Costs and gains of porcelain crab suspension feeding in different flow conditions

Porcellana longicornis is a filter feeder that changes its feeding mechanism in relation to water flow conditions. In still water and slow flow, it actively sweeps the water with its maxillipeds. As flow speed increases, it switches from active suspension feeding to passive feeding. In low frequency oscillating flow, the crabs rhythmically reorient their suspension feeding fans so that both cup-shaped maxillipeds are upstream. In high frequency oscillating flow, the crabs perform alternate movements of left and right feeding fans, so that 1 fan is extended and the concave face is directed upstream, while the other is withdrawn. In the present study, we estimated the clearance rate and energy absorption of P. longicornis in the different flow regimes. The average clearance rate for active feeding, in slow unidirectional flow, was 0.267 1 crab^-1 h^-1, while in passive feeding, in fast unidirectional flow, the clearance rate was 0.142 1 crab^-1 h^-1. In oscillating flow at 0.5 Hz, the clearance rate was 0.158 1 crab^-1 h^-1, while at 0.1 Hz it was 0.201 1 crab^-1 h^-1. Using the algal concentrations and the energy conversion factor we calculated the amount of energy gained by the crabs during the different feeding behaviors. The energy absorbed was computed and presented as J crab^-1 h^-1. Using the dry weight of the crabs, this was converted to energy absorbed by a 'standard' crab of 25 mg. The oxygen consumption of crabs was estimated as 1.122 μmol O₂ (25 mg dry wt)^-1 h^-1 for active feeding; 1.640 μmol O₂ (25 mg dry wt)^-1 h^-1 for passive feeding; 0.921 μmol O₂ (25 mg dry wt)^-1 h^-1 for low frequency oscillating current; and 0.829 pmol O₂ (25 mg dry wt)^-1 h^-1 for high frequency oscillating flow. However there was no significant difference between oxygen consumption during the different flow conditions. The average heat production for all the experiments was 0.5454 ± 0.3096 J crab^-1 h^-1 or 0.5353 ± 0.3734 J (25 mg dry wt)^-1 h^-1. The energy budget of P. longicornis during the different behaviors was computed, and it was found that the highest energy gain is during active feeding. The higher energy input is contributed by grazing, and not by low heat production. It is suggested that this behavior is an adaptation to life under stones where flow is limited and particulate organic matter is supplied by the activity of the crabs.