ABSTRACT: Scleractinian corals are the major builders of the complex structural framework of coral reefs. They live in tropical waters around the globe where they are frequently exposed to potentially harmful ultraviolet radiation (UVR). Coral eggs and early embryonic stages are thought to be the most sensitive life stages of corals to UVR given that they are highly buoyant and remain near the sea surface for prolonged periods of time. Here we analyzed gene expression changes in different larval stages of the Caribbean coral Montastraea faveolata to natural levels of UVR using high-density cDNA microarrays (10,930 clones). We found that larvae exhibit low sensitivity to natural levels of UVR during most time points analyzed as reflected by comparatively few transcriptomic changes in response to UVR. However, we identified a time window of high UVR sensitivity that coincides with the motile planula stage and the onset of larval competence. These processes have been shown to be affected upon UVR exposure, and the transcriptional changes we identified explain these observations well. Our analysis of differentially expressed genes indicates that UVR induces a stress response and affects the expression of neurogenesis-related genes that can be linked to swimming and settlement behavior at later stages. Taken together, our study provides further data to the impact of natural levels of UVR on coral larvae. Furthermore, our results might allow a better prediction of settlement and recruitment rates after coral spawning events based on UVR climate data. Gamete capture and larval rearing Montastraea faveolata gametes were captured and reared as described in Voolstra et al. 2009 (2009a). Briefly, gametes from 10 colonies were captured during a spawning event on the night of the 10th of September 2009 at approximately 22:00 hours using collecting nets attached to plastic enclosures at “La Bocana Chica” (20º50´N, 86º52´W) located in the “Parque Nacional Arrecife de Puerto Morelos”. Within 10 minutes the gametes were brought to the research vessel “Carybdea”, where they were placed in 5 µm filtered sea water (FSW), large zooplankton was removed, and the egg-sperm mixture was mixed gently to enhance the process of fertilization during transportation to the research station (Unidad Académica Puerto Morelos). After 1 hour, the egg-sperm mixture was repeatedly washed with 5 µm FSW to ensure that all unused sperm and remaining zooplankton were removed. The embryos were placed in round, bottomless, incubation bins fitted with 100 µm mesh, which were housed in abundant 5 µm FSW. Fertilization success, measured 6 hours after the washing procedure, was estimated at 95% by counting the number of eggs undergoing division as a proportion of the total number of eggs (dividing + non-dividing). Experimental procedure After 12 hours, the majority of the embryos were in the late blastula stage. A subsample of embryos was taken from different incubation bins and divided into twelve 1 liter containers with 5 µm FSW added to the brim. Each container held approximately 3,000 embryos. All of the containers were placed in 800 L fiber glass aquaria filled with flowing sea water and exposed to natural solar radiation from 9am to 3pm (6h) at 29ºC and 3.5% salinity. Six of the containers were placed under a sheet of 6mm thick Plexiglass G UVT that has a full width at half maximum (FWHM) at 282 nm and is therefore transparent to UVR. The other six containers were placed under a sheet of 4 mm thick Plexiglass G UF-3 that has a FWHM at 390 nm and is therefore opaque to UVR. At the end of the exposure period, the embryos were harvested and preserved in RNAlater (Ambion), placed at 4ºC for 24 hours to ensure infiltration of the fixative and frozen at -80ºC until further processing. The same exposure and fixation procedures were repeated on 36 hours old embryos (late gastrula stage), on 60 hours old non-motile, floating larvae, on 84 hours old motile planulae, and on 132 hours old planulae that are motile, diving, and ready for settlement.
