Evaluating whether velar lobe size indicates food limitation among larvae of the marine gastropod Crepidula fornicata

Disproportionately large feeding structures have been used to infer food li mitation in some marine invertebrate larvae, but few studies have investiga ted whether other factors alter larval morphology in similar ways. In this study, larvae of Crepidula fornicata were reared either at five different f ood concentrations of Isochrysis galbana (clone T-ISO) at a single temperat ure (22 degrees C) (Experiments I and II); or on three different phytoplank ton species (Isochrysis galbana, Dunaliella tertiolecta, and Pavlova luther i) at both high and low concentrations at a single temperature (22 degrees C) (Experiment III); or at high and low concentrations of Isochrysis galban a at four different temperatures between 16 and 25 degrees C (Experiment IV ). Shell lengths and velar lobe dimensions were determined for individual l arvae at intervals to monitor relative rates of velar and shell growth, in addition (Experiment V), fast growing and slow growing larvae in Experiment I were examined separately to determine whether velar lobes developed at s imilar rates (relative to shell growth) for fast and slow growing larvae wi thin individual cultures. In general, velar lobes grew significantly larger , relative to shell length, when larvae were reared at low food concentrati ons (P < 0.0001); for larvae of similar shell length, the velar lobes of th ose fed 1 x 10(4) cells ml(-1) were on average 17.7% larger than those of l arvae feel 18 x 10(4) cells ml(-1) of T-ISO. In contrast, larvae fed differ ent phytoplankton species at equivalently high food concentrations did not differ in relative velum size (P = 0.2666), even though shell growth rates differed significantly for larvae raised on the different diets, indicating substantial variation in food quality. We also found that relative rates o f velum and shell growth differed among fast and slow growing individuals w ithin treatments. Temperature had no significant effect on relative rates o f velar and shell growth within the 16-25 degrees C range tested (P = 0.121 ), but may have altered the relationship between food concentration and rel ative velar growth. These results indicate that dramatically reduced food c oncentration induces disproportionate growth in the velar lobes of C. forni cata, but that interpretation of data from held-collected individuals of th is species will be made difficult by the potentially confounding effects of temperature, food quality, and differences in individual growth potential. Assessments of food limitation using morphological measurements for field- collected larvae will need to be supplemented with other indicators before convincing conclusions about the extent of food limitation in C. fornicata can be drawn. (C) 2000 Elsevier Science B.V. All rights reserved.