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.
As described a while back, all steps in the culturing process seems to be going pretty well, except for one bottleneck. The adult broodstock is (are?) happy to lay eggs, the eggs hatch consistently, and the veligers settle. However, they will not develop much farther after settling in a controlled environment. Oddly, the settled veligers will develop if left on their own in a large tank full of algae. Although I have now reared E. clarki from egg to adult, it is not really possible to plan experiments based on when slugs may or may not decide to develop in a display aquarium. A more systematic approach was needed.
The hatchery is an attempt at making the juveniles happier during and after settling. Egg masses will still develop in glass crystallization dishes, but they will be placed in the new setup just before hatching. There were a few issues that may have impeded development, and they should be addressed by the new setup.
The tank is an acrylic “half-ten” from Glasscages.com. It is essentially a half-height 10-gallon tank (10″ W X 20″ L X 6″H). I had originally planned on using a standard 10 gallon, but it became clear that it would be clumsy and result in a lot of wasted space.
One problem that arises with free-swimming veliger larvae is that they are positively phototactic (attracted to light). This may not be a problem in the open sea or a large aquarium, but in the small dishes I was using for hatching, it meant that they would swim to the surface and promptly get stuck in the air-water interface. This would leave little rafts of veligers on the surface. These floaty veligers were capable of settling, so it was not a complete disaster, but it could not be good for them. Some authors (e.g., Dionisio et al, 2013) go so far as suggest rearing them in the dark. My solution is to illuminate from below. Marineland makes a nice little submersible LED light that can be used to keep the veligers swimming downward. Below are views of a prototype hatching chamber (2″ PVC pipe, with 50 micron nylon mesh to retain the veligers and larvae), showing the light coming from underneath.
Secondly, the presence of a relatively large food plant (large enough to be certain all of the little slugs can climb on) might have altered water chemistry, either through the process of photosynthesis (raising pH, e.g.) or by releasing chemicals that inhibit the slugs’ feeding or development. Continuously recirculating ASW (artificial seawater) from the larger system through their hatching chambers should reduce or eliminate this problem. In order to keep voracious invertebrates from entering the chambers, ASW will pass through a UV sterilizer before being distributed in the hatchery.
The manifold for distributing the ASW to the chambers is made from a few PVC pipe fittings. Once the cement has cured, it will be drilled to accommodate valves to control the flow to each chamber.
Naturally, water arriving in the tank needs to leave, so I drilled it and added a bulkhead to drain to the sump.
Once the manifold is finished, and the UV unit arrives, it will be ready to hook up and accommodate the next available brood. Stay tuned.