Vertical movements and distribution of planktonic larvae of the serpulid polychaete Spirobranchus polycerus (Schmarda); effects of changes in hydrostatic pressure

Abstract

Vertical distribution, in a 60 cm water column, was recorded for 1, 2, 3, 4 and 6 day larvae of the tropical serpulid Spirobranchus polycerus (Schmarda), in the dark and under low level overhead illumination. All age classes of larvae were distributed unevenly under both light conditions. One-day larvae concentrated at both the bottom and the top of the water column; 2- and 3-day larvae concentrated at the top, as did 4-day larvae in the dark. Four-day larvae in the light and 6-day larvae under both light regimes were equally concentrated at the top and the bottom. Upward and downward vertical displacement rates were measured for swimming 1-day larvae; sinking speed was measured for anaesthetized larvae. One day larvae swim up or down more frequently than horizontally. One-day larvae respond to an increase in hydrostatic pressure with an increase in the percent of larvae moving downward; they respond to a decrease in pressure with an increase in the percent moving upward. The same overall change in pressure was applied at three rates; in all cases the change in the percent of larvae moving up or down was significant at the p<0.05 level. Maximum calculated rates of change in pressure experienced by larvae swimming up and down are close to the lower end of the range of experimentally applied rates of change of pressure. The results are discussed in terms of earlier models for the effects of changes in hydrostatic pressure on decapod larvae.