus spp. (6.4), Cypridina sinuosa (5.6), Oithona spp. (4.8), caridean larvae (4.0), Sagitta spp. (3.8), Euterpina (2.1), Lucifer (1.2), Oncea (1.2), calyptopis larvae (1.0), Hydromedusae (1.0), Euconchoecia chierchiae (1.0), Creseis acicula (1.0), brachyuran zoeae (0.8), Ctenophora (0.5), Mysidacea (0.5), fish eggs (0.5), postlarval bivalves (0.5), postlarval gastropods (0.5), Cumacea (0.1), Gammaridea (0.1) and Hyperiidea (0.1). Evadne tergestina and Thalia democratica abound in waters at certain times of the year only and then virtually disappear. The remaining groups were numerically unimportant most of the time. Almost every major group showed an annual cycle of abundance; greater numbers were recorded either for the entire or part of the period February to August, compared to the period September to November. From the end of January to early August, 1969, the average numbers per haul of total zooplankton were about three times greater than for the period mid-August to mid-November. During February-March and July-August, several oceanic indicators were observed together in the neritic waters. Artificial light induced the following changes in night zooplankton: Mysidacea, Leptochela sp., Hyperiidea, Cypridina sinuosa, brachyuran megalopae and fish larvae were attracted towards artificial light and aggregated densely under the lamp; Calanoida, Corycaeus spp., Macrosetella, Microsetella and Euterpina avoided regions of strong illumination and aggregated in dim-lit areas; Lucifer, Creseis acicula and postlarval gastropods were more abundant in dim-lit samples compared to dark hauls; caridean larvae, brachyuran zoeae and larvaceans were less abundant in high-intensity light samples compared to dark hauls on the same nights.This work was performed at the Department of Zoology, University of Dar es Salaam, Tanzania, S.E. Africa.     

β-glucanase activity in the seastar Pisaster ochraceus

Specimens of Pisaster ochraceus, an intertidal carnivorous seastar, were collected in northern Puget Sound at Post Point, near Bellingham, Washington, USA, in November 1993, and used in experimentation through September 1994. Ammonium sulfate fractions (60% precipitates) from excised pyloric caeca (digestive glands) possessed -glucanase activity directed toward such -linked substrates as carboxymethylcellulose (CMC), partially-digested cellulose (cellodextrins) and the synthetic substrate p-nitrophenyl--D-glucoside (PNDG). pH-activity and pH-stability maxima were at pH 6 and 5, respectively. No hydrolytic activity was detectable on native cellulose, native chitin or on cationic-modified celluloses such as DEAE-cellulose. Gel filtration (Sephadex G-100) of this 60% precipitate followed by DEAE-Sephadex column chromatography revealed heterogeneity of -glucanase activity, with an indication of up to 11 distinct fractions with variable activity directed toward the substrates indicated above (CMC, cellodextrins and PNDG). These fractions displayed differing ratios of hydrolytic activity on these substrates. None of the isolated fractions displayed activity toward either native cellulose or native chitin. Gut extracts from the edible mussel (Mytilus edulis), a common prey species of P. ochraceus, were shown to possess partially digested cellulose (cellodextrins). These cellodextrins were further degraded by seastar -glucanases in vitro. The digestion of the gut contents of ingested prey provides a possible adaptive explanation for the presence of -glucanase activity in a carnivore.     

Feeding rates and diel vertical migration of copepods near South Georgia: comparison of shelf and oceanic sites

Seventeen Longhurst Hardy Plankton Recorder profiles were taken over a diel cycle in January 1990 to study the feeding of four major copepods over the South Georgia shelf. Ontogenetic changes in vertical migration were followed and feeding cycles determined by gut fluorometry for Calanoides acutus Stage CV, Calanus sinillimus CV and CVI, C. propinquus CV and Rhincalanus gigas CV and CVI. In common with a neighbouring oceanic site visited two weeks later and reported elsewhere, all four species had a diel cycle of feeding and migration. The vertical distributions of C. simillimus (all stages), R. gigas (nauplii) and Euphausia frigida (postlarvae) were similar at both sites, the night being spent within the chlorophyll maximum at 15 to 30 m. However, the biomass dominants, C. acutus and R. gigas, dwelt below the chlorophyll maximum, about 30 m deeper than their oceanic counterparts. Unlike the oceanic site, feeding at the shelf site was not restricted to darkness, but increased 6 to 10 h before nightfall and finished at dawn; the intervening period coincided with sinking and digestion. Daylight feeding may have been induced by the shorter night, lower light levels or greater food requirements at the shelf site, despite planktonic predators being over three times more abundant. Daily ration estimates for R. gigas at both sites were only 2% body carbon per day. These low values contrast with its smaller competirors, whose rations were in the range 5.6 to 27%.     

