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In the Sea Scallop Placopecten Magellanicus (Mollusca: Bivalvia)

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In the Sea Scallop Placopecten Magellanicus (Mollusca: Bivalvia) MARINE ECOLOGY PROGRESS SERIES Vol. 108: 251-264,1994 Published May 19 Mar. Ecol. Prog. Ser. I Variation in food quality and particle selectivity in the sea scallop Placopecten magellanicus (Mollusca: Bivalvia) Bruce A. MacDonald, J. Evan Ward Marine Research Group, Department of Biology, University of New Brunswick, Saint John. New Brunswick. Canada E2L 4L5 ABSTRACT: The sea scallop Placopecten rnagellanicus (Gmelin) is a suspension-feeding bivalve, com- mon in subtidal regions in the northwest Atlantic. In this study, we examined the feeding responses of scallops to changes in the quantity and quality of natural seston under field and laboratory conditions. Concentrations of natural suspended particles at our study site were < 5 mg I-', and chlorophyll a con- centrations were < 1.4 pg mg-'. Under these seston quantities and qualities, clearance rates were posi- tively correlated with the total amount of chlorophyll-containing particles in the water (pg I-'), while ingestion rates increased with increasing concentration of total suspended particulate matter (mg I-'). Scallops were able to select high-quality chlorophyll-a-containing particles and significantly improve the quality of material ingested. Pseudofaeces production increased slightly with increasing particle concentration, and selection was not based on the size of particles. We suggest that during conditions when the ingestive capacity of scallops is not exceeded, qualitative factors of the seston are more important than quantitative ones in mediating feeding processes. KEY WORDS: Suspension-feeding . Selection. Pseudofaeces . Food quality . Bivalve INTRODUCTION In nature, suspension-feeding bivalves are exposed to a food supply that fluctuates unpredictably in both Bivalve molluscs are dominant members of many quantity and quality. Many previous studies, however. infaunal and epibenthic marine communities. At high have examined bivalve feeding responses in the labo- densities, suspension-feeding bivalves may alter the ratory using only monocultured phytoplankton (for quantity and composition of seston in the overlying review see Winter 1978, Bayne & Newel1 1983).While water column, and they represent an important mech- a reasonable starting point, there have always been anism for coupling pelagic and benthic processes technical and conceptual difficulties in predicting an (Dame et al. 1980, Kemp et al. 1990, Dame 1993). organism's response in the natural environment from Depletion of suspended food particles can suppress data measured in the laboratory (Bayne et al. 1977, growth rates of other suspension-feeders in the com- Doering & Oviatt 1986, Bayne & Hawkins 1992). One munity (Peterson 1982, Frechette & Bourget 1985),and reason for this is that the natural food supply consists of can also significantly influence population dynamics of a complex mixture of organic and inorganic particles the plankton (Cloern 1982, Peterson & Black 1987). that is difficult to mimic in the laboratory. Another is Establishing the relationship between the concentra- that laboratory studies often use formulated seston tion and quality of suspended particulate matter and concentrations that are much higher than natural con- the uptake of this material by bivalves is important to centrations available in the field. Despite these differ- the understanding of energy flow not only through this ences, few workers have conducted studies on the major group of marine primary consumers (Kisrboe et feeding strategies of bivalves using natural seston, and al. 1980), but also through the benthic community as a even fewer have evaluated feeding activity when food whole (e.g. Herman 1993). varies temporally (Stenton-Dozey & Brown 1992, O Inter-Research 1994 Resale of full article not permitted Mar. Ecol. Prog. Ser. Newell & Shumway 1993). Information regarding Sciences Research Laboratory (MSRL), Memorial Uni- changes in feeding behaviour under natural conditions versity of Newfoundland, Logy Bay, in 1990. Scallops is critical to the analysis of bivalve energetics (Bayne et were collected at Sunnyside by SCUBA divers from a al. 1988). water depth of 10 to 15 m. Epibiota were cleaned from In general, suspension-feeding bivalves have the shell surface of 9 haphazardly selected individuals adopted several strategies for controlling the ingestion ranging in size from 120 to 160 mm in shell height. Col- of particulate matter including regulation of (1) feed- lection and cleaning of scallops were always carried ing duration (Foster-Smith 1975), (2) clearance rates out at least 1 d prior to the experiment, and animals (Bayne & Newell 1983), and (3) pseudofaeces produc- were held at the experimental