17 – Sponges
183
in most sponges), produced by varying degrees of
infolding of the body wall and complexity of water
canals throughout the sponge. Adults are asymmetrical
or radially symmetrical, and have evolved an amazing
range of growth forms best described as highly irregular and sometimes completely plastic, frequently
altered by prevailing external conditions (currents,
turbidity, salinity etc.). Sponges also have evolved an
amazing array of colours, some linked to dietary carotenoid proteins and others with a photoprotection
functionality.
The current classification of the Porifera is based
primarily on features of the organic (collagen fibres and
filaments) and inorganic skeletons (discrete and/or
fused spicules composed of calcium carbonate or silicon dioxide), with some species also having a hypercalcified basal skeleton of solid limestone. The taxonomic
scheme is primarily morphologically-based, and as
complex as the diversity of sponges—the study of
sponge taxonomy is not for the faint-hearted. Applying
taxonomic principles to sponges is made even more
difficult by the occurrence of frequent character losses,
modifications and apparently convergent features reappearing within the classification. No attempt is made
here to provide more than a very basic summary, with
a list of further reading provided. There are three distinct classes of living sponges (plus a fourth extinct
one): Calcarea, having calcitic spicules with three or
four rays; Hexactinellida, with discrete and/or fused
siliceous spicules, the larger ones three or six rayed;
and Demospongiae, with siliceous spicules in many
(but not all) species, and/or a fibrous skeleton, and
spicules with one, two or four rays divided into megasclere and microsclere categories. Only Calcarea and
Demospongiae have so far been recorded from the
GBR, although Hexactinellida live in deeper waters on
the continental slope and shelf adjacent to the GBR. An
overview of the phylum, including a taxonomic revision and identification keys for approximately 25 orders, 127 families and 700 genera, has recently been
undertaken but species-level identifications remain appallingly difficult, with few easily accessible taxonomic
publications that would be useful to a non-specialist
audience. Further useful reading is listed below, including general reading on sponge biology, sponge cell
biology, a web checklist of the published Australian
sponge fauna (including Queensland species) with
keys to genera, a web list of all published sponge species worldwide, and sponge higher classification. The
recent escalation of the molecular study of sponges will
certainly have a major impact on our current ideas of
the phylogeny and classification of Porifera, and to this
end a Sponge Barcoding Project (based on a systematic
use of molecular tools) has commenced and is also
available on the web.
N REPRODUCTION AND LIFE HISTORY
Sponges utilise a number of reproductive strategies
based around their characteristic cellular totipotency.
Asexual reproduction involves the production of propagules such as buds and fragments containing a sufficient number of cells from which complete sponges can
develop. A few euryhaline species of the genus Mycale
produce gemmule-like bodies, but true gemmules are
restricted to freshwater sponges of the Order Haplosclerida. Most groups have considerable means of
asexual propagation, such as fragmentation from storm
events, which is thought to be an important mechanism
for sponge recruitment, and all have extensive regenerative powers that appear to be vital for sustaining local populations. Sponges have sexes that are separate,
or sequencially hermaphroditic, producing eggs and
sperm at different times. Although there are no gonads
or reproductive ducts, sexual reproduction involves
the production of gametes by the choanocytes and
totipotent archaeocytes, with fertilisation often (but not
always) internal. Individuals release sperm externally
via the exhalant current, whereas their oocytes reside
in the incurrent aquiferous system to minimise self fertilisation. Sperm are engulfed by choanocytes, which
become amoeboid, travelling to and transferring them
to the oocytes. Cleavage leads to a solid steroblastula or
hollow coeloblastula, with internally brooded, viviparous embryonic development in many cases, and larvae leaving the parent for dispersal. Other sponges are
oviparous, with females shedding their eggs externally
as zygotes or early embryo stages, rarely as unfertilised
oocytes, although the details of embryology still remain
unknown for most species. Other forms of development
183
in most sponges), produced by varying degrees of
infolding of the body wall and complexity of water
canals throughout the sponge. Adults are asymmetrical
or radially symmetrical, and have evolved an amazing
range of growth forms best described as highly irregular and sometimes completely plastic, frequently
altered by prevailing external conditions (currents,
turbidity, salinity etc.). Sponges also have evolved an
amazing array of colours, some linked to dietary carotenoid proteins and others with a photoprotection
functionality.
The current classification of the Porifera is based
primarily on features of the organic (collagen fibres and
filaments) and inorganic skeletons (discrete and/or
fused spicules composed of calcium carbonate or silicon dioxide), with some species also having a hypercalcified basal skeleton of solid limestone. The taxonomic
scheme is primarily morphologically-based, and as
complex as the diversity of sponges—the study of
sponge taxonomy is not for the faint-hearted. Applying
taxonomic principles to sponges is made even more
difficult by the occurrence of frequent character losses,
modifications and apparently convergent features reappearing within the classification. No attempt is made
here to provide more than a very basic summary, with
a list of further reading provided. There are three distinct classes of living sponges (plus a fourth extinct
one): Calcarea, having calcitic spicules with three or
four rays; Hexactinellida, with discrete and/or fused
siliceous spicules, the larger ones three or six rayed;
and Demospongiae, with siliceous spicules in many
(but not all) species, and/or a fibrous skeleton, and
spicules with one, two or four rays divided into megasclere and microsclere categories. Only Calcarea and
Demospongiae have so far been recorded from the
GBR, although Hexactinellida live in deeper waters on
the continental slope and shelf adjacent to the GBR. An
overview of the phylum, including a taxonomic revision and identification keys for approximately 25 orders, 127 families and 700 genera, has recently been
undertaken but species-level identifications remain appallingly difficult, with few easily accessible taxonomic
publications that would be useful to a non-specialist
audience. Further useful reading is listed below, including general reading on sponge biology, sponge cell
biology, a web checklist of the published Australian
sponge fauna (including Queensland species) with
keys to genera, a web list of all published sponge species worldwide, and sponge higher classification. The
recent escalation of the molecular study of sponges will
certainly have a major impact on our current ideas of
the phylogeny and classification of Porifera, and to this
end a Sponge Barcoding Project (based on a systematic
use of molecular tools) has commenced and is also
available on the web.
N REPRODUCTION AND LIFE HISTORY
Sponges utilise a number of reproductive strategies
based around their characteristic cellular totipotency.
Asexual reproduction involves the production of propagules such as buds and fragments containing a sufficient number of cells from which complete sponges can
develop. A few euryhaline species of the genus Mycale
produce gemmule-like bodies, but true gemmules are
restricted to freshwater sponges of the Order Haplosclerida. Most groups have considerable means of
asexual propagation, such as fragmentation from storm
events, which is thought to be an important mechanism
for sponge recruitment, and all have extensive regenerative powers that appear to be vital for sustaining local populations. Sponges have sexes that are separate,
or sequencially hermaphroditic, producing eggs and
sperm at different times. Although there are no gonads
or reproductive ducts, sexual reproduction involves
the production of gametes by the choanocytes and
totipotent archaeocytes, with fertilisation often (but not
always) internal. Individuals release sperm externally
via the exhalant current, whereas their oocytes reside
in the incurrent aquiferous system to minimise self fertilisation. Sperm are engulfed by choanocytes, which
become amoeboid, travelling to and transferring them
to the oocytes. Cleavage leads to a solid steroblastula or
hollow coeloblastula, with internally brooded, viviparous embryonic development in many cases, and larvae leaving the parent for dispersal. Other sponges are
oviparous, with females shedding their eggs externally
as zygotes or early embryo stages, rarely as unfertilised
oocytes, although the details of embryology still remain
unknown for most species. Other forms of development