Ecology and bioenergetics of two species of Asellus in Rutland Water
Populations of A. aquaticus and A. meridianus in Rutland Water, England, were sampled sequentially at monthly intervals from November 1979 to November 1981. The population densities of A. aquaticus ranged from 3758.7 m-2 to 238.8 m-2. A. meridianus was only present in the south arm where densities ranged from 999.9 m-2 to 42.9 m-2. The densities in the south arm followed divergent patterns during the study; A. aquaticus increased while A. meridianus decreased. Ovigerous females were present from early spring to late summer, giving rise to spring and autumn cohorts. A. aquaticus was most abundant on an algal substrate (Cladophora glomerata), and in the 'littoral' (< 2m depth), while A. meridianus occurred most frequently in the mud-detritus substrate. Respiration rates were determined at 4°, 10° and 16°C. The two Asellus species had similar rates of respiration, except at 16°C, where A. aquaticus showed a slightly higher rate of respiration than A. meridianus. Consumption, assimilation and faecal production rates on both decayed sycamore leaves and Cladophora were determined gravimetrically. For both species, the consumption and assimilation of decayed sycamore leaves were higher than those of Cladophora and increased with temperature for both species. The consumption and assimilation rates of Cladophora were rather higher in A. aquaticus than in A. meridianus. The annual population energy budget (KJ m-2) for A. aquaticus (mean biomass 15.086) was: consumption 705.8, production 48.0, respiration 132.3, and faeces 525.5. The significance of these energy values, and the ecological efficiencies calculated from them, are discussed in relation to previous reports. Both laboratory and field investigations of predation by invertebrates (using serological techniques) suggest that the intensities of predation on both Asellus species were severe. Intensities vary according to the predatory species, their abundance, and the time of the year. Further, these studies indicate that predation on A. meridianus is proportionally much greater than that on A. aquaticus, by a factor of 2.75.