Feeding ecology of the seagrass-grazing nerite Smaragdia souverbiana (Montrouzier, 1863) in subtropical seagrass beds of eastern Australia

Abstract

By amalgamating all seagrass-associated grazing invertebrates into an epiphyte-feeding guild, the currently accepted model of seagrass trophic dynamics ignores the diverse range of invertebrates that feed directly on, and do considerable damage to, seagrasses. Of the wide range of invertebrates documented to damage seagrass directly, the gastropod genus Smaragdia has adaptations and ecology that suggests it could be a specialized seagrass-feeding group, of which at least two species are known preferentially to consume seagrass. This paper investigated the dietary associations of Smaragdia souverbiana, one of the most widely distributed but least studied species of the genus, in the subtropical eastern Australian part of its range. Using field-based assessments of grazing damage and targeted laboratory feeding trials, we assessed the dietary associations, digestive ability and feeding preferences of S. souverbiana with local seagrasses (Halophila ovalis, Zostera capricorni and Cymodocea serrulata). We found that this species consumed and damaged all available species, but showed a strong preference for the most abundant and moderately digestible Z. capricorni. Although it avoided seagrass bearing a high epiphyte load in a laboratory context, considerable amounts of epiphytic material were found in the faeces of field-caught individuals. Grazing and digestibility of seagrass cells was higher in Z. capricorni and H. ovalis, and the former was preferred when both were available. This study adds to the growing body of literature demonstrating that S. souverbiana—and potentially many other grazing invertebrates—cause considerable damage to seagrasses directly, rather than targeting epiphytes.