Sponges of the New Caledonian lagoon - IRD

44
Sponges
of the New
Caledonian
Lagoon
Spongin fibres
and filaments
In several orders of sponges
the mineral skeleton has
been last completely, and
lar these groups fibre
characteristics are importont
in their classification. In
other groups, where there is
both spongin fibres and
spicules, the latter may be
portiallyor fully contained
inside the former, and thus
the skeletal architecture is
predominontly dictated by
the form of the organic
skeleton. In some groups
(e.g. some Hoplosclerida)
there ore no fibres but
spicules ore cemented
together with granular
collagen. Mostly, though,
spongin fibres are useful
in identification.
Spongin fibres vary both in
o hierarchy of size and
construction. Three size
categories of fibres are
generolly recognised
[primory, secondory, tertiory
fibres!. sometimes
differentiated by both size,
construction, and
the moteriol contoined
within eoch type of fibre.
In addition to these fibres
some groups hove collagen
filaments (eg Ircinio!.
which are long, thin,
convoluted, terminally
swollen collagenous
structures dispersed within
the mesahyl.
Several other classes of fibre
construction are recognised,
based an the amount of
spongin protein deposited
when the fibre was
secreted, and whether or
not this spongin was
deposited evenly
Ihomogeneous fibres) or
periodically Istrotilied fibresl
Sponges with heavy
spongin fibres, often termed
'horny' or 'kerotose'
sponges, belong to
the orders Dictyocerotido,
Dendrocerotido ond
Verongido. The most simple
fi bres are homogeneous in
cross section without
o centrol core [or visible
pithlle.g. Spongia!.
whereas the most'complex'
fibres are stratified in cross
section, composed of
concentric rings of protein
I'bork'!, with on optically
diffuse pith in the centre
of eoch libre
le.g. Aplysinal. Intermediate
forms are also common,
such os Iaund in species
of Thorec/a with slightly
stratified [laminated) fi bres
Inot bark-like!.
with 0 granular pith.
Mineral skeleton
The inorganic or mineral
skeleton is traditionally
the most important feature
for identifying sponges.
Th is skeleton moy consist of
a fused, coral-like basal
skeleton and/or individual
components called spicules
o Basal skeleton
Some groups of sponges
secrete 0 secondory,
colcareous Ihypercolcified!.
spicular bosol skeleton, in
addition to free siliceous or
calcitic spicules. This feature
was once considered
diagnostic for a class of
sponges known as
"sclerosponges", but is now
interpreted as a grade of
construction lound within
both Calcarea ond
Demospongiae. The species
concerned le.g. As/roscleral
usually live in coral reefs
and their calcareous
skeletons contribute in a
minor way to the overall
accretion of these reefs.
o Spicules
These are classified according
to five major criteria:
1 Chemical composition ­
These may be silicate or
calcitic, indicating division
between the classes
Demospongioe
ond Calcarea.
2. Spicule size -
larger spicules, called
megosderes, contribute to
the skeletal fromework within
the sponge, whereas smaller
ones, microscleres,
are packed between tracts
of megoscleres, supporting
the soft parts. Spicule sizes
are essentiol criterio in
defining species, in some
examples providing the only
easy clues to distinguishing
related species, whereas
absolute spicule dimensions
are less important at higher
taxonom ic levels.
3. Spicule fusion -
Most spicules ore free within
the mesohyl or bound
together by the orgonic
skeleton, whereas some are
characteristically fused
together, producing
an interlocking or articuloted
skeleton. These spicules
consist of rigid monospicular
skeletons composed of
Hypercalcified I)asal ('sclerosponge') skeleton
(photo J.N.A. Hooper)