Detecting sex‐related differences in mesograzer feeding experiments: An often overlooked source of intraspecific variation in herbivory

Abstract

Nutritional demands of males vs. females can be markedly different, leading to potential differences in food utilization within natural populations. However, widely-used experimental approaches to determine food choice and feeding rates of marine consumers seldom detect sex-related differences in feeding. Such studies have expressed consumption results largely per experimental replicate or per individual. By using small modifications of these methods, and alternative ways of expressing consumption, we tested for sex-related differences in feeding in the amphipod Gammarus aequicauda and the crab Omalacantha bicornuta. Males and females of the amphipod showed similar preferences for Ulva cf. prolifera over Cladophora glomerata in pairwise choice assays. In contrast, females of the crab significantly preferred the alga Acanthophora spicifera over Ulva fasciata, but males expressed no preference. While choice was not affected by different ways of expressing feeding, we showed that standardizing results by consumer mass highlighted significant differences in total consumption and feeding rates in both choice and no-choice conditions. For both mesograzers, females consumed significantly more algae per mass than males when total consumption from the choice experiments was analyzed. Similarly, when confined to single diets (no-choice), feeding rates of females standardized by female mass were significantly higher for both consumers. Expressing consumption as algal mass eaten by individual, individual length, or individual width, failed to detect any sex-related differences for these two consumers. Using consumer mass in analyzing feeding experiments can help elucidate poorly understood patterns of intraspecific variation in marine grazers.