Feeding ecology of the grooved tiger shrimp <Emphasis Type="Italic">Penaeus semisulcatus</Emphasis> De Haan (Decapoda: Penaeidae) in inshore waters of Qatar,Arabian Gulf

The feeding ecology of the green tiger shrimp Penaeus semisulcatus was studied in inshore fishing grounds off Doha, Qatar, using a combination of stable isotope (δ¹³C and δ¹⁵N) analysis and gut contents examination. Samples of post-larvae, juvenile and adult shrimp and other organisms were collected from intertidal and subtidal zones during the spawning season (January–June). Shrimp collected from shallow water seagrass beds were mostly post-larvae and juveniles and were significantly smaller than the older juveniles and adults caught in deeper macroalgal beds. Gut content examination indicated that post-larvae and juvenile shrimp in seagrass beds fed mainly on benthos such as Foraminifera, polychaetes, benthic diatoms and small benthic crustaceans (amphipods, isopods and ostracoda), whereas larger shrimp in the macroalgal beds fed mainly on bivalve molluscs and to a lesser extent polychaetes. In shrimp from both seagrass and algal beds, unidentifiable detritus was also present in the gut (18, 32%). δ¹³C values for shrimp muscle tissue ranged from −9.5 ± 0.26 to −12.7 ± 0.05‰, and δ¹⁵N values increased with increasing shrimp size, ranging from 4.1 ± 0.03 to 7.7 ± 0.11‰. Both δ¹⁵N values and δ¹³C values for shrimp tissue were consistent with the dietary sources indicated by gut contents and the δ¹³C and δ¹⁵N values for primary producers and prey species. The combination of gut content and stable isotope data demonstrates that seagrass beds are important habitats for post-larvae and juvenile P. semisulcatus, while the transition to deeper water habitats in older shrimp involves a change in diet and source of carbon and nitrogen that is reflected in shrimp tissue stable isotope ratios. The results of the study confirm the linkage between sensitive shallow water habitats and the key life stages of an important commercially-exploited species and indicate the need for suitable assessment of the potential indirect impacts of coastal developments involving dredging and land reclamation.     

Tolerance of Mediterranean seagrasses (<Emphasis Type="Italic">Posidonia oceanica</Emphasis> and <Emphasis Type="Italic">Cymodocea nodosa</Emphasis>) to hypersaline stress: water relations and osmolyte concentrations

The present study examines for the first time the effects of increased salinity on water relations and osmolyte (carbohydrates and amino acids) concentrations in two Mediterranean seagrass species, Posidonia oceanica and Cymodocea nodosa, which are adapted to growth in environments with contrasting salinity and have a known differential sensitivity to alterations in ambient salinity. The specific aim was to obtain insights into their respective capacities to cope with natural or anthropogenically induced (e.g. desalination plants) hypersaline stress and its ecological implications. To this end, large plant fragments of both seagrass species were maintained for 47 days in a laboratory mesocosm system under ambient salinity (37 psu; control) and three chronic hypersaline conditions (39, 41 and 43 psu). Analyses of leaf-tissue osmolality indicated that both species followed a dehydration avoidance strategy, decreasing their leaf water potential (Ψ_w) as the external salinity increased, but using different physiological mechanisms: whereas P. oceanica leaves exhibited a reduction in osmotic potential (Ψ_π), C. nodosa leaves maintained osmotic stability through a decrease in turgor pressure (Ψ_p) probably mediated through cell-hardening processes. Accordingly, the concentrations of soluble sugars and some amino acids (mainly Pro and Gly) suggested the activation of osmoregulatory processes in P. oceanica leaves, but not in C. nodosa leaves. Osmotic adjustments probably interfered with leaf growth and shoot survival of P. oceanica under hypersaline stress, whereas C. nodosa showed a more efficient physiological capacity to maintain plant performance under the same experimental conditions. These results are consistent with the more euryhaline ecological behaviour of C. nodosa and contribute to understanding the high vulnerability shown by P. oceanica to even mild increments in seawater salinity.     

