Differential utilization of species of phytoplankton by

Acta Oecologica 24 (2003) S299–S305 www.elsevier.com/locate/actoec

Original article

Differential utilization of species of phytoplankton by the mussel Mytilus edulis G. Rouillon *, E. Navarro Departamento Biología Animal y Genética, Facultad de Ciencias. Universidad del País Vasco, Apdo. 644-48080, Bilbao, Spain

Abstract The phytoplankton community includes a variety of species differing in size and other structural characteristics that are likely subjected to a differential processing by filter-feeding bivalves. These processes would include: (a) The differential retention in the gills, (b) the preingestive selection at the level of gills and palps associated to the process of pseudofaeces formation, and (c) the selective digestion of phytoplankton cells ingested. The present experiments were conducted using a mixed diet of a diatom (Phaeodactylum tricornutum) and a naked flagellate (Tetraselmis suecica) at low particle concentration (below the pseudofaeces threshold) in order to check for the differential retention and/or digestive selection of any microalgae component by the mussel Mytilus edulis. To this effect, changes in the proportion of both components between the feeding suspensions, stomach contents and faeces over time were ascertained. Simultaneous clearance rate measurements allowed quantifying the ingestion rate of each microalgal component which, compared with the amounts retained inside the stomach, served to compute their respective rates of gut processing. Filtering activity reduced the abundance of P. tricornutum (reduced Phaeodactylum/Tetraselmis (P:T) ratio) in the surrounding water, but differences between the control and remaining water were found to be non-significant; consequently, we failed to demonstrate a differential retention by the gill of any component of the diet. P:T ratio was greatly reduced in the stomach contents and, to a lesser extent, in faecal particles, for comparison with particle suspension in controls and remaining water; these differences being significant. This result strongly suggests the preferential utilization of the diatom within the stomach of mussels. The rates of evacuation from the stomach were clearly different, showing a fast and slow component represented by P. tricornutum and T. suecica, respectively. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Filter-feeding bivalves; Mussels; Digestive selection; Phytoplankton

1. Introduction Mussels Mytilus edulis L. often occur in different habitats, from shallow coastal waters to estuaries (Seed and Suchanek, 1992), with different environmental characteristics, including large variations in food supply. Suspension feeding bivalves have phytoplankton as the main component of their diet (Bricelj and Shumway, 1991; Mac Donald and Ward, 1994; Shumway et al., 1987), and this component includes a variety of species differing in cell size, shape and other structural features. Consequently, one important source of environmental food variability appears to be associated to changes in both the concentration and species composition of the phytoplankton community that may result from different change scales, from seasonal successions to short term tidedriven resuspension events (Bayne, 1993). * Corresponding author. E-mail address: [email protected] (G. Rouillon) © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. DOI: 1 0 . 1 0 1 6 / S 1 1 4 6 - 6 0 9 X ( 0 3 ) 0 0 0 2 9 - 8

Previous studies have shown that bivalve molluscs deal with particle diversity in the seston in a variety of ways. These include (Shumway et al., 1985): (a) the differential clearance due to a selective retention at the level of gills, (b) the preingestive selection in gills and palps that occurs associated with the process of pseudofaeces formation, and (c) the selective digestion or differential utilization of ingested particles within the gut. While selection of living phytoplankton against inert particles have been broadly ascertained, particularly at the preingestive level (Bricelj and Malouf, 1984; Kiørboe and Møhlenberg, 1981; Newell and Jordan, 1983; Prins et al., 1991; Urrutia et al., 1996), there are not many reports concerning the differential utilization of species of phytoplankton. Most of these studies made use of flow cytometry to compare the ratios of different microalgae in the food suspension; faeces and pseudofaeces having revealed a complex situation where different groups of microalgae have been reported to be preferentially cleared from the suspension, rejected within the pseudofaeces or subjected

