Sponges of the New Caledonian lagoon - IRD

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66 Sponges of the New Caledonian Lagoon Sexual reproductive products are undoubtedly effective in recruitment of reef sponges, over Sllort distances and short periods of time. However, it is suspected that asexual (clonal) modes of dispersal are Widespread, particularly in tropical sponge populations, where adult sponges fragment into small pieces of tissue (called propagules) and reattach. Pelagic rafting of sponge fragments is thought to be minimal, but this is not yet certain. From our limited knowledge the longevity of propagules is variable and possibly not extensive, in which case the potential for rafting may be limited to very few species, if any. Similarly, there appears to be very little mixing between deeper water and shallow water populations, and it is unlikely that sponges can migrate across major depth contours. Consequently, we currently think that the dispersal of sexual reproductive products and asexual propagules may be effectively blocked by wide oceans and deep water barriers, so that shallow-water populations of sponges surrounding islands like New Caledonia may be truly isolated and species consequently evolve into new forms over time. Conversely, there are other species of shallow-water sponges, possibly up to 5% of species, that have relatively wide distributions throughout the Indo-west Pacific - or more accurately, populations of some species from widely separated, isolated localities do not appear to differ from each other in their morphology. However, there is not yet any chemical or genetic data to confirm or refute these ideas concerning conspecificity of Widely distributed populations. Interestingly though, many of these apparently Widespread species are associated with the specialised coral reef fauna, and it is speculated that perhaps these may have some special ability to disperse over longer distances (e.g. through asexual methods), but there is still very little empirical support for this idea. Survivorship and growth Soft-bodied sponges are susceptible to breakage and fragmentation through natural causes (storms, currents), with fragmentation probably a predominant mechanism for local recruitment of shallow-water species. Predators including fishes and turtles are also effective in dispersing fragments within the local area, altllough it is not cer tain what proportion of these fragments remain viable once voided by the predators. Their fixed, sedentary lifestyle does not enable sponges to actively evade
predators. Nor do many have adequate physical protection such as an armoured surface, although some are thought to be capable of defending themselves using a diverse array of noxious chemicals. These chemicals are also possibly active agents in competition for space against other sedentary animals, such as corals and ascidians. Growth rates have been documented for a number of species, and are generally slow in comparison to other sessile marine invertebrates. This is at least partially due to the fact that most sponges are heterotrophs, obtaining their nutrients from filter feeding suspended particles in the water column. Heterotrophic species may be efficient in surviving high silt, high energy environments, like those found on the floor of the coral reef lagoon with strong current and high sedimentation, but they are poor competitors against the fast-growing autotrophic (phototrophiC) reef building corals. In clear waters corals utilise nutrients produced by the photosynthetic activities of their symbiotic zooxanthellae. There is also a special fauna of autotrophic sponges that obtain some of their nutrients from the photosynthetic by-products of symbiotic cyanobacteria (blue-green algae). Like reef building corals these autotrophic sponges grow relatively quiCkly, being the dominant sedentary organisms in some clear water reef habitats (e.g. reef flats), but this fauna has a very restricted distribution confined to shallow, clear waters. Consequently, the mode of nutrition and the presence or absence of symbiotic microