The Great Barrier Reef
184
have also been recorded, such as elimination of the
free-swimming larval stage and embryos brooded in
the maternal sponge before being expelled as young
adults. Eight different larval types are known but few
of these have been adequately investigated. Most embryos develop into free-swimming (lecithotrophic) or
demersal crawling larvae, ciliated to a greater or lesser
extent, 0.05–5.00 mm long, with a brief planktonic
phase, short longevity (maximum of 72 hrs recorded),
and, unlike most marine invertebrates, have no planktotrophic stage.
Release of propagules (gametes, zygotes or embryos) is asynchronous in viviparous species but
highly synchronous in oviparous sponges, triggered
by factors such as temperature and lunar cycle. A
prominent member of the GBR sponge community,
Xestospongia testudinaria, is oviparous and broadcasts
eggs in spawning events that were synchronised
among populations of the same species, with timing
found to be correlated with the lunar cycle. Molecular
studies of individuals in local sponge populations
show that most have high levels of genetic variability,
not high genetic relatedness as would be expected if
asexual recruitment was predominant, with some evidence that both asexual and sexual propagules are important for population structure, whereby sponge
fragments that disperse and reattach may contain incubated sexual propagules.
Larval settlement and metamorphosis is thought
to be influenced by a variety of environmental stimuli
(such as light, gravity, physical and chemical features
of the substrate), with the former best studied to date.
There are examples of both photonegative and photopositive responses among the phylum. Larval competence (the threshold and duration of larval maturity
required for settlement) is not thought to be as important for sponges as for many invertebrates since the
high cellular totipotency allows fragmented larvae to
attach unselectively. Growth rates, regenerative abilities after damage, and longevity is still poorly understood, but what little is known to date demonstrates
that these vary considerably across groups of sponges
and the habitats they occupy. Using various direct
(C 14 ) and indirect measurements (e.g. growth rate
extrapolation indices), some species are known to
reproduce and die in less than one year (such as some
soft-bodied Haliclona spp.), or are highly seasonal in
their growth, biomass and ultimately survival (Chondrilla australiensis); some species live for many decades
(Aplysilla sp.), to over 400–500 years (Astrosclera willeyana) and it is claimed that sponges belonging to the
hexactinellid family Rosellidae living in Antarctica are
among the oldest living animals on the planet, with
individuals estimated at 1515 years old.
N FEEDING
Sponges filter sea water to eat, exchange gases and excrete waste products. Filtration is an active process
involving choanocytes lining chambers. Each choanocyte has a central flagellum that actively beats to create a water current, surrounded by a collar of cilia that
traps food particles such as plankton and bacteria, as
well as detritus. A water current containing food enters the sponge through an osculum and is initially
filtered through a series of sieve-like pores (diminishing in size), finally ending up at the collar cells. Food
particles are actively carried across the cell wall, engulfed by archaeocytes, and are transferred throughout the mesohyl to other cells. Filtered water leaves
the sponge via the exhalant canal system. Unlike most
multicellular animals, digestion and excretion of
waste products occurs within cells, not within any
common body cavity. There may be 7000–18 000 choanocyte chambers per cubic millimetre of sponge, and
each chamber may pump approximately 1200 times
its own volume of water per day. Thus, a sponge is
capable of pumping around 10 times its body volume
each hour, making them the most efficient vacuum
cleaners of the sea. Some sponges, particularly those
growing on coral reef flats, also have a unique symbiosis with cyanobacteria, providing the sponge with
nutrients derived from photosynthesis to supplement
those obtained by the sponge from normal filter feeding activities (phototrophy or autotrophy). These extra nutrients greatly augment sponge growth rates
and competitive ability in coral reef systems. There
are also often huge populations of bacteria and Archaea living within sponge cells and/or within the
sponge mesohyl (hence the term ‘sponge hotels’), with
184
have also been recorded, such as elimination of the
free-swimming larval stage and embryos brooded in
the maternal sponge before being expelled as young
adults. Eight different larval types are known but few
of these have been adequately investigated. Most embryos develop into free-swimming (lecithotrophic) or
demersal crawling larvae, ciliated to a greater or lesser
extent, 0.05–5.00 mm long, with a brief planktonic
phase, short longevity (maximum of 72 hrs recorded),
and, unlike most marine invertebrates, have no planktotrophic stage.
Release of propagules (gametes, zygotes or embryos) is asynchronous in viviparous species but
highly synchronous in oviparous sponges, triggered
by factors such as temperature and lunar cycle. A
prominent member of the GBR sponge community,
Xestospongia testudinaria, is oviparous and broadcasts
eggs in spawning events that were synchronised
among populations of the same species, with timing
found to be correlated with the lunar cycle. Molecular
studies of individuals in local sponge populations
show that most have high levels of genetic variability,
not high genetic relatedness as would be expected if
asexual recruitment was predominant, with some evidence that both asexual and sexual propagules are important for population structure, whereby sponge
fragments that disperse and reattach may contain incubated sexual propagules.
Larval settlement and metamorphosis is thought
to be influenced by a variety of environmental stimuli
(such as light, gravity, physical and chemical features
of the substrate), with the former best studied to date.
There are examples of both photonegative and photopositive responses among the phylum. Larval competence (the threshold and duration of larval maturity
required for settlement) is not thought to be as important for sponges as for many invertebrates since the
high cellular totipotency allows fragmented larvae to
attach unselectively. Growth rates, regenerative abilities after damage, and longevity is still poorly understood, but what little is known to date demonstrates
that these vary considerably across groups of sponges
and the habitats they occupy. Using various direct
(C 14 ) and indirect measurements (e.g. growth rate
extrapolation indices), some species are known to
reproduce and die in less than one year (such as some
soft-bodied Haliclona spp.), or are highly seasonal in
their growth, biomass and ultimately survival (Chondrilla australiensis); some species live for many decades
(Aplysilla sp.), to over 400–500 years (Astrosclera willeyana) and it is claimed that sponges belonging to the
hexactinellid family Rosellidae living in Antarctica are
among the oldest living animals on the planet, with
individuals estimated at 1515 years old.
N FEEDING
Sponges filter sea water to eat, exchange gases and excrete waste products. Filtration is an active process
involving choanocytes lining chambers. Each choanocyte has a central flagellum that actively beats to create a water current, surrounded by a collar of cilia that
traps food particles such as plankton and bacteria, as
well as detritus. A water current containing food enters the sponge through an osculum and is initially
filtered through a series of sieve-like pores (diminishing in size), finally ending up at the collar cells. Food
particles are actively carried across the cell wall, engulfed by archaeocytes, and are transferred throughout the mesohyl to other cells. Filtered water leaves
the sponge via the exhalant canal system. Unlike most
multicellular animals, digestion and excretion of
waste products occurs within cells, not within any
common body cavity. There may be 7000–18 000 choanocyte chambers per cubic millimetre of sponge, and
each chamber may pump approximately 1200 times
its own volume of water per day. Thus, a sponge is
capable of pumping around 10 times its body volume
each hour, making them the most efficient vacuum
cleaners of the sea. Some sponges, particularly those
growing on coral reef flats, also have a unique symbiosis with cyanobacteria, providing the sponge with
nutrients derived from photosynthesis to supplement
those obtained by the sponge from normal filter feeding activities (phototrophy or autotrophy). These extra nutrients greatly augment sponge growth rates
and competitive ability in coral reef systems. There
are also often huge populations of bacteria and Archaea living within sponge cells and/or within the
sponge mesohyl (hence the term ‘sponge hotels’), with