Elysia is an “opisthobranch” sea slug famous on the internet for its remarkable ability to photosynthesise, giving it the nickname of “solar-powered sea slug”. It does this by kleptoplasty – stealing plastids from its algal food. If you note the greenish colour in E. asbecki above (Wägele et al., 2010), the green comes from the harvested chloroplasts. This post will look at this process, and generally at the biology of the genus.
Before starting, it’s worth noting that while Elysia is the most famous of these examples on the internet, it’s definitely not the only animal that can photosynthesise by symbiosis with algae. Falkowski & Knoll (2007) have a three page-long table (pp. 90-92) listing various eukaryotes where this has been documented, including sponges, corals, ascidians, flatworms, and bivalves.
E. chlorotica, if internet fame is anything to go by, is probably the poster child for the phenomenon, and also the one that exhibits it the best. It can harvest the chloroplasts from any number of ulvophycean and xanthophyte algae, but the most successful relationship is with the xanthophyte alga, Vaucheria litorea. The chloroplasts get sequestered intracellularly in the digestive epithelium, and stay functional for over 14 months (Rumpho et al., 2006). This isn’t just an opportunistic relationship, but one that has evolved into a real symbiosis, as evidenced by the identification of lateral transfer of V. litorea nuclear genes into the E. chlorotica genome (Schwartz et al., 2010), most tellingly the transfer of the oxygenic photosynthesis gene, psbO, and fcp, associated with light-harvesting complexes (Rumpho et al., 2008). This is also the reason why naming it a symbiosis may be a mistake, as it doesn’t involve two genetically independent organisms (Law & Lewis, 1983). Nomenclature aside, the chloroplast is hugely beneficial for the slug, as it provides “free” carbohydrates at times when food isn’t available (e.g. in winter, when host algae don’t grow).
Interestingly, the algal chloroplast loses its host membrane and outer two of the four chloroplast membranes (Rumpho et al., 2000) and is found free-living in the cytoplasm in adults after getting phagocytosed (Rumpho et al., 2000). In juveniles, they are bound within a special membrane.
Elysia contains over 120 species, accounting for ~40% of all sacoglossans. In fact, the phenomenon was first discovered in E. atroviridis, not E. chlorotica (Kawaguti & Yamasu, 1965), with functionality of chloroplasts documented by Taylor (1967) and evidence that the chloroplasts contribute to the animal’s metabolism by Trench et al. (1972). Other Elysia species do not nearly have E. chlorotica‘s efficiency, with most retaining the chloroplast for a couple of months at best, after which the chloroplasts degrade and get enveloped in phagosomes and presumably digested (Marín & Ros, 1993). Feeding allows chloroplasts to get replaced, but this is limited by the presence of algae. It is however notable that chloroplasts from multiple species can get sequestered (Curtis et al., 2006), so it’s not necessarily a specific interaction.
Elysia belongs to the Plakobranchidae, a family where long-term plastid retention is autapomorphic (Händeler et al., 2009) and lasts over a couple of weeks generally, but outside of the family, the association lasts for no more than a week. Individuals behaviourally try to control light intensity in order to maximise the lifespan of the inherited chloroplasts and to regulate photosynthesis rate (Casalduero & Muniain, 2008), but at some point, the chloroplasts just stop working.
A very valid question to ask is how such an association can arise. The ability to acquire plastids from food is most probably an ancestral one in sacoglossans, and is linked to one of the main autapomorphies of the Sacoglossa, a radula with only one row of teeth and one median tooth (Mikkelsen, 1996; pictured above from E. asbecki from Wägele et al. (2010)), which is what allows the animal to pierce the algal cell and suck out the plastids (Jensen, 1997). This is a feeding style that is highly-specialised and not found anywhere else in the molluscs, and underlies the reason why sacoglossans feed only on septate and siphonous algae, with only some also feeding additionally on other plants such as seagrass (Jensen, 1981). Each species, depending on their specificity, can also have further modifications to fit the radula perfectly to its host plant (Jensen, 1994), and it’s likely that host shifts can play a role in diversification and speciation, as hinted at by Trowbridge & Todd (2001)‘s work on the shifting of a Scottish subpopulation of E. viridis to feeding on an invasive alga within the past 50 years.
The retention of plastids can be imagined as having considerable fitness benefits, for example a free food source in the winter when algae don’t grow, or conferring a defensive adantage as camouflage – once they ingest the chloroplasts, the slugs turn green, identical to the background, and this is a tremendous advantage considering their lack of shell. There is also some evidence that their food gives them defensive compounds to sequester as well as chloroplasts, for example chlorodesmin, a fish repellent (Hay et al., 1989).
On to Elysia‘s general biology. The monophyly of the genus is not known for sure, and is strongly supported only by molecular phylogenies (Händeler et al., 2009). Truly diagnostic macroscopic features are not to be found, as is clear from field guides, where sacoglossans are all lumped together as unidentified morphospecies. So if you happen to catch one (your best bet is to look carefully in algal meadows, at any depth, keeping in mind that they will be well-camouflaged), your best bet is to consult an expert or trawl through seaslugforum.net.
As all gastropods, Elysia mating involves “love darts”. Schmitt et al. (2007) describe the process in E. timida, where it basically amounts to a synchronised shooting of hypodermic love darts, followed by a short period of standard vaginal impregnation aided by glandular fluids. You can view3 videos from that paper here. Elysia‘s life cycle involves a planktonic larval stage, with the veliger larva having a sinistral shell and dispersing for a couple of weeks or more. The metamorphosis to the adult takes place when the veliger attaches itself to a film of microorganisms growing in an alga-rich habitat.
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Katharina Händeler1, Yvonne P Grzymbowski, Patrick J Krug, & Heike Wägele (2009). Functional chloroplasts in metazoan cells – a unique evolutionary strategy in animal life Frontiers in Zoology DOI: 10.1186/1742-9994-6-28
Mary E. Rumpho, Elizabeth J. Summer, & James R. Manhart (2000). Solar-Powered Sea Slugs. Mollusc/Algal Chloroplast Symbiosis Plant Physiology DOI: 10.1104/pp.123.1.29
Mary E. Rumpho, Farahad P. Dastoor, James R. Manhart, & Jungho Lee (2006). The Kleptoplast Advances in Photosynthesis and Respiration DOI: 10.1007/978-1-4020-4061-0_23