Abstract
Photosensitive behaviors and circadian rhythms are well documented in reef-building corals and their larvae, yet photoreceptive structures and opsins have not been described in these organisms. Here I provide evidence for red sensitivity in several species of coral larvae. Behavioral experiments with two Caribbean corals, Porites astreoides and Acropora palmata demonstrated that larvae settle and metamorphose at a greater frequency on red substrata than on similar substrata of other colors. Attachment to red substrata was not observed when larvae were maintained in the dark, suggesting that red sensitivity was responsible for the observed behavior. Extracellular recordings confirm photosensitivity and indicate that the peak sensitivity of coral photoreceptors are shifted towards the orange-red region of the visible light spectrum, similar to the spectra (fluorescence and reflectance) of preferred artificial (plastic) and natural (crustose coralline algae) settlement substrata. Using Blast analyses and a PCRbased approach, I have identified, sequenced and cloned two full-length opsin cDNAs from A. palmata larvae. One cDNA (Acropsin 1) encodes an opsin protein that is similar to a vertebrate melanopsin; the second (Acropsin 2) encodes a protein that is most similar to cephalopod rod opsin. I have successfully developed synthetic peptide antibodies against each Acropsin 1 and Acropsin 2. Western blots of adult A. palmata and A. cervicornis protein detect a 37kDa and 40kDa band, corresponding to the predicted molecular weights of Acropsins 1 and 2, respectively. Immunohistochemistry confirms expression of both opsins in A. palmata larvae. Staining of sectioned larvae demonstrates that Acropsin 1 is localized in the larval gastroderm while Acropsin 2 is localized in solitary epithelial cells, scattered throughout the larval ectoderm but with a polarized distribution and higher concentration in the aboral epidermis. This research provides several lines of evidence to support the existence, and demonstrate one potential ecological function, of opsin-based photosensitivity in corals.