Slug Makes New Species Top 10 List
The Washington Post reported that a species of photosynthetic nudibranch has made the SUNY Environmental Science and Forestry list of the Top 10 New Species of 2015. The field was large, about 18,000 species in all, but Phyllodesmium acanthorhinum made the list based on what the animals tell us about the evolution of the symbiosis between the slugs and the photosynthetic algae they host.
Like Elysia, species of Phyllodesmium steal the ability to perform photosynthesis from their food organisms and maintain the required components in sacs extending from the gut called digestive diverticula. There are some important differences, though. Unlike Elysia, Phyllodesmium is a true nudibranch, and it feeds on corals rather than macroalgae. Another important difference arises from the different biology of the algae that Elysia eat and the corals upon which Plyllodesmium feeds. Photosynthetic corals, such as Xenia, contain symbiotic algae (dinoflagellates, actually) called zooxanthellae, which provide the corals with most of their nutritional needs. When Phyllodesmium feeds on Xenia (or other coral species, depending on the species of Phyllodesmium), it steals the zooxanthellae and stores them in the diverticula. In this way, Phyllodesmium has it a bit easier, the stolen algae are autonomous cells, and the slugs do not need to worry about maintaining isolated chloroplasts.
So how did this species end up in the top 10? A recent paper describing Phyllodesmium acanthorhinum and analyzing the interrelationships of species within the genus (E. Moore and T.Gosliner, 2014, The Veliger 51:237) provides some new insight into how the ability to maintain zooxanthellae evolved within the group. Earlier work had suggested that the branching of the diverticula, and their extension into the cerata (the frills on the back of the nudibranch) increases with the increased ability to sequester and maintain zooxanthellae. In other words, species that simply digest the zooxanthellae have minimal branching, while those that maintain large collections of active zooxanthellae have more elaborate diverticula that branch deeply into the cerata. Based on the descriptions of P. acanthorhinum and another species, P. undulatum, both of which are relatively less specialized for maintaining zooxanthellae, Moore and Gosliner provide additional support for this hypothesis. Further, they suggest that the larger body sizes achieved by more derived species, i.e., those that are better able to maintain populations of zooxanthellae, result from the additional nutrients produced by the symbionts.
Once again, slugs find a way of hijacking photosynthesis from their food. Because Elysia and Phyllodesmium are only distantly related, and their biology and that of their food are so different, the two forms of theft-based photosynthesis must have evolved independently. The similarities are striking, though. It does make one wonder if there is some aspect of the biology of sea slugs that predisposes them to separate chloroplasts or entire zooxanthellae from their food and maintain them in digestive diverticula.