S300

G. Rouillon, E. Navarro / Acta Oecologica 24 (2003) S299–S305

to differential digestion and absorption depending on the bivalve species tested (Cucci et al., 1985; Shumway et al., 1985) or other variables such as the balance between species of phytoplankton in the diet (Bougrier et al., 1997). To deal with the complexity characteristic of natural phytoplankton communities, a gradual approach based on the studying of the feeding behaviour of bivalves with diets of a more simple composition might be convenient. Consequently, this work aimed to establish, in the mussel Mytilus edulis, the differential utilization of two phytoplankton species presenting different structural properties (a diatom and a naked flagellate). Experiments were performed at seston concentrations below the threshold for pseudofaeces production and preferential processing of any species by mussels was tested by checking the possible variations in the relative abundance of both phytoplankton components between suspended and retained material, stomach contents and faeces. With respect to digestive selection, conclusions based on observations of proportions of each phytoplankton species in the faeces might be misleading if the retention time of each phytoplanktonic component in the stomach is assumed to be different. To deal with this possible difficulty, direct observation of the stomach content composition, in addition to faecal composition, was undertaken. On the other hand, clearance rate measurements allowed quantifying ingestion rate of each microalgal component which, compared with the amounts retained inside the stomach, served to compute their respective rates of processing by this digestive organ. Besides, the clearance rate for monoalgal diets were recorded in order to test the hypothesis that adjustments in pumping rate might result from the selective digestion of food particles, in the same manner as these adjustments have been reported to be promoted by preingestive selection (Iglesias et al., 1998). 2. Materials and methods Mussels were collected in June 2000 in Plentzia Bay (43°25’LN; 02°75’LE) from Biscay’s Gulf. Fifty-four individuals measuring 50-60 mm in shell length were transported to the laboratory where they were cleaned from epizoa and maintained within a recirculating sea water system regulated at the temperature recorded during collection (14 ± 1 °C). After 24 h, the mussels were fed with three different diets:

two monoalgal diets based on the naked flagellate T. suecica (range of particle diameter: 5.4-9.2 µm) and the diatom P. tricornutum, (range of particle diameter: 3.1-5.9 µm, and one combined diet comprising the mixing up of the two algal species in the proportion 1:1 as expressed in terms of particulate volume. Both the species were chosen on account of their different structural properties see (Hasle and Syvertsen, 1996; Loret et al., 2000; Throndsen, 1993) and (Table 1) for further information. Six hours before the experiment began, diets were transferred to the feeding tanks from concentrated stocks of algal cultures, by means of peristaltic pumps, at appropriate rates to provide uniform particle concentrations of 1.5 mm3 l–1, as measured with a Coulter Multisizer fitted with a 100 µm aperture diameter tube. In the mixed diet, both algal species were clearly distinguishable as two separate populations of particles in the size-frequency distribution provided by the above electronic particle counter. 2.1. Clearance rate (CR: l h– 1) CR was measured in a closed system regulated at a maintenance temperature (14 ± 1 °C). Nine mussels per diet were individually disposed in glass flasks, with 0.8 l filtered sea water, provided with gentle aeration. At the start of the experiments, each flask received the appropriate volume of microalgal stock to provide an initial concentration of 1.5 mm3 l–1, and particle concentration measured in terms of particle number were recorded at t = 0 (C0) and t = 15 min. (C15) on the Coulter Multisizer. Following this, the initial concentration was restored by adding new stock and the same procedure was repeated three times. Following Coughlan (Coughlan, 1969), CR was estimated in each occasion using the formula: ln 共 C0 C15 兲 − ln 共 C′0 C′15 兲 CR = × 0.8 (1) 0.25 C' representing the particle concentration recorded in a control (flask without animal) set to correct for particle sedimentation. Individual CR was computed as the mean of the three determinations replicated over time. The shell lengths of mussels used in CR determinations were measured to the nearest mm and individual rates were standardized to an equivalent mean size of 55 mm shell length, using a value of b = 1.85 for Mytilus (Pérez and González, 1984). The re-

Table 1 Characteristics of the phytoplankton species incorporated to diets fed to mussels M. edulis Microalgaea Taxonomic group Common name Cell shape Cell size (microns) Equivalent spherical diameter (microns)b Marker pigmentc Monoalgal diet name a

