Phylum: Porifera (sponges)
Porifera (sponges) differ from other animals in that they do not exhibit
true tissue development but rather have a cellular level of
organisation. They are simple in body form, having a sac-like body which
is perforated by many pores, yet complex at the cellular level having
several types of cells. The outer layer of the cells wall contains
flattened epidermal cells, some of which have contractile fibers; the
middle layer is a semifluid matrix with wandering amoebocytes; and the
inner layer is composed of flagellated cells called collar cells (or
choanocytes) that look like protozoans. One could thus argue that
sponges are composed of colonies of protozoans living in a symbiotic
relationship with each other.
sponges are sessile, suspension feeding animals with radial or no
symmetry. Only their sperm and larvae are motile and once a larva
settles it attaches itself to a substrate and develops into an adult.
Sponges are filter feeders and are the only animals in which digestion
occurs within cells. They may be as tiny as a finger nail to as large as
a person. Even though many evolutionary biologists believe them to be an
evolutionary dead end, sponges are one of nature’s success stories.
There are over 5000 species of sponges known globally and are generally
classified according to their skeletal structures. Their body shape is
maintained by possessing spicules, made of calcium carbonate or
silicate, or collagenous fibers known as spongin. The class Calcarea
possesses calcium carbonate spicules and members thereof are exclusively
marine. The class Hexactinellida or glass sponges, primarily found in
deep marine habitats, possesses spicules composed of silica. The class
Demospongiae, found in marine, brackish and fresh water environments,
possesses both spicules and spongin. At present 314 Poriferan species
are known from South Africa. This species diversity compares well with
most regions that have well documented sponge faunas but is
unexceptional when compared to Australia (2426 species), although this
later comparison cannot strictly be made due to the vast differences in
sampling effort, area and habitat diversity.
are largely predated upon by nudibranchs. In turn they filter out
microscopic food particles from the water (taken in through pores in
their body walls) that gets pumped out via larger exhalent openings
called oscula. As a group they are generally protected against predators
by their spicule skeletons, while others slime up or may produce toxins.
In fact, some nudibranchs, which feed on certain sponges, have the
ability to deter predators because they assimilate these sponge toxins
into their own flesh after feeding on them.
Sponges are able to form unusual
interactions and associations with other
They may bore into the shells of bivalves, gastropods, and the colonial
skeletons of corals by slowly etching away chips of calcareous material.
Larvae may settle on mollusc shells that are being used by hermit crabs
(eg., sponge living on the hermit crab ) and then develop into adult
sponges by engulfing the shell completely. Crustaceans like the cryptic
sponge crab, Cryptodromiopsis spongiosa, may cut off pieces of
unpalatable sponge and hold it over its body with its modified fifth
pair of legs. Sponge/crab associations result in sponges that are no
longer sessile. These exclusive, species specific, associations can
accurately be used by field observers to identify certain species in
situ (in the field).
The original bath sponge was
derived from specimens of sponge that have skeletons composed entirely
of spongin fibers. However, most modern sponges are made of synthetic
substances, but in certain areas of the world sponges are still
harvested for this industry. More recently sponges, along with many
other organisms, have been discovered to be sources of biomedical
compounds. Spongistatin 1 is one such compound which has exhibited
antimitotic properties, stunting the growth of cancerous tumors. Sponges
are also valuable indicators of marine environmental health.
Classification of southern African sponges
Branch, G. and Branch, M. 1981. The Living Shores
of Southern Africa. C. Struik Publishers, Cape Town.