Kim Lundgreen

Growth of mussels Mytilus edulis at algal (Rhodomonas salina) concentrations below and above saturation levels for reduced filtration rate

Abstract Average filtration and growth rates of groups of juvenile Mytilus edulis (n=25–45 of 22–35 mm shell length) were measured at different concentrations of an algal cell monoculture in 9 laboratory experiments of duration 14–30 days, 4 experiments below and 5 above the limit of incipient saturation concentration (C sat≈6000–7000 Rhodomonas salina cells ml−1). From a nearly constant filtration rate (F≈30 ml min−1 for a 30 mm shell length) at measured algal concentrations below C sat the steady-state filtration rate decreased approximately as 1/C for increasing algal concentrations (C) above C sat to levels as low as 12–9% of the former value. Corresponding calculated gross ingestion rates (I=F×C) increased linearly below C sat and remained nearly constant above C sat. However, the measured weight-specific growth rates (µ) decreased sharply above C sat from a maximal value of about 9.5% day−1 to about 1.5% day−1. Below C sat on the other hand, measured µ values increased linearly with increasing algal concentration, which was in good agreement with an earlier advanced bioenergetic growth model. The overall functional response of M. edulis resembles a Type I in terms of gross ingestion, but with a rapid decrease instead of a constant above C sat in terms of actual ingestion and growth. The physiological implications of the functional response remain uncertain. The response to increasing food concentration with possible regulation of net ingestion appears only to come into play when C sat is exceeded and then as partial valve closure and reduced filtration and growth rates along with production of pseudofaeces. A survey of naturally occurring phytoplankton biomass in the sea shows that this is generally below C sat except for the short spring bloom periods; hence, mussels generally feed at optimal rates depending on the composition and concentration of biomass exceeding the minimal concentration below which the mussels close their valves and reduce or cease filtering.

Physiological response in the European flounder (Platichthys flesus) to variable salinity and oxygen conditions

Physiological mechanisms involved in acclimation to variable salinity and oxygen levels and their interaction were studied in European flounder. The fish were acclimated for 2 weeks to freshwater (1‰ salinity), brackish water (11‰) or full strength seawater (35‰) under normoxic conditions (water Po2 = 158 mmHg) and then subjected to 48 h of continued normoxia or hypoxia at a level (Po2 = 54 mmHg) close to but above the critical Po2. Plasma osmolality, [Na+] and [Cl−] increased with increasing salinity, but the rises were limited, reflecting an effective extracellular osmoregulation. Muscle water content was the same at all three salinities, indicating complete cell volume regulation. Gill Na+/K+-ATPase activity did not change with salinity, but hypoxia caused a 25% decrease in branchial Na+/K+-ATPase activity at all three salinities. Furthermore, hypoxia induced a significant decrease in mRNA levels of the Na+/K+-ATPase α1-subunit, signifying a reduced expression of the transporter gene. The reduced ATPase activity did not influence extracellular ionic concentrations. Blood [Hb] was stable with salinity, and it was not increased by hypoxia. Instead, hypoxia decreased the erythrocytic nucleoside triphosphate content, a common mechanism for increasing blood O2 affinity. It is concluded that moderate hypoxia induced an energy saving decrease in branchial Na+/K+-ATPase activity, which did not compromise extracellular osmoregulation.

Field data and growth model for mussels Mytilus edulis in Danish waters

Abstract Growth rates of blue mussels, Mytilus edulis, in suspended net-bags were measured in field experiments conducted in Limfjorden and Great Belt, Denmark, for extended periods of time (22–69 days) with up to 8 intermediate data samplings during the period. The resulting time series of growth of different overlapping size-classes of M. edulis were analysed individually and after being assembled to cover the full range of sizes, as full time series in terms of weight specific growth rate (µ=(1/W) dW/dt,% day−1) as a function of dry weight of soft parts (W, g). The results were compared to a simple bioenergetic growth model for mussels (µ=aW b , a=0.871×C−0.986, b=− 0.34), and observed power-law relations of growth data were in good agreement with the model, which takes into account the prevailing average chlorophyll a (chl a) concentration (C, µg chl a l−1) at field sites. Thus, the b-values of data were close to the predicted −0.34 while the a-value was in one case lower than estimated due to suboptimal conditions during part of the growth period. As a supplementary interpretation the growth data have been expressed in terms of the time to double the dry weight of soft parts for a given size of mussel and at a given experimental site.

Biomass Composition of Blue Mussels, Mytilus edulis, is Affected by Living Site and Species of Ingested Microalgae

We have investigated changes in specific contents of protein, glycogen and lipid, and fatty acids of blue mussels, Mytilus edulis, under different conditions in the field and in laboratory feeding experiments using different microalgae. Specific contents of glycogen and lipid increased in mussels relocated to net bags at a location in Kerteminde Bay (Great Belt, Denmark) in contrast to mussels relocated to a location in Sallingsund (Limfjorden, Denmark). The polyunsaturated fatty acid, eicosapentaenoic acid, reached 3 times higher values in the mussels in Kerteminde Bay. Mussels fed pure cultures of Crypthecodinium cohnii, which is rich in the polyunsaturated fatty acid, docosahexaenoic acid, and glycogen, gained the highest specific contents of this fatty acid and glycogen. Mussels feeding on the most protein rich of the microalgae, Bracteacoccus sp., gained the highest protein contents. The specific glycogen content of the mussels was influenced by their “condition” (body dry weight/shell length ratio) while specific protein and lipid contents were not. Starvation affected mainly the specific glycogen content. These results show that biomass composition of blue mussels is affected by living site and local phytoplankton species and that the fatty acids composition of mussels reflects the content of fatty acids in the diet.