Tetraselmis suecica Prasinophyceae–Chlorophyta Naked flagellate Ovate or quadrangular 5 × 10 7.5 Chlorophyll b T

See, Throndsen (1993), Hasle and Syvertsen (1996) for review. Equivalent spherical diameter (ESD) as defined by electronic Coulter Multisizer. c Marker pigments are reported by Loret et al. (2000). b

Phaeodactylum tricornutum Bacillariophyceae–Chromophyta Diatom Ovate, fusiform or triadiate 3 × 8 (ovate), 25–35 (fusiform) 4.5 Fucoxanthin P

G. Rouillon, E. Navarro / Acta Oecologica 24 (2003) S299–S305

S301

Table 2 (a) Summary of ANOVA for testing the effects of diets on the clearance rate of M. edulis. (b) Tukey’s multiple comparison test (a) ANOVA Parameter Clearance rate (l h–1)

(b) Tukey’s test T P T+P

Source Between groups Within groups Total T – 0.4632 2.1283*

df 2 24 26 P 0.4632 – 16 651

Sum of squares 22.55 57 968 80 519 T+P 2.1283* 16 651 –

F 4 668

P 0.019*

* Significant differences.

ported CR values for every diet were computed as the mean of nine size-standardized individual determinations. 2.2. Differential retention After CR measurements were completed, water samples of tanks provided with the mixed diet were analysed for particle size distribution in the range of 2-10 µm, using 64 channels of a Coulter Multisizer fitted with a 100 µm aperture tube. Packed volumes of particle populations assigned to P. tricornutum and T. suecica were separately computed and proportions of each of these components in the whole diet were then compared between the controls and the water remaining in the experimental tanks after CR measurements, in order to check for differential retention in the gill of any microalgal species. 2.3. Stomach contents, faeces and digestive selection Following CR measurements, the nine mussels provided with the mixed diet were transferred to flasks with filtered sea water, and three mussels sampled at every 0, 1 and 3 h, for the analysis of stomach contents. Faeces produced by these mussels at the end of 1 and 3 h were also collected for analysis. Mussel valves were separated by cutting the adductor muscle in order to allow stomach content collection by means of a hypodermic syringe. After recording the total volume collected, diluted aliquots of these samples were analysed for particle size distribution under the above conditions. Faecal pellets were disaggregated by forceful ejection from a syringe through a filtering device fitted with a 30 µm net, and diluted aliquots of the resulting particle suspensions were similarly analysed for particle size distribution. Differential digestion was estimated by recording any change in the proportions of microalgal components of the diet between controls, stomach contents and faeces.

The evacuation of stomach contents was computed as the difference between the amount of particles (mm3) ingested and the amount remaining inside the stomach at different times. Also, with respect to this measurement the two microalgal components were separately computed. 2.5. Statistical analysis Before performing statistical analysis, data were tested for normality and variance homogeneity. When required (i.e., ratios of both microalgal components), data were normalized using square root arcsine transformation. Statistical significant differences were then tested following analysis of variance (ANOVA) and multiple comparison test (Tukey) (Sokal and Rohlf, 1979), using the statistical software SPSS.

3. Results Size-standardized clearance rates (CR) of 55 mm shell length mussels from the Plentzia estuary ranged between 1.8 and 2.9 l h–1 (Fig. 1). The results of one-way ANOVA indicated significant differences between diets ( P = 0.019) and Tukey’s test showed this significance applied to differences between maximum rates with the mixed diet and minimum rates with T. suecica diet (Table 2). Fig. 2 displays particle size distribution in the various compartments sampled (water of controls, remaining water,

2.4. Ingestion rates and evacuation of stomach contents Total ingestion and ingestion of every microalgal component of the mixed diet were individually computed (in terms of particulate volume: mm3) in two ways: (1) As a balance between the amount of stock added along CR measurements and the amount remaining in the flasks at the end of such measurements. (2) As the product of CR and the particulate volume concentration (mm3 l–1) in the control.

