Sponges of the New Caledonian lagoon - IRD

whiCh are always present irrespective of other components, and often also with
spongin fibres. Commercial "bath" sponges have a reticulate skeleton of spongin
fibres and once treated and cleaned of sand and other foreign particles, this is all
that remains in the commercial product.
The mineral skeleton in Porifera is composed of hydrated or opaline silica (Si0 2 ),
characteristic of the Demospongiae and Hexactinellida, or calcite (CaC0 3 ) found
in the Calcarea. There are some demosponges, previously referred to as
"sclerosponges", with hypercalcified skeletons containing both a rigid calcitic basal
skeleton as well as free siliceous spicules. During bygone eras hypercalcified
species were particularly important reef builders, as can be seen easily in the exposed
Devonian fossil reefs of central Australia (older than 360 mya). Today hypercalcified
sponges remain important contributors to the process of reef accretion, especially
in reef cave and deeper water faunas.
Apart from the relatively few living hypercalcified species sponge skeletons are
mostly composed of discrete elements called spicules, with amazing morphologies
and size ranges, and the skeletal patterns sponges construct have been used as
models of strength by many architects. Very often there is a mixed skeleton, with
spicules associated in many ways with spongin, enveloped within it, protruding
from it or completely outside of it. Some spicules are clearly functional, especially
the large ones that provide structure and strength to the body, whereas the functionality
of other smaller spicules is often less obvious. Some sponges are able to
leave part of their skeleton behind during growth, bUilding a new one in a new area,
and this gives the impression of mobility. Other species may change from a basal
skeleton to an apical-based growth strategy. Some hypercalcified sponges have
lost their free siliceous spicules and only produce a basal, compacted, reticulate
skeleton of calcite or aragonite. Other species have only collagen in their skeleton,
lacking a mineral skeleton completely.
Sponges have a great range of both sexual and asexual reproductive mechanisms.
In sexual reproduction some species are gonochoristic with separate sexes,
with individuals producing either eggs or sperm, and some have alternating sexes
(protandrous hermaphrodites). Many sponges are OViparous, synchronously releasing
male and female gametes into the sea, where fertilisation and development are
external to the parent. Other species have internal cross-fertilization, whereby
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Sponge
structure