Just back from a week diving in Bonaire. The focus was more on recreation than slugs, but we came across a few Elysia crispata along the way. As usual, we found the highest numbers in the more degraded sites to the north, but the slugs were always on long-dead coral regardless of where we found them.

For this post, I wanted to highlight the intense blue color of the slugs we found this year at Karpata, a site near the north end of the island. I have previously posted photos of blue slugs that have turned up at nearby sites, and a quick Google search will provide many more examples.

This seemed to be a common color pattern. Each photo in this post shows a different individual, but their colors are remarkably similar.

Even though the colors look downright artificial, I did little more than adjust the contrast of the photos. These slugs really look like this. I emphasize this point, because the range of colors of most species of Elysia is limited to shades of green.

Despite the striking appearance, the color pattern may provide camouflage. When the slugs were curled up and their rhinophores hidden, they did passable impressions of sponges. E. crispata are almost invariably scrunched up when we come across them on the reef, so this may be a successful way of hiding in plain sight.

The color of many Elysia species is derived from pigments taken from their food plants. For example, my E. crispata and E. clarki hatchlings have little color until they start feeding, and then take on the green color of the chloroplasts they sequester in their digestive diverticula. Costa et al, (2012), provide a nice example of this effect. They showed that E. timida could be either green or brown, depending on whether they were fed Acetabularia algae that had taken on different colors based on having been acclimated to low light (green) or high light (brown).

So what could be making these slugs so blue?

One possibility is that they are feeding on Dictyota, a brown alga that grows abundantly on the dead coral in this area. Although the Dictyota in the photo at the top of this post is not impressively blue, the alga is certainly capable of producing intensely blue color. Most species of Elysia feed on green, rather than brown algae, but it is not unheard of for them to branch out (e.g., Trowbridge et al., 2010). It is also possible that the slugs are consuming one of their more usual food plants that happen to be producing high levels of blue pigments. It would be interesting to take a small tissue sample and find out what the slugs have been eating.

It also brings up an interesting question regarding the coloration of Elysia in general. In addition to their green background, many species have distinctive patterns, such as the colorful markings on the parapodia and rhinophores of E. diomedea, below, or the colorful edges of the parapodia in E. clarki.

At a higher magnification, one can see concentrations of pigment spots, such as those shown below in the parapodium of E. clarki. Are these spots of concentrated pigment derived from their food plant, or are they synthesized by the slugs themselves?

As far as I can tell, there is no answer in the literature, but who knows what will turn up next.