Coexistence of Hydroid Eating Nudibranchs: Do Feeding Biology and Habitat Use Matter?

The feeding biologies and habitats of four nudibranchs in colonies of the hydroid Obelia geniculata were investigated to determine whether these factors contribute to the coexistence of the nudibranchs. The radulae and feeding behaviors showed species-specific traits. Dendronotus frondosus has a multi-seriate radula; when small (<5 mm), individuals are suctorial feeders, whereas larger nudibranchs (>5 mm), bite whole polyps. Doto coronata uses a flat, uniseriate radula to penetrate stolons. Eubranchus exiguus penetrates hydrothecae with a triseriate radula. Tergipes tergipes has a curved, uniseriate radula and rakes naked tissue. Each species of nudibranch occupied a distinct area within the hydroid colony, suggesting that the micro-habitats are dictated by feeding biology. D. frondosus occupies hydrocauli towards the center of the colony, D. coronata occurs along the edge of the colony on the kelp surface, E. exiguus is found on hydrocauli at the edge of the colony, and T. tergipes sits atop tall hydrocauli in the center of the colony. Separation in the hydroid food resource exists among these nudibranchs and equilibrial coexistence could have operated, but equilibrial conditions necessary for exclusion are unlikely to occur or persist significantly long. Thus, this assemblage of nudibranchs appears structured by non-equilibrial processes perhaps similar to populations of phytophagous insects.