Clearance rates of the great scallop (<Emphasis Type="Italic">Pecten maximus</Emphasis>) and blue mussel (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) at low natural seston concentrations

Great scallop, Pecten maximus, and blue mussel, Mytilus edulis, clearance rate (CR) responses to low natural seston concentrations were investigated in the laboratory to study (1) short-term CR variations in individual bivalves exposed to a single low seston diet, and (2) seasonal variations in average CR responses of bivalve cohorts to natural environmental variations. On a short temporal scale, mean CR response of both species to 0.06 μg L⁻¹ chlorophyll a (Chl a) and 0.23 mg L⁻¹ suspended particulate matter (SPM) remained constant despite large intra-individual fluctuations in CR. In the seasonal study, cohorts of each species were exposed to four seston treatments consisting of ambient and diluted natural seston that ranged in mean concentration from 0.15 to 0.43 mg L⁻¹ SPM, 0.01 to 0.88 μg L⁻¹ Chl a, 36 to 131 μg L⁻¹ particulate organic carbon and 0.019 to 0.330 mm³ L⁻¹ particle volume. Although food abundance in all treatments was low, the nutritional quality of the seston was relatively high (e.g., mean particulate organic content ranged from 68 to 75%). Under these low seston conditions, a high percentage of P. maximus (81–98%) and M. edulis (67–97%) actively cleared particles at mean rates between 9 and 12 and between 4 and 6 L g⁻¹ h⁻¹, respectively. For both species, minimum mean CR values were obtained for animals exposed to the lowest seston concentrations. Within treatments, P. maximus showed a greater degree of seasonality in CR than M. edulis, which fed at a relatively constant rate despite seasonal changes in food and temperature. P. maximus showed a non-linear CR response to increasing Chl a levels, with rates increasing to a maximum at approximately 0.4 μg L⁻¹ Chl a and then decreasing as food quantity continued to increase. Mean CR of M. edulis also peaked at a similar concentration, but remained high and stable as the food supply continued to increase and as temperatures varied between 4.6 and 19.6°C. The results show that P. maximus and M. edulis from a low seston environment, do not stop suspension-feeding at very low seston quantities; a result that contradicts previous conclusions on the suspension-feeding behavior of bivalve mollusks and which is pertinent to interpreting the biogeographic distribution of bivalve mollusks and site suitability for aquaculture.     

Habitat use,urea production and spawning in the gulf toadfish <Emphasis Type="Italic">Opsanus beta</Emphasis>

Field studies were conducted in Johnson Key Basin, Florida Bay, USA from September 2002 through September 2004 to examine physiological, ecological, and behavioral characteristics of the gulf toadfish, Opsanus beta (Goode and Bean in Proc US Natl Mes 3:333–345, 1880), in relation to nitrogen metabolism, habitat usage, and spawning. Fish collected 5 cm above sediments in experimental shelters (epibenthic) were compared with those collected by throw traps which were found on or burrowing within sediments. The relationship between microhabitat ammonia and urea excretion, as determined by the enzymatic activity of glutamine synthetase (GS), was examined. The hypothesis tested was that O. beta occupying epibenthic nests were less ureotelic with lower GS activities than non-nesting individuals on/in sediments, due to a decreased environmental ammonia burden. Porewater total ammonia (T _Amm) concentrations at a sediment depth of 5 cm, i.e., the approximate depth of burrowing toadfish, ranged from 0 to 106.5 μmol N l⁻¹ while the pH ranged from 7.48 to 9.14. There was a weak but significant correlation between environmental ammonia (NH₃) concentration and hepatic GS activity for epibenthic toadfish (P < 0.001, r ² = 0.10), but not for burrowing toadfish. Mean urea-N and T _Amm concentrations within shelters occupied by toadfish (n = 281) were 9.8 ± 0.83 μmol N l⁻¹ and 13.0 ± 0.7 μmol N l⁻¹, respectively. As predicted, hepatic GS activity was significantly lower in epibenthic toadfish captured in shelters (4.40 ± 0.24 μmol min⁻¹ g⁻¹; n = 281) as compared to individuals on/in sediments (6.61 ± 0.47 μmol min⁻¹ g⁻¹; n = 128). Glutamine synthetase activity generally peaked in March (spawning season) and was lowest in July. Gender differences in hepatic and branchial GS activity were also found during the spawning season, which is attributable to the fact that males brood and guard offspring in their epibenthic nests while females often rest on or burrow into the sediments. Finally, hepatic and branchial GS appeared to have different patterns of enzymatic activity suggesting functional differences in gene expression.     

