Foraging theory and partial consumption in a tardigrade-nematode system

We investigated foraging behavior of a carnivorous eutardigrade, Macrobiotus richtersi, living in soil or fresh water sediments, where the distribution of nematode prey is heterogeneous and where, consequently, the value of information about prey availability should be high. We directly observed diet choice in various mixtures of 2 size classes of nematodes Acrobeloides nanus, A1 (small prey) and A2 (large prey), differing in profitability (biomass divided by handling time) but not in digestive quality. At various prey densities of A2, we measured how much of each prey item was consumed as a function of search time. Additionally, we derived cumulative biomass uptake rates per single A2 prey from 154 complete feeding acts, and we used the marginal value theorem (MVT) to predict optimal residence time and prey exploitation as a function of environmental quality and search time. Macrobiotus richtersi did not preferentially select the more profitable size class A2 but increased its biomass uptake rate by modifying the amount consumed per prey item (partial consumption): when encounter rates were high and there were more high-quality prey, the tardigrade abandoned food more quickly and consumed less biomass per captured prey. We conclude that 1) hungry tardigrades under food-rich or food-limited conditions maximize biomass gain according to MVT and 2) tardigrades differentiate at least 2 prey situations, high supply (at high encounter rates) and low supply (at moderate and low prey densities). 3) Partial consumption, performed under food-rich conditions or by a satiated predator, leads to a reduction of prey numbers up to 3 times as much as under food-poor conditions.