17 – Sponges
173
distributions include small scale random events, such
as patterns and timing of arrival and survival of larvae
and asexual propagules, effects of severe storm events
on fragmentation and dispersal, and the history of
(and changes to) current patterns and other barriers to
larval/propagule dispersal. From our current understanding, sponges appear to have very limited sexual
reproductive dispersal capabilities (an absence of any
pelagic larval stage), with short larval lives (with a reported maximum of 72 hrs in the water column before
settlement). Oviparous species, like the ubiquitous
Xestospongia testudinaria (see Fig. 17.7G) that broadcast
eggs and sperm into the water, last only several days
at most. Conversely, at smaller spatial scales at least, it
is thought (also now with some molecular support)
that clonal dispersal and larval recruitment are predominant, and small scale endemism appears to be
common among sponges, possibly through genetic
isolation of remnant populations of once widespread
species.
At larger spatial scales (i.e. from biogeographic
provinces to ocean basins), factors such as historical
changes to physical barriers and current patterns, climate change impacts, presence or absence of carbonate
platforms have had large scale influences on present
day sponge faunas. Unlike some other marine invertebrate phyla there are no apparent latitudinal gradients
of sponge species richness from temperate to tropical
waters (both have patchy mosaics of very rich faunas,
on both sides of the Australian continent). The sponge
fauna composition changes substantially, however,
along the east coast of Australia, with subtropicaltropical faunal transition zones (or species turnover
points) occurring in the vicinity of the Tweed River,
Hervey Bay-Fraser Island, the Mackay-Townsville region, Cape Flattery north of Cooktown, and on the
eastern side of Cape York.
N REEF HABITATS
Once thought of as predominantly niche generalists, opportunistically scattered over reefs wherever larvae settled or propagules landed, sponges are increasingly
recognised as being predominantly niche specialists
with marked habitat preferences. Distinct species
assemblages characterise particular habitats, although
there are a number of ubiquitous species found vicariously throughout the reef. Some of the more prominent
sponge habitats occurring on the GBR are described here.
Reef flats and rock pools
Of all the habitats, the reef flats and shallow parts of lagoons contain probably the most significant of the coral
reef sponge faunas, at least in terms of their visibility
and provision of ecological services. These are the phototrophic (or autotrophic) species, belonging primarily
to two orders of Demospongiae, the Dictyoceratida and
Haplosclerida, that derive most of their nutrition from
the photosynthetic products of their resident symbiotic
cyanobacteria, and in the process contribute significantly to overall net primary productivity of entire reef
systems. Phototrophic species (which derive most of
their energy from sunlight through photosynthetic symbionts, see Chapter 7) include representatives from several orders, with the common species on the GBR being
Phyllospongia papyracea, Carteriospongia foliascens, Strepsichordaia lendenfeldi, Haliclona cymaeformis, Cymbastela
coralliophila and Lamellodysidea herbacea (Fig. 17.1A–H).
Some of these species can be found in very large populations, especially in the clear waters of outer reefs.
These phototrophic sponges are also unusual in having
(probably truly) widespread geographic distributions,
appearing to be very similar on both sides of the continent, with some also common in the Indo-Malay archipelago and south-western Pacific islands, although no
molecular study of these widespread populations has
yet been attempted to test their alleged conspecificity.
Another highly diverse fauna on the reef flat and in
the lagoon shallows are the coral rubble, under-rubble
and boulder sponges, living in crowded, encrusting
and sciaphilic communities. These sponges range from
thin crusts of no more than several millimetres in thickness, competing with each other for space and other
resources using an arsenal of chemicals (Box 17.1 and
Fig. 17.2), to massive slimy sponges that bind the coral
rubble together and form the paving substrate. Examples of these include: Myrmekioderma granulata, Clathria
aceratoobtusa, Leucetta microraphis, Aplysinella rhax,
Neopetrosia exigua, Gelliodes fibulatus and Hyrtios erecta
(Fig. 17.3A–H, Fig. 17.4A).
