A comparative study of feeding and growth in two coexisting species of carnivorous gastropods

The effects of variable food supply on growth rate and feeding behaviour in two coexisting intertidal gastropods were examined through field and labor atory experiments involving mark and recapture procedures, growth experimen ts under controlled food supply and video observations of feeding. All coho rts of both Japeuthria species grew most in summer, with the highest growth rate being achieved by the youngest cohort of both sexes. In contrast, alm ost all cohorts (except the youngest one) showed zero to negative growth in winter. The growth rates of Japeuthria species clearly varied depending on feeding frequencies and the initial size of individuals. Females of both s pecies had a significantly higher growth rate than males, though the differ ence became less marked with decreasing food supply. The total time spent o n feeding and the length of the first feeding bout generally increased as t he duration of the interval between meals was increased across treatments. From both the field growth experiment and the observations of feeding behav iour it is clear that J. ferrea had a higher frequency of feeding than J. c ingulata in the field; 1- to 3-d intervals of feeding frequency in the form er and 3- to 6-d intervals in the latter. Most notably, the two species sho wed a reversal in growth rates with changing conditions of food supply. Whe n food was relatively abundant J. ferrea achieved better growth than J. cin gulata, but J. cingulata grew better than J. ferrea under low food supply. In view of the fact that J. ferrea seems to be competitively superior to J. cingulata under normal levels, the present observation that J. cingulata c an perform better than J. ferrea when food availability is low gives an imp ortant advantage to the latter in terms of its coexistence with the former.