The keratose sponges (1.e. those in which the mineral skeleton is replaced by a collagenous skeleton) are generally restricted to shallow-water habitats, but the causes of this distinct bathymetric pattern remain unclear. Sharp pycnoclines at the depth of the upper slope may hinder colonization of deep waters because of thermal stress or reduced light and particulate food below the pycnoclines. It is also possible that oligotrophy and loss of symbiotic cyanobacteria below the pycnocline may lead to a nutritional stress. Using manned submersibles in Exuma Sound, Bahamas, we determined that the pycnocline lies between 70 and 100 m. We transplanted individuals of 2 keratose sponges (Aplysina fistulans and Ircinia felix) from their natural habitat on a shallow reef (4 m deep) to 3 depths (100, 200, 300 m) within or below the pycnocline to investigate mortality and changes in body size, shape and histology as a function of depth. We also recorded changes in populations of photosynthetic and heterotrophic symbiotic bacteria, as well as the parasitic polychaete Haplosyllis spongicola. By transplanting individuals of A. fistularis bearing buds for asexual propagation (fistules) and individuals of I. felix brooding embryos, we also tested the viability of reproductive propagules in deep-water environments. We found that, although these 2 sponges do not naturally occur at depths below 40 m, 62.5% of A. fistularis and 42.8% of I. felix survived at 100 m for 12 mo. No A. fistularis survived at 200 m, whereas 28.5 % of I. felix did. All sponges transplanted to 300 m died within 2 mo. Water temperature was the most likely cause of sudden mortality at this depth. There were no significant differences in growth between individuals at the slope and controls on the shallow reef. Cyanobacteria were lost in individuals of I. felix that survived at 100 and 200 m, and these sponges repositioned oscules and formed chimney-like processes, probably to enhance water flow through the sponge and compensate for nutritional stress. By contrast, cyanobacteria were still abundant in individuals of A. fistularis surviving at a depth of 100 m, and these sponges did not change shape significantly, apart from the loss of fistules. It appears, therefore, that the loss of cyanobacteria and the increasing oligotrophy with depth do not set the lower bathymetric limits of species. Removal of sponge tissues by the parasitic polychaete H. spongicola also appears not to aggravate significantly the nutritional stress experienced by sponges transplanted to deep water, at least to the extent that it may restrict the bathymetric distribution of the host. Despite the facts that only the species I. felix was heavily parasitized and that parasites survived within hosts at all depths, there was no significant difference in survival with depths between sponge species. A TEM (transmission electron microscope) examination of the mesohyl did not reveal significant cytological differences among sponges transplanted to various depths. At all depths, surviving individuals of both species showed archeocytes engaged in phagocytosis and digestion of cyanobacteria and/or heterotrophic bacteria. Similarly, collencytes and spongocytes were apparently secreting collagen, indicating that temperatures at 100 and 200 m do not inhibit the formation of the skeleton. Sponge recruitment derived from either asexual or sexual propagules was never observed at slope depths. Since adult sponges survived when they were artificially transported to deep waters, the inhibition of larval dispersal or settlement success (perhaps caused by the sharp decrease in temperature With increasing depth) emerges as the most plausible explanation for the shallow-water confinement of these keratose spongesThis research was funded by a NOAA grant (CMRC-95-3047) and has also benefited from funds of a Postdoctoral Fulbright fellowship to M.M. and a DCCYT grant (PB-94-0015-C02-01). This is Harbor Branch contribution number 1236Peer reviewed