Determinants of Recruitment, Juvenile Growth, and Spatial Distribution of a Shallow‐Water Gorgonian

Abstract

In the northern Gulf of Mexico, the gorgonian Leptogorgia virgulata (Cnidaria: Octocorallia) recruited onto a shallow limestone outcropping that was covered by sand of various depths. To test the hypothesis that sand limited the recruitment of L. virgulata at this site, I placed cement patio stones as settlement surfaces in different microhabitats. Sand scour reduced recruitment by 50% on buried stones compared with unburied controls. Recruitment was also inhibited by the presence of an algal mat on unburied controls. Initial recruitment patterns persisted for 10 mo on artificial substrata. By that time, dead, intact skeletons were more common on buried than on raised stones. Overall mortality rates did not differ between raised and buried treatments. In a 55—d transplant experiment, buried juvenile colonies grew significantly more than unburied colonies. Rapid growth may enhance survival of buried colonies by raising polyps out of the sand. I mapped the microhabitats in an untouched 24—m2 plot and constructed an index of habitat quality for each contiguous 1—m2 block. This index was highly correlated with numbers of both adult and juvenile Leptogorgia virgulata colonies. Sixty—seven percent of the 1984 recruitment cohort died the following year, but the spatial pattern established at the time of recruitment persisted. Although the abundance of recruits was correlated with habitat quality, there was no relationship between recruitment and adult abundance or the percent cover of encrusting organisms. Finally, recruitment was highest and least variable in patches of clean limestone compared with recruitment in randomly selected patches. On both natural and artificial substrata, the recruitment of Leptogorgia virgulata was affected by the distribution of sand. In spite of heavy 1st—yr mortality, the spatial pattern established at the time of recruitment persisted in the adult population. Mortality after recruitment did not modify the distribution of colonies to any great extent.