Assimilation of trace elements by the mussel Mytilus edulis: effects of diatom chemical composition

Mussels have been widely used as bioindicators of coastal contamination, and recent reports have demonstrated that metals are accumulated from both the dissolved phase and from ingested food. In the winter and spring of 1995, we examined the influence of the chemical composition of food (protein content, trace element concentrations and ratios in the diatom Thalassiosira pseudomana) on the assimilation of six trace elements (Ag, Am, Cd, Co, Se and Zn) in the mussel Mytilus edulis (L.). Differences of up to 38% in diatom protein content had no major influence on the assimilation of any trace element or carbon. Protein assimilation in M. edulis examined with a ³⁵S radiotracer was also independent of protein content in the diatoms. Similarly, Se assimilation in mussels was not affected by the different Se concentrations in the diatoms. Cd assimilation increased with increasing Cd concentration, presumably due to higher desorption of Cd under acidic conditions typical of the mussel gut. Zn assimilation was inversely related to Zn concentration in the food particles, implying a partial regulation of this metal in the mussels. There was no evidence of any interaction of Cd and Zn in their assimilation by the mussels. These results suggest that mussels are highly responsive, in an element-specific way, to some components of ingested food (e.g., metal concentration), but other food components (such as the biochemical composition of the algae) have little effect on assimilation.     

Feeding responses of the bivalves Crassostrea gigas and Mytilus trossulus to chemical composition of fresh and aged kelp detritus

The chemical composition of kelps (e.g. polyphenolics) deters grazing by herbivores, but kelp detritus is potentially a source of nutrition for suspension feeders. The effects of kelp detritus derived from two species [Agarum fimbriatum Harvey and Costaria costata (Turner) Saunders] on feeding of oysters, Crassostrea gigas Thunberg, and mussels, Mytilus trossulus Gould, were examined in feeding experiments. Fresh and aged kelp particles were sequentially presented in combination with the microalga Rhodomonas lens at an initial total concentration of 5᎒^-4 ml^-1. Aging of kelp particles for 4 days in seawater significantly reduced the concentration of polyphenolics without changing the total carbon or nitrogen content. Clearance rates of both mussels and oysters were significantly lower in the presence of fresh versus aged kelp particles, and clearance rates declined overall with declining polyphenolic concentrations. Video endoscopy was used to examine feeding selectivity at the level of the gill in oysters in the same food treatments used in the clearance rate experiments. Comparison of particle composition in the water versus the pseudofeces in both oysters and mussels was also used as a measure of feeding selectivity. When presented with R. lens in combination with fresh and aged kelp particles selectivity for R. lens tended to be greater against fresh than aged particles, and there was some indication that this was stronger for A. fimbriatum than for C. costata particles. The ability to select was lower at very high polyphenolic concentrations, which may reflect poisoning of sensory binding sites. These data suggest that bivalves distinguish among particles of varying chemical composition and respond by changing their clearance rates and their selectivity.     

Ultrastructural,biochemical, and immunological characterization of two populations of the mytilid mussel <Emphasis Type="Italic">Bathymodiolus</Emphasis><Emphasis Type="Italic">azoricus</Emphasis> from the Mid-Atlantic Ridge: evidence for a dual symbiosis

