Sponges of the New Caledonian lagoon - IRD

predators. Nor do many have adequate physical protection such as an armoured
surface, although some are thought to be capable of defending themselves using a
diverse array of noxious chemicals. These chemicals are also possibly active
agents in competition for space against other sedentary animals, such as corals
and ascidians.
Growth rates have been documented for a number of species, and are generally
slow in comparison to other sessile marine invertebrates. This is at least partially
due to the fact that most sponges are heterotrophs, obtaining their nutrients from filter
feeding suspended particles in the water column. Heterotrophic species may be
efficient in surviving high silt, high energy environments, like those found on the
floor of the coral reef lagoon with strong current and high sedimentation, but they
are poor competitors against the fast-growing autotrophic (phototrophiC) reef building
corals. In clear waters corals utilise nutrients produced by the photosynthetic activities
of their symbiotic zooxanthellae. There is also a special fauna of autotrophic
sponges that obtain some of their nutrients from the photosynthetic by-products of
symbiotic cyanobacteria (blue-green algae). Like reef building corals these autotrophic
sponges grow relatively quiCkly, being the dominant sedentary organisms in some
clear water reef habitats (e.g. reef flats), but this fauna has a very restricted distribution
confined to shallow, clear waters. Consequently, the mode of nutrition and the
presence or absence of symbiotic microorganisms clearly influences the competitive
advantage that a particular sponge may have, and where it can effectively grow
and survive.
Habitat
availability
Sponges are generally not evenly distributed within all habitats. Some sponges are
Widely distributed throughout adjacent habitats and are called opportunistic or
generalist species, whereas the greatest diversity of species appear to be 'specialists',
having very strict ecological requirements (e.g. only found in HaJimeda beds; on
shallow-water beach rock; burrOWing into mud; bioeroding dead coral; reef-bUilding
coralline species, etc.). Within any coral reef system, like New Caledonia, there are
many different habitats, and thus patterns of species distributions are generally
heterogeneous or patchy, largely determined by the presence or absence of these
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Sponge
diversity,
distribution ond
biogeography