Sponges of the New Caledonian lagoon - IRD

32
Sponges
of the New
Caledonian
logoon
functional integration, such that any physicochemical information received by a cell
is transported within the sponge by contact reactions between non-specialised
cells. This process is very slow and is often localised. However. the sponge is not
completely without organisation as there is a lot of continuous local controls that
can have an effect on the whole sponge, such as contractile oscules and temporarily
halting water flow.
Because they have no organised tissues or central nervous system sponges are
usually less impacted by minor damage than other animals. A sponge partially
eaten by a predator may heal rapidly, with archaeocytes migrating to the site of
damage and transforming into specialised cells as required, and normal operation
will eventually resume in this region. It is also very easy to isolate sponge cells and
let them settle individually. Within a few hours these cells will reassemble randomly
and form small spheroid heterogeneous aggregates. These spheroidal aggregates
slowly reorganise themselves and spread out to form a new sponge. This remarkable
capability of Porifera to reorganise from dissociated cells is unique in the animal
world. It is an interesting experiment to combine dissociated cells from two or more
species and watch them sort themselves out into distinct species, illustrating the
molecular specificity of cell systems, and "self versus not-self" recognition (an
important concept in human immunology). This ability to recognise "self" is a key
phenomenon in the biological evolution of multicellular animals.
There are no "characteristic" shapes in sponges. as in many other animals. The
diverse morphological shapes seen amongst sponges are relatively consistent
within individual species and are largely genetically determined. Although these
growth forms are subject to phenotypic modification, in response to prevailing
environmental conditions, with few exceptions variability in shape is relatively well
defined. That is, modifications to 'typical' growth forms in particular species are
usually still recognisable as being those species. Sponges grow by marginal
extensions in all directions, vertically off the substratum or horizontally across it.
Encrusting sponges cover the substrate, with most growth directed horizontally,
maximising surface area, whereas massive sponges. including spherical and
cushion-shaped species, have predominantly vertical growth with a greater
volume to surface area ratio. Between these forms is a whole continuum of
shapes, erect, lobate, digitate, cups and fans. Shape is maintained by a mineral
and/or organic skeleton. The organic skeleton is supported by collagen fibrils,