Functional response of the appendicularian Oikopleura dioica

Abstract

Different concentrations of 14C radiolabeled cultures of the prymnesiophyte Isochrysis galbana (5.5 µm in size), the prasinophyte Tetraselmis suecica (9.5 µm), and the chlorophyte Chlorella sp. (3.5 µm) were offered as food to groups of 2–5 Oikopleura dioica to determine the response of clearance (CR) and ingestion (IR) rates to food concentration (FC). At high FCs of I. galbana, IR and CR of O. dioica decreased with age of the house. Aging had little effect on the functional response (FR) because most of the decrease in IR and CR occurred during the initial 10% of the house lifespan. The FR resembled a type II model for the IR and presented saturation and inhibition at high FC (>100 µg C L−1)of I. galbana. Fits of FR curves of the IR of animals weighing ca. 3 µg C ind−1 to a Michaelis Menten model yielded Imax (maximum IR) of 198 ± 28, 498 ± 171, and 489 ± 150 ng C ind−1 h−1 and Km (half saturation FC) of 38 ± 21, 225 ± 177, and 290 ± 205 µg C L−1 for the prey I. galbana, T. suecica, and Chlorella sp., respectively. The Km values are high and in two of three cases exceed the maximum concentration of ingestible particles found in the natural habitat, which suggests that O. dioica is adapted to high phytoplankton concentrations. In contrast with classical FR models, CR remained nearly constant for all concentrations. Particles cleared from suspension by O. dioica are ingested and transformed into fast‐sinking fecal pellets at low FC, while they are mainly accumulated into slow to fast‐sinking filter houses at high FC, which implies considerable shifts in the biogeochemical role of these animals with changing particle concentrations.