Evolution of fatty acid profiles of subtidal and rocky shore mussel seed (Mytilus galloprovincialis, Lmk.). Influence of environmental parameters

Abstract

In the present study fatty acid profiles of Mytilus galloprovincialis mussel seeds originating from two habitats with different environmental conditions (rocky shore and subtidal) were compared after transfer to the same environmental habitat (subtidal). The aim of the research was to investigate the influence of various environmental parameters on the relative percentage of fatty acids. The study was based in the Arosa Rı́a, Northwest Spain, between 27th November 1995 and 3rd July 1996. The location of the mussels suspended from the raft, the rope density (1.6 kg m⁻¹) and cultivation depth (1.5–5.0 m) were common parameters for both mussel groups. Our results show that during the first 36 days of the experimental period the mussel origin participated significantly in the model explaining the variance of various fatty acids of physiological importance in marine bivalves, namely the acids 18:0, 16:1n−7, 18:1n−9, 18:1n−7, 18:2n−6, 18:3n−3, 18:4n−3, 20:2NMID1, 20:5n−3 and 22:6n−3. In addition, other environmental parameters related to food availability, such as the ratio chl-a/POM and TPM, only participated in the explanation of two and three of these acids, respectively. In contrast, 50 days into the experiment the mussel origin did not participate in the model of variance of the fatty acids studied, and the ratio chl-a/POM participated significantly in the model explaining the variance of 11 of the total (16) selected fatty acids studied. Moreover, the coefficients were only positive in the fatty acids of known energetic importance in marine bivalves, namely the acids 14:0, 16:1n−7, 18:1n−7, 20:5n−3 and the ratio PUFAs n−3/n−6. The influence that mussel origin and various environmental parameters could exercise on the variability of diverse fatty acids of both mussel groups is discussed.

Introduction

Numerous studies have demonstrated that the lipid levels and composition of marine bivalves clearly reflect the biochemical and environmental conditions of seed development De Moreno et al., 1980, Napolitano et al., 1992, Fernández-Reiriz et al., 1998, Soudant et al., 1998. From this standpoint, the influence of contrasting or extreme environmental conditions on the biochemical composition of bivalves distributed in such environments would be an interesting study. In this sense, mussels distributed in the rocky shore zone have had to adapt to extreme environments as a consequence of the large tidal ranges. Among these environmental conditions, one of the most important is the reduction in the time available for food acquisition imposed by periods of air exposure (Bayne et al., 1988).

With regards to the influence that such periods of starvation usually exercise on lipids, some authors have noted a drop in the triacylglycerols of larval and juvenile stages of different marine bivalves Fraser, 1989, Caers et al., 2000. The consequence of starvation on different fatty acids in marine bivalves has been highlighted in the work published by Langdon and Waldock (1981), who studied the essential fatty acids PUFAs n−3 and n−6 of oyster juveniles Crassostrea gigas. The authors observed greater decreases in the fatty acid levels of the n−3 family belonging to the triacylglycerol fraction.

Bivalves distributed in coastal environments where detritus, bacteria and nanozooplankton contribute substantially to the composition of an abundant food load (Langdon and Newell, 1990) display a relatively high proportion of saturated fatty acids such as 14:0, 16:0 and 18:0 (Perry et al., 1979) compared to bivalves mostly nourished by phytoplankton which are dominated by polyunsaturated fatty acids Chu et al., 1990, Galap et al., 1999. Various authors have suggested that the seasonal variations observed in the levels of total lipids, neutral lipids and fatty acids of various species of marine invertebrates is intimately related to the nature of their diet. Accordingly, in spring when the available food source is dominated by material of phytoplanktonic origin, an increase in polyunsaturated fatty acids of 18, 20 and 22 carbons has been described De Moreno et al., 1976b, De Moreno et al., 1980, Langdon and Waldock, 1981, Fernández-Reiriz et al., 1996, Fernández-Reiriz et al., 1998, Soudant et al., 1999.

In the present study, the fatty acid profiles of Mytilus galloprovincialis mussel seeds were analysed over a seasonal cycle in order to ascertain how environmental conditions influence the changes observed in the fatty acid composition. Moreover, comparison of seed from two different habitats (rocky shore and subtidal) will provide information about adaptation delay.