On the ecology of Heterocypris salinus,H. incongruens and Cypridopsis aculeata (Crustacea: Ostracoda) from Baltic brackish-water rockpools

The ostracod fauna of Baltic brackish-water rockpools is made up of two groups: permanent members of the pools, and occasional guests from the littoral zone. The former group consists of Heterocypris salinus, H. incongruens and Cypridopsis aculeata. These species are characterized by rapid development (which starts when the water temperature approaches 15°C), a short life span, and 2 or 3 separate generations during the summer and autumn. The number of generations is determined by water temperature. Hibernation always takes place as eggs. Hatching and development during the late spring or early summer has been found to be mostly simultaneous. The spawning of the 3 species always starts epidemically. Reproduction is entirely parthenogenetic in the investigated area. Under natural conditions, C. aculeata may be found with either H. salinus or H. incongruens, but these two latter species have never been recorded together. H. incongruens is less tolerant to high salinities than the other 2 species and, even after successive adaptation, it does not resist salinities higher than 16. H. salinus has been found in 35.2 S in the field, and has been kept in 30 S in the laboratory after successive adaptation. The optimum salinity-temperature range for this species is 5 to 10 S and 15°C, when both survival and development are considered. Corresponding figures for C. aculeata are 0.5 to 20 S and 15°C, although this species, like H. salinus, survives longest at 5°C. The very rapid development, parthenogenetic reproduction and short life span of these species must be considered as favourable adaptations to the variable and unstable environment of the rockpool ecosystems.     

Capillary supply and mitochondrial volume density in the axial muscles of the mesopelagic teleost Argyropelecus hemigymnus

Quantitative measurements have been made on the ultra-structure and capillary supply to the axial muscles of the mesopelagic hatchet fish Argyropelecus hemigymnus (Cocco, 1829). Fish were collected at Eastern North Atlantic Ocean Station 10244, 32°48N; 31°15W during November 1980, from a depth of 480 to 550 m. Mitochondria with densely packed cristae occupy 44.3% of slow-fibre volume. Each myofibril is in direct contact with a mitochondrion. Compared with other fishes studied, the capillary supply to A. hemigymnus slow fibres is poorly developed. The average number of capillaries per fibre is 0.9, such that each m of capillary contact supplies 0.011 m² of fibre cross-sectional area. The capillary surface area (m²) supplying 1 m³ of slow-fibre mitochondria is 0.17 in anchovy (Engraulis encrasicolus), 0.14 in rat-fish (Chimaera monstrosa), 0.14 in tench (Tinca tinca), 0.16 in catfish (Clarias mossambica), and only 0.025 for A. hemigymnus. It is suggested that, relative to the former species, some modifications in factors determining tissue oxygenation (e.g. myoglobin concentrations, blood flow, perfusion distribution or haemoglobin) and/or mitochondrial respiration rate are required in order to match oxygen supply and demand to the slow muscle in A. hemigymnus.     

Eulerian sampling of zooplankton from Marseilles' Vieux Port over a 24 h period

A 24 h continuous study of hydrological parameters and zooplanktonic populations was carried out in Marseilles' Vieux-Port, a semi-closed area supposed to be a fairly homogeneous body of water. Sampling was performed in spring 1980 with a pump system. Samples were taken every 30 min over a period of 24 h, during which time only slight temperature variations were recorded (min.=14.5°C, max.=15.1°C); salinities were high and fairly stable (min.=37.49, max.=37.63%.). It can thus be assumed that the water was homogeneous. Concentrations of suspended particulate matter were high and fairly stable (x = 11.8 mg 1^-1). In most samples, nitrate levels were fairly low (x = 0.84 g-at 1^-1); conversely, phosphate contents fluctuated greatly (min.=0.24 g-at 1^-1, max.=2.24 g-at 1^-1). As a result of these fluctuating nutrient-salt values, the N:P ratios were often unbalanced (between 0.5 and 2.0). Detergent values were low (often less than 0.2 ppm). Zooplanktonic populations were made up of a few eurybiotic species, the most thriving of which were the cladoceran Podon polyphemoides and the copepod Acartia clausi. These two species perform nychthemeral migrations towards the surface: P. polyphemoides during the day and A. clausi at night. As a result of these opposite patterns of migrations, the planktonic biomass remained fairly stable near the surface. There were also some meroplanktonic species (especially cirripede larvae) and a small number of other copepods present. This study revealed a regular hydrological and faunal cycle with a return to initial conditions at the end of the 24 h period.     

