Date of Award

Fall 1990

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

Larry G Harris


Kelp blades (Laminaria spp.) with colonies of Obelia geniculata were collected over a 28 month period from Cape Neddick, ME to document the epifaunal community and to determine the possible mechanisms allowing four nudibranch species to coexist. Regression analyses correlated the abundances of epifauna with habitat parameters. The feeding biology of nudibranchs was described. Density manipulations of nudibranchs in pair-wise behavioral interactions tested the effects of interference on the location of nudibranchs in the colony.

Obelia geniculata provides habitat and food for many invertebrate epifauna. The hydroid colony acts as the island for most species. Hydrocauli provide structure to motile epifauna. Denser and taller colonies have higher abundances of motile epifauna.

O. geniculata is a nursery habitat for nudibranchs. Adults of Dendronotus frondosus, Doto coronata and Eubranchus exiguus were infrequent inhabitants of the hydroid colony. In contrast, Tergipes tergipes consistently occupied the colony for its entire life cycle suggesting that O. geniculata is the primary habitat of Tergipes.

Each nudibranch species utilized a separate portion of the hydroid colony as food and habitat. Feeding behavior of Dendronotus was size dependent; small nudibranchs ($$5 mm) bit polyps. Doto drilled through stolons and Eubranchus penetrated hydrothecae. Tergipes rasped naked tissue from polyps. Dendronotus was found throughout the colony on hydrocauli. Doto was on the kelp surface and Eubranchus on hydrocauli at the edge of colonies. Tergipes occupied the central area of colonies and atop hydrocauli.

Behavioral interactions among the nudibranchs occurred frequently but did not influence the habitat or feeding locations of nudibranchs. The vast majority (71%) of reactions of nudibranchs in pair-wise encounters between nudibranchs were non-aggressive. This suggests that nudibranchs are unaware of each other, similar to many insect communities.

Although some separation of the hydroid food resource is present, resource partitioning is unlikely to be a major factor allowing coexistence of the nudibranchs. Interference competition is too rare or weak to cause the observed patterns. Differential recruitment by all four nudibranch species may overwhelm the hydroid but equilibrium conditions necessary for exclusion are unlikely to occur or persist for long periods.