Page 278 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 265
Despite the flexibility of a typical bath
sponge, sponges are skeletal organisms. Skel-
etons are composed of a colloidal jelly or
spongin, a horny organic material; calcareous
or siliceous spicules may occur with or without
spongin. These structures support the body
shape and provide a framework for the rather
disparate cells of the sponge. In simple terms,
the sponge animal functions as a colonial,
loosely-integrated protist, but with a higher
degree of physiological integration.
Three basic levels of chamber organization
have been recognized among the sponges (Fig.
11.2), and these provide a useful guide to
their shape. The simple ascon sponges are sacs
with a single chamber lined by fl agellate cells,
whereas the sycon grade has a number of
simple chambers with a single central para-
gaster. The leucon grade is the most common
where a series of sycon chambers access a
large central paragaster.
Autecology: life as a sponge
Sponges are part of the sedentary benthos,
with large exhalant openings, communicating
upwards with the water column. When not
resting, the sponge sucks in water through its
upward-facing ostia, forming inhalant cur-
rents; material is then pumped out of the
animal though the exhalant opening. The
group is entirely aquatic, living attached in a
range of environments from the abyssal depths
of oceans to the moist barks of trees in the
humid tropics. Most Paleozoic and early
Mesozoic forms have been collected from
shallow-water environments, although like
many other groups they expanded into deep-
water environments during the Ordovician
where they remained an important part of the
benthos.
Today, sponges occupy a wider range of
environments than in the past. Modern hexac-
tinellids prefer depths of 200–600 m, proba-
bly extending down onto the abyssal plains
and into submarine trenches, whereas the cal-
careous sponges are most common in depths
of less than 100 m. The modern calcifi ed
sponges are either deep-reef or, more often,
cave dwellers, lurking in the shadows of sub-
marine crevices at depths of 5–200 m, mainly
in the Caribbean although the group occurs
elsewhere, including the Mediterranean. The
meadows of Antarctic cold-water sponges can
comprise up to 75% of the living benthos in
seas under the ice sheets.
Sponges use a variety of substrates. Clionid
sponge borings, producing the trace fossil
Entobia (see p. 523) in mollusk shells, have a
long geological history and today Cliona is
commonly associated with many oyster beds.
Spicules themselves can form mat-like sub-
strates that when colonized form local pockets
of biodiversity. Although almost all sponges
are fi xed filter feeders, some deep-water forms
are carnivorous: their long barbed spicules
entangle fish and arthropods, and the sponge
tissue rapidly grows over the prey to digest it.
Moreover some encrusting sponges can crawl
slowly over the surface in search of food. Few
predators attack sponges, although some
fishes, snails, starfish and turtles have been
observed eating their soft tissues in the tropics;
and some organisms have used sponges as a
refuge, including hermit crabs, while dolphins
sometimes use sponges to protect their snouts
when investigating crevices.
Synecology: sponges and
sponge reefs through time
Sponges and corals are the major components
of modern and ancient reefs (Wood 1990).
The first sponges probably appeared in the
Late Proterozoic as clusters of fl agellate cells.
But the evolution of the main groups of fossil
sponges is intimately related to their partici-
pation in reef ecosystems (Fig. 11.6). Particu-
lar grades of organization were suited to
special environmental conditions and sponges
can possess a rigid, reef-building skeleton by
the fusion of strong spicules or by the devel-
opment of an additional basal calcareous
skeleton. The Cambrian sponge fauna, of
thin-walled and weakly-fused spiculate demo-
sponges and hexactinellids together with early
calcisponges, is mainly cosmopolitan, having
a wide geographic distribution. In contrast,
Ordovician sponge faunas are characterized
by the heavier, thick-walled demosponges that
continued to dominate Silurian faunas in car-
bonate environments; siliciclastic facies were
dominated by the hexactinellids. The demo-
sponges, however, became less important as
the stromatoporoids together with rugose and
tabulate corals began to sneak into these sorts
of niches. Hexactinellids were locally abun-
dant during the Late Devonian, and in the