Photosynthetic activity of the non-dormant <Emphasis Type="Italic">Posidonia oceanica</Emphasis> seed

The photosynthetic adaptive features of non-dormant seeds in Posidonia oceanica were studied in order to evaluate the effects of light on germination success. Transmission electron micrographs showed the presence of chloroplasts in the epidermal cells, close to the nucleus at the periphery of the cytoplasm. The well-developed thylakoid membranes and the presence of starch granules indicated that the chloroplasts were photosynthetically active. The relationship between photosynthesis versus irradiance in P. oceanica seeds incubated at 15 and 21°C was analysed. The net photosynthesis in the non-dormant seed of P. oceanica was positive and compensated its respiration demand (90 μmol quanta m⁻² s⁻¹) at both temperatures. Net photosynthesis was negative at the other irradiance values. To test the effects of light on germination success, seeds were placed both in dark and light conditions. Germination success was significantly higher in light rather than in dark condition. The characteristics observed in the photosynthesis in P. oceanica seed could be a mechanism to guarantee seedling survival in temperate waters, demonstrating though the specialized nature of this species.     

Trophic variation with length in two ommastrephid squids, <Emphasis Type="Italic">Ommastrephes bartramii</Emphasis> and <Emphasis Type="Italic">Sthenoteuthis oualaniensis</Emphasis>

From 1998 to 2001 a total of 200 Ommastrephes bartramii (27 paralarvae) and 170 Sthenoteuthis oualaniensis (14 paralarvae) were sampled from the Central North Pacific. One group of non-paralarval O. bartramii (n = 30) was sampled from farther northwest in 1996. The δ¹⁵N of mantle muscle of non-paralarval O. bartramii ( = 12.4‰) was significantly greater than that of non-parlarval S. oualaniensis ( = 8.1‰) (P < 0.001). The δ¹⁵N of whole paralarvae of O. bartramii ( = 6.4‰) was not significantly different than parlarvalae of S. oualaniensis ( = 6.1‰) (P = 0.528). There was no significant difference between the mantle muscle δ¹⁵N values of male (n = 95, = 13.3‰) and female (n = 18, = 12.9 ‰) O. bartramii greater than 300 mm mantle length (ML) (P = 0.15). There was also no significant difference between the mantle muscle δ¹⁵N values of male (n = 15, = 7.2‰) and female (n = 26, = 7.3 ‰) S. oualaniensis in the same size range (P = 0.41). Overall there was a distinct logistic increase in δ¹⁵N with mantle length for O. bartramii, whereas S. oualaniensis showed an exponential increase in δ¹⁵N with mantle length that was stronger within individual years than with all samples combined. In general, adult O. bartramii are more than a trophic level above S. oualaniensis (4.3‰, 1.3 TLs). Because of the nature of the sampling protocol, this study could not separate spatial and temporal effects on the δ¹⁵N signals from each squid species. This study demonstrates the ability of stable isotope analyses to differentiate trophic levels between squid species as well as track trophic changes across size ranges from paralarvae to adults. Additional research is needed to validate these trophic changes across size within individuals.     

Food web analysis in intertidal <Emphasis Type="Italic">Zostera marina</Emphasis> and <Emphasis Type="Italic">Zostera noltii</Emphasis> communities in winter and summer

The food web of two intertidal seagrass (Zostera marina and Zostera noltii) beds that may be influenced by the seasonal variation in food source abundance was studied in winter and in summer with δ¹³C and δ¹⁵N analysis. In spite of high relative variation of abundance of main primary producers at the two sites, the food web did not vary between winter and summer. The δ¹³C range of primary producers was wide. Zostera leaves, the most ¹³C-enriched source, were not consumed directly by grazers. Deposit and filter feeders have a similar δ¹³C and could use a mix of suspended and sedimented organic particulate matter, largely composed of detritus from macroalgae to seagrass. This trophic pathway allows the local incorporation of the high biomass produced by seagrasses. The wide δ¹⁵N range of predators was linked either to a large variety from omnivore to carnivore predators or to the also wide ranges of δ¹⁵N of primary consumers.     