173
distributions include small scale random events, such
as patterns and timing of arrival and survival of larvae
and asexual propagules, effects of severe storm events
on fragmentation and dispersal, and the history of
(and changes to) current patterns and other barriers to
larval/propagule dispersal. From our current understanding, sponges appear to have very limited sexual
reproductive dispersal capabilities (an absence of any
pelagic larval stage), with short larval lives (with a reported maximum of 72 hrs in the water column before
settlement). Oviparous species, like the ubiquitous
Xestospongia testudinaria (see Fig. 17.7G) that broadcast
eggs and sperm into the water, last only several days
at most. Conversely, at smaller spatial scales at least, it
is thought (also now with some molecular support)
that clonal dispersal and larval recruitment are predominant, and small scale endemism appears to be
common among sponges, possibly through genetic
isolation of remnant populations of once widespread
species.
At larger spatial scales (i.e. from biogeographic
provinces to ocean basins), factors such as historical
changes to physical barriers and current patterns, climate change impacts, presence or absence of carbonate
platforms have had large scale influences on present
day sponge faunas. Unlike some other marine invertebrate phyla there are no apparent latitudinal gradients
of sponge species richness from temperate to tropical
waters (both have patchy mosaics of very rich faunas,
on both sides of the Australian continent). The sponge
fauna composition changes substantially, however,
along the east coast of Australia, with subtropicaltropical faunal transition zones (or species turnover
points) occurring in the vicinity of the Tweed River,
Hervey Bay-Fraser Island, the Mackay-Townsville region, Cape Flattery north of Cooktown, and on the
eastern side of Cape York.
N REEF HABITATS
Once thought of as predominantly niche generalists, opportunistically scattered over reefs wherever larvae settled or propagules landed, sponges are increasingly
recognised as being predominantly niche specialists
with marked habitat preferences. Distinct species
assemblages characterise particular habitats, although
there are a number of ubiquitous species found vicariously throughout the reef. Some of the more prominent
sponge habitats occurring on the GBR are described here.
Reef flats and rock pools
Of all the habitats, the reef flats and shallow parts of lagoons contain probably the most significant of the coral
reef sponge faunas, at least in terms of their visibility
and provision of ecological services. These are the phototrophic (or autotrophic) species, belonging primarily
to two orders of Demospongiae, the Dictyoceratida and
Haplosclerida, that derive most of their nutrition from
the photosynthetic products of their resident symbiotic
cyanobacteria, and in the process contribute significantly to overall net primary productivity of entire reef
systems. Phototrophic species (which derive most of
their energy from sunlight through photosynthetic symbionts, see Chapter 7) include representatives from several orders, with the common species on the GBR being
Phyllospongia papyracea, Carteriospongia foliascens, Strepsichordaia lendenfeldi, Haliclona cymaeformis, Cymbastela
coralliophila and Lamellodysidea herbacea (Fig. 17.1A–H).
Some of these species can be found in very large populations, especially in the clear waters of outer reefs.
These phototrophic sponges are also unusual in having
(probably truly) widespread geographic distributions,
appearing to be very similar on both sides of the continent, with some also common in the Indo-Malay archipelago and south-western Pacific islands, although no
molecular study of these widespread populations has
yet been attempted to test their alleged conspecificity.
Another highly diverse fauna on the reef flat and in
the lagoon shallows are the coral rubble, under-rubble
and boulder sponges, living in crowded, encrusting
and sciaphilic communities. These sponges range from
thin crusts of no more than several millimetres in thickness, competing with each other for space and other
resources using an arsenal of chemicals (Box 17.1 and
Fig. 17.2), to massive slimy sponges that bind the coral
rubble together and form the paving substrate. Examples of these include: Myrmekioderma granulata, Clathria
aceratoobtusa, Leucetta microraphis, Aplysinella rhax,
Neopetrosia exigua, Gelliodes fibulatus and Hyrtios erecta
(Fig. 17.3A–H, Fig. 17.4A).