Testing the Janzen-Connell hypothesis with fine-scale spatial and genetic analyses of the distribution of Caribbean giant barrel sponges
The Janzen-Connell hypothesis was developed to explain the processes that allow some ecosystems to support high levels of biodiversity; it states that the distribution of individuals is controlled by species-specific pathogens or predators that are attracted to adults or to their reproductive output. The distribution of the affected species displays a distinct pattern, with survivorship increasing at a given distance from the conspecific adult (negative density dependence), leaving a vacant area around the adult where other species can survive. While it was developed for tropical forests, this hypothesis has recently been applied to coral reef habitats. We are investigating Janzen-Connell predictions on Conch Reef, Florida, USA by examining the distribution of giant barrel sponges (Xestospongia muta) as a function of size using spatial point pattern analysis and developing a new method of object pattern analysis. To address the genetic relatedness of sponges as a function of distance and age, microsatellite data will be linked to the spatial distribution data.
Sponges now surpass corals as the dominant benthic organisms on many Caribbean reefs, and may provide a better system than corals for testing the Janzen-Connell hypothesis on tropical reefs. Despite the fact that X. muta is a long-lived and abundant species that is actively grazed by parrotfish and is susceptible to disease, neither the point pattern nor object pattern analyses supported Janzen-Connell predictions of sponge distribution. Polymorphic microsatellite regions have been identified for X. muta, providing the appropriate genetic variability to examine small-scale genetic relatedness of individuals across a single reef. The combination of spatial and genetic analyses will reveal the fine-scale distribution of X. muta while providing information on the mechanisms that are controlling sponge distribution.