Fig. 1. Clearance rate of standard sized (55 mm shell length) M. edulis measured with two monoalgal diets ( T. suecica and P. tricornutum ) and a mixed diet (T + P). The values are means ± 95% CI. ( n = 9).

S302

G. Rouillon, E. Navarro / Acta Oecologica 24 (2003) S299–S305

Fig. 3. Percentages of microalgal components in control (n = 3), remaining water (n = 9), stomach contents (n = 9) and faeces (n = 6). The values are means ± 95% CI. Percentage algae in relation to packed volume in both control and remaining water, stomach content and faeces of M. edulis.

Fig. 2. Particle size distribution in samples of control, remaining water and different comportments of M. edulis. Algue components of diet are indicated by black colums (P. tricornutum) and white columns (T. suecica).

stomach contents and faeces), showing the two component populations present in mixed diets: P (P. tricornutum) and T (T. suecica). According to the aims of the experiment, these two components presented similar proportions in samples

from controls. However, proportions clearly departed from the 1:1 ratio in the rest of compartments (Fig. 3). The comparison of P. tricornutumand T. suecica ratios between compartments by means of ANOVA (Table 3a) showed significant differences ( P = 0.002). Compared with the controls, P:T ratio in the remaining water was reduced, but these differences were found to be non-significant according to Tukey’s test (Table 3b). P:T ratio was greatly reduced in the stomach contents and, to a lesser extent, in faecal particles, for comparison with controls and remaining water, these differences being significant (Table 3b). Ratio differences between stomach contents and faeces were also found to be significant (Table 3b). However, such differences appear to be time-dependent as the ratios for these compartments tend to approach each other from 1 to 3 h (Fig. 4) i.e., as a function of the degree of stomach evacuation. Results of a two-way ANOVA (Table 4) confirmed that this interactive effect was significant. Two independent measurements of the particulate volume ingested during CR recording, based, respectively, on a balance of particulate volumes (Method I) and on CR determinations (Method II) (reported in Section 2) were computed. Values obtained through Method I proved to be less variable (C.V. = 3.74% vs. 16.88% with Method II), as corresponding to a measurement performed over a longer period of time (i.e., more stable); consequently, the ingestion values obtained with Method I were applied for further computation of stomach evacuation. Amounts of each algal species evacuated from the stomach are shown in Fig. 5, in different moments of the evacuation process. Under steady-state conditions (i.e., continuously fed animals), the rates of evacuation are clearly different for the two species (Fig. 6), with a fast component represented by P. tricornutum and a slow one represented by T. suecica. With suppression of feeding, complete evacuation of the slow component would account for the time-dependent change of P:T ratio in both the evacuated materials (statistically significant P = 0.002; Table 5) and the faeces over the time.

G. Rouillon, E. Navarro / Acta Oecologica 24 (2003) S299–S305

S303

Table 3 (a) Summary of ANOVA for testing the differences in the ratio of particulate volumes of both microalgae, between compartments. (b) Tukey’s multiple comparison test (a) ANOVA Parameter Ratio

(b) Tukey’s test Control Remaining water Stomach content Faeces

Source Between groups Within groups Total Control – 0.0114 0.1279* 0.1393*

df 3 8 11 Remaining water 0.0114 – 0.2351* 0.096

Sum of squares 0.19 0.041 0.231 Stomach content 0.1279* 0.2351* – 0.1070*

F 12 224

P 0.002*

Faeces 0.1393* 0.096 0.1070* –

* Significant differences. Table 4 Summary of two-way ANOVA for testing the effects of compartment and time of stomach evacuation on the ratio of particulate volumes of both microalgae Parameter Ratio

Factors Time Compartment Interaction

df 1 1 4

Sum of squares 0.036 0.028 0.039

F 1 964 3 104 4.21

P 0.191 0.109 0.03*

* Significant differences Table 5 (a) Summary of ANOVA for testing the effects of time of evacuation on the ratio of both microalgaein the particulate material evacuated from the stomach (b) Tukey’s multiple comparison test (a) ANOVA Parameter Evacuation rate (mm3 h–1)

(b) Tukey’s test 0 (1.3 h) 1 (2.3 h) 3 (4.3 h)

Source Between groups

Df 2

Sum of squares 0.256

Within groups Total 0 (1.3 h) – 0.229* 0.412*

6 8 1 (2.3 h) 0.229* – 0.18

0.0353 0.292 3 (4.3 h) 0.412* 0.18 –

F 21 813

P 0.002

* Significant differences.

