Life history biology of deep-sea gammaridean amphipods from the rockall trough ne atlantic

Abstract

Gammaridean amphipods were examined from collections made during the Scottish Marine Biological Associations time-series sampling programme in the Rockall Trough, NE Atlantic. Twenty three samples were sorted, producing five thousand and thirty four specimens, representing eighty one species. Amphipods comprised 3.56% of the total fauna of the sample site. Multivariate analysis reveal all the samples come from one statistical population, and there was no significant change in composition over the length of sampling programme. There is a seasonal flux of phytodetrital material to the study region, representing a significant input of organic matter to the deep benthic community. Two of the more common species were examined to evaluate any effect of this deposition of phytodetritus on their life history biology. Reproductive studies revealed the ampeliscid Ampelisca sp nov to be a quasi-continuous breeder, with a seasonal increase in reproductive intensity. In contrast the lysianassid Tryphosella biloba is a seasonal breeder. Both species show a seasonal increase in juvenile recruitment during the summer, coincident with the influx of detrital material. Brood size and embryo diameter were positively correlated with female length for all species examined in this study. Brood sizes were smaller, and embryo diameters larger in deep-sea amphipods than those predicted for a shallow water amphipod of similar size. Studies of the amount and composition of gut contents ^or Ampelisca sp nov reveal it feeds on the phytodetrital flux. Morphological studies on the mandible of this species show it is adapted for a microphagous detritivore diet. The mandible of T. biloba reveals morphological adaptions for a scavenging and/or omnivorous diet. Gut content analysis for this species indicates it may supplement its diet with phytodetrital material. Phytodetritus seems to provide amphipods in this study with an energy source for vitellogenesis and/or a food resource for newly released juveniles. Experimental trap studies off the Bahamas attracted scavenging lysianassid amphipods to another important organic input to the deep-sea, macrophyte debris, this attraction was shown to be a feeding, rather than a shelter response. This study is the first to report seasonality in the reproductive and feeding biology of deep-sea gammaridean Amphipoda. Amphipods form a significant portion of the deep-sea macro-invertebrate fauna, as such their feeding activities may have important structuring effects on the rest of the community.

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