Diurnal cycle and kinetics of ammonium assimilation in the green alga <Emphasis Type="Italic">Ulva pertusa</Emphasis>

The kinetics of ammonium assimilation was investigated in Ulva pertusa (Chlorophyceae, Ulvales) from northeastern New Zealand. Ammonium assimilation exhibited Michaelis–Menten kinetics with a maximum rate of assimilation (V _max) of 54 ± 5 μmol g⁻¹ dry weight h⁻¹ and half-saturation constant (K _m) of 23 ± 8 μM. In contrast, values for ammonium uptake were considerably higher with a V _max of 316 ± 59 μmol g⁻¹ dry weight h⁻¹ and K _m of 135 ± 46 μM. At environmentally relevant ammonium concentrations (5 μM), assimilation accounted for most (70%) of the ammonium taken up. Darkness decreased the maximum rate of ammonium assimilation by 83%. We investigated the hypothesis that rates of biosynthetic processes are greater in the early part of the day in Ulva. Consistent with this hypothesis, the maximum rate of ammonium assimilation in U. pertusa peaked in the morning and coincided with low levels of the photosynthetic product sucrose, which peaked in the afternoon. There was a diurnal cycle in the rate of ammonium uptake and assimilation in light and dark, but the amplitude was much greater for assimilation than uptake. Moreover, our data suggest that net ammonium assimilation only occurs during the day in U. pertusa. We suggest that two major roles for diurnal cycles are minimisation of interspecific competition for resources and metabolic costs.     

Inter-polyp genetic and physiological characterisation of <Emphasis Type="Italic">Symbiodinium</Emphasis> in an <Emphasis Type="Italic">Acropora valida</Emphasis> colony

Corals harbouring genetically mixed communities of endosymbiotic algae (Symbiodinium) often show distribution patterns in accordance with differences in light climate across an individual colony. However, the physiology of these genetically characterised communities is not well understood. Single stranded conformation polymorphism (SSCP) and real time quantitative polymerase chain reaction (qPCR) analyses were used to examine the genetic diversity of the Symbiodinium community in hospite across an individual colony of Acropora valida at the spatial scale of single polyps. The physiological characteristics of the polyps were examined prior to sampling with a combined O₂ microelectrode with a fibre-optic microprobe (combined sensor diameter 50–100 μm) enabling simultaneous measurements of O₂ concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface as a function of increasing irradiances. Both sun- and shade-adapted polyps were found to harbour either Symbiodinium clade C types alone or clades A and C simultaneously. Polyps were grouped in two categories according to (1) their orientation towardps light, or (2) their symbiont community composition. Physiological differences were not detected between sun- and shade-adapted polyps, but O₂ concentration at 1,100 μmol photons m⁻² s⁻¹ was higher in polyps that harboured both clades A and C symbionts than in polyps that harboured clade C only. These results suggest that the acclimatisation of zooxanthellae of individual polyps of an A. valida colony to ambient light levels may not be the only determinant of the photosynthetic capacity of zooxanthellae. Here, we found that photosynthetic capacity is also likely to have a strong genetic basis and differs between genetically distinct Symbiodinium types.     

Intraguild predatory interactions between the jellyfish <Emphasis Type="Italic">Cyanea capillata</Emphasis> and <Emphasis Type="Italic">Aurelia aurita</Emphasis>

While qualitative observations of jellyfish intraguild predation abound in the literature, there are only few rate measurements of these interactions. We quantified predation rates among two common jellyfish in northern boreal waters, Cyanea capillata and its prey Aurelia aurita, both of which also feed on crustacean zooplankton and fish larvae. A series of incubation experiments using a wide range of prey concentrations (0.38–3.8 m⁻³) in large containers (2.6 m³) was carried out. By replenishing the prey continuously as they were captured we maintained a nearly constant prey concentrations. Ingestion rates increased linearly up to prey concentrations of 1.92 m⁻³, yielding maximum clearance rates of ∼2.37 ± 0.39 m³ predator⁻¹ h⁻¹ for C. capillata predators 16 ± 2.3 cm in diameter. Mean ingestion rate at saturated prey concentrations (1.92–3.85 m⁻³) was 4.01 ± 0.78 prey predator⁻¹ h⁻¹. Behavioral observations suggested that predators did not alter their swimming behavior during meals, and thus that feeding rates were generally handling limited rather than encounter limited. Predators captured more prey than needed, and semi-digested prey was often discarded when fresh prey was encountered.     

How does salinity tolerance influence the distributions of <Emphasis Type="Italic">Brachionus plicatilis</Emphasis> sibling species?

