This week's paper is in Biology Letters, lead authored by Dr. Trevor Willis who is a Senior Lecturer at the University of Portsmouth in the UK. Willis and his co-authors studied the relationship between nudibranchs and their hydroid prey off the coast of Sicily. Nudibranchs are spectacular little animals. Often referred to as "sea slugs" (which does them no justice) they comprise about 2,300 species of shell-less mollusk distributed worldwide. They vary enormously in color, including bright polka-dots and stripes, as well as in shape. The plumes on their backs are called cerata and are where much of their gas exchange occurs (gills) and also where they defend themselves through nematocysts, stinging cells that they acquire from their prey which includes anemones and the hydroids (more on those soon). Many nudibranch species are also toxic, either via the consumption of toxic sponges or de novo synthesis of toxins. These defenses likely explain the extraordinary warning colorations of many nudibranchs.
Many nudibranch species, including Cratena peregrina, the subject of this paper, consume hydrozoans. Hydrozoans are small animals related to jellyfish. They have different life stages, one of which is the polyp stage in which they are called hydroids. Hydroids are most commonly colonial, which is to say they live in groups. Hydroids are a bit like upside-down jellyfish that live in little tubes all connected to each other in a branching pattern (which makes them look a bit like a plant).
Hydroids use the stinging nematocysts on their tentacles to capture zooplankton prey. Hydroids themselves are then prey to nudibranchs. In this week's paper Willis and his colleagues examined this relationship between nudibranchs, hydroids, and the hydroids' feeding on zooplankton. In particular, they tested the hypothesis that nudibranchs prefer to feed on hydroids that have just captured a zooplankton, than on hungry hydroids. To do this they brought nudibranchs and into flowing seawater tanks in the lab and provided them with the choice between feeding on hydroids that were starved versus hydroids that had just been fed zooplankton. They measured nudibranch preferences for fed and starved hydroid polyps as well as how long it took the nudibranchs to consume fed and starved polyps. They also examined the stable isotope ratio of nudibranchs, hydroids, and zooplankton (see my post on "Smashing & Spearing Stomatopods" for more explanation of stable isotope analysis) to assess what nudibranchs are eating in the wild.
Willis and his colleagues found that nudibranchs preferred to attack hydroid polyps that had just consumed zooplankton over starved hydroids, and it took nudibranchs longer to consume those fed hydroids than starved hydroids. In addition, Willis showed with stable isotope analysis that nudibranchs were not solely consuming hydroids, but that zooplankton made up at least half of their diet. This data suggests that not only are nudibranchs preferentially consuming hydroids that have just captured prey in the lab, they also do so in the field. Willis and his colleagues introduce a novel term for this behavior: "kleptopredation". Kleptopredation is a combination of kleptoparasitism (when animals steal food from other individuals) and predation, as the nudibranch in this case is doing both. It is not clear why nudibranchs do this. The most obvious hypothesis is that polyps with captured zooplankton deliver more calories (and potentially nutritional diversity?) than starved polyps. Willis also suggests that the consumption of fed polyps results in nudibranchs consuming less polyps total, and may prevent them from driving their hydrozoan prey locally extinct.