Fig. 4. Percentages of microalgal components in both stomach contents and faeces of M. edulis at different times. The values are means ± 95% CI.

4. Discussion Differential processing of both microalgae species by the feeding system of M. edulisis summarized in Fig. 7 96.4% of T. suecica and 95.7% of P. tricornutum processed by the gill, respectively, are retained and ingested. Differences, however, were found to be non-significant, suggesting the absence of a

Fig. 5. Accumulated evacuation volumes of each microalgal component from the stomach of M. edulis at different times. The values are means ± 95% CI.

differential retention by the gill of any microalgal component of the diet. This result would be consistent with a previous report on M. edulis (Cucci et al., 1985), that also failed to demonstrate any differential retention of three species of phytoplankton (a diatom, a dinoflagellate and a cryptomonad), but contrasts with the preferential retention of

S304

G. Rouillon, E. Navarro / Acta Oecologica 24 (2003) S299–S305

report. If residence time of food particles in the stomach, digestion and absorption is a collective limiting process to the entry of food into the digestive canal, then particles readily processed would allow higher clearance rates, as is the case for the diets including P. tricornutumin their composition.

Fig. 6. Rates of evacuation of each microalgal component from the stomach of M. edulis at different times. The values are means ± 95% CI.

T. suecica over the similarly sized diatom Skeletonema costatum (Bougrier et al., 1997). The retention efficiency of bivalves has long been reported to increase with increasing size of particles, due to the importance of the distance between dorsal cirri in determining selectivity properties of the gill (Jørgensen, 1990). M. edulis has been shown to retain particles greater than 3 µm with 90% efficiency (Møhlenberg and Riisgård, 1978; Vahl, 1972), and thus no differences in retention are likely to occur between most phytoplankton cells. In spite of having ingested a similar proportion of both microalgae, the P:T ratio dramatically changed in the stomach contents. A greater abundance of T. suecica indicated a faster disposal of the diatom P. tricornutum by this organ involved in extracellular digestion. For instance, over a 78 min. period computation, 95.3% of ingested P. tricornutum had been evacuated from the stomach, but only 83.7% of ingested T. suecica had been evacuated (Fig. 7); these differences being statistically significant (Table 5). Results attained in comparing the clearance rates of monoalgal and mixed diets are consistent with the above

Particles processed by the stomach may follow two alternative ways: either they may be taken up by the ducts of the digestive gland to undergo intracellular digestion or they may by-pass these ducts to the intestine to be voided directly with the so-called intestinal faeces (Decho and Luoma,1991; Widdows et al., 1979). Now, a higher evacuation rate of P. tricornutum from the stomach (see Fig. 6) combined with a greater abundance of T. suecica in the faeces is unequivocally pointing out to the preferential incorporation of P. tricornutum into the more efficient process of intracellular digestion. Regarding the mechanisms underlying this selective diges tion, it can be hypothesized that a prerequisite for the incorporation of these food particles to the digestive gland relies on some fragmentation by means of extracellular digestion since intact phytoplankton cells are hardly observed inside the diverticula. That provided, diatom cells protected inside a rigid siliceous frustule would be, in spite of its refractory nature, more accessible to mechanical treatment by the cristaline style than flagellates enclosed by a more flexible cell wall. This could be consistent with previous reports pointing to a beneficial grinding effect of mineral silt particles in the gut of Mytilus as regards the digestibility of phytoplankton, and that appeared particularly effective when gastric function was partly impaired by parasitization (Navarro et al., 1996). On the other hand, less digestible flagellates would be kept recirculating in suspension thus enlarging its residence time inside the stomach, a behaviour described for large light beads inside the stomach of Placopecten magellanicus (Brillant and MacDonald, 2000).

