Photosynthetic animals and where to find them: abundance and size of a solar-powered sea slug in different light conditions

Three populations of the solar-powered sea slug Elysia crispata on reefs from the southern Gulf of Mexico and the Caribbean were analyzed. The aim was to describe and compare the changes in abundance and size of this species in different localities, as a function of depth and time-of-day. We hypothesized that differences in abundance would be related to locality, time of the day and depth, and differences in size would be related to locality and time of the day. Using snorkeling and SCUBA diving, all individuals within quadrats were counted and measured. A total of 680 organisms were recorded at Verde, Arcas and Puerto Morelos (PM) reefs at five times of the day (sunrise, morning, zenith, evening, and night) and depths of 0–13 m. Zero inflated negative binomial (ZINB) regressions adjusted to abundance data showed that E. crispata in Arcas and Verde reefs is expected to be more abundant (> 50) in shallow depths (< 2 m) at any time of the day except sunrise, whereas a low abundance (≤ 1 organism) is predicted in PM regardless of depth and time-of-day. According to linear models, size was not related to depth, but was related to locality and time-of-day, with sea slugs from Arcas and Verde having similar size, and both larger than those in PM. This information suggests that this sea slug is capable of moving within the reefs and helps to understand the unique biological phenomena of kleptoplasty.