Grazing of two common appendicularians on the natural prey assemblage of a tropical coastal ecosystem

The clearance rates of co-occurring appendicularian species, Oikopleura longicauda and O. fusiformis, in Kaneohe Bay, Hawaii, were investigated to evaluate and compare their roles in a tropical food web. Individual appendicularians were captured in situ and allowed to feed on the natural plankton assemblage for 60 to 180 min. Feeding rate estimates were based on flow-cytometry analyses of cell-density changes for heterotrophic bacteria (Hbact), Synechococcus spp. (Syn) and <13.0 μm autotrophic eukaryotes (Aeuks). Despite morphological differences, O. longicauda and O. fusiformis cleared the largest prey size-fraction at statistically indistinguishable rates. For the 3 prey categories (Hbact, Syn and Aeuks), mean clearance rates (±95% CI) were 12 ±7, 27 ±6 and 34 ± 18 ml individual (ind.)–1 h–1 and 25 ± 12, 26 ± 15 and 38 ± 20 ml ind.–1 h–1 for O. longicauda and O. fusiformis, respectively. The mean clearance rates of these 2 species on total sub-micron cells in Kaneohe Bay were not significantly different; however, O. fusiformis cleared Hbact at a marginally higher rate (p = 0.07). Only O. longicauda exhibited significantly different retention efficiencies as a function of prey size, clearing the smallest prey (Hbact) at approximately 36% the rate of the largest (Aeuks) (p < 0.01). Despite reduced efficiencies on the smallest prey categories, at high abundances in Kaneohe Bay (often 2 ind. l–1), O. longicauda is capable of removing >60% of the picoplankton standing stock from the water column daily. While generally much less abundant, during occasional peaks of 1 ind. l–1, O. fusiformis can remove an almost equivalent amount (> 50%). Nevertheless, the consistently higher abundances of O. longicauda make this species a more significant link between picoplankton production and higher-level consumers (chaetognaths and fishes) in this coastal tropical embayment.