The role of larval supply and competition in controlling recruitment of the temperate coral Oculina arbuscula

Abstract

Recruitment success in sessile benthic invertebrates that produce pelagic larvae is an emergent property of larval supply, settlement rates, and post-settlement survival. While intuitive that larval supply and settlement rates should be positively correlated, previous studies have demonstrated that many factors, such as competition for limited space on hard substrata, may decouple this relationship. Oculina arbuscula is a temperate, broadcast spawning coral and is the only structurally complex Scleractinian occurring on hard bottom reefs of the South Atlantic Bight (SAB), a region located in the North Atlantic Ocean off the southeastern coast of the U.S.A. Planula larvae of this species must settle on hard-bottom substrate that is densely colonized by a diversity of sessile benthic invertebrates, including sponges, bryozoans, and ascidians. To investigate the relative roles of larval supply and competition for space on the recruitment dynamics of O. arbuscula, settlement to and survival on 30 × 30 cm plots was monitored over 5 years on a reef off the coast of Georgia, U.S.A. Treatments consisted of 10 replicates of each of the following: 1) unmanipulated natural substrate, 2) natural substrate initially cleared of encrusting competitors such as sponges and tunicates, and 3) artificial substrate composed of concrete paving tiles. These plots were photographed 3–5 times per year between July 2004 and June 2009. These images showed that O. arbuscula recruits throughout the year in the SAB with a peak in September/October. While recruitment rates were higher than death rates in all treatments and resulted in a net gain of O. arbuscula colonies, recruitment to artificial substrate far-exceeded that found on natural surfaces. The high recruitment rates observed on artificial substrate ruled out the possibility that maintenance of O. arbuscula populations on natural surfaces is limited by larval supply. Competition with other sessile invertebrates, such as ascidians and sponges, also appeared to contribute to differences in recruitment among treatments, however, comparisons between cleared and unmanipulated control plots on natural substrate suggested that the competition effect was small. Based on these findings as well as anecdotal observations suggesting that sediment accumulates less on the artificial substrate, the alternative hypothesis that O. arbuscula populations in this system are controlled by physical factors, especially sedimentation, is proposed and warrants further investigation.