clones, and uptake of chlorophyll vs. non-chlorophyll ceils, both within and between scallop species. Significant differences in clearance rates of individual algal ...
Aquaculture International 5, 89-99
Particle clearance and selection in three species of juvenile scallops S.E. S h u m w a y 1,4,* T.L. CuccP ,s, M.P. Lesser ~, N. Bourne 2 and B. Bunting 3 ~Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, ME 04575 USA 2Fisheries and Qceans, Pacific Biological Station, Nanaimo, British Columbia, Canada V9R 5K6 3Island Scallops Ltd, 5552 West Island Highway, R.R. 3, Site 327, C-1, Qualicum Beach, British Columbia, Canada VOR 2TO 4Current address: Natural Science Division, Southampton College, LIU, Southampton, NY 11968, USA; 5Current address: Zoology Department, Spaulding Life Sciences Center, University of New Hampshire, Durham, NH 03824-3544, USA
Juvenile scallops ( < 2 mm shell height) of three species (Placopecten magellanicus, Patinopecten yessoensis, Argopecten irradians) were fed mixed, unialgal cultures. Scallops were fed a total of six algal clones simultaneously and clearance rates were monitored using flow cytometric techniques. In another experiment, scallops were presented with natural assemblages of particulate matter as a food source. Data are presented on differences in clearance rates for the individual algal species as well as size-related differences of algal clones, and uptake of chlorophyll vs. non-chlorophyll ceils, both within and between scallop species. Significant differences in clearance rates of individual algal species have been found within and between scallop species. Particle selection does not appear to be based upon size alone and is apparently based on other characteristics of the algae as well. The results demonstrate pre-ingestive sorting. KEYWORDS: Feeding, Scallops (Placopecten, Patinopecten, Argopecten), Selection
INTRODUCTION A suitable algal diet is crucial for the d e v e l o p m e n t , growth and survival of cultured bivalves. Diets t h a t s u p p o r t larval growth h a v e b e e n d e t e r m i n e d for s e v e r a l bivalve species, but p o s t - m e t a m o r p h i c d i e t a r y r e q u i r e m e n t s are still p o o r l y u n d e r s t o o d . In addition, v e r y little is known a b o u t the functioning of the feeding organs in juvenile scallops (Beninger et al., 1994). Rapid g r o w t h and high s u r v i v a l of t h e early juveniles a r e critical to t h e e c o n o m i c viability of bivalve h a t c h e r i e s . P h y t o p l a n k t o n s p e c i e s cultured for bivalve food are generally s e l e c t e d on the basis of size, nutritive value a n d a m e n a b i l i t y to m a s s cultivation. Recent investigations on t h e selection of particles b y juvenile scallops (Lesser et al., 1991) h a v e d e m o n s t r a t e d the imp o r t a n c e of u n d e r s t a n d i n g species-specific p r e f e r e n c e s as well, especially w h e n *Author to whom correspondence should be addressed. 0967-6120 © 1997 Chapman & Hall
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providing mixed algal diets. This knowledge is essential for determining the algal species and rations that optimize growth and efficiency in aquaculture systems. MATERIALS AND METHODS
Juvenile specimens of Placopecten magellanicus (Gmelin, 1791) (0.9-1.6 mm), Argopecten irradians irradians (Lamarck, 1819) (1.2-1.6 mm) and Patinopecten yessoensis Jay (0.8-1.6 mm) were obtained from The Great Maritime Trading Company, New Brunswick, Canada, Shinnecock Tribe Oyster Project, Southampton., New York, and Island Scallops Ltd, British Columbia, Canada, respectively. Animals were maintained in filtered seawater (0.45 ~m; 15 °C) and fed Isochrysis (TISO) prior to use in experiments. Water was changed daily and animals were used within 3 days of acquisition. Experiments were carried out at 15°C. Scallops (approximately 5-10 per vial, attached since settlement to monofilament net) were placed in Coulter vials containing varying volumes (10-20 ml) of algal culture mixtures (see below) and gently aerated. Control vials, without animals were run simultaneously to correct for algal cell division during the experiment. Experiments lasted up to 20 h, depending upon the feeding rate (depletion of experimental medium). Algal cultures (Table 1) were supplied from the Culture Center for Marine Phytoplankton, Bigelow Laboratory for Ocean Sciences. Algal species were chosen based on their size, fluorescence characteristics and prior use as food sources in scallop culture. Cultures were grown in Guillard's f/2 media at 15°C on a 14:10 light/ dark photoperiod. For all experiments using cultures, a final concentration of 10 4 cells m1-1 was used to avoid any density-dependent changes in clearance rates (Riisgard and Randlov, 1981). Two sets of experiments were run using different mixtures of algal species. The first mixture included: Omega 48-23, PHAEO, PLATY1, 3C and AMPHI; the second contained Omega 48-23, TISO, 3C, PHAEO, CHGRA and AMPHI. Experiments to determine the clearance of natural particulates were set up
