Perilous proximity: Does the Janzen–Connell hypothesis explain the distribution of giant barrel sponges on a Florida coral reef?

Abstract

One popular concept used to explain the high biodiversity of some ecosystems is the Janzen–Connell hypothesis, which states that the distribution of conspecifics is controlled by species-specific pathogens or predators that are attracted to adults or to their reproductive output. The distribution of the affected species would then display a distinct pattern, with survivorship increasing at greater distance from the conspecific adult (negative density dependence), leaving a vacant area around the adult where other species can survive. The giant barrel sponge, Xestospongia muta, is an abundant and long-lived sponge on Caribbean coral reefs that is actively grazed by sponge-eating fishes and is susceptible to disease. We tested the Janzen–Connell hypothesis on barrel sponges on Conch Reef, Florida, by examining their distribution as a function of size using spatial point pattern analyses. Clark and Evans tests and a series of Ripley’s K function analyses revealed no consistent distribution pattern, with most analyses resulting in a random pattern of sponge distribution. While predation by sponge-eating fishes has recently been discovered to structure sponge communities on reefs across the Caribbean, these top-down effects do not translate to spatial distributions of X. muta that support Janzen–Connell predictions.