settlement

ABSTRACT  Coral and oyster reefs have declined greatly due to anthropogenic stressors. Low recruitment rates from larvae hamper recovery of these important ecosystems. Although much is known about factors affecting larval settlement, a detailed understanding of their swimming and substrate selection behaviour is lacking. Here, we present an approach to study coral and oyster larval behaviour in unprecedented detail, using a high resolution camera, choice chambers and behavourial analysis software. From second-by-second spatial data, we extracted variables such as swimming pattern, swimming speed and distance travelled using larvae between 0.2 and 3 mm in length. We applied this to larvae of the Caribbean brooding coral Favia fragum and show they locate their major settlement cue, coralline algae, within 90 min when placed in a choice chamber. Oyster (Ostrea edulis) larvae exhibited reduced swimming speed with age, suggesting pre-settlement behaviour. With the presented real-time high resolution tracking approach we can address new questions related to the behaviour of coral, oyster and other marine larvae, with applications in ecology, aquaculture and coastal engineering. Most notable is future development of “flypaper” substrates with cues to promote larval settlement on reefs, to aid restoration efforts.

Compiled abundances of juvenile corals revealed no change over time in the Pacific, but a decline in the Caribbean. Using these analyses as a rationale, we explored recruitment and post-settlement success in determining coral cover using studies in the Caribbean (St John, Bonaire) and Pacific (Moorea, Okinawa). Juvenile corals, coral recruits, and coral cover have been censused in these locations for years, and the ratio of juvenile (J) to recruiting (R) corals was used to measure post-settlement success. In St John and Bonaire, coral cover was stable but different between studies, with the ratio of the density of juveniles to density of recruits (J : R) ~0.10; in Moorea, declines in coral cover were followed by recovery that was related to the density of juvenile corals 3 years before, with J : R ~0.40; and in Okinawa, a decline in coral cover in 1998 was followed by a slow recovery with J/R ~0.01. Coral cover was associated positively with juvenile corals in St John, and in Okinawa, the density of juvenile corals was associated positively with recruits the year before. J : R varied among studies, and standardised densities of juvenile corals declined in the Caribbean, but increased in the Pacific. These results suggest that differences in the post-settlement success may drive variation in coral community structure.