scientific chapter |
Sponaugle S, Cowen RK
Mesophotic coral ecosystems (MCEs) have the potential to supply larvae to help sustain spatially discrete shallow-reef populations (Deep Reef Refugia Hypothesis); however, for this to be viable, mesophotic populations must be ecologically connected to shallow-reef populations. Three primary criteria for successful connectivity are: (1) robust populations of shallow-reef species must co-occur at mesophotic depths, (2) shallow and mesophotic habitats must be physically connected by currents, and (3) life history traits of organisms must enable successful delivery of viable larvae from mesophotic to shallow reefs. One such MCE, Pulley Ridge, is located on the west Florida shelf and supports populations of algae, sponges, corals, and fishes, some of which co-occur in mesophotic and shallow reefs. For organisms with short larval durations (hours to <10 days), such as many corals, Pulley Ridge is less likely to function as a larval source for shallow reefs. Genetic differentiation among depth-stratified coral populations in the Florida Keys confirms this reduced connectivity. In comparison, a population of a model fish species, the bicolor damselfish, at Pulley Ridge invests more heavily in reproduction than their shallow-reef counterparts, and biophysical modeling demonstrates that they can seed downstream, shallow reefs. Thus, the degree to which Pulley Ridge can serve as a refuge varies by taxon. For many key reef species, such as corals, population connectivity may be insufficient for Pulley Ridge to serve as a regular larval source. However, for fishes, including the invasive lionfish, Pulley Ridge may already be serving as a source of larvae for shallow-reef populations.