A recently described species of mytilid mussel, Bathymodiolus azoricus Von Cosel et al., 1999, was observed to be the dominant organism at the hydrothermal vents off the Azores, at both the Lucky Strike and Menez Gwen sites. Evidence suggests this species of Bathymodiolus represents yet another example of the intriguing dual symbiosis known in three other species of deep-sea mytilid mussels. Transmission electron micrographs (TEM) show the majority of gill bacteriocytes in mussels sampled from both populations to contain two distinct symbiont morphotypes. One morphotype is characterized by large size (mean diameter, 1.25 µm), coccoid shape, and stacked intracytoplasmic membranes that are consistent with the morphology of type I methanotrophs. The second morphotype is smaller (mean diameter, 0.35 µm) and was observed in coccoid or rod shapes. Immunoblots revealed the presence of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) and methanol dehydrogenase (MeDH) in both populations of mussels. Activities of these enzymes, as well as sulfate adenylyl transferase (ATP sulfurylase) and adenylyl sulfate reductase (APS reductase), were detected in gill extracts. The activities measured for the two populations were highly variable, though the population sampled from Lucky Strike showed higher RubisCO activity. Stable carbon isotope values (Lucky Strike, '¹³C=-32.6ǂ.3‰; Menez Gwen, '¹³C=-22.8ǂ.4‰) are in the range of previously reported stable carbon isotope measurements for mytilid mussels hosting a dual symbiosis. Collectively, these results provide evidence for the activity of both sulfur-oxidizing and methane-oxidizing metabolic pathways in B. azoricus. Furthermore, evidence for a greater dependence on methanotrophy in the Menez Gwen mussel population is offered by analysis of cell counts from TEMs. Higher methanotroph numbers, and putatively activity, in this population of mussels are further supported by published geochemical data indicating higher methane concentrations in the vent fluids at Menez Gwen. This finding suggests that environmental conditions may regulate a balance between the physiological activities of different symbiont populations associated with these mussels. The existence of a dual symbiosis could thus confer greater environmental tolerance and increased niche space to the mytilid host in the stochastic hydrothermal vent habitat.     

Feeding biology and carbon budget of the sediment-burrowing heart urchin Brissopsis lyrifera (Echinoidea: Spatangoida)

This study of the burrowing heart urchin Brissopsis lyrifera includes measurements on feeding and food selection, ingestion rate, absorption efficiency, ventilation and respiration. B. lyrifera regulated feeding depth, ingestion rate and absorption efficiency in relation to food source. When food was added to the top layer of sediment, B. lyrifera burrowed closer to the surface and ingested mainly enriched surface material, whereas it burrowed deeper and ingested deep-seated sediment in the controls. In non-enriched sediment, the feeding rate was 0.04 g sediment DW h^-1 ind.^-1, and, in macro- and microalga-enriched sediment the feeding rate was 0.06 and 0.08 g sediment DW h^-1 ind.^-1, respectively. Absorption efficiency of TOC was about 43% in non-enriched sediment and in microalga-enriched sediment, but was significantly lower (34%) in macroalga-enriched sediment. Absorption efficiency of N varied between 48% and 55%, and was independent of food source. B. lyrifera feed selectively, enriching the gut TOC content about 2-fold and N content about 2.5-fold. The C/N ratio was therefore lower within the gut compared to the surrounding sediment, while the faecal C/N ratio was almost equal to the surrounding sediment. The faeces were, however, slightly richer in TOC and N compared to the surrounding sediment. For 3-5 cm long B. lyrifera, water current rate varied between 4 and 24 ml water h^-1, with a mean of 11 ml h^-1. Mean respiration rate was 205 µl O₂ h^-1 ind.^-1. The water current rate was not sufficient for B. lyrifera to sustain itself by filter feeding only. However, organic-rich particles from the surface are suggested to be an important contribution to the diet. A carbon budget was calculated for B. lyrifera from measured values of consumption, absorption efficiency and respiration, in order to estimate annual production of B. lyrifera. Compared to literature values, growth was overestimated about tenfold in the budget. A large proportion of the absorbed carbon was suggested to leave the animal as dissolved carbon, through mucus production or through anaerobic pathways, either by the heart urchin or by micro-organisms in the gut.     

