BENTHIC MACROALGAE AS A DISPERSAL MECHANISM FOR FAUNA - INFLUENCE OF A MARINE TUMBLEWEED

Effective dispersal is problematic for benthic organisms without plank tonic larvae; rafting and vertical migrations are mechanisms that can potentially be employed by such fauna, but these strategies entail con siderable predation risk as well as other disadvantages. Unattached, b ut non-floating, ''drift'' algae harbor large numbers of fauna and may serve as an alternative dispersal mechanism in some systems. This pap er reports field manipulations in Florida Bay, Florida, USA designed t o determine (1) if such algae can disperse benthic animals, and (2) if dispersal efficiency varies as a function of two common substrata typ es: seagrass and bare sediment. A live immersion stain was used to mar k faunal associates of Laurencia spp. algal clumps in situ. The fideli ty of molluscs, decapods, ophiuroids, and fishes to stationary algal c lumps was then compared with the fidelity of these animals to clumps t hat were forced to tumble over a given distance with a blower apparatu s; these experiments were performed over both sand and seagrass substr ata. Measurements of frequency, spatial extent, and rate of algal drif t were made to aid in assessing the potential importance of benthic al gae as a dispersal mechanism. Algal clumps often rolled in a manner si milar to that of terrestrial tumbleweeds; mark-recapture work showed t hat algal clumps can move up to 0.5 km/day and that algal drift is a f requent phenomenon. The algal masses were effective transporters of be nthic fauna, including mobile shrimps and fishes; dispersal was more e fficient over sand than over seagrass. Dispersal of fauna via this mob ile habitat should entail lower risk than other adult dispersal strata gems such as vertical migration or rafting; this mechanism would be mo st advantageous for brooding species or those with limited planktonic phases. Differential fidelity to clumps tumbling across seagrass versu s sand suggests that the algae could facilitate exchange of fauna betw een isolated seagrass patches.