Fig. 7. Fractions of the two phytoplankton species in the composition of the mixed diet (T + P) and fractions of each microalgal component that are ingested and evacuated from the stomach of M. edulis.

G. Rouillon, E. Navarro / Acta Oecologica 24 (2003) S299–S305

Differences in assimilation efficiency of trace elements incorporated to M. edulis by different species of phytoplankton (Wang and Fisher, 1996) indirectly confirms the present results as to the preferential digestion of Phaeodatcylum vs. Tetraselmis. However, these findings do not allow generalization concerning the behaviour of other diatoms and flagellates upon digestion, since direct evidence of digestive selection based on flow cytometry showed the digestion of a cryptomonad flagellate in preference to a diatom and a dinoflagellate in M. edulis (Cucci et al., 1985) and other species of filter-feeding bivalves (Shumway et al.,1985). The ability exhibited by benthic filter-feeders for the differential utilization of seston components is of great importance in both exploiting food environments characterized by complex assemblages of particles of variable food value and regulating intraspecific competence. Particularly, digestive selection of food particles could be considered as the main process by which differential utilization of species of phytoplankton takes place. Contradictory results discussed here concerning the preferential utilization by the gut of structurally different groups of phytoplankton (diatoms and naked flagellates) would thus require further research. Acknowledgements This work was supported through the Research Group Project UPV 154.320-G07/99. G.R. was funded by an AECI grant. The authors would like to thank the research group of the Animal Physiology Lab for help given in the course of the present experiments. References Bayne, B., 1993. Feeding physiology of bivalves: time-dependence and compensation for changes in food availability. In: Dame, R.F. (Ed.), Bivalve Filter Feeders in Estuarine and Coastal Ecosystem Processes, NATO ASI Series G: Ecological Sciences, vol. 33. Springer-Verlag, Heilderberg, pp. 1–24. Bougrier, S., Hawkins, A.J., Héral, M., 1997. Preingestive selection of different microalgal mixtures in Crassostrea gigas y Mytilus edulis analysed by flow cytometric. Aquaculture 150, 123–134. Bricelj, V., Malouf, R., 1984. Influence of algal and suspended sediment concentration of the feeding physiology of the hard clam Mercenaria mercenaria. Mar. Biol. 84, 155–165. Bricelj, V., Shumway, S., 1991. Physiology. Energy acquisition and utilization. In: Shumway, S. (Ed.), Scallops: Biology and Aquaculture. Elsevier Science Publishers B.V, Amsterdam, pp. 305–345. Brillant, M.G.S., MacDonald, B.A., 2000. Postingestive selection in the sea scallop, Placopecten magellanicus (Gmelin): the role of particle size and density. J. Exp. Mar. Biol. Ecol. 253, 211–227. Coughlan, J., 1969. The estimation of filtering rate from the clearance of suspension. Mar. Biol. 2, 356–358. Cucci, T., Shumway, S., Newell, R.C., Selvin, R., Guillard, R., Yentsch, C., 1985. Flow cytometry: a new method for characterization of differential ingestion, digestion and egestion by suspension feeders. Mar. Ecol. Prog. Ser. 24, 201–204.

