The suspension-feeding activity of Cei-astoderrna edule (L ) was investigated in response to a wide range of experimental seston concentrations which approximated the changes in seston quality and quantity found in the natural environment over tidal cycles of sediment resuspension. The different seston concentrations were produced by adding increasing quantities of resuspended fine sediment to a relatively low and constant algal concentration (0.59 mg dry wt I-'). The total seston concentrations (total particulate matter) varied over a wide range from 1.6 to 570 mg l", and the organic fraction (particulate organic matter) between 0.62 and 79.8 mg 1-' The organic content of the diets decreased with the increasing seston concentration, from 38.8% at the lowest concentration to 14 % at the highest concentration. A significant negative relationship was found between clearance rate of C. edule and seston concentration, with very low values at 570 mg 1-' of total seston. Filtration rate increased with seston concentration from a minimum value of 2.67 mg h-' at the lowest concentration to a maximum of 84.0 mg h-' at a concentration of 300 mg I-'. Above 300 mg 1-' there was an abrupt decline in filtration rate. Pseudofaeces production started at a concentration of 4.8 mg 1 ', showing its highest values at around 300 mg I-' The organic content of pseudofaeces was significantly lower than the organic content of the seston, suggesting that C. edule is able to preferentially select organic particles for ingestion instead of inorganic particles. Selection efficiency was maintained at a high and constant level at seston concentrations between 4.8 and 97 mg 1-', declining at the highest seston concentrations with the lowest organic content. Thus the physiological sorting and selection mechanism appears to be very effective at seston concentrations below 100 mg 1-l, this ability declining abruptly at higher concentrations. Ingestion rate was constant in the lower range of seston concentration (1.6 to 34 mg 1-l), increased at concentrations around 250 mg I-', and declined abruptly at the higher food rations. Absorption efficiency appeared relatively independent of seston concentrations over a large range, but it was reduced at concentrations above 250 mg 1-' We found a very effective digestive mechanism to regulate absorption rate at seston concentrations between 1.6 and 250 mg I-' The results suggest that C. edule can compensate efficiently for a decrease in seston quality over a wide range of seston concentration (1.6 to 300 mg I-') by maintaining an effective preingestive mechanism of selection for organic particulate matter, as well as increasing filtration and rejection rates. As a consequence, this species is well adapted to living in turbid environments, such as intertidal mudflats, which are characterised by marked fluctuations in seston quality and quantity, caused by resuspension of fine sedirnents during periods of high current velocities on the flood or ebb tides.

Seasonal changes In the size spectrum and biochemical composition of the suspended particulate matter were recorded for 2 yr in Logy Bay, southeast Newfoundland. Canada. The seston load was lower than in many shalloiv marine bays, owing partly to a relatively small terrigenous input and little silt, and the quantity of the parhculate organic matter primarily depended on phytoplankton productivity. The ratio chlorophyll a:phaeopigments was high during the phytoplankton bloom, due to the living nature of the organic seston at this time, compared with the rest of the year, when lower values were associated with the highly degraded suspended material. The size-frequency distribution of the suspended particulate matter reflected the succession of the phytoplankton community throughout the year. The spnng peaks were composed mainly of single diatoms and chain-forming diatoms in the size range 20 to 60 pm (equivalent spherical diameter). Variations in carbohydrate, lipid and protein in the seston were primarily associated with the periods of high and low standing stocks of phytoplankton observed in this environment. The seasonal trends were very similar for each biochemical component. and values were highly correlated with those for other properties of the seston, such as organic matter, chlorophyll a and particulate organic carbon and nitrogen. The nutritional quality of the seston was expressed by a food index, calculated as the ratio of food material (protein + carbohydrate + lipid) to total seston. This index followed the cycle of the phytoplankton bloom, with maximum values during spring and summer, and much lower values during winter. The results suggest that the gross analysis of the suspended particulate matter or the measurement of a single chemical variable cannot describe fully the nutritive value of the seston to a suspension-feeder. An analysis of the size spectrum and biochemical composition of the material is required for this purpose.

