A new species of
Dendronotus, discovered in the San Juan Islands, Washington, is described. The description is based on color, external morphology, structure of the jaws, number and shape of radula teeth, and structure of the reproductive system. In the laboratory, this species copulated and spawned in February. It showed a swimming escape response to the asteroid,
Pycnopodia helianthoides, a potential predator in natural situations. The prey of this new species of
Dendronotus is probably one or more species of hydroids.
Dendronotus nanus is provisionally accepted as a valid species distinct from
D. iris. However, the similarities between the two species are discussed and it is suggested that further studies will show that the two species are the same with the name
D. iris taking precedence. Some features of the radula, jaw, and digestive system of
D. robustus are discussed briefly. The reproductive system is described in detail and figured for the first time. Some aspects of ecology, especially feeding, are discussed. Although most specimens examined had eaten hydroids, some had ingested whole one or more sabellid polychaetes.