Phytoplankton Composition of the Stomach Contents of the Mussel Mytilus
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PHYTOPLANKTON COMPOSITION OF THE STOMACH CONTENTS OF THE MUSSEL MYTILUS EDULIS L. FROM TWO POPULATIONS: COMPARISON WITH ITS FOOD SUPPLYAuthor(s) :G. ROUILLON, J. GUERRA RIVAS, N. OCHOA, and E. NAVARRO Source: Journal of Shellfish Research, 24(1):5-14. 2005. Published By: National Shellfisheries Association DOI: 10.2983/0730-8000(2005)24[5:PCOTSC]2.0.CO;2 URL: http://www.bioone.org/doi/full/10.2983/0730-8000%282005%2924%5B5%3APCOTSC%5D2.0.CO %3B2
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Journal of Shellfish Research, Vol. 24, No. 1, 514, 2005.
PHYTOPLANKTON COMPOSITION OF THE STOMACH CONTENTS OF THE MUSSEL MYTILUS EDULIS L. FROM TWO POPULATIONS: COMPARISON WITH ITS FOOD SUPPLYG. ROUILLON,1* J. GUERRA RIVAS,1 N. OCHOA2 AND E. NAVARRO1 Departamento de Gentica, Antropologa Fsica y Fisiologa Animal, Facultad de Ciencia y Tecnologa. Universidad del Pas Vasco/Euskal Herriko Unibertsitatea. Apdo. 64448080, Bilbao, Espaa; 2Laboratorio de Ecologa Acutica, Facultad de Ciencias Biolgicas. Universidad Nacional Mayor de San Marcos, Ciudad Universitaria, Avda. Venezuela, cuadra 32 s/n Lima, Per
ABSTRACT Seasonal data on phytoplankton composition of seston and stomach contents of the mussel (Mytilus edulis Linnaeus, 1758) from two contrasting sites, an estuarine mud flat and a rocky open shore, were compared to ascertain: (a) the extent to which differential characteristics of both sites affect this composition and (b) the degree of similarity between stomach contents and microalgal composition of seston of these sites as an index reflecting the complex processes of selection taking place within the feeding-digestive system of mussels. Individuals and water samples were collected monthly from November 2001 to December 2002, when salinity, temperature, and total and organic particulate matter concentration were also recorded in the water column. Preserved samples of seston and stomach contents were analyzed by inverted microscopy according to the Utermhl method. Phytoplankton cells were counted and the different species grouped, taxonomically and, according to the habitat, into pelagic and tychopelagic. These data served to compute abundance (total cell count) and frequency index. Relative abundances of each group were compared for similarity between sampling sites and stomach and water samples in each site. Similarity analyses were performed using the index of Bray-Curtis, significant differences between samples being determined by the non parametric test of ANOSIM. Results of this test for the comparison between water and stomach contents resulted in significant differences: R 0.68 in the estuary and R 0.75 in the open shore area. Stomach contents presented a reduced average number of species (n 6 in mussels from both sites) and a greater proportion of tychopelagic forms for comparison with the water samples (n 20 and 24 in the estuary and open shore, respectively). Maximum phytoplankton density in water samples occurred in the May to October period, the group responsible for this increment being the diatoms. The stomach contents of marine mussels displayed two peaks of phytoplankton concentration in May (caused by the dinoflagellate Ensiculifera sp.) and in July (caused by the diatoms Pseudo-nitzschia pungens and Licmophora sp.). In the case of stomach contents of estuarine mussels, a single peak of abundance was recorded in the month of May and was mainly produced by Ensiculifera sp. To conclude, the main result coming from these comparisons is the increased abundance of dinoflagellates in the stomach contents relative to the corresponding seawater samples in the estuarine and open shore media. This result is discussed in the light of previous data concerning the differential utilization of species of phytoplankton by bivalve molluscs. KEY WORDS: phytoplankton, food supply, stomach content, mussels, Mytilus edulis INTRODUCTION
Marine coastal areas are characterized by great space-temporal fluctuations in both phytoplankton abundance and specific composition. In the temperate area, dominant species of phytoplankton exhibit seasonal variations and a natural succession between phytoplankton groups that accompany the occurrence of blooms (Berg & Newell 1986, Varela 1996). Superimposed on these long-term cycles, short-term pulses of microphytobenthos resuspension caused by wind- and tide-driven currents are also relevant regarding the chlorophyll concentration and microalgal composition of the seston (Roman & Tenore 1978, Baillie & Welsh 1980, Muschenheim 1987, Grant et al. 1990, de Jonge & van Beusekom 1992, Newell & Shumway 1993). This stands particularly true for soft-bottomed shallow tidal flats common to estuaries and sheltered areas of the coast, where tidal variability of chlorophyll concentration can exceed the seasonal range (Grant et al. 1993, Smaal & Haas 1997). Hummel (1985) studying the diatom composition of the water column above a tidal mud-flat in the Wadden Sea reported a greater presence of benthic species at the most landward sampling points for comparison with deeper points. Consequently, microalgal composition at these sites would appear, to a great extent, affected by short-term tidal changes in contrast with the open shores where seasonal variability would be dominant.