Pools of chlorophyll and live planktonic diatoms in aphotic marine sediments

Chlorophyll a and numbers of live pelagic diatoms were recorded from sediment depth profiles at 11 stations in the oligotrophic Øresund, Denmark, in late-June. Extraction efficiency of chlorophyll a analysed fluorometrically did not differ significantly between paired samples of frozen-thawed and fresh sediment. The depth profiles of chlorophyll a could be explained by a diagenetic model involving two different chlorophyll pools: one reactive pool declining exponentially with core depth, and one non-reactive pool, of about 1 µg Chl ml^-1 wet sediment, being constant with depth. The number of live diatoms, quantified by the dilution-extinction method, and expressed in terms of most probable number (MPN), declined from an average of about 300,000 g^-1 in the surface sediment to zero values at a depth of 13 cm. The number of live cells was significantly correlated with the sediment chlorophyll a, and the profiles of live cells as well as reactive chlorophyll followed the same exponential decline with core depth, suggesting that the main source of chlorophyll in the sediment was live pelagic diatoms. Taxonomic composition of diatoms in the sediment, dominated by the pelagic genera Chaetoceros, Thalassiosira and Skeletonema, matched the species composition in the water column 3 months earlier during the spring bloom. Regular recordings of the phytoplankton community in the water column showed that only these specific bloom species could be the source of the sediment content of diatoms and chlorophyll a. Further, the ratios between live cells and chlorophyll a were similar in the sediment and in the spring bloom. A conservative estimate of depth-integrated pools of diatoms in the sediment suggested that about 44% of the total phytoplankton biomass during the spring bloom was still present as live cells in the sediment after 3 months. This indicates that the spring bloom input to the sediment is not degraded immediately by the benthic fauna.     

Effects of substrate type on growth and mortality of blue mussels (<Emphasis Type="Italic">Mytilus</Emphasis><Emphasis Type="Italic">edulis</Emphasis>) exposed to the predator <Emphasis Type="Italic">Carcinus</Emphasis><Emphasis Type="Italic">maenas</Emphasis>

Structure and complexity of the substrate are important habitat characteristics for benthic epifauna. The specific growth and mortality rates and inducible defence characters on medium-sized blue mussels (Mytilus edulis L.) exposed to shore crabs (Carcinus maenas L.) were examined on three different substrate types in combined field and laboratory experiments. The experiments showed that complexity of the substrate increased blue mussel survival significantly, through a decrease in predation pressure. However, increased intraspecific competition for food on the complex substrate resulted in significantly lower growth rates of the mussels. Inducible defence characters were also influenced by substrate type. Blue mussels were more affected by predators on the structurally simple substrate, where they developed thicker shells and a larger posterior adductor muscle.     

Particle sorting in bivalves: in vivo determination of the pallial organs of selection

Benthic particle feeders are exposed to a food supply varying in both quantity and quality. Previous studies have shown that bivalve molluscs deal with such fluctuating particle regimes in a variety of ways, including adjustments in pumping and ingestion rates, and selective rejection of non-nutritive particles as pseudofeces. The actual site of particle selection within the pallial cavity, however, has remained a topic of speculation. During August 1995 and January and August 1996, we exposed the oysters Crassostrea virginica (Gmelin) and C. gigas (Thunberg), and the mussel Mytilus trossulus Gould to a mixture of ground, aged Spartina alterniflora Loisel and similar-sized phytoplankton at three concentrations (10³, 10⁴, 10⁵ particles ml⁻¹). We then examined the ctenidia and labial palps by means of endoscopy and sampled, in vivo, the particulate material from various ciliated tracts, and analyzed the samples with a flow cytometer. We found that in oysters, the ctenidia are responsible for particle sorting, whereas the labial palps play an accessory role in particle selection, or function to control the volume of material to be ingested. In mussels, however, the ctenidia play little role in particle selection and simply transport particulate matter to the palps for further processing. We suggest that selection by the ctenidia of oysters is a function of their architecture (plicate, heterorhabdic). Received: 4 June 1997 / Accepted: 23 January 1998     

Burrowing behaviour and sediment reworking in the heart urchin Brissopsis lyrifera Forbes (Spatangoida)

Sediment reworking due to burrowing and feeding was studied in the spatangoid Brissopsis lyrifera, at two different temperatures (7°C and 13°C). Spine activity and burrowing behaviour were recorded with a real-time video camera. Reworked sediment volume was calculated from tracks produced by the heart urchin. Ingestion rates were measured by feeding the heart urchins with luminophores. Temperature had a significant effect on the bioturbation activity of B. lyrifera. At 13°C reworked sediment volume due to burrowing was 22 ml sediment h^-1 and at 7°C 14 ml sediment h^-1. The ingested amount of sediment was 0.08 and 0.02 g dry sediment h^-1 in 13 and 7°C, respectively. Reworked sediment volume due to burrowing was 60-150 times higher than the volume ingested. The large reworked volume is a consequence of B. lyrifera moving with a rocking motion through the sediment. The spines were continuously transporting sediment around the test with 5-min metachronal wave cycles.     

