Effects of food concentration on the behaviour of Oikopleuradioica

Feeding behaviour of the pelagic tunicate Oikopleuradioica presented with a range of food concentrations (FC, 4–670 µg C l–1) of the flagellate Isochrysisgalbana was observed using video. Simultaneous bottle incubations provided clearance rate estimates. Tail beats per minute (TB), number of tail arrests or pauses (TA) and swimming speeds were recorded.

TB decreased with increasing FC, from 521 to 355 min–1, while TA increased from 17 to 35 min–1. Aberrant behaviour with repeated TA was found if inlet filters were obstructed by larger particles or aquarium walls. Swimming speed was 1.0±0.4 mm s–1, and the time spent swimming decreased with FC from 38% to 11% of the time.

Clearance rate decreased sharply with FC, from 4.3 to 0.11 ml ind.–1 h–1, resembling a type II functional response with Michaelis–Menten parameters Imax=113 µg C ind.–1 h–1 and Km=27 µg C l–1. Using TB, TA and a residual error term, a simple model to predict clearance rate from behavioural parameters was developed.

Power functions of FC were fitted to each parameter, resulting in exponents of –0.047, –0.183 and –0.38, respectively. The error term includes passive clogging and food rejection at the mouth, and was according to the model the most important for shaping the functional response.