The Great Barrier Reef
172
frequently visited localities in the vicinity of the major
marine research stations, Heron and One Tree Islands
in the Capricorn-Bunker Group off Gladstone (387
species), the vicinity of Orpheus and Palm Islands off
Townsville (107 species), Low Isles off Port Douglas
(134 species), and Lizard and the Direction Islands NE
of Cairns (212 species known so far). We also know of
other species-rich ‘hotspots’ in more remote areas on
the GBR that are less frequently visited (and hence not
necessarily biased by collection effort), including the
Swain Reefs (304 species) at the southern end of the
GBR, the Ribbon Reefs (204 species), and the HowickTurtle Island group (210 species known so far) in the
northern region of the GBR. In between these ‘hotspots’
is a variable mosaic of diversity and species richness,
with the central region of the GBR generally less rich
than either the northern or southern sectors. These
observations also have some genetic support from
phylogeographic analysis of rDNA ITS sequences
of a widely distributed calcareous sponge, Leucetta
chagosensis (see Fig. 17.4C), which shows clear genetic
divergence between northern and southern GBR populations, both of which are genetically more closely related to Indonesian populations than they are to each
other, suggesting weak connectivity between northern
and southern regions and indicative of significant
exogenous larval recruitment and colonisation from
regions outside the GBR.
Although we now know the GBR contains a highly
diverse sponge fauna, and that sometimes sponges occur in dense local populations (‘sponge gardens’),
there still remains a significant challenge to place these
faunas into an international context through the processes of rigorous taxonomy. Only by this strategy can
we accurately determine which of these species are
unique/ endemic to the GBR (or to a particular reef
system within the GBR), and which are truly widespread and distributed over large (international) spatial scales. Like many marine invertebrate taxa, a
number of sponge species have long been perceived to
be widely distributed, ranging from the Red Sea to the
central western Pacific islands (reported as 5–15% of
regional faunas). This notion of cosmopolitanism is
now gradually diminishing with increased application
of molecular techniques at population levels, with the
outcome being that many so-called widely distributed
morphospecies may consist of several sibling species
with high genetic diversity that is not, or only barely,
manifested at the morphological level across their
wide geographic ranges. This problem is exacerbated
by the high plasticity of growth form renowned among
Porifera, challenging even the most experienced taxonomists to differentiate ‘regional variants’ of widespread morphospecies. Estimates of sponge diversity,
therefore, based on morphospecies, may be grossly
underestimated.
N DISTRIBUTION AND ABUNDANCE
Sponges may live in all types of coral reef habitats, but
in reality they exhibit very patchy distributions, such
that in one particular area they may form the dominant
structural benthos, whereas in another adjacent area
they may be practically absent. This is not unusual for
marine invertebrates, where at small (local) spatial
scales (i.e. encompassing different habitats within a
single reef, up to groups of adjacent reefs tens of kilometres apart), spatial heterogeneity is common (in
terms of both species diversity and abundance/biomass), and has been widely reported for sponges across
all ocean basins. Many factors may significantly influence local sponge distributions. Terrestrial influences,
such as freshwater input, turbidity, sedimentation, light
penetration, nutrient levels, food particle size availability and so forth have been found to explain differences
between sponge faunas in the lagoon, closer to the land,
and those living on the outer reefs. Geomorphological
differences between reefs may also markedly influence
the composition and distribution of their resident
sponge faunas, including factors such as microhabitat
availability, the nature and quality of the substrate (coralline v. non-coralline, soft v. hard), aspect of the seabed, exposure to waves and currents, depth and other
factors. In fact, adjacent reef systems (only tens of kilometres apart) have been reported to have as little as
15% similarity in their species compositions, with the
presence or absence of particular niches (such as caves,
a reef-flat, a lagoon, spurs and grooves) showing strong
correlation with the presence or absence of particular
species. Other factors that influence patchy sponge
172
frequently visited localities in the vicinity of the major
marine research stations, Heron and One Tree Islands
in the Capricorn-Bunker Group off Gladstone (387
species), the vicinity of Orpheus and Palm Islands off
Townsville (107 species), Low Isles off Port Douglas
(134 species), and Lizard and the Direction Islands NE
of Cairns (212 species known so far). We also know of
other species-rich ‘hotspots’ in more remote areas on
the GBR that are less frequently visited (and hence not
necessarily biased by collection effort), including the
Swain Reefs (304 species) at the southern end of the
GBR, the Ribbon Reefs (204 species), and the HowickTurtle Island group (210 species known so far) in the
northern region of the GBR. In between these ‘hotspots’
is a variable mosaic of diversity and species richness,
with the central region of the GBR generally less rich
than either the northern or southern sectors. These
observations also have some genetic support from
phylogeographic analysis of rDNA ITS sequences
of a widely distributed calcareous sponge, Leucetta
chagosensis (see Fig. 17.4C), which shows clear genetic
divergence between northern and southern GBR populations, both of which are genetically more closely related to Indonesian populations than they are to each
other, suggesting weak connectivity between northern
and southern regions and indicative of significant
exogenous larval recruitment and colonisation from
regions outside the GBR.
Although we now know the GBR contains a highly
diverse sponge fauna, and that sometimes sponges occur in dense local populations (‘sponge gardens’),
there still remains a significant challenge to place these
faunas into an international context through the processes of rigorous taxonomy. Only by this strategy can
we accurately determine which of these species are
unique/ endemic to the GBR (or to a particular reef
system within the GBR), and which are truly widespread and distributed over large (international) spatial scales. Like many marine invertebrate taxa, a
number of sponge species have long been perceived to
be widely distributed, ranging from the Red Sea to the
central western Pacific islands (reported as 5–15% of
regional faunas). This notion of cosmopolitanism is
now gradually diminishing with increased application
of molecular techniques at population levels, with the
outcome being that many so-called widely distributed
morphospecies may consist of several sibling species
with high genetic diversity that is not, or only barely,
manifested at the morphological level across their
wide geographic ranges. This problem is exacerbated
by the high plasticity of growth form renowned among
Porifera, challenging even the most experienced taxonomists to differentiate ‘regional variants’ of widespread morphospecies. Estimates of sponge diversity,
therefore, based on morphospecies, may be grossly
underestimated.
N DISTRIBUTION AND ABUNDANCE
Sponges may live in all types of coral reef habitats, but
in reality they exhibit very patchy distributions, such
that in one particular area they may form the dominant
structural benthos, whereas in another adjacent area
they may be practically absent. This is not unusual for
marine invertebrates, where at small (local) spatial
scales (i.e. encompassing different habitats within a
single reef, up to groups of adjacent reefs tens of kilometres apart), spatial heterogeneity is common (in
terms of both species diversity and abundance/biomass), and has been widely reported for sponges across
all ocean basins. Many factors may significantly influence local sponge distributions. Terrestrial influences,
such as freshwater input, turbidity, sedimentation, light
penetration, nutrient levels, food particle size availability and so forth have been found to explain differences
between sponge faunas in the lagoon, closer to the land,
and those living on the outer reefs. Geomorphological
differences between reefs may also markedly influence
the composition and distribution of their resident
sponge faunas, including factors such as microhabitat
availability, the nature and quality of the substrate (coralline v. non-coralline, soft v. hard), aspect of the seabed, exposure to waves and currents, depth and other
factors. In fact, adjacent reef systems (only tens of kilometres apart) have been reported to have as little as
15% similarity in their species compositions, with the
presence or absence of particular niches (such as caves,
a reef-flat, a lagoon, spurs and grooves) showing strong
correlation with the presence or absence of particular
species. Other factors that influence patchy sponge