Variability of biochemical composition and size distributions of seston in the euphotic zone of the Bay of Biscay: implications for microplankton trophic structure

Particle-size distributions and several biochemical components of seston were studied from October 1986 to December 1987 in surface waters of the Bay of Biscay. Variance partitioning of hydrographic and seston parameters indicates that, although seasonal variability related to the thermal cycle was important, most of the differences in seston concentration were caused by spatial components of variance. Changes in the vertical structure of the water column appeared to be the principal source of variability. Phytoplankton blooms in spring and fall and the effects of the coastal upwelling and the thermocline during summer were traced using seston concentration and biochemical composition. The different seston measurements were scaled according to their correlations with total concentration and particle size. Photosynthetic pigment biomass was related to both large and small partieles. However, concentrations of particulate proteins, lipids and carbohydrates were more related to small particles. Although not specifically analysed, low concentrations of inorganic particles and detritus can be expected in the study area, thus most of the particulate organic matter was associated with small, non-pigmented organisms, particularly in surface waters during the period of thermal stratification. The variability in concentrations of total seston and particulate organic matter was mainly due to variations in particulate proteins, lipids and carbohydrates; pigment concentrations were of secondary importance. Expressed as the ratio chlorophyll a: particulate protein-nitrogen, phytoplankton constituted an important fraction of the microplankton biomass only during spring blooms, when it averaged 75% of the particulate protein-nitrogen. In constrast, <30% of protein-nitrogen was related to chlorophyll a during the summer. These results suggest that an oligotrophic structure based on the microbial-loop prevails in microplankton assemblages of surface waters for most of the year.     

Comparison of bacterial and algal utilization of orthophosphate in an estuarine environment

Bacterial utilization of orthophosphate in an estuarine environment has been differentiated from algal utilization by using flow-filters of 5.0, 1.2 and 0.45 m poresize. Examination by light microscopy showed that most of the bacterial population passed through a 5.0-m filter, whereas most algae were retained. In all experiments, bacterial and algal cell numbers and biomass were estimated. P-uptake by algae and bacteria was closely correlated with cell biomass. P-uptake by algae was high only in the summer months, whereas P-uptake by bacteria was high throughout the year. Neither algal nor bacterial P-uptake, however, was correlated with temperature or dissolved orthophosphate, total organic phosphate or total phosphate concentrations. Cell biomass of algae at a given time had a high correlation with dissolved organic phosphate levels in 2 weeks prior to sampling (r=0.830) and a low correlation in the 2 weeks following sampling (r=0.0005). Algal cell numbers had a high correlation with bacterial cell numbers (r=0.950). The biomass of algae and bacteria also had a high correlation (r=0.902). The rate of P-uptake from the water by algae and bacteria varied with season and with the species composition of the natural population.     

Grazing and ingestion rates of nauplii,copepodids and adults of the marine planktonic copepod Calanus helgolandicus

The average grazing and ingestion rates of all stages of the marine planktonic copepod Calanus helgolandicus (Calanoida) from nauplius stage IV to adults were measured experimentally at 15°C in agitated cultures. The chain-forming diatom Lauderia borealis and the unarmoured dinoflagellate Gymnodinium splendens were offered as food. The food concentrations were close to natural conditions and ranged from 36 to 101 g of organic carbon per liter. The medium body weights expressed in g of organic carbon of almost all larval stages raised at 49 g C/1 were identical with the weight of the same stages caught in the Pacific Ocean off La Jolla, California, USA. In a log-log system, grazing and ingestion rates increased almost linearly with increasing body weight. Grazing rates ranged from 4 to 21 ml/day/nauplius stage IV to 286 ml to 773 ml/day/female. Ingestion rates increased from 0.2 g to 0.8 g C/day/nauplius stage IV to 18 g to 69 g C/day/female. Grazing and ingestion rates per unit body weight decreased gradually with increasing body weight. The daily ingested amount of food decreased from 292 to 481% of the body weight (g C) of nauplius stage V to 28–85% of the body weight of adult females. Grazing and ingestion performances of all stages increased with increasing particle size. Grazing rates decreased and ingestion rates increased with increasing food concentrations. The published data on food intake of the different age groups of C. helgolandicus show that the young stages of herbivorous planktonic copepods can play a major part in the consumption of phytoplankton in the sea due to their high grazing and ingestion rates.     