A population of Modiolus modjolus (horse mussel) inhabiting a sub-arctic environment in Logy Bay, Newfoundland, Canada, was studied for 2 yr. The main objective was to gain insight into the relationship between environmental factors (temperature and components of the seston) and the physiological response of this species. The highest values for energy acquisition (ingestion and absorption rates) by M. rnodiolus coincided with the spring phytoplankton bloom occurring during April-May in Logy Bay, whereas energy expenditure (oxygen uptake and ammonia excretion rate) was greatest during the summer (July and August). The result was a clear seasonal fluctuation in the 2 physiological integrations, scope for growth (SFG) and net growth efficiency (K,), for which lower values were associated with a high metabolic rate, high temperature and low quality of the food supply. Conversely, higher values of SFG and K2 were associated with a low metabolic rate, low temperature and a n energy-rich food supply provided by the phytoplankton bloom. M. modiolus appears to be a species which is able to compensate physiologically for the poor food conditions occurring in Logy Bay during a large part of the year. It does this by reducing feeding activity during fall and winter, when there is a poor food supply. Conversely, it increases clearance rate during spring and early summer, when the seston is mainly composed of phytoplankton. Furthermore, M modiolus has a high absorption efficiency, which may be an adaptation to the low seston concentration in Logy Bay Such physiological compensations minimise the periods dunng which SFG is negative, and enable the horse mussel to survive in a n environment characterised by an intermittent and often inadequate food supply.

Digestive enzyme activities (amylase, cellulase, laminarinase and protease) were analysed in mussels (Mytilus chilensis) from intertidal and subtidal habitats in Yaldad Bay, Chiloé, Chile. In order to analyse the effects of the past-feeding history (origin) and new nutritional conditions (habitat) on these enzymatic activities, a cross-over transplant was carried out and the analysis performed after a 7-day acclimation period. Crystalline styles showed higher carbohydrase and lower protease activities than digestive glands, with the highest differences recorded for subtidal mussels. Cellulase is the enzyme with the highest activity in both the digestive gland and crystalline style in all the experimental conditions. Intertidal mussels transplanted to a subtidal habitat showed enzyme resources significantly higher than in their original habitat. In the inverse case, mussels transferred from an original subtidal habitat to an intertidal one, a significant decrease in carbohydrase and protease activities was observed. The "past feeding history' is involved in the specific and total carbohydrase and protease activities, with a highly significant effect on amylase and cellulase activities in both the crystalline style and the digestive gland. Laminarinase activity can be interpreted considering the habitat (trophic regime), either individually or interacting with mussels' origin, in relation with the feeding periods. The results establish that in M. chilensis, an investment in enzyme resources is one of the mechanisms employed to optimise the acclimated response in terms of energy gains when variations in the food regime occur.

Active predators obtain energy and nutrients from prey through complex processes in which the energy gained must exceed the energy invested in finding and ingesting the prey. In addition, the amount of energy available will vary with the prey that are selected for consumption. The muricid gastropod Acanthina monodon inhabits rocky shores, where it routinely feeds on the mytilids Semimytilus algosus and Perumytilus purpuratus. In this study, S. algosus was highly preferred by the predator (over 90% were eaten) versus P. purpuratus (only 9% were eaten) when offered a mixed diet. The energetic cost of attacking one S. algosus individual was 91 J bivalve-1 while for P. purpuratus it was slightly higher: 95 J bivalve-1. Also, whereas A. monodon required on average 19 h to consume S. algosus, successful attacks on P. purpuratus required about 32% more time (25 h). In addition, a longer resting time was needed by the predator after preying on P. purpuratus before it initiated another attack. Moreover, the active metabolic costs associated with successfully attacking the prey increased 3.2 times over the basal metabolic costs when attacking S. algosus, but only by 2.5 times when attacking P. purpuratus. The calculations associated with preying on each species showed that the energetic gain per unit time likely accounts for the predator’s preference for attacking S. algosus, even though predation on both species provided net energy gains for the predator. However, as S. algosus occurs seasonally at our study site, P. purpuratus would probably also be consumed due to its constant availability throughout the whole year.