Geographical and seasonal variation of trace metal bioavailabilities in the Gulf of Gdansk,Baltic Sea using mussels (<Emphasis Type="Italic">Mytilus trossulus</Emphasis>) and barnacles (<Emphasis Type="Italic">Balanus improvisus</Emphasis>) as biomonitors

The barnacle Balanus improvisus and the mussel Mytilus trossulus have been used as biomonitors of the trace metals Cu, Zn, Cd, Fe, Pb, Mn and Ni at five sublittoral sites in the Gulf of Gdansk (Baltic Sea) between February 2000 and September 2001. The study has established a benchmark against which future biomonitoring programmes will be able to establish changes in local metal pollution, particularly if metal loadings in the river Vistula (draining into the Gulf) alter in the future. The study highlighted differences in trace metal bioavailabilities to both barnacles and mussels, geographically and over time. Accumulated metal concentrations of Cu, Zn, Fe, Pb and Ni, but not Cd or Mn, were correlated in the barnacles and mussels, suggesting that the bioavailabilities of the former metals to the two biomonitors were similar. The barnacles showed greater discriminatory power than the mussels as trace metal biomonitors. Concentrations of trace metals in surficial sediments (<63 m) did not correlate significantly with accumulated metal concentrations in either barnacles or mussels, indicating that sediment metal concentrations are not necessarily good proxy measures of ambient trace metal bioavailabilities to the local coastal filter feeders.Communicated by J.P. Thorpe, Port Erin     

Toxic metal health risk by mussel consumption

Seafood is a major dietary food worldwide. However, seafood consumption by humans can induce health risk because seafood may be contaminated by various pollutants. The mussel Mytilus galloprovincialis is widely distributed in the coastal waters of Montenegro, SouthEast Adriatic Sea. Here, Zn, Fe, Cu, Ni, Cd, Pb, As, and Hg contents in M. galloprovincialis from ten sites were analyzed to investigate health risks associated with the consumption of wild and cultivated mussels. Since there is a lack of data on the mussel consumption rate in Montenegro, the amount of mussels that can be ingested weekly over a lifetime with no risk of negative health effects was calculated using provisional tolerable weekly intakes (PTWI). We found that Cd concentrations were the limiting factor for mussels as a food. The weekly consumptions of 0.64–1.2 kg of fresh wild and 0.84–1.2 kg of fresh cultivated mussel would be sufficient to reach the PTWI_Cd, which may result in a risky weekly intake of Cd for long-term exposure. Moreover, weekly intake of 125 g mussels was used to calculate the dietary intake of trace elements by mussel consumption and compared with the prescribed PTWIs. Here, we found that there is no risk for human health for all investigated elements. In this case, the highest Cd level obtained in wild and in cultivated mussels represents 19.8 and 14.9% of the PTWI_Cd, respectively. This is the first study in Montenegro giving an assessment of the health risk from trace elements via the consumption of wild and cultivated M. galloprovincialis.     

