The assimilation efficiency of Asellus aquaticus L. (Crustacea, Isopoda)

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Abstract
Summary: (1) Comparative studies of the ash‐ratio and gravimetric methods of determining assimilation efficiency in Asellus aquaticus showed that the differential use of minerals by this species rendered the ash‐ratio method unreliable. Results obtained by the gravimetric method were therefore employed in the analysis of further experiments.(2) The assimilation efficiencies estimated for A. aquaticus ranged between 26 and 44%, and varied according to population density and reproductive condition.(3) Individual winter males had a significantly lower assimilation efficiency (26%) than grouped animals (35%), but the assimilation efficiency of summer males (33%) did not differ significantly from that of winter males at the same density. It is concluded that density affects assimilation efficiency in A. aquaticus.(4) The assimilation efficiency of summer males (33 %) is significantly different from that of summer females (non‐ovigerous, 41 %; ovigerous, 44%). A mean assimilation efficiency of 40% is proposed for the summer period whereas an overall annual mean of 30% is suggested.(5) Despite the various assimilation efficiencies reported within any one season consumption rate per unit weight is fairly constant (winter, 0·04–0·05 cal/24 h; summer, 0·36–0·38 cal/24 h) and it is suggested that the different assimilation rates are a mechanism whereby the additional energy and material requirements of females for breeding can be met without increasing food intake.