TABLE 1. Algal species, clone designations, and approximate sizes of ceils used in feeding experiments Species
Clone
Size range (#m)
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3C
Particle selection by scallops
91
in the same way as those for cultures, and natural seawater from Boothbay Harbor, Maine, was collected and characterized the day of the experiment. Samples were analysed by flow cytometry using the differences in fluorescing intensities from cells' respective photosynthetic pigments and/or cell size from forward angle light scatter (Fig. 1) as described previously (Cucci et al., 1985; Shumway et al., 1985)• Statistical
analyses
Clearance rates of each group of algal or natural particulate assemblage by the three scallop species were analysed using an analysis of variance (ANOVA) on the fixed effect of clearance rate of total cells• Additionally, an A_NOVA was conducted for each species of scallop to determine which species of algae had significantly higher clearance rates. For experiments using natural assemblages of particles a two-way
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S.E. Shumway et al
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ANOVA with species and particle size as t r e a t m e n t effects and total particles, nonchlorophyll-containing particles, and chlorophyll-containing particles as measurem e n t s was conducted. All experimentaI d a t a were normally distributed and no unequal variances detected. Significant t r e a t m e n t effects were followed with posthoc multiple c o m p a r i s o n testing using a Student-Neuman-Keuls (SNK) analysis at a significance level of 5%. Results are expressed as clearance rate (ceils 1--1 h -1) per individual. RESULTS AND DISCUSSION
Typical flow cytometric results are p r e s e n t e d in Figs. 2 and 5 for scallops fed pure algal clones and natural particulate assemblages, respectively. There was a significant effect (ANOVA: P = 0.0001) of species on the clearance rate of total cells (algal cultures) for juvenile scallops (Fig. 3). Multiple comparison testing revealed that all pairwise comparisons were significantly different (SNK: P < 0.05). For each species of juvenile scallop a significant effect (ANOVA: P < 0.05) of individual algal species on clearance rate (Fig. 4) was observed. Multiple comparison testing for each species revealed a specific pattern. For Patinopecten yessoensis, Omega 48, TISO and CHGRA were preferred significantly less than the other clones (Fig. 4, top panel). For both Placopecten magellanicus (Fig. 4, middle panel) and Argopecten irradians (Fig. 4, bottom panel), Omega 48 was the least preferred while all other clones were c o n s u m e d at similar rates. Lastly, the experiments utilizing natural particles revealed a significant effect of both species (ANOVA: P = 0.0001) and particle size (ANOVA: P = 0.001) for the m e a s u r e m e n t of clearance rates of total particles, non-chlorophyll-containing particles, and chlorophyll-containing particles. Patinopecten yessoensis exhibited significantly lower clearance rates of all size ranges tested while Placopecten magellanicus and Argopecten irradians exhibited significantly higher rates except for the clearance rates of > 8 ~m particles for P. mageUanicus. This species exhibited significantly lower clearance rates of all particle types in this size range. It is clearly s h o w n in Fig. 2 that two clones, AMPHI and PHAEO, were cleared by Patinopecten yessoensis prior to removal of other cells. Both Placopecten and Argopecten also d e m o n s t r a t e d a similar selection of algal species but the differences were not as dramatic. Further, Argopecten also selectively depleted the population of TISO. It should be noted that AMPHI and PHAEO are not the same size and, in fact, other clones (CHGRA and TISO) are of similar size to PHAEO and yet were not removed from s u s p e n s i o n at the same rate. These choices are probably not, in fact, in the scallop's best interest, as PHAEO is known to be a poor food source for bivalves and AMPHI is suspected to be toxic to shellfish (Guillard, 1958 and personal communications). All three species of scallops appear to clear the larger particles first when p r e s e n t e d with natural assemblages of particles (Fig. 5). However, these d a t a are more difficult to interpret and being analysed further, these results indicated that: • scallops fed on algal mixtures s h o w e d clear differences in overall clearance rates (ceils 1-1 h -x individual-l): Patinopecten < Argopecten < Placopecten (Fig.
