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/bio-mass), 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 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 1 arval/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 subtropical-Iropical 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.

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