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

Abstract

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.

Effect of temperature on lower feeding thresholds, gut evacuation rate, and diel feeding behavior in the copepod Acartia hudsonica

“…The present study demonstrates that a lower feeding threshold exists in Acartia hudsonica, consistent with earlier reports of reduced clearance and ingestion rates in other copepod species at low food abundance (Frost 1975, Ki~rboe et al 1985a, Paffenhofer & Stearns 1988, and direct observations of reduced swimming and feeding activity in copepods when food was absent or of low abundance [Piontkovskii & Petipa 1976(cited in Paffenhofer & Stearns 1988), Cowles & Strickler 1983, Price & Paffenhofer 1986, Gill & Poulet 1988. Although a lower feeding threshold may be a widespread phenomenon, the response over the same food concentration range differs markedly among copepod species.…”

“…For example, as food concentration declined below 0.22 pg C I-', the clearance rate in Eucalanus pileatus and Paracalanus sp. increased, whereas that in A. tonsa sharply declined (Paffenhofer & Stearns 1988). Thus there will be no single absolute feeding threshold that is generally applicable to different copepod species.…”

Prey switching by Acartia clausi:experimental evidence and implications of intraguild predation assessed by a model

“…Acartia spp. tend to reduce their clearance rates for algae below some threshold food abundance (Deason 1980, Kerboe et al 1985, Paffenhofer & Stearns 1988, Durbin & Durbin 1992, Kierboe et al 1996. For diatoms, the clearance rate decreases when the abundance is below -100 to 500 cells ml-l, or -4 to 40 pg C 1-l. For smaller flagellates the level where clearance rate starts to decrease seems to be higher (Kerboe et al 1985: maximum clearance rate at 150 1-19 C 1-' for Rhodomonas baltica; Gismervik unpubl.…”

“…results: maximum clearance rate at about 5000 cells ml-' for Tetraselmis damir). According to Paffenhofer & Stearns (1988) the decrease in clearance rate in A. tonsa at low algal food concentrations may be due to few chemoreceptors, and a n inability to change the flow field in order to re-route cells that are displaced towards the copepod. Thus, this initial increase in clearance rates for algae found by some investigators is generated by switching from non-feeding to feeding activities (Marten 1973).…”

“…A tonsa's ability to feed at high rates at such low concentrations of ciliates may be due to specialization for carnivory by numerous mechanoreceptors on the first antenna (Paffenhofer & Stearns 1988. Prey moving in the vicinity of the copepod will be detected and captured.…”