Über den einfluß des hydrostatischen druckes auf die ultrastrukturen von zellen und geweben. VI. Reversibilitätsversuche druckinduzierter schädigungen an membranen und mikrotubuli in Zellen mariner Tiere

Gill epithelia of Modiolus modiolus and Mytilus edulis (Baltic and North Seas), and the skin, pharynx and muscles of Branchiostoma lanceolatum (Pallas) have been investigated at the ultrastructural level, employing hydrostatic pressure, to explore the possibility of structural regeneration following decompression. Restoration of all cell structures seems possible to a certain extent, depending on the degree of disintegration and the time period given after decompression. Structural reorganization after release of pressure begins immediately, but no regenerated organelle is comrable with control species, since structural scars remain. Non-reversible destruction to some systems is apparent near the respective LD₅₀-values of the species.     

Juvenile growth of the polychaete Nereis virens feeding on a range of marine vascular and macroalgal plant sources

A laboratory experiment was conducted to examine variation of juvenile growth (% d^-1) of the polychaete Nereis virens (Sars) in relation to tidal flat plant species as food sources. We used vegetable materials (algae and vascular plants) which are carried along by tidal currents and are found abundantly at the upper tidal level. Juveniles (2-yr-old) markedly increased in wet weight with the algae Laminaria longicruris (weight-specific growth rate : 1.7% d^-1) and Enteromorpha intestinalis (: 1.6% d^-1) as food sources. A higher value of assimilation efficiency was observed for algal species (L. longicruris 55.1±7.9%; E. intestinalis 54.8±0.5%; Fucus vesiculosus 40.6%) than for marine vascular plants (Spartina alterniflora 26.8±10.9%; Zostera marina 1.4%). The digestion of marine vascular species lasts longer (19 to 38 h) than that of algal species. All these characteristics (growth, assimilation efficiency and duration of the digestive process) seem to correlate positively with lignin and cellulose concentrations in the plant structure.     

Food size spectra,ingestion and growth of the copepodAcartia tonsa during development: Implications for determination of copepod production

Clearance rates on different sizes of spherically shaped algae were determined in uni-algal experiments for all developmental stages (NII through adult) of the copepodAcartia tonsa, and used to construct food size spectra. Growth and developmental rates were determined at 7 food levels (0 to 1 500 g C l^-1 ofRhodomonas baltica). The lower size limit for particle capture was between 2 and 4 m for all developmental stages. Optimum particle size and upper size limit increased during development from 7 m and 10 to 14 m for NII to NIII to 14 to 70 m and 250 m for adults, respectively. When food size spectra were normalized (percent of maximum clearance in a particular stage versus particle diameter/prosome length) they resembled log-normal distributions with near constant width (variance). Optimum, relative particle sizes corresponded to 2 to 5% of prosome length independent of developmental stage. Since the biomass of particulate matter is approximately constant in equal logarithmic size classes in the sea, food availability may be similar for all developmental stages in the average marine environment. Juvenile specific growth rate was exponential and increased hyperbolically with food concentration. It equaled specific female egg-production rate at all food concentrations. The efficiency by which ingested carbon in excess of maintenance requirements was converted into body carbon was 0.44, very similar to the corresponding efficiency of egg-production in females. On the assumptions that food availability is similar for all developmental stages, and that juvenile and female specific growth/egg-production rates are equal, female egg-production rates are representative of turnover rates (production/biomass) of the entireA. tonsa population and probably in other copepod species as well. Therefore, in situ estimates of female fecundity may be used for a rapid time- and site-specific field estimate of copepod production. This approach is shown to be fairly robust to even large deviations from the assumptions.