The Phylum Porifera is the most primitive of the multi-cellular animals, with a most ancient geological history. Porifera appeared early in the history of life on Earth, established in the Proterozoic, with the major Class Demospongiae first appearing in the Precambrian Ediacaran age (about 750 million years (My) ago). By the Middle Cambrian (about 500 My) demosponges were thriving, and today represent about 85% of all living species. During the massive radiation of life forms in the Ordovician (490-460 My), and for the next 100 million years or so, sponges formed extensive barrier and fringing reefs around the ancient continents and were the primary reef builders in these ancient oceans. Sponges declined as the primary reef-builders at the end of the Devonian (350 My), and today they are not significant reef-builders in shallow waters, unable to compete with the faster growing zooxanthella-bearing corals, although several hard-bodied (lithistid and hy-percalcified) reef-building groups of sponges persist in modern day reefs. Although sponges are still a major component of modern-day coral reef ecosystems they are often overlooked by the curious naturalist because they are frequently hidden among the more prominent corals on the reef, or live in the less frequently visited deeper waters surrounding reefs. Nevertheless, in terms of their species diversity, they outnumber both the hard and soft corals combined. In some habitats they provide pivotal ecological services, such as significant contributions to coral reef primary productivity, filtration of the waste products and toxins from other reef organisms, and major recycling of calcium carbonate back into the reef system through bioerosion.
Sponges are predominantly marine, living from the intertidal to the abyssal zone, with a small number also found in freshwater habitats. Worldwide there are approximately 8500 species described in the literature that are considered to be 'valid', with about twice this number estimated for all oceans. Approximately 1500 species have been described among the Australian fauna so far, although an escalated collection effort over the past couple of decades, mainly from tropical and subtropical waters and spurred on by drug discovery from marine organisms, has discovered a fauna three times this size. This work leads to an estimated Australian fauna of at least 5000 species. So far only about 400 species are described in the literature for all Queensland waters, including the coast, the GBR and Coral Sea island territories. Extensive surveys over the past two decades, however, reveal that more than 2500 sponge species actually live here, with the majority remaining undescribed (new species).
Amongst the better known sponge faunas on the GBR are, not surprisingly, those from the most 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 Howick-Turtle 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 tax-onomists to differentiate 'regional variants' of widespread morphospecies. Estimates of sponge diversity, therefore, based on morphospecies, may be grossly underestimated.
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