Environmental salinity is important in defining Brachionus plicatilis sibling species distributions. However, while salinity influences distributions, sibling species often co-exist. Three different mechanisms potentially account for the partial co-occurrence of sibling species: (1) siblings have differing salinity tolerances that partially overlap; (2) siblings physiological tolerances may be commonly broad, but relatively small differences in tolerances differentiate distributions via interactions e.g. competition; or (3) siblings distributions may be influenced by physical factors other than salinity. Here, we assess the extent of salinity tolerance in three B. plicatilis sibling species (B. plicatilis 6TUR, B. plicatilis IOM and B. rotundiformis 6TOS) by measuring population growth rate (μ, day⁻¹) and egg development time in response to salinity (5–60‰) and salinity fluctuations (≤ Δ40‰). Sibling species were identified by analysis of the mitochondrial COI gene, and salinity responses were compared by regression analysis. Responses differed significantly between siblings, although the broad trends were similar. Positive growth occurred at all salinities, and highest growth rates ranged between 0.93 and 1.08 day⁻¹ at 16–18‰. Rapid changes in salinity reduced growth rates, but net mortality occurred only in one treatment (100% mortality on transfer from 10 to 40‰). Egg development time was largely invariant with salinity except for B. plicatilis IOM and where rotifers were transferred from 30 to 60‰. We indicate that several siblings are similarly euryhaline and tolerate salinity fluctuations. Undoubtedly, wide tolerances in B. plicatilis are adaptations to ephemeral and seasonally variable habitats. Given common broad salinity tolerances, it is unlikely that the differential distributions of sibling species are a direct result of physiological constraints. Instead, we illustrate using a simple model that subtle differences in physiological tolerances may have important impacts on interactions between sibling species, which may in turn influence distributions.     

Seasonal variations in the growth rates of euphausiids (<Emphasis Type="Italic">Thysanoessa inermis</Emphasis>, <Emphasis Type="Italic">T. spinifera</Emphasis>, and <Emphasis Type="Italic">Euphausia pacifica</Emphasis>) from the northern Gulf of Alaska

The euphausiids Thysanoessa inermis (Kroyer 1846), Thysanoessa spinifera (Holmes 1900), and Euphausia pacifica (Hansen 1911) are key pelagic grazers and also important prey for many commercial fish species in the Gulf of Alaska (GOA). To understand the role of the euphausiids in material flows in this ecosystem their growth rates were examined using the instantaneous growth rate (IGR) technique on the northern GOA shelf from March through October in 2001–2004. The highest mean molting increments (over 5% of uropod length increase per molt) were observed during the phytoplankton bloom on the inner shelf in late spring for coastal T. inermis, and on the outer shelf in summer for T. spinifera and more oceanic E. pacifica, suggesting tight coupling with food availability. The molting rates were higher in summer and lower in spring, for all species and were strongly influenced by temperature. Mean inter-molt periods calculated from the molting rates, ranged from 11 days at 5°C to 6 days at 8°C, and were in agreement with those measured directly during long-term laboratory incubations. Growth rate estimates depended on euphausiid size, and were close to 0 in early spring, reaching maximum values in May (0.123 mm day⁻¹ or 0.023 day⁻¹ for T. inermis) and July (0.091 mm day⁻¹ or 0.031 day⁻¹ for T. spinifera). The growth rates for E. pacifica remained below 0.07 mm day⁻¹ (0.016 day⁻¹) throughout the season. The relationship between T. inermis weight specific growth rate (adjusted to 5°C) and ambient chlorophyll-a concentration fit a Michaelis–Menten curve (r ² = 0.48) with food saturated growth rate of 0.032 day⁻¹ with half saturation occurring at 1.65 mg chl-a m⁻³, but such relationships were not significant for T. spinifera or E. pacifica.     

