Abstract

The functional response of the appendicularian Oikopleura dioica was investigated with a unialgal diet (Isochrysis galbana, 0-1,600 ?g C 1-1) and with additions of specific concentrations of algae either smaller than the incurrent filter mesh size (Rhodomonas baltica, 90 ?g C 1-1; Thalassiosira weissflogii, 60 ?g C 1-1) or larger algae (Ceratium lineatum, 8 ?g C 1-1; C. tripos, 52 ?g C 1-1). Clearance ranged from 0.13 to 3.3 ml ?g C-1 h-1 and followed a Michaelis-Menten curve corresponding to a maximum filtration rate (particles collected by both animal and house), FRmax = 164 ng C ?g C-1 h-1 and Km = 64 ?g C 1-1. Maximum pellet production was 9 pellets ind-1 h-1 and occurred at 100 ?C 1-1. Pellet production was proportional to filtration rate below this level, but declined at higher concentrations. House production ranged from 0.5 to 5.5 houses ind-1 day-1 and the corresponding curve fit resulted in Hmax = 4.6 houses ind-1 day-1 and Km house = 32 ?g C 1-1. Addition of edible small algae caused increased internal clogging of the houses, evidenced by significantly higher pigment concentrations in abandoned houses than in houses produced with I. galbana as a single food. House production was not affected. In contrast, larger algae caused a significantly higher house production, but the algae were not collected on the houses. Increased levels of edible algae will not have negative effects on O. dioica at ecologically realistic concentrations. Ambient bloom concentrations of larger algae may, however, have a negative effect on the growth of O. dioica populations, owing to the compensatory increased house production.