MEPS prepress abstract  -  DOI:

Functional diversity among herbivorous sea urchins on a coral reef: grazing rate, dietary preference, and metabolism

Levi Steele Lewis*, Jennifer E. Smith


ABSTRACT: Herbivores influence the structure and function of ecosystems, especially in the marine environment where ecosystems have rapidly transformed due to the presence or absence of a single important grazer or grazing community. Intraguild variation in the ecological functions of herbivores, however, likely determines their ultimate effects on benthic dynamics. For example, echinoids (sea urchins) can facilitate the growth of stony corals by consuming fleshy algal competitors, yet our understanding of taxonomic variation in their grazing behaviors remains limited. Here, we examined the trophic functions of five herbivorous echinoids on a coral reef in Maui, Hawaii. We conducted field-based assays to compare grazing rates and consumption profiles using several key algal functional groups and contrasted the results with reported differences in echinoid metabolism. Grazing rates varied among species by up to 10-fold, with taxonomic differences explaining 77-85% of the variation among individual urchins and metabolic rate explaining 81-98% of the taxonomic variation in mean biomass and energy ingestion rates. Though all species consumed several algae, they also exhibited distinct grazing behaviors. Species with lower metabolic rates exhibited the largest intraspecific variation in diets and no clear algal preferences. In contrast, species with higher metabolic rates consistently consumed or avoided specific macroalgae, indicating a positive relationship between metabolic rate and diet specificity. This phylogenetic variation in grazing and metabolism aligns with classic metabolic and foraging theory and suggests that species identity, community structure, and complementarity are likely key to understanding the functional roles of herbivorous echinoid communities on coral reefs.