Postingestive selection in the sea scallop (Placopecten magellanicus) on the basis of chemical properties of particles

Suspension-feeding bivalves are adapted to use a highly variable mixture of particles by sorting them before and after ingestion. Postingestive sorting in bivalves has been confirmed for several bivalve species, but few studies have attempted to isolate the factors influencing postingestive selection among different particles presented simultaneously. The ability of the sea scallop, Placopecten magellanicus (Gmelin), to sort a mixture of organic (¹⁴C-labeled dinoflagellates, Prorocentrum minimum) and inorganic (⁵¹Cr-labeled beads, diameter 16-18 µm) particles of similar size and shape within the stomach was examined. The study was carried out in August 1997 using scallops collected near Deer Island, New Brunswick, Canada. Sorting of the two particle types was measured by dissecting the scallops at intervals after feeding and comparing the ¹⁴C:⁵¹Cr ratios within the stomach, digestive gland, and feces. The ¹⁴C:⁵¹Cr ratio in the stomach decreased over time, indicating that sea scallops were sorting organic from inorganic particles. The ability of P. magellanicus to sort particles solely on the basis of chemical properties was tested in a second study by presenting them with a mixture of protein-coated and uncoated beads of two different colors. This study was carried out in February 2000 using scallops collected near Maces Bay, New Brunswick, Canada. Gut retention times of the two types of beads were measured using flow cytometry. Scallops retained protein-coated beads in the gut longer than uncoated beads, indicating postingestive selection by chemical properties. This study provides the first evidence for simultaneous postingestive sorting by bivalves of particles based solely on chemical properties. This would potentially enable P. magellanicus to preferentially retain particles of higher food quality longer than those of poor quality, thereby enhancing digestive efficiency.     

Sexual conflict,sexual selection and sperm competition in the spawning decisions of bitterling, <Emphasis Type="Italic">Rhodeus</Emphasis><Emphasis Type="Italic">sericeus</Emphasis>

Sexual selection may operate through either direct selection on preference genes or indirect genetic benefits. However, in many species both direct and indirect selection may operate and can influence female mate and oviposition choice. Adaptations by males for sperm competition can also affect female mate and oviposition choice and can lead to sexual conflict. We investigated the role of direct and indirect benefits and male adaptation to sperm competition for female oviposition decisions in the bitterling, Rhodeus sericeus, a freshwater fish that spawns on the gills of living unionid mussels. Field experiments showed that females chose mussels for spawning on the basis of the number of embryos already in mussels and the body size and/or extent of red coloration of the eyes of territorial males. There was also a significant interaction between these direct and indirect benefits of oviposition choice. Territorial male bitterling avoided leading females to mussels into which rivals had already released sperm. Males also increased their rate of sperm release into mussels in response to sperm release by sneaking males. We discuss the possibility that sexual conflict in oviposition choice occurs in this species.     

Characterization of urease activity in three marine phytoplankton species,<Emphasis Type="Italic"> Aureococcus anophagefferens</Emphasis>,<Emphasis Type="Italic"> Prorocentrum minimum</Emphasis>, and<Emphasis Type="Italic"> Thalassiosira weissflogii</Emphasis>

The availability of different forms of nitrogen in coastal and estuarine waters may be important in determining the abundance and productivity of different phytoplankton species. Although urea has been shown to contribute as much as 50% of the nitrogen for phytoplankton nutrition, relatively little is known of the activity and expression of urease in phytoplankton. Using an in vitro enzyme assay, urease activities were examined in laboratory cultures of three species: Aureococcus anophagefferens Hargraves et Sieburth, Prorocentrum minimum (Pavillard) Schiller, and Thalassiosira weissflogii (Grunow) Fryxell et Hasle. Cultures of P. minimum and T. weissflogii were grown on three nitrogen sources (NO₃^m, NH₄⁺, and urea), while A. anophagefferens was grown only on NO₃^m and urea. Urease was found to be constitutive in all cultures, but activity varied with growth rate and assay temperature for the different cultures. For A. anophagefferens, urease activity varied positively with growth rate regardless of the N source, while for P. minimum, urease activity varied positively with growth rate only for cultures grown on urea and NH₄⁺. In contrast, for T. weissflogii, activity did not vary with growth rate for any of the N sources. For all species, urease activity increased with assay temperature, but with different apparent temperature optima. For A. anophagefferens, in vitro activity increased from near 0-30°C, and remained stable to 50°C, while for P. minimum, increased in vitro activity was noted from near 0-20°C, but constant activity was observed between 20°C and 50°C. For T. weissfloggii, while activity also increased from 0°C to 20°C, subsequent decreases were noted when temperature was elevated above 20°C. Urease activity had a half-saturation constant of 120-165 wg atom N l^у in all three species. On both an hourly and daily basis, urease activity in A. anophagefferens exceeded nitrogen demand for growth. In P. minimum, urease activity on an hourly basis matched the nitrogen demand, but was less than the demand on a daily basis. For T. weissflogii, urease activity was always less than the nitrogen demand. These patterns in urease activity in three different species demonstrate that while apparently constitutive, the regulation of activity was substantially different in the diatom. These differences in the physiological regulation of urease activity, as well as other enzymes, may play a role in their ecological success in different environments.     

