Sponges of the New Caledonian lagoon - IRD

30
Sponges
of the New
Caledonian
Lagoon
water slows down thus enabling the choanocytes to absorb available nutrients and
oxygen. Similarly, waste products are expelled into excurrent canals which have a
larger diameter, and finally jettisoned into the surrounding seawater through the
oscules which are larger than inhalant pores. The water flow in the inhalant and
exhalant canals varies from place to place, being fastest in the area of pores and
oscules and slowest in the areas of absorption (choanocyte region). Water circula­
tion can be further controlled by the sponge by diverting the flow into additional,
non-choanocytic chambers and sphincter-like sieves or filters, to slow down or
increase flow rates as required. Water flow can also be halted completely or redirected
to other parts of the body, if there is a particle overflow in the network, for
example. It is easy to observe this water flow using fluorescein injected into the
water system and subsequently ejected by the sponge. The generation of a strong
exhaled current through the oscule is important as it minimises contamination
between inhaled and exhaled water, the latter containing no food and molecular
waste of no further use to the sponge.
There is a great diversity of aquiferous systems in Porifera. Some are relatively
simple with short inhalant canals, a single choanocyte chamber and a single
oscule, but most are much more complex reflecting particular ecophysiological
adaptations. A hierarchy of complexity of the aquiferous system has its own terminology
asconoid, syconoid, leuconoid, from simple to complex. These various
modes of water canal systems were at one time considered to be the most important
criteria in the taxonomy and evolution of the class Calcarea, whereas it is now
recognised that these different grades of body organisation have developed independently
in several lines of calcareous sponges, now haVing little importance to
their taxonomy. In the oemospongiae all known species are of the complex (Ieuconoid)
construction, although recent discoveries indicate that some deep sea species
lack choanocytes and associated water canal systems completely, being
instead carnivorous.
The distribution and size of pores and oscules, and the drainage canals associated
with these are frequently related to the general shape of the sponge. The morphology
and organisation of the aquiferous system often clearly reflect adaptation of the
species to environmental hydrodynamic conditions. For example, in foliose
sponges the pores are often on one side and the oscules on the other. In cup or
vase-shaped sponges the pores are on the external surface and the oscules are