Energy and ash contents of sandy beach macrofauna from the West Coast of South Africa

Abstract

The energy values and ash contents of 15 species of macrolauna found on three exposed sandy beaches on the west coast of South Africa were examined over an 18-month period. The values obtained ranged from a mean of 26,65 kJ.g⁻¹ ash-free dry weight (AFDW) in the haustoriid amphipod Urothoe grimaldii to 19,83 kJ.g⁻¹ for the bivalve Donax serra and the values obtained are applied to biomass data for the three beaches. A pattern of decreasing energy content with increasing mean body weight was observed. No patterns of seasonal or regional variation were detected in either energy values or ash contents. Ash contents depended on taxonomic group (owing to morphological effects) and the mean size of the species; in the case of the Crustacea increasing with size within a given taxonomic grouping. The mean energy value for the 15 species was 23,81 kJ.g⁻¹, higher than the 23,09 kJ.g⁻¹ conversion factor obtained by Brey, Rumohr & Ankar (1988) for aquatic macrobenthic invertebrates in general, and in accordance with the suggestion by Slobodkin & Richman (1961) that energy contents are affected by the extent of storage for adverse conditions.