The trophic link between squid and the emperor penguin <Emphasis Type="Italic">Aptenodytes forsteri</Emphasis> at Pointe Géologie,Antarctica

Cephalopod beaks retrieved from stomachs of dead emperor penguin chicks at Pointe Géologie, Terre Adélie, provide information on taxonomic and size composition of the penguin’s squid diet, on the trophic range of the squid species preyed upon and on the fractional trophic impact of the penguin on the whole food web. Emperor penguins prey upon four squid species (Psychroteuthis glacialis, Kondakovia longimana, Gonatus antarcticus, Alluroteuthis antarcticus) and do not take squid larger than 480 mm mantle length. Larger squid live either below the penguin’s diving range or are beyond its handling capacity. Nitrogen stable isotope ratios indicate that squids cover a range of about two trophic levels (2.5–8‰ δ¹⁵N). The impact of the emperor penguin, however, concentrates on the upper part of this range, about 68% of its squid prey being >6‰ δ¹⁵N. The principal components of the emperor’s diet, fish, krill and squid, differ distinctly in average trophic level. Consequently the trophic position of the emperor penguin changes accordingly with diet composition and may differ by almost one trophic level between different emperor penguin colonies.     

Time-course uptake and elimination of benzo(a)pyrene and its damage to reproduction and ensuing reproductive outputs of Pacific oyster, <Emphasis Type="Italic">Crassostrea gigas</Emphasis>

The time-course of uptake and elimination of benzo(a)pyrene (BaP) for the Pacific oyster, Crassostrea gigas and reproduction damage and reproductive outputs were studied. Sexually immature C. gigas broodstock were fed for 28 days with live algae grown in four BaP solutions of 0, 50, 500, and 5,000 μg L⁻¹ (hereafter, control, 50, 500, and 5,000 oysters) and were subsequently conditioned to maturation by a feeding with BaP-free live algae under temperature manipulation for another 28 days. The 5,000 μg L⁻¹ oysters gained a steady state concentration, around 30,000 ng g⁻¹ d.w. for digestive gland, a week earlier compared to the 500 μg L⁻¹ oysters. The earlier gain or longer persistence of the steady state concentration influenced elimination of BaP, with an eliminating trend for 500 μg L⁻¹ oysters, while no elimination for 5,000 μg L⁻¹ oysters. The maternal persistence of the steady state concentration resulted in significant damages in the reproductive success and their reproductive outputs in terms of the hatching rate and larval growth, survival, and settlement. The 50 μg L⁻¹ oysters remained far below the steady state concentration, and showed a manifest eliminating behavior during the subsequent BaP-free 28 day maturation period. The reproductive success and initial larval events of 50 μg L⁻¹ oysters were comparable to those of control. However, the damage potential of the 50 μg L⁻¹ oysters might be more significant if their maternal exposure continued beyond 28 days, since the accumulation profile at this dose was linear.     

Stable isotopes document winter trophic ecology and maternal investment of adult female southern elephant seals (<Emphasis Type="Italic">Mirounga leonina</Emphasis>) breeding at the Kerguelen Islands

Individual specialisation is widespread and can affect a population’s ecological and evolutionary dynamics. Whether intra-specific niche differences can influence reproductive investment was examined in a marine mammal, the southern elephant seal (Mirounga leonina), whose females were known to forage in two different areas during the austral winter. The study was conducted at Kerguelen Islands (49°21′S, 70°18′E), southern Indian Ocean, in late winter–early spring 2006. Pups were used as proxies of their mothers’ biology and combined information on their weaning mass (a proxy of females’ foraging success and short-term fitness) together with their blood δ¹³C value (a proxy of female foraging zone). First, the use of isotopic signature of pups was validated to study the female foraging ecology during their pre-breeding trip by demonstrating that δ¹³C and δ¹⁵N values of pups and their mothers were positively and linearly correlated. Then, blood samples were taken from a large number of newly-weaned pups, which were also weighed, to provide information at the population level. Estimated δ¹³C values of female seals encompassed a large range of values (from −23.7 to −19.1‰) with an unimodal frequency distribution, suggesting no contrasted foraging areas within the population. No significant relationship was found between pup weaning mass and their carbon signature, indicating no link between female foraging areas and maternal foraging success and investment. Finally, blood δ¹³C and δ¹⁵N values gave new insights into southern elephant seal ecology, suggesting that females mainly foraged north of the Polar Front where they preyed upon myctophid fish in late winter.     