Doubly uniparental transmission of mitochondrial DNA length variants in the mussel <Emphasis Type="Italic">Mytilus</Emphasis><Emphasis Type="Italic">trossulus</Emphasis>

Doubly uniparental inheritance of mitochondrial DNA (mtDNA) is an unusual feature found in marine mussels and a few other bivalve species. In addition to a mitochondrial genome (F) inherited through the female line, heteroplasmic individuals (males) contain a second highly diverged genome (M) that is inherited through the paternal line. The Baltic mussel Mytilus trossulus is characterized by the presence of two phylogenetically close female and male genomes. We examined M. trossulus sampled from the Gulf of Gdansk, southern Baltic. Somatic tissues and gametes were surveyed for the presence of different types of mtDNA. Length variants were identified using PCR (polymerase chain reaction) amplification of the mtDNA fragment containing the major noncoding region. Seventeen length variants present in homo- and heteroplasmic individuals were found in a sample of 343 individuals analyzed. Two length variants of the major noncoding region (PCR product lengths: 1520 and 1370 bp) were the most frequent. Length heteroplasmy was found in 46.8% of males, and was mostly caused by the presence of the short length variant (PCR product length: 1370 bp) of the major noncoding region. MtDNA variants were also detected by restriction analysis of a 1.3 kb segment of coding region (ND2-COIII) amplified by PCR. This study provides evidence that the short length variant is always transmitted to sperm and has taken over the role of the M genome. The remaining length variants were present in both males and females. Longer length variants were transmitted mainly through the female line. A possibility of much higher incidence of genome role reversals in Baltic M. trossulus, in comparison with other populations of mussels is discussed.     

Reproductive toxicity of metals in calanoid copepods

This study investigated the effect of exposure route on metal accumulation, tissue distribution, and toxicity in the marine copepods Acartia hudsonica and A. tonsa. Sublethal toxicity was measured as decreases in egg production, hatching rate, ovarian development and protein (yolk) content of the egg. When algal food, exposed to Hg at 1 nM or Cd at 5 nM resulting in cells containing 34 and 64 nmol metal g^-1 dry weight, respectively, was ingested over a 4-h period by copepods, the total copepod body burden increased nine-fold for Hg and two-fold for Cd over background concentrations, and egg production decreased by 50%. Sublethal concentrations of metals were >2 orders of magnitude lower than LC₅₀ concentrations. Hatching rate, ovarian development and egg protein content all decreased following trophic exposure to metals, implying that the process of yolk accumulation (vitellogenesis) was affected. Exposure to dissolved Cd had no effect, but dissolved Hg at concentrations as low as 0.25 nM did affect egg production. Different toxic effects following different exposure routes were related to different metal distributions in the copepods: exposure to dissolved metal resulted in metal deposition in the exoskeleton, whereas exposure to dietary metal resulted in metal deposition in internal tissues. These findings indicate that enrichment of metal concentrations in internal tissues, which occurs primarily after exposure to dietary metal, affects vitellogenesis. The reproduction rate decreases by about 75% at metal concentrations only moderately higher than levels in coastal waters. Toxicity tests involving aquatic animals need to consider effects following uptake by different pathways, including the trophic transfer of metals.     