Fluorometer Controlled Apparatus Designed for Long-Duration Algal-Feeding Experiments and Environmental Effect Studies with Mussels

Experimental feeding and growth studies on filter-feeding organisms often rely on constant algal concentrations maintained over extended periods of time. Here we present a fluorometer controlled apparatus (FCA) designed for feeding experiments with suspension-feeding mussels at naturally low chlorophyll concentrations above 0.5 µg L−1. The principle used is feedback regulation of the algal concentration based on continuous monitoring of the fluorescence intensity of chlorophyll in water pumped through the apparatus from an aquarium with mussels. The filtration rate is monitored continuously as the rate of change of measured volume of an algal stock added to the aquarium for keeping a constant algal concentration. As an example, the FCA has been used to study the filtration rates of blue mussels (Mytilus edulis) at algal concentrations both near and above the incipient saturation level for reduced filtration activity. As another example to put the FCA into perspective as a reliable method for environmental effect studies, the apparatus has been used to demonstrate the acute effect of changing salinity on the filtration rate of M. edulis.

Jellyfish and Ctenophores in the Environmentally Degraded Limfjorden (Denmark) During 2014 - Species Composition, Population Densities and Predation Impact

Species composition, population densities and size of jellyfish and ctenophores were recorded during 5 cruises in the heavily eutrophicated Limfjorden in 2014. No or very few ctenophores (Pleurobrachia pileus) and jellyfish (Aurelia aurita, Cyanea lamarckii) were recorded in April and June 2014, whereas in August and September numerous small individuals of the invasive ctenophore Mnemiopsis leidyi were found on all 4 locations studied, which were strongly reduced in population density during November. M. leidyi exerted a notable predation impact, most pronounced in Logstor Bredning and Skive Fjord in August when the estimated half-lives of zooplankton were 4.8 and 7.3 d, respectively, and in late September, when the half-life in Skive Fjord was only 2.2 d. Severe oxygen depletion in Logstor Bredning and Skive Fjord between June and September resulted in a release of nutrients. This was followed by a bloom of the dinoflagellate Noctiluca scintillans and a subsequent peak in the abundance of copepods which decreased rapidly after the introduction of M. leidyi into Limfjorden from the North Sea (between early April and mid- July) to become virtually absent during the rest of the season. This subsequently resulted in starvation and decay of the M. leidyi population. The small predatory ctenophore Beroe gracilis was recorded on most locations during August and September 2014 but although B. gracilis eats small M. leidyi, their low number suggested a negligible predation impact on the M. leidyi population. Our present understanding of the many biological and environmental factors that control the species composition, abundance and predation impact of jellyfish and ctenophore populations in Limfjorden are discussed. It is concluded that there are many unsolved questions, e.g. how gelatinous predation of zooplankton may reinforce anoxia and further habitat degradation in eutrophicated waters.

Coelomic transport and clearance of durable foreign bodies by starfish (Asterias rubens)

Echinoderms have excellent healing and regeneration abilities, but little is known about how they deal with the related challenge of durable foreign bodies that become lodged within their bodies. Here we report a novel mechanism for foreign body elimination in starfish. When injected into the arm of a starfish, passive integrated transponder tags and magnets of similar dimensions are eliminated at a rate approximating 10% per day. These objects are forcefully ejected through the body wall at the distal tip of an arm. Ultrasound images reveal that foreign bodies are moved within the body cavity, and tracking of magnets injected into starfish suggests that the movements are haphazard rather than directed. Constrictions of the body wall near the foreign object are the likely mechanism for this transport process. Open questions include the ecological relevance of this behavior, why clearance occurs through the distal tips of the arms, the neurological and muscular control of this behavior, what other animals use this mechanism, and the range of objects starfish can eliminate in this way.

UV fluences required for compliance with ballast water discharge standards using two approved methods for algal viability assessment

This study investigates the extra UV fluence needed to meet the International Maritime Organisations ballast water discharge standards for the 10-50 μm size-class using the approved vital stain (VS) method compared to the Most Probable Number (MPN) method for organism viability assessment. Low- and medium pressure UV collimated beam treatments were applied to natural algae collected in temperate and tropical water environments and analysed using both methods. About 10 times higher UV fluence was required to meet discharge standards when using VS compared to MPN. Implementing a dark-hold period after UV treatments decreased algal viability. Length of dark-hold period to meet discharge standards decreased with increasing UV fluence. No significant differences between temperate and tropical samples were observed. The results showed that UV treated algae assessed using the VS method could meet discharge standards by increasing fluence and/or introducing a dark-hold period.