The Urch- to Eat: Effect of encrusting bryozoans on the growth and feeding behavior of green urchins

Abstract

Symbiotic relationships are a fluid part of all ecosystems; two species’ biological relationship contributes to their role in shaping the dynamics of the ecosystem. Sometimes a symbiotic connection can help both species (mutualism), one species can benefit at the expense of another (predation or parasitism), or in some cases, one species can benefit while the other is unaffected (commensalism). Kelp forests are biodiverse habitats that are home to a multitude of vertebrate and invertebrate species. One of the most common and dominant invertebrate grazers of kelp forests is Strongylocentrotus droebachiensis, the green sea urchin. S. droebachiensis feed on various seaweed species, including the kelp Agarum fimbriatum. The epiphytic bryozoan, Membranipora membranacea, often lives on the same blades of the kelp on which green sea urchins feed. We quantified S. droebachiensis’ herbivorous relationship with kelp, investigating the symbiotic interaction between A. fimbriatum and M. membranacea. We used laboratory feeding experiments to assess urchin growth and feeding on kelp with and without encrusting bryozoans. In no-choice feeding experiments, the urchins consumed more of the bryozoan and kelp diet than those offered a kelp-only diet. This observation of increased urchin feeding on bryozoan-encrusted kelp blades suggests that M. membranacea may have a parasitic relationship with the kelp A. fimbriatum, possibly deriving benefit at the kelp’s expense by making it more likely to be consumed. Still, we did not detect a statistically significant difference in urchin growth between the two diets. Continued analysis of the interaction between A. fimbriatum and M. membranacea will help further explain this symbiotic relationship and how it has affected and may continue to influence kelp forest ecosystems in a changing ocean. Carballosa