Why is Acartia tonsa (Copepoda: Calanoida) restricted to nearshore environments?

The copepod Acartia tonsa is adapted to high food concentrations which it encounters in estuaries and upwelled waters.It cannot obtain sufficient food for reproduction on the middle and outer shelf, where food concentrations are usually low, because it decreases clearance rates when concentrations of Thalassiosira weissflogii fall below 0.25 mm3 1-' In comparison, the offshore copepod Paracalanus sp.continues to increase its clearance rate when food levels are below the abovementioned concentration.Several factors are thought to be responsible for this reduction of clearance rates of A. tonsa feeding on T. weissflogii: ( l ) The proportion of time during which water is transported towards the copepod decreases with decreasing food concentration.(2) The efficiency of capturing food particles decreases below 22 pg C I-' (= 0.28 mm3 I-' of T. weissflogb].(3) A. tonsa does not seem to reroute phytoplankton cells individually towards its median, and therefore cannot use a, hypothesized, increased sensitivity of its chemoreceptors at low chlorophyll concentrations to increase clearance rate.