site overnight. This pro- tion (particles cleared but rejected before ingestion; cedure allowed scallops to recover from any stress Kiarboe et al. 1980, Newell &Jordan 1983). Regulation associated with handling and pre-exposed them to of clearance rates and duration of feeding can only experimental conditions for at least 12 h before mea- alter the quantity of material ingested by the bivalve. surements were made. Experiments were conducted in Regulation of pseudofaeces production, however, can the field using natural assemblages of particles on prevent particle overload, eliminate material that April 18 and 19, at the beginning of the spring phyto- exceeds the animal's ingestive capacity (Beninger et plankton bloom, and on August 14 and 15. Experi- al. 1992), and facilitate selective rejection of less nutri- ments were conducted at the MSRL using natural ses- tious particles and enhance the quality of material ton supplemented with cultured microalgal cells on ingested (e.g. Ki~rboeet al. 1980, Newel1 & Jordan May 3, and supplemented with a mixture of micro- 1983).Thus, production of pseudofaeces can alter both algae and silt on November 15. the quantity and quality of material ingested, and it Measurements of clearance rate and retention effi- allows certain species to compensate for dilution of ciency. At Sunnyside, seawater from approximately food quality due to increases in the proportion of par- 5 cm above the bottom of a scallop bed was pumped ticulate inorganic matter (PIM; Widdows et al. 1979, into a 25 1 header tank of the experimental apparatus, Kiarboe et al. 1980, Kisrboe & Mahlenberg 1981, situated on the deck of a small stationary boat. Analy- Bncelj & Malouf 1984). This strategy is viewed as an ses of particle size distributions of unfiltered seawater, adaptive feature that enables the bivalve to maintain a using an electronic particle counter/sizer (Coulter positive flow of energy into growth and reproduction Multisizer) fitted with a 280 pm aperture, revealed that even during periods of poor seston conditions (Bayne & particles > 60 pm were very rare in the seston. There- Newel1 1983). The importance of such behavioural fore, the water was passed through a 100 pm screen responses to the overall feeding strategy in many semi-submerged in the header tank to prevent sus- bivalves, however, is not clear. This is especially true pended debris or flocs from clogging the apparatus. for bivalves exposed to natural seston at low particle Seawater from the header tank was distributed via concentrations (< 5 mg I-'). Tygon tubing to a series of 10 chambers (MacDonald The sea scallop Placopecten magellanicus (Gmelin) 1984). Flow rates were held fairly constant, between is a benthic suspension-feeder that is exposed to a 200 and 300 m1 min-', by maintaining head pressure wide range of pelagic and resuspended benthic mate- and using plastic flow-restricting plugs inserted in the rial (Shumway et al. 1987).Although scallops consume Tygon delivery lines. Within 1 h of placing scallops a variety of organisms and detritus, phytoplankton and in individual 'experimental' chambers, quantitative benthic microalgae seem to be a major food resource water samples were simultaneously collected from the (Shumway et al. 1987, Cranford & Grant 1990). The standpipe drain of each chamber. This was done over a goal of thls research was to determine the pre-inges- recorded time interval to allow calculation of flow tive feeding strategies of P. magellanicus when rates. Samples were taken hourly over a 4 to 8 h exper- exposed to quantitatively and qualitatively different imental period. The concentration of particles in the assemblages of natural particles. Both field and labora- outflowing water of an empty 'reference' chamber and tory experiments were conducted using natural seston, 9 experimental chambers was determined using the or natural seston supplemented with microalgae and particle counter equipped with a 100 pm aperture. Dif- silt. ferences in the concentration of particles between the reference and experimental chambers were used to calculate clearance rates (see below). The high water MATERIALS AND METHODS flow and exchange rates in the chambers prevented scallops from removing more than -30% of the parti- General procedures. Feeding activity in scallops cles. Analysis of water collected inside the containers was measured seasonally in the field at Sunnyside, suggested that dilution of the inflowing water by out- Trinity Bay, Newfoundland, Canada, and at the Marine flowing water (i.e.partially filtered) was minimal. MacDonald & Ward: Placopecte~lrnagellan~cus response to food quality 253 The size-frequency distribution of the particulate 47 mm GF/C grade filters under vacuum. Filters were matter in the reference chamber was determined peri- washed with approximately 5 m1 of isotonic ammo- odically using
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