Feeding ecology of the mangrove crab <Emphasis Type="Italic">Ucides cordatus</Emphasis> (Ocypodidae): food choice,food quality and assimilation efficiency

Food preferences, consumption rates and dietary assimilation related to food quality were investigated for the large semi-terrestrial and litter-consuming mangrove crab Ucides cordatus cordatus (Ocypodidae, L. 1763) in northern Brazil. Stomach contents were composed of mangrove leaves (61.2%), unidentified plant material and detritus (28.0%), roots (4.9%), sediment (3.3%), bark (2.5%), and animal material (0.1%). U. cordatus prefers Rhizophora mangle over Avicennia germinans leaves despite a higher nitrogen content, lower carbon to nitrogen (C/N) ratio and lower tannin content of the latter. Also, assimilation rates for senescent R. mangle leaves (C: 79.3%, N: 45.4%) were higher than for A. germinans leaves (C: 40.6%, N: 9.1%). Faeces composition indicates that A. germinans leaves were more difficult to masticate and digest mechanically. The leaf-ageing hypothesis, according to which crabs let leaves age in burrows to gain a more palatable and nutritive food, was rejected for U. cordatus since N content, C/N ratio and the abundance of microorganisms did not differ significantly between senescent leaves and leaves taken from burrows. The low microbial biomass on leaf surfaces and in the sediment indicates its minor importance for the nutrition of U. cordatus. It is concluded that high ingestion and assimilation rates of a R. mangle diet together with the consumption of algae allow for a high intake of C, N, and energy. The data suggest that the digestibility of mangrove leaves by U. cordatus is not hampered by tannins. This may have provided a competitive advantage over other leaf-consuming invertebrates unable to digest mangrove litter with high tannin concentrations. Due to the large stock biomass of U. cordatus in the study area, a great amount of finely fragmented faeces is produced (about 7.1 ton dry matter ha⁻¹ year⁻¹ in a R. mangle forest) which is enriched in C, N and bacterial biomass compared to the sediment. The decomposition of mangrove litter, and thus nutrient and energy transfer into the sediment, is greatly enhanced due to litter processing by U. cordatus.     

High reproduction of <Emphasis Type="Italic">Calanus finmarchicus</Emphasis> during a diatom-dominated spring bloom

Feeding, egg production, hatching success and early naupliar development of Calanus finmarchicus were measured in three north Norwegian fjords during a spring bloom dominated by diatoms and the haptophyte Phaeocystis pouchetii. Majority of the copepod diet consisted of diatoms, mainly Thalassiosira spp. and Chaetoceros spp., with clearance rates up to 10 ml ind⁻¹ h⁻¹ for individual algae species/groups. Egg production rates were high, ranging from ca 40 up to 90 eggs f⁻¹ d⁻¹, with a hatching success of 70–85%, and fast naupliar development through the first non-feeding stages. There was no correlation between the egg or nauplii production and diatom abundance, but the hatching success was slightly negatively correlated with diatom biomass. However, the overall high reproductive rates suggested that the main food items were not harmful for C. finmarchicus reproduction in the area, although direct chemical measurements were not conducted. The high population egg production (>1,20,000 eggs m⁻² d⁻¹) indicated that a large part of the annual reproduction took place during the investigation, which stresses the importance of diatom-dominated spring phytoplankton bloom for population recruitment of C. finmarchicus in these northern ecosystems.     

Population dynamics and growth of <Emphasis Type="Italic">Nassarius reticulatus </Emphasis>(Gastropoda: Nassariidae) in Rhosneigr (Anglesey,UK)

Seasonal changes in catch rate, growth and mortality of Nassarius reticulatus from an intertidal lagoon and a wave-exposed beach at Rhosneigr (Anglesey, North Wales, UK) are described. The number of N. reticulatus caught in baited traps from the lagoon was significantly higher (>125 individuals trap⁻¹) during the summer (>18°C), than at <12°C (<65 individuals trap⁻¹), and the numbers caught in the lagoon were an order of magnitude greater than on the beach, >13 individuals trap⁻¹ in July (>16°C), and <5 individuals trap⁻¹ between December and April (<9.5°C). Predictions of shell growth attained by N. reticulatus annually in the lagoon using graphical modal progression analysis (MPA) of length frequency data, were similar to the growth of marked and recaptured lagoon N. reticulatus. Predictions of shell growth using computerised length frequency distribution analysis (LFDA), however, did not reflect the growth as accurately as MPA. Modal progression analysis demonstrated that N. reticulatus from the lagoon achieved a higher asymptotic maximum shell length (L _∞) and a lower growth constant (K) than animals from the beach. Shell growth was seasonal with growth of the lagoon individuals slowing down towards the end of September and resuming in early April, about a month later than the beach individuals. Mortality of N. reticulatus was greater during the summer, and survival was lower in the lagoon than on the beach. Recruitment patterns were similar in the lagoon and on the beach, and MPA and LFDA predicted that larval N. reticulatus settled between late summer and early autumn, with juveniles (7–8.9 mm) appearing in the population the following year, between February and April. Growth of male and female N. reticulatus in the laboratory was similar and was temperature and size dependent. The different growth patterns between N. reticulatus from the two habitats, predicted using MPA, were maintained when individuals were reared under laboratory conditions for ∼6 months; N. reticulatus <21 mm from the beach grew faster than individuals from the lagoon, although N. reticulatus >21 mm from the lagoon grew faster and attained a larger length (26 mm) than individuals from the beach (24 mm). Low food availability did not affect N. reticulatus survival in the laboratory but significantly suppressed shell growth.     

