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The feedlng response of the blue mussel Mytllus trossulus exposed to environmentally relevant seston matnces were studied in the laboratory under flow-through conditions Twelve seston matnces varying In quality and quantity were prepared by mixlng 3 microalga Thalassloslra pseudonand (5, 20 and 150 X 106 cells 1 ' ) and 4 silt concentrations (0 5 20 and 50 m g 1 ') to represent an increasing seston organic quallty of 10 to 71 and dn increaslilg seston quantity of 1 4 to 56 6 mg I ' Clearance rates (CRs) pseudofaeces (PF) production, sorting efficiency (SEF) ingestion rates (IRs) of particulate organic matter (POhl) and particulate inorganic matter (PIM) and apparent and true carbon ass~milation efflciencies (C-AE0 ,) were determined for the varlous exposure regimes Under conditions of 0 dnd 5 mg 1 ' silt CRs decreased by 3-fold 114 4 to 4 9 1 h ' g ' dry wt (gdw ' ) I and 6-fold (18 0 to 3 3 1 h ' cldw ' ) respectively with Increasing seston quality Under condit~ons of hlgh silt loads (20 and 50 mg I-') CRs were independent of increasing seston quallty with maximum CRs (21 8 & 2 2 1 h ' gdw l ) observed at the 20 x lob cells 1 ' and 20 mg l I silt exposures PF production was dependent on seston quantity (r2 0 63 p < 0 05) with mussels preferentially rejecting the inorganic versus the organic component of the seston This SEF was optlrnized at a seston quality of 40% organic matter As a consequence of this selective feedlng strategy, determined POM IRs under high a lgae /h~gh s ~ l t (high quantity/quality) exposure reglmes were comparable to those of the high quality (lust algae) exposures (POM IRs of 48 0 as compaied to 38 1 and 91 3 mg h ' gdbv ' for high quality and high quanti ty/qual~ty respectively) In contrast mussels exposed to low quahty/quantity seston Ingested both seston components (SEF <24%) I e the mussel was non-select~ve possibly ingesting both seston components to meet nutrient requirements Apparent C AE",, expressed as C  C , ,,,,., /C  correlated with seston quality ( '0 POM) (r2 = 0 64, p < 0 05) However, carbon assin~ilation expressed as a true C-AE"o, C C,,,, ,,, iC \vas Independent of seston quality as was carbon assimilation rate ( the product of the amount of Ingested organic matter and carbon assim~lation efficiency) Hence through a dynamic interplay between CRs and cai bon a s s~m~la t ion effic~ency the blue mussel was able to malntain a constant late of carbon assim~lation regardless of the quality and quantity of sebton to which it was exposed This compensatory feeding strategy displayed by the blue mussel has Impoi tant implications for predictive models w h ~ c h relate contaminant uptake to a diet/energy pathway Contaminant uptake may be either unde~est imated or overestimated if the abi l~ty of the blue mussel to optlmize nutrlent gain under d vanety of seston compositions has not been taken into account KEY LVORDS Myhlus trossulus Feeding behaviour Sorting process Pseudofaeces Ingestion rate Ass~milatlon of carbon INTRODUCTION portance, is a species complex of blue mussel (Mytilus edulis L., M. galloprovincialis Lamarck, and M. trossuWithin estuarine environments, one of the main suslus Gould). As a result of its global distribution and its pension feeders, of both economic and ecological imability to concentrate metals from its environment, the blue mussel has been used world-wide for pollution 'Addressee for correspondence E-mail. bendell@sfu.ca monitoring programs. Given its importance, much 0 Inter-Research 1997 Resale of full a r t~c l e not pe l rn~t ted 242 Mar Ecol Prog SE study has focused on how biotic factors such as size. sex and growth rates influence tissue metal burdens (Cossa et al. 1979, Poulsen et al. 1982, Lobe1 et al. 1991). More recent studies have been directed at determinlng the underlying physiological mechanisms that influence metal accumulation by filter-feeding organisms. For example, Wang et al. (1995) related the amount of metal accum~~lation by bivalves to gut passage time and digestive partitioning. Decho & Luoma (1996) noted that bivalves were capable of modifying the digestive processing of food to reduce exposure to biologically available chromium concentrations. Wang & Fisher (1996) suggested that for some metals, assimilation is related to the degree of carbon assimilation by the mussel, i.e. that metal assimilation and hence accumulation by the mussel follows a diet/energy pathway. We know of no studies, however, which have assessed the possible role of feeding behaviour in determinlng the amount of metal to which the mussel is exposed to via its diet and hence amounts of metal that are potentially available for uptake and accumulation by the filter-feeding mussel. Currently, there is much controversy in the literature in regards to whether filter feeders have a selective feeding strategy (e.g. Bayne et al. 1993) or whether filter-feeding is a highly automated process characterized by the full exploitation of the filter-pump to process the ambient water under