Predation vulnerability of small invertebrates varies among and within major prey groups in eelgrass beds

Small invertebrates in eelgrass beds form an important prey base for fish and larger invertebrates, thereby supporting trophic transfer in coastal food webs. Although previous studies indicate that mobile crustaceans generally experience higher predation pressure than gastropods in eelgrass ecosystems, it remains uncertain whether this broad taxonomic contrast sufficiently characterizes relative prey vulnerability, as vulnerability may also differ among species within the same broad prey group. We conducted standardized field tethering experiments at three eelgrass beds along the Pacific coast of Japan (approximately 34-43°N) to compare relative predation pressure among focal small benthic invertebrates. Predation pressure varied between major prey groups, with crustaceans generally more vulnerable than gastropods. Notably, considerable species-level variation was observed within these groups, particularly among crustaceans, indicating that biologically meaningful variation in prey vulnerability can occur within broad taxonomic categories. Body size alone did not account for this pattern, suggesting that species identity may capture additional aspects of prey biology not fully represented by size, such as morphology, behavior, microhabitat association, detectability, and defensive characteristics. These results indicate that incorporating species-level resolution, alongside broad prey-group identity, improves the characterization of relative prey vulnerability in eelgrass beds.