Effect of different time of salt stress on growth and some physiological processes of <Emphasis Type="Italic">Avicennia marina</Emphasis> seedlings

Growth and physiological characters of Avicennia marina seedlings cultured under different levels of salinity were compared at 45 and 100 days after sowing. Based on the growth and physiological responses, the levels of salinity were grouped into two kinds, moderate (5–30‰) and extreme (40 and 50‰ as well as 0‰). Root and shoot length, leaf area, biomass of different organs, and net photosynthesis rate all showed a similar trend: the seedlings grew better at moderate levels of salinity but were adversely affected by extreme levels. Longer exposure (100 days) to salinity markedly enhanced the difference between the effects of the two levels on growth. By 45th day, the cotyledons had withered and fallen off. The concentration of ions (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻) and ash content of the cotyledons were determined before sowing and 45 days later. Ion concentrations and ash content of cotyledons were markedly lower at 45 days—lower than the initial levels—in seedlings irrigated with water at 0‰ salinity level. This suggested that the poor growth of these seedlings at 100 days may be due to lack of ions provided by the cotyledons. The high ion concentrations in the cotyledons grown at moderate salinity levels suggest that these organs may function as ion sinks at this stage, reducing the concentration of ions and consequent toxicity caused by excessive concentrations. Root biomass was higher than shoot biomass 45 days after sowing, whereas after 100 days, shoot biomass was higher. At the early stage of growth (45 days), the rate of photosynthesis at lower levels of salinity (0–30‰) was limited mainly by stomatal closure but at higher levels of salinity (40–50‰), other factors came into play. Later, at 100 days, the causes of reduced photosynthetic rate were other than stomatal closure at both low and high levels of salinity. This indicates that photosynthesis is affected by prolonged exposure to salt stress—including that caused by 0‰ salinity, as shown by poor growth of the seedlings.     

Lethal and sublethal effects of naphthalene and 1,2-dimethylnaphthalene on the marine copepod <Emphasis Type="Italic">Paracartia grani</Emphasis>

Here we evaluate the effects of two quantitatively very important components of the water soluble fractions of fuel oils (naphthalene and 1,2-dimethylnaphthalene, hereafter NAPH and C2-NAPH, respectively) on the survival, feeding and egg production rates, and viability of eggs of the coastal copepod Paracartia (Acartia) grani. Acute toxicity responses resulted in lethal concentrations (LC50) of 2,535 and 161 μg l⁻¹ for NAPH and C2-NAPH, respectively, with no evidence of narcotic effects. Hydrocarbon-specific differences in the toxicity response indicate that sublethal effects (EC50) on feeding by C2-NAPH were likely driven by induced mortality, whereas NAPH has direct negative effects on feeding. Sublethal effects on egg production rates followed a similar detrimental pattern to the one exhibited by feeding rates, suggesting that the lower egg production rates were mediated by the decrease in feeding rates. At the exposure time tested (24 h), the 50% reduction effective concentrations (EC50) determined for sublethal effects were relatively high in comparison with hydrocarbons’ concentrations found under natural circumstances. Long exposure (4 days) of P. grani adults to the tested hydrocarbons at concentrations well below the recorded EC50, however, had no significant effects on feeding, egg production and hatching rates. The viability of the eggs was either not affected or only slightly influenced when healthy eggs were incubated under very high concentrations (up to 6,400 and 700 μg l⁻¹ NAPH and C2-NAPH, respectively). The significance of the effects of oil spills on marine zooplankton communities is discussed in light of the results presented in this study.