Meeting Abstract
A comparative taxonomic approach is used to test the phototactic Sensitivity Hypothesis (SH), i.e. that light response reflects habitat photic conditions. Alternatively, if a species has relatively recently adopted a new habitat, light responses may reflect ancestral photic conditions. A series of laboratory experiments were conducted to determine phototactic polarity, spectral responses and intensity thresholds for four species of copepods, each representing different habitat-lineage combinations. Phototaxis was examined for two calanoid and two harpacticoid species. Parvocalanus crassirostris is a typical calanoid in that it lives in the water column, while Pseudodiaptomus pelagicus, also calanoid, is unusual in having a benthic association. In an analogous cross-comparison of species, Tisbe biminiensis is a typical harpacticoid in living on the benthos, while Euterpina acutifrons, also harpacticoid, lives in the water column. This is the first phototaxis lab study to include harpacticoid copepods. The benthic copepods, regardless of taxonomic lineage, displayed negative phototaxis. The pelagic species, in contrast, showed positive phototaxis. The polarity of phototaxis for all species was consistent with the SH in that the direction of movement is likely to sustain them in their respective habitats. Spectral sensitivity, however, may reflect ancestral preferences rather than the most readily available wavelengths for copepods in derived habitats. Pseudodiaptomus pelagicus had a peak response to 540 nm, not too dissimilar from the other calanoid in this study and many from the literature. Euterpina acutifrons had peak responses at 500 nm, similar to the other harpacticoid in this study when light-adapted. Intensity thresholds were highly variable and could reflect a complex combination of ancestry, depth distribution, habitat preference, and water quality conditions.
