Comparative phototaxis of calanoid and harpacticoid copepods

Abstract

A comparative taxonomic approach was used to examine copepod phototaxis. The sign of phototaxis, spectral responses, and intensity thresholds were examined for two species each from the orders Calanoida and Harpacticoida. Parvocalanus crassirostris is a typical calanoid, living in the water column, while P. pelagicus (Calanoida) is unusual in having a benthic association. Tisbe biminiensis is a typical harpacticoid, living on the benthos, while Euterpina acutifrons (Harpacticoida) is atypical, living in the water column. This is the first empirical study to include harpacticoid phototaxis. Tisbe and Pseudodiaptomous displayed negative phototaxis, and the sign of phototaxis for all species is likely to help them persist in their respective habitats. We suspect that the sign of phototaxis is either flexible or a prerequisite before changing habitats. Spectral sensitivity, however, is more complex and, we hypothesize, may be partially constrained by ancestry rather than fully adapted for the modern/current habitat. Pseudodiaptomous pelagicus, benthic calanoid, had a peak spectral response similar to other calanoids. E. acutifrons, pelagic harpacticoid, had a peak spectral response similar to light-adapted T. biminiensis. E. acutifrons exhibited positive or negative photoresponses dependent upon spectrum, a dual strategy unobserved in other copepods but potentially effective for maintaining vertical position. We suggest a Constrained Sensitivity Hypothesis where ancestral habitat may influence the extent and nature of modern responses.