MEPS 303:213-223 (2005)  -  doi:10.3354/meps303213

ABSTRACT: Swimming behaviour of copepodite stage CI to adults of the planktonic copepod Acartia clausi were observed at different food levels (0, 60, 500 and 2000 cells ml^–1) of the diatom Thalassiosira weissflogii. Concurrent measures of clearance in bottle incubations were performed for CI and adult females. All indivuduals exhibited typical jump–sink behaviour. At low food concentrations early copepodites filtered 36% of the time, and later stages 18%, yet the increased suspension-feeding effort did not enhance their clearance rate. Older stages and adults did not show feeding-bouts at low food concentrations, which is interpreted as a switch to the ambush-feeding mode, whereby the copepods are assumed to be searching for potentially abundant larger and motile prey, for instance, ciliates. When offered no food, frequent and long jumps were observed in all stages except for CI. Search volumes during ambush feeding were estimated, and the slow sinking of early stages resulted in a small search volume as compared to that in later stages. Early copepodite stages depended to a higher degree on suspension feeding, regardless of food concentration, due to their undeveloped food searching ability. High food availability is therefore crucial for the growth and survival of early copepodite stages of A. clausi. The heavier body of later stages enhanced foraging efficiency both during suspension feeding (by gravity tethering) and in ambush feeding (by faster sinking).

KEY WORDS: Swimming behaviour · Suspension feeding · Ambush feeding · Switching behaviour · Lower feeding threshold · Life history · Calanoid