optimal conditions (e.g. Jargensen 1990). Bayne et al. (1993) found that blue mussels fed at high concentrations of seston of relatively low organic content (40% particulate organic matter) increased their filtration rate, rejected a higher proportion of filtered material as pseudofaeces, and increased the efficiency with which filtered matter of higher organic content was selected for ingestion. These changes in response to a changing food environment by the mussel resulted in higher rates of carbon absorption than would have been predicted based on the assumptions of a non-compensating feeding behaviour. In contrast, based on their studies of growth, filtration and respiration in the blue mussel, Clausen & Riisgdrd (1996) conclu.ded that there was no evidence for physiological regulation 01 the filtration rate to meet nutritional needs and that as claimed by Jsrgensen (1990), food uptake in nature was characterized by the full exploitation of the capacity of the bivalve filter-pump. If mussels are highly selective as suggested by Bayne et al. (1993), then the 'diet' of the mussel [i.e. that component of the seston that the animal is actually ingesting) and the subsequent absorption of carbon from the diet have the potential of being completely different from that which would be expected based on in situ measurements of seston within the environment and what the mussels are supposedly feeding on. This has important implications for predictive models of metal accumulat~on by seston-ingesting organisms that link metal assimilation to the organic content of the 'diet' (e.g. Wang & Fisher 1996). If mussels are capable of a compensatory feeding strategy, as proposed by Bayne et al. (1993), which leads to a relative enhancement of net energy and nutrient acquisition over that which would be predicted based on the organic/inorganlc composition of the seston matrix, then predictive models of metal (and organic) contaminant uptake, based on a diet/energy pathway, may underestimate amounts of contaminants to whlch the mussels are actually exposed. Hence, the objective of our study was to determine the feeding response of the blue mussel to environmentally relevant seston matrices which varied in both quality and quantity. Specifically we were interested, first, in determining the degree to which the mussel could alter its food environment (i.e. wa.s the 'diet' that the mussel ingested the same as the seston matrix to which it had been exposed) given a wide range of environmentally relevant seston matrices, and second, in determining the corresponding carbon assimilation efficiencies for the 'diet' as selected for by the mussel. To meet these objectives, changes in the following physiological attributes in the blue mussel Mytilus trossulus exposed to the various seston matrices were assessed: (1) clearance rate, (2) pseudofaeces production, (3) sorting efficiency, (4) particulate organic and inorganic matter ingestion rates, (5) apparent and true carbon assimilation efficiencies and (6) carbon assimilation rates. Seston quality and quantity were chosen to represent seasonal and tldal patterns in seston quality and quantity found under natural conditions (i.e. within the estuarine environment) (Berg & Newell 1986, Smaal et a1 1986, Fegley et al. 1992, Galois et al. 1996, Stecko 1997). Ultimately this information will be used in predictive models of metal accumulation in bivalves based on a selective feeding strategy which optimizes for the assimilation of carbon under a range of environmental conditions. MATERIALS AND METHODS Experimental mussels. Mussels Mytilus trossulus Gould, 1850 were collected from an intertidal area on the west coast of British Columbia (within Howe Sound), Canada. Collected mussels were placed in a cooler, and transported to the laboratory at the Department of Biological Sciences, Simon Fraser University. Mussels 144.9 + 2.07 mm in shell length (SL)] were acclimatized to experimental conditions (temperature, 13 * 1°C; salinity: 28 ppt) for 2 wk prior to use in each experiment. Throughout the acclimation period, the Arifin & Bendell-Young: Feeding response of blue mussel 24 3 mussels were fed the diatom alga Thalassiosira pseudonana daily, and the seawater was changed on a regular basis. Prior to the feeding experiments, mussels were separated from their basal attachments to one another, brushed clean and kept for approximately 15 min under dry air. This procedure ensured that only live mussels were used in the experiments, as those mussels that were not viable did not respond to being submerged in seawater following the 15 min exposure to air. Experimental apparatus. Feeding behaviour experiments were conducted under flow-through conditions in 500 m1 plexiglass tanks. An 18 l source of filtered seawater was provided by sequentially filtering supplied seawater through 5.0 and 1.0 pm cartridge filters (Labcor Inc.). The seston matrices were prepare

Feeding response and carbon assimilation by the blue mussel Mytilus trossulus exposed to environmentally relevant seston matrices