Elysia rufescens
Elysia rufescens is a species of sea slug, a marine gastropod mollusc in the family Plakobranchidae. This sea slug resembles a nudibranch but is not classified in that order of gastropods, instead belonging to a closely related clade, Sacoglossa, the "sap-sucking" sea slugs. This species was first described by Pease from Tahiti in 1871.

Description
Elysia rufescens is olive green with large white spots, often in a reticulated pattern, and grows to about . The parapodia are somewhat convoluted and have dark blue edges and a submarginal orange line. The rhinophores are rolled and have blunt, greyish ends.<ref name=SSF>[http://www.seaslugforum.net/showall/elysrufe Elysia rufescens (Pease, 1871)] The Sea Slug Forum. Retrieved 2012-01-27.</ref>

DistributionElysia rufescens is found in the Pacific Ocean. Its range includes the coastlines of South Africa, Réunion, Thailand, Myanmar, the Philippines, Japan, Guam, Samoa, Tahiti, Hawaii and Australia.

BiologyElysia rufescens feeds on green filamentous algae such as Bryopsis pennata, which it rasps with each of a series of rachidian teeth. E. rufescens grazes on Bryopsis'' sp., an alga that defends itself from predators by using peptide toxins with fatty acids, called kahalalides.  A bacterial obligate symbiont produces many defensive molecules, including kahalalides, in order to protect the alga.  This bacteria is able to use substrates derived from the host in order to synthesize the toxins.  The Hawaiian Sea Slug grazes on the alga in order to accumulate kahalalide.  This uptake of the toxin, which the slug is immune to, allows it to also become toxic to predators.  This shared ability, both originating from the bacteria, provide protection within the marine ecosystems.

References

Plakobranchidae
Gastropods described in 1871