Effects of a habitat-modifying eelgrass epibiont on predator success and epifaunal survival

Abstract

Structural complexity plays an important role in mediating predator-prey interactions. Structural elements can limit predator mobility and ability to detect prey, increasing prey survival. Seagrasses are marine plants that provide refuge for many fishes and invertebrates by increasing the structural complexity of the benthos. Though prey survival often is positively correlated with seagrass biomass or shoot density, seagrasses frequently are colonized by epibiotic organisms such as algae and colonial invertebrates, which could affect this relationship. In San Diego Bay, Thalamoporella californica Levinsen is a common rigid branching bryozoan that creates a three-dimensional structure within eelgrass (Zostera marina L.) canopies. We conducted laboratory mesocosm experiments in which we examined fish Heterostichus rostratus predator foraging behaviors and shrimp Hippolyte californiensis prey escape behavior with (i) different densities of artificial seagrass, both with and without artificial bryozoans, and (ii) constant seagrass density, but varying amounts of artificial bryozoans. We also conducted a laboratory habitat selection experiment to determine if H. rostratus prefer to forage in artificial eelgrass with or without the presence of artificial bryozoans, and a field experiment to determine whether bryozoan, eelgrass and algal biomasses influenced survivorship of tethered H. californiensis. In the lab, we found that increasing eelgrass density reduced predator foraging efficiency, but bryozoans did not alter this relationship. We also found that fish preferred foraging in artificial eelgrass without artificial bryozoans. In the field, the biomass of bryozoans, eelgrass and algae did not influence tethered shrimp relative survival. Our results suggest that it should not be assumed that alterations to habitat complexity alter the outcomes of predator-prey interactions in seagrass habitat.