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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 sponges |
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