3); • there is apparent preference for particular clones (see above) (Fig. 4);
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• there may be some differentiation between chlorophyll-containing and nonchlorophyll-containing particles (Fig. 6); however, more individual experiments need to be done to establish significance levels; • no scallop species readily ingested Omega 48-23; this is the smallest of the clones used in experiments; • when scallops were fed natural assemblages of particles, clearance rates varied between scallop species, with Placopecten generally having higher uptake rates for all three size classes of particles (3-5 ~m; 5-8 ~m; > 8 ~m) followed by Argopecten and Patinopecten. For the 8 t~m size class of particles, Argopecten had the highest clearance rate. Results obtained here for Placopecten support a previous study of juveniles (6.5-9.5 tJ,m) by Lesser et al. (1991), where highest clearance rates were recorded for AMPHI, DUN (Dunaliella tertiolecta, 9-11 i~m) and UW442 (Pyraminonas parkeae, 11-I6
Fig. 4. Mean clearance rates (note differing scales) for each algal clone fed to Patinopecten yessoensis, Argopecten irradians and Placopecten magellanicus in mixtures (initial concentration of cell mixture: 104 cells ml-1). Error bars represent 95% confidence limits and like letters indicate no significant difference in clearance rates of individual algal clones. Note that neither Placopecten nor Argopecten exhibited the same marked preference for particular clones as did
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98
S.E. Shumway et al
C o n s i d e r a b l e attention has been paid to the d e v e l o p m e n t of 'artificial', non-living diets for larval and post-set bivalves (Langdon and Siegfried, 1985; Langdon, 1989) and efforts are u n d e r w a y to develop s u c h a diet for P magellanicus (J. Kean-Howie, pers. comm.). Most aquaculture facilities, however, still rely on cultured microalgae for the bulk of their feed requirements. Traditionally, culturists used only a few species of algae as 'tried and true' food sources, e.g. lsochrysis, Thallassiosira, Tetraselmis, and it is generally a c c e p t e d that the nutritional effects of two or more species of algae are superior to m o n o c u l t u r e s as a food s o u r c e (Epifanio, 1979). While o u r e x p e r i m e n t s do not a d d r e s s the nutritional quality of any of t h e algal s p e c i e s investigated, it must be r e m e m b e r e d that before any alga can be utilized it must be ingested. Particle selection is an o b v i o u s a d v a n t a g e to suspension-feeding organisms, not only in sorting organic from inorganic particles, but also in c h o o s i n g particles of easier digestibility or higher nutritive value o v e r t h o s e m o r e difficult or impossible to digest. Growth rates of scallops need to be d e t e r m i n e d using various algal diets and it is h o p e d that t h e s e studies, in collaboration with t h o s e of r e s e a r c h e r s working to d e t e r m i n e the biochemical r e q u i r e m e n t s for growth of bivalves, will lead to i m p r o v e d selection of new food s o u r c e s for scallop culture.
CONCLUSIONS 1. Significant differences in c l e a r a n c e rates of individual algal species were found within and b e t w e e n three species of juvenile scallops. 2. Particle selection did not a p p e a r to b e b a s e d u p o n size alone but was influenced b y o t h e r characteristics of the algae as well. 3. Pre-ingestive sorting by scallops was d e m o n s t r a t e d .
ACKNOWLEDGEMENTS This is Bigelow L a b o r a t o r y for Oceap. Sciences c o n t r i b u t i o n no. 95013.
REFERENCES Cucci, T.L., Shumway, S.E., Newell, R.C., Selvin, R., Guillard, R.R.L. and Yentsch, C.M. (1985) Flow cytometry: a new method for characterization of differential ingestion, digestion and ingestion by suspension feeders. Marine Ecology Progress Series 24, 201-204. Epifanio, C.E. (1979) Growth in bivalve molluscs: nutritional effects of two or more species of algae in diets fed to the American oyster Crassostrea virginica (Gmelin) and the hard clam Mercenaria mercenaria (L.). Aquaculture 18, 1-12. Guitlard, R.R.L. (1958) Some factors in the use of nannoplankton cultures as food for larval and juvenile bivalves. Proceedings of the National Shellfisheries Association 48, 134-142. Langdon, C.J. (1989) Preparation and evaluation of protein microcapsules for a marine suspension-feeder, the Pacific oyster Crassostrea gigas. Marine Biology 102, 217-224. Langdon, C.J. and Siegfried, C.A. (1985) Progress in the development of artificial diets for bivalve filter feeders. Aquaculture 39, 135-153. Lesser, M.P., Shumway, S.E., Cucci, T., Barter, J. and Edwards, J. (1991) Size-specific selection of phytoplankton by juvenile filter-feeding bivalves: comparison of the sea scallop Placopecten magellanicus (Gmelin, 1791) with Mya arenaria Linnaeus, 1758 and Mytilus
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edulis Linnaeus, 1758. In: An International Compendium of Scallop Biology and Culture, (ed S.E. Shumway) World Aquaculture Society: Baton Rouge pp. 341-346. Riisgard, H.U. and Randlov, A. (1981) Energy budgets, growth and filtration rates in Mytilus edulis at different algal concentrations. Marine Biology 61, 227-234. Shumway, S.E., Cucci, T.L., Newell, R.C. and Yentsch, C.M. (1985) Particle selection, ingestion and absorption in filter-feeding bivalves. Journal of Experimental Marine Biology and Ecology 91, 77-92.
