The Effects of Anthropogenic Stressors on Reproduction and Recruitment of Corals and Reef Organisms
The persistence of populations of marine organisms depends on the success of the dual processes of reproduction and recruitment. The production of offspring alone is inconsequential unless larvae and propagules can recruit, which often entails a period of development and distribution in the water column and subsequent selection of appropriate habitats. For fish, this may mean drifting in currents before responding to particular habitat cues. For corals and other benthic invertebrates, larvae must undergo site selection, settlement and metamorphosis into the juvenile form, and survivorship is directly linked to site choice and environmental conditions. Both biotic and abiotic factors affect population replenishment success, and hence, anthropogenic influences such as pollution, sedimentation and climate change can negatively affect critical processes such as reproductive synchronization in spawning species, successful embryological development, appropriate site selection, settlement, metamorphosis and in the case of reef building corals, acquisition of the required zooxanthellae partner. Effective management practices are essential for ensuring the persistence of populations of coral reef organisms of economic, cultural and ecological value.
Reproduction and Recruitment
Reproduction is the process by which new individuals are formed from parental stock, and can occur through asexual or sexual means. In corals, the most common type of asexual reproduction is via fragmentation, and often involves physical separation of a portion of the tissue-covered skeleton. This can occur as a result of fish feeding on associated crabs, shrimp, bivalves or barnacles, and the driver of recruitment “behavior” is gravity, that is, the fragment sinks, settles, attaches and hopefully survives. Other means of asexual reproduction can occur via tissue sloughing or polyp bail out, and the resulting ciliated tissue bits can disperse over greater distances and exhibit selective settlement behavior (Sammarco, 1982). Larvae can also be produced via parthenogenesis, and while genetically identical to the parent colony, exhibit classic larval behaviors. Sexual reproduction involves the fusion of sperm and egg (gametes) produced by the male and female of the species respectively. Sexual reproduction results in two opportunities for increasing genetic variability: one through the contributions of two different parents to the offspring and another through the “crossing over” among chromosomes that occurs during the prophase one stage of meiosis, the reduction division that makes haploid (1n) gametes that fuse to become a diploid (2n) organism. No matter how the progeny are formed, they do not contribute to the population unless they successfully recruit, and that is where behavior comes in.
The persistence of populations depends on the success of both reproduction and recruitment processes. With a focus on the importance of larval behavior for successful recruitment, the following discussion will center on sexual reproduction as the primary means of producing this critical life history stage for the perpetuation of coral reefs and related ecosystems. There are six chemically mediated steps involved in the replenishment of reef populations: gamete development within mature coral colonies, synchronization of gamete release, successful fertilization of eggs by sperm, complete embryological development to the competent larval stage, settlement and metamorphic induction, and acquisition of symbiotic zooxanthellae in recruits that do not have them vertically transmitted by the parent colony (Richmond, 2014; Figure 1). These key processes are affected by both density dependent and density independent factors.