Why Nudibranchs are Partial Predators: Intracolonial Variation in Bryozoan Palatability

Abstract

Intraindividual variations in morphology as well as variations in population density of bryozoan (Dendrobeania lichenoides) affect foraging patterns of their nudibranch predators in the field and laboratory in Puget Sound, Washington. Three nudibranch species characteristically graze Dendrobeania lichenoides colony perimeters and leave the center unconsumed: °50% of surveyed colonies showed evidence of nudibranch predation. The proportion of a colony consumed is density dependent. When prey density was low, nudibranchs could consumed entire colonies, but ate less per colony as prey density increased in laboratory experiments. Nudibranchs apparently assess intracolonial variations in quality, and when prey are abundant, select the preferred parts (perimeter) of each colony. In paired palatability tests, nudibranchs rejected strips cut from colony centers, yet readily ate pieces from other regions. Three features of the colonies were associated with the nonpalatable center of the colony: attachment rhizoids, brown bodies, and embryo. The attachment rhizoids do not account for the decreased palatability observed, which thus may result from either distasteful embryos of brown bodies, which both occur in highest numbers in the colony center. Brown bodies are products of degenerated polypides retained within zooecia in some bryozoan taxa. The function of brown body retention has never been established, despite considerable debate among invertebrate biologists. This study indicates that colonial invertebrate colonies are not necessarily homogeneous resources. Intracolonial palatability may generally vary with ontogeny which, in a colonial animal (or plant) results in intracolonial variation. Nudibranchs primarily eat colonial invertebrates (coelenterates, ascidians, and bryozoans), and consequently their foraging patterns may generally reflect intracolonial variations in the palatability of their resources.