Differential effects of Aureococcus anophagefferens isolates ("brown tide") in unialgal and mixed suspensions on bivalve feeding

Picoplanktonic brown tides of Aureococcus anophagefferens have had devastating effects on production of commercially exploited bivalve populations in shallow, mid-Atlantic estuaries in the United States. The toxin produced by this alga has not been chemically characterized. This study develops a bioassay using juvenile mussels, Mytilus edulis, based on the inhibitory effect of brown tide on bivalve suspension-feeding, to compare the cellular toxicity of three Long Island, New York, clonal isolates of A. anophagefferens. Two recent (1995) isolates (CCMP 1707 and 1708) from Peconic Bay proved highly toxic and caused greater than 100-fold reduction in clearance rates (CR) of juvenile mussels in unialgal and mixed suspensions with a nutritious alga, Isochrysis galbana (clone T-iso), relative to controls. A third 1986 isolate from Great South Bay (CCMP 1784) showed no detectable toxicity in 24-h trials, and may have lost its initial potency over more than a decade of laboratory culture. Identification of a non-toxic strain provides a useful tool for future research. Cultures of the toxic isolate CCMP 1708 in late-stationary growth phase were significantly more toxic than those in early-exponential phase. The threshold concentration of toxic A. anophagefferens cells that inhibits clearance on co-occurring phytoplankton species was determined for juvenile (10-mm) hard clams, Mercenaria mercenaria. Relatively low concentrations (⣃᎒³ to 50᎒³ cells ml^-1) of isolate CCMP 1708 were sufficient to sharply reduce clam CR of I. galbana. Calculations based on these results suggest that, at peak historical densities of M. mercenaria in Great South Bay, removal of A. anophagefferens at low cell densities by suspension-feeding benthos could provide an effective top-down grazing control mechanism to prevent the initiation of brown tide in shallow, inner bays.     

Escape responses of copepod nauplii in the flow field of the blue mussel,<Emphasis Type="Italic"> Mytilus edulis</Emphasis>

Bivalves are important grazers on phytoplankton in shallow waters. However, very little is known about their ability to capture actively moving zooplankton. We investigated the escape response and success of early and late nauplii of three copepod species (Acartia tonsa, Temora longicornis and Eurytemora affinis) in the flow field of a blue mussel, Mytilus edulis, using both video observations and incubation experiments. An empirical model was created to describe the spatial distribution of the fluid deformation rate. Nauplii responded with escape jumps at mean fluid deformation rates of 0.6-1.9 s^у. Escape success differed between taxa. T. longicornis was the poorest escaper, while A. tonsa and E. affinis were more efficient and similar to one another. Deformation rates differed in different parts of the flow field, which resulted in differences in escape success between the sectors. Nauplii were caught most often in the sector furthest away from the exhalent siphon, where the deformation rate was the weakest. There the nauplii were unable to detect an escape signal in time to react and flee.     

A fractal approach for detecting spatial hierarchy and structure on mussel beds

Within beds of blue mussel (Mytilus edulis L.), individuals are aggregated into small patches, which in turn are incorporated into bigger patches, revealing a complex hierarchy of spatial structure. The present study was done to find the different scales of variation in the distribution of mussel biomass, and to describe the spatial heterogeneity on these scales. The three approaches compared for this purpose were fractal analysis, spatial autocorrelation and hierarchical (or nested) analysis of variances (ANOVA). The complexity (i.e. patchiness) of mussel aggregations was described with fractal dimension, calculated with the semivariogram method. Three intertidal mussel beds were studied on the west coast of Sweden. The distribution of wet biomass was studied along transects up to 128 m. The average biomasses of blue mussels on the three mussel beds were 1825낚, 179ᆩ and 576ᇖ g per0.1 m², respectively, and the fractal dimensions of the mussel distribution were 1.726ǂ.010, 1.842ǂ.014 and 1.939ǂ.029 on transects 1-3, respectively. Distributions of mussels revealed multiscaling behaviour. The fractal dimension significantly changed twice on different scales on the first bed (thus showing three scaling regions), the second and third beds revealed two and three scaling regions, respectively. High fractal dimension was followed by significant spatial autocorrelation on smaller scales. The fractal analysis detects the multiple scaling regions of spatial variance even when the spatial structure may not be distinguished significantly by conventional statistical inference. The study shows that the fractal analysis, the spatial autocorrelation analysis and the hierarchical ANOVA give complementary information about the spatial variability in mussel populations.     