*Corresponding author. E-mail: [email protected]
This fact constitutes one of the sources of spatial variation along the coast. Mussels Mytilus edulis Linnaeus, 1758 are conspicuous inhabitants of both open-shore and estuarine habitats where they form important populations. The interaction of mussel populations with phytoplankton as food source has long been documented. For example, Blanton et al. (1987), van der Veer (1989) and Hickman et al. (1991) reported that phytoplankton blooms were accompanied by increased growth and production and improved condition index of various mussel species in culture plots. All these observations support the idea that mussels generally rely on phytoplankton as their main food supply. However, different species of phytoplankton seems to behave in a different way as regards its food value for bivalves under culture conditions (Epifanio 1979, Enright et al. 1986, Laing & Millican 1986). When available, field observations appear to confirm the above extremes: Beukema and Cadee (1991) compared the patterns of growth of Macoma balthica Linnaeus, 1758 with the patterns of abundance of different phytoplankton components over 15 y, to conclude faster growth and better condition associated with higher abundance of diatoms for comparison with years when flagellates were dominant. Given that, the ability exhibited by several bivalves (mussels included) for the preferential utilization of species of phytoplankton (Cucci et al. 1985, Shumway et al. 1985, Bougrier et al. 1997, Rouillon & Navarro 2003) should be considered a useful mechanism ensuring a complete exploitation of this resource under the conditions of variability prevailing in the marine coastal habitat. 5
which spatial differences in phytoplankton composition reflect the particular conditions of both sites, specially in which concerns to the balance between pelagic and tychopelagic microalgae and (b) the degree of similarity between stomach contents of mussels from the two sites and the microalgal composition of the water column at these sites as an index reflecting the possible occurrence of differential retention, ingestion, and selective digestion of species of phytoplankton within the feeding apparatus of mussels.MATERIALS AND METHODS
Figure 1. Map of the study area. Sampling sites are indicated
In dealing with this phenomenon of differential utilization, a useful approach under field conditions would be the comparison of gut contents, and the water column or superficial sediments, in terms of phytoplankton species composition. Using these procedures, Kamermans (1994) reported that species composition in gut contents of various epifaunal and infaunal bivalves living in mud flats resembled more the composition of the water column than that in superficial sediments, but no evidence of selective feeding can be concluded from her data. Shumway et al. (1987) and Muetn-Gmez et al. (2001) also reported similar patterns of seasonal variation of species composition in seawater and stomachs of Placopecten magellanicus Gmelin, 1791 and Spondylus leucacanthus Broderip 1833, respectively. However, Ciocco and Gayoso (2002) observed that stomach content composition of ribbed mussels (Aulacomya atra Molina, 1782) did not reflect the occurrence of diatom blooms recorded in the area of study. The present study compares seasonal data on phytoplankton composition of seston and stomach contents of mussels (M. edulis) sampled along a year. The study was conducted at two sites presenting different characteristics, a muddy flat in an estuary and a rocky shore from an exposed beach, to ascertain (a) the extent to
Two sampling sites were chosen to represent open shore and estuarine habitats: a rocky area at Atxabiribil beach (4323 29 LN; 0259 28 LW) and a muddy-flat in the Ra de Plencia (4324 39 LN; 0256 52 LW), respectively (Fig. 1). Water temperature and salinity were recorded each time with a LF330/SET Standard Conductivity Cell (TetraCon 325, Germany). Sampling was performed during the ebb tide from a point situated approximately 3 m. above the chart datum. Mussels recently emerged (