Back from Bonaire, with a fresh puzzle.
In research, as in life, there are things that don’t make sense. Often these things make enough sense that you ignore them, choosing to focus on other mysteries. One such little small, nagging issue is the question of what draws Elysia crispata to hard-bottom coral reefs, which lack obvious growth of green algae known to be their food. Based on observations of many years, the slugs are not in transit, most are just sitting there.
My knowledge of the habits of Elysia in the wild is far from encyclopedic, but the species I know best have hearty appetites and stay close to their food. E. diomedea are found on or near Codium in Bahia de los Angeles, and E. clarki spend most of their time face down in their food in aquaria. This tends to hold true in the literature as well. For example, E. tuca is generally found on its favorite food, Halimeda incrassata (Rasher et al., 2015, PNAS 112: 12110). As a counter example, Middlebrooks et al. (2014) found that E. clarki were often found at sites that contained few or no specimens of their food plants (Penicillus, Halimeda, Bryopsis) determined via DNA barcoding.
In any case, I think I am justified in being puzzled by the lack of an obvious food source on the reef. The photos in this post are all from a single dive at The Cliff, a site in the north-ish part of Bonaire. We found maybe a dozen slugs, most in the face-down posture, which makes them look like large blobs of colorful frosting on the rocks. The area had a lot of dead coral, which possibly serves as a substrate for the growth of food algae. However, there were no obvious growths of green algae anywhere nearby, although algae such as Halimeda and Caulerpa are plentiful in mangroves on the island.
Rather than snap a few photos of the more photogenic slugs, I thought it might be useful to document as many of the slugs as I could, with emphasis on the substrate. Honestly, what you see is what you get; there are no large clumps of Bryopsis or Halimeda hiding around the corner.
What are these gals eating? The most prominent alga is Dictyota, a brown alga which, based on known feeding habits, is an unlikely food.
Are they grazing on the little strands of green algae that can be seen if one expands the photos and looks really hard? Is this a late life stage that does not feed as much? Are E. crispata truly crawling leaves, getting their energy from photosynthesis? Is the much lighter color of E. crispata, compared to related species, like E. clarki and E. diomedea, a clue?