Alarm responses of the green sea urchin, Strongylocentrotus droebachiensis, induced by chemically labelled durophagous predators and simulated acts of predation

The green sea urchin, Strongylocentrotus droebachiensis, exhibited immediate behavioural responses to waterborne chemosensory cues from two durophagous predators, the Atlantic wolffish, Anarhichas lupus, and the edible crab, Cancer pagurus; as well as from crushed conspecifics and crushed blue mussels, Mytilus edulis. The response patterns were dose dependent and diet dependent. Strong responses were elicited by water conditioned by echinivorous wolffish (97.5%), undiluted urchin extract (73%), undiluted mussel extract (52.5%), and by water conditioned by echinivorous crab (45%). In contrast, urchin extract diluted to 1%, mussel extract diluted to 10%, and water conditioned by predators on a mussel diet elicited weak responses (~20%). Infection by the endoparasitic nematode Echinomermella matsi had no significant effect on the response pattern of S. droebachiensis. There was no conclusive evidence of an alarm response to the predators per se, as the weak response to stimuli from non-echinivorous wolffish and crab, as well as from the seastar Asterias rubens, may have been caused by chemical cues transmitted from their mussel diet. The diet-dependent response to predators suggests that active predators were labelled by chemical cues from their echinoid prey. The chemical cue from echinivorous wolffish acted as both an arrestant and as a repellent, whereas the response to other cues was predominantly or exclusively repellent. The response to echinivorous wolffish was quantitatively stronger than, and qualitatively different from, the response to other stimuli, including undiluted urchin extract. The wolffish is apparently being labelled by a latent chemical cue which derives its potency from activation by, or interaction with, substances in the digestive system of the wolffish. We interpret this novel phenomenon as evidence of alarm signal magnification. The induced behavioural modifications demonstrate the green sea urchins' ability to detect chemical cues associated with active durophagous predators, and would therefore seem to have adaptive potential as a predator defence mechanism.     

Assimilation of dietary free amino acids,peptides and protein in post-larval Atlantic halibut (<Emphasis Type="Italic">Hippoglossus</Emphasis><Emphasis Type="Italic">hippoglossus</Emphasis>)

With the aim of improving the formulation of diets for early stages of marine fish, post-larval Hippoglossus hippoglossus (56 days after first feeding; 23 mg dry weight) were tube-fed liquid diets of radiolabelled L-(¹⁴C) free amino acids (FAA diet), peptides (PEPT diet) and protein (PROT diet), in order to compare the kinetics of transfer from gut into the liver and carcass. The test diets were prepared using equal concentrations of amino acids (AA) and fed as a single pulse (0.3 µl, 2.2 µg AA). The PROT diet consisted of methylated-¹⁴C-labelled (mainly lysine) bovine serum albumin (BSA), the PEPT diet was the PROT diet digested with trypsin and the FAA diet consisted of an amino acid mixture based on BSA composition with added ¹⁴C-lysine. The common labelling with ¹⁴C-lysine permitted a comparison of the three diets. Based on dissection and distribution of the tracer in each compartment, the FAA diet was almost completely absorbed 1 h after the tube-feeding; PEPT and PROT diets required more time before they were processed by the digestive tract. The FAA diet was absorbed about eight times as fast as protein and about six times as fast as peptides from the gut. A low retention of label in the liver for all diets at all sampling times suggests that the liver does not store the AA at these absorption rates. The FAA diet had a higher assimilation efficiency (about 90%) than the PEPT diet (about 12%) and PROT diet (about 32%). FAAs, thus, appear to be superior to protein and peptides as a dietary source of amino acids in post-larval Atlantic halibut.