S305

Decho, A.W., Luoma, S.N., 1991. Time-course in the retention of food material in the bivalves Potamocorbula amurensis and Macoma balthica: significance to the absorption of carbon and chromium. Mar. Ecol. Prog. Ser. 78, 303–314. Hasle, G., Syvertsen, E., 1996. Marine Diatoms. In: Tomas, C.R. (Ed.), Identifying Marine Diatoms and Dinoflagellates. Academic Press Inc., USA, 395 p. Iglesias, J., Urrutia, M., Navarro, E., Ibarrola, I., 1998. Measuring feeding and absorption in suspension-feeding bivalves: an appraisal of the biodeposition method. J. Exp. Mar. Biol. Ecol. 219, 71–86. Jørgensen, C.B., 1990. Bivalve Filter Feeding: Hydrodynamics, Bioenergetics, Physiologyand Ecology. Olsen & Olsen, Denmark, 140 p. Kiørboe, T., Møhlenberg, F., 1981. Particle selection in suspension-feeding bivalves. Mar. Ecol. Prog. Ser 5, 291–296. Loret, P., Pastoureaud, A., Bacher, C., Delesalle, B., 2000. Phytoplankton composition and selective feeding of the pearl oyster Pinctada margaritifera in the Takapoto Lagoon (Tuamoto Archipielago, French Polynesia): in situ study using optical microscopy and HPLC pigment analysis. Mar. Ecol. Prog. Ser. 199, 55–67. Mac Donald, B., Ward, J., 1994. Variation in food quality and particle selectivity in the sea scallop Placopecten magellanicus (Mollusca: Bivalvia. Mar. Ecol. Prog. Ser. 108, 251–264. Møhlenberg, F., Riisgård, M.V., 1978. Efficiency of particle retention in 13 species of suspension feeding bivalves. Ophelia 17, 239–246. Navarro, E., Iglesias, J.I.P., Pérez Camacho, A., Labarta, U., 1996. The effect of diets of phytoplankton and suspended bottom material on feeding and absorption of raft mussels ( Mytilus galloprovincialis Lmk). J. Exp. Mar. Biol. Ecol. 198, 175–189. Newell, R.I.E., Jordan, S.J., 1983. Preferential ingestion of organic material by the American oyster Crassostrea virginica. Mar. Ecol. Prog. Ser. 13, 47–53. Pérez, A., González, R., 1984. La filtración del mejillón (Mytilus edulis L.) en laboratorio. Actas do Primeiro Seminario de Cencias do Mar, As Rías Galegas. Cuaderno da Area de Ciencias Marinas, Seminario de Estudos Galegos, 427–437. Prins, T.C., Smaal, A.C., Pouwer, A.J., 1991. Selective ingestion of phytoplankton by the bivalves Mytilus edulis L. and Cerastoderma edule (L.). Hydrobiol. Bull. 25 (1), 93–100. Seed, R., Suchanek, T., 1992. Population and community ecology of Mytilus. In: Gosling, E. (Ed.), The Mussel Mytilus : Ecology, Physiology, Genetics and Culture. Elsevier Science Publishers B.V, Amsterdam, pp. 87–169. Shumway, S., Cucci, T., Newell, R., Yentsch, C., 1985. Particle selection, ingestion, and absorption in filter-feeding bivalves. J. Exp. Mar. Biol. Ecol. 91, 77–92. Shumway, S., Selvin, R., Shick, D., 1987. Food resources related to habitat in the scallop Placopecten magellanicus (Gmelin, 1791): a qualitative study. J. Shellfish Res. 6, 89–95. Sokal, R., Rohlf, F., 1979. Biometría. Principios y métodos estadísticos en la investigación biológica. In: Blume, H. (Ed.). Spain, 832 p. Throndsen, J., 1993. The planktonic marine flagellates. In: Tomas, C.R. (Ed.), Marine Phytoplankton. Academic Press Inc., USA, 263 p. Urrutia, M.B., Iglesias, J.I.P., Navarro, E., Prou, J., 1996. Feeding and absorption in Cerastoderma edule under environmental conditions in the bayof Marennes-Oleron (Western France). J. Mar. Boil. Assoc. UK 76, 431–450. Vahl, O., 1972. Efficiency of particle retention in Mytilus edulis L. Ophelia 10, 17–25. Wang, W.X., Fisher, N.S., 1996. Assimilation of trace elements and carbon by the mussel Mytilus edulis: effects of food composition. Limnol. Oceanogr. 42 (2), 197–207. Widdows, J., Fieth, P., Worral, C.M., 1979. Relationships between seston available food and feeding activity in the common mussel Mytilus edulis. Mar. Biol. 50, 195–207.