Impact of Crustose Clionid Sponges on Caribbean Reef Corals

Abstract

Live and dead thalli of crustose coralline algae were examined to evidentiate their endolithic flora. As it occurs in corals, there is a great difference between endolithic microorganisms observed in live thalli and those observed in dead thalli. During our study live thalli were found to have few (Plectonema terebrans and Ostreobium quekettii) or no endolithic microorganisms, whereas a more numerous number of microorganisms (cyanobacteria, chlorophyta and fungi) was found in dead thalli. Some species of limestone-excavating Porifera (Clionidae, Hadromerida) cover their substrate as a thin, continuous, veneer-like crust (beta stage). This film of tissue is the result of fusion of the initially discrete incurrent and excurrent papillae (alpha stage) that are the common and lifelong morphological feature of most other representatives of the family. In the tropical and subtropical western Atlantic, at least four species of clionids encrust reef-coral skeletons (Scleractinia, Hydrozoa): Cliona caribbaea CARTER (including C. aprica PANG and C. langae PANG), C. delitrix PANG, C. lampa de LAUBENFELS, and C. varians (DUCHASSAING and MICHELOTTI) (=Anthosigmella varians of authors). One conspicuous feature of these encrusting sponges is that many border live coral or cover recently dead coral as indicated by the clear outline of the coral calicular structure under the thin sponge veneer. Field experiments and histological study conducted on Cliona caribbaea in Belize and Cliona lampa in Bermuda indicate that the sponges overpower stressed coral which they overgrow at a fast rate. Stress parameters include extended periods of above-average water warming or below-average water cooling, excess of suspended sediments, organic pollution, and physical damage inflicted by fish bites, anchors, and other means. Clionids do not seem to produce toxic compounds that affect virile coral colonies and are repelled by healthy coral polyps. Overgrowth is accomplished by excavating coral calyces from below the surface, thus depriving the polyps of their support, or by covering skeletons of dying coral. Overpowering corals by bioerosion is a successful competitive mechanism that works also on unstressed coral, albeit at a much slower pace. On the other hand, also non-boring encrusting sponges may overgrow coral by lateral spreading, for instance the symbiotic (with cyanobacteria) species of Chondrilla and Terpios, particularly if sponge growth is stimulated and coral resistance weakened by elevated levels of pollution. There are indications that encrusting clionids and other sponges may dramatically change the community structure and physical stability of shallow reefs that are readily compromised by natural or anthropogenic pressures.