Sponges of the New Caledonian lagoon - IRD

.,
Matrix cyanobacleria
(photo J. Vacelet)
within the cavity on the concave side. In most encrusting and massive species
oscules are placed on the ends of small stalks or conules, above the level of the
surface and away from the pores, to allow the excreted water to be flushed away
from the sponge.
Within the sponge the living 'tissue', bounded on all sides by the pinacoderm, is
called the mesohyl, which includes all the area between the pinacocyte layers, or
between pinacocytes and choanocytes, and between the canals and choanocyte
chambers. The mesohyl contains a matrix or ground substance composed of a
striated protein called collagen, an organic skeleton composed of spongin fibres,
and/or an inorganic skeleton composed of mineral spicules. Within this mesohyl
are found mobile totipotent cells, capable of changing function as required. Cell
types can vary between taxonomic groups, although their recognition and cytological
taxonomy is still poorly understood. These cells include generalised amoebocytic
'stock' cells (archaeocytes) with large lobopods capable of active phagocytosis,
as well as many other types that have become specialised to carry out particular
functions within the sponge. Cells that produce the precursors of spongin
fibres (collencytes) have filopods; those that secrete spicules (sclerocytes) are
capable of incorporating silica or provoking a calcium deposit, and these migrate
to the area within the mesohyl where the mineral skeleton is being deposited;
contractile cells occur around excurrent oscules (myocytes). Of the many sorts of
cells in sponges, known by their characteristic shape and organelles, only few so
far have a known function or chemical structure. Attempts have been made to
use these cytological characters in a taxonomic framework but with limited
success. In addition to these native cells many species have symbiotic bacteria
and cyanobacteria which play an important role in sponge nutrition in these
species.
The distribution of cells within the mesohyl, the deposition of collagen and location
of water current canals are more or less homogeneous within the sponge. In some
massive sponges, however, it is possible to see a differentiated external layer several
millimetres thick, which contains no choanocyte chambers. This ectosomal
layer is strengthened by collagen fibrils and often also a special mineral skeleton,
clearly differentiated from the deeper choanosomal region. AS a consequence of
lacking developed tissues or organs sponges also lack a nervous system, nerve
cells, coordination centre, head end or brain. They have only a low level of cellular
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Sponge
struc:ture