Limits on the bathymetric distribution ofkeratose sponges: a field test in deep water.

The keratose sponges (i.e. those in which the mineral skeleton is replaced by a collagenousskeleton) are generally restricted to shallow-water habitats, but the causes of this distinct bathymetricpattern remain unclear. Sharp pycnoclines at the depth of the upper slope may hinder colonizationof deep waters because of thermal stress or reduced light and particulate food below thepycnoclines. It is also possible that oligotrophy and loss of symbiotic cyanobacteria below the pycnoclinemay lead to a nutritional stress. Using manned submersibles in Exuma Sound, Bahamas, we determinedthat the pycnocline lies between 70 and 100 m. We transplanted individuals of 2 keratosesponges (Aplysina fistularis and Ircinia felix) from their natural habitat on a shallow reef (4 m deep) to3 depths (100, 200, 300m) within or below the pycnocline to investigate mortality and changes in bodysize, shape and histology as a function of depth. We also recorded changes in populations of photosyntheticand heterotrophic symbiotic bacteria, as well as the parasitic polychaete Haplosyllis spongicola.By transplanting individuals of A. fistularis bearing buds for asexual propagation (fistules) and individualsof 1. felix brooding embryos, we also tested the viability of reproductive propagules in deep-waterenvironments. 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 100m for 12 mo. No A. fistularis survived at200m, whereas 28.5% of I. felix did. All sponges transplanted to 300m died within 2 mo. Water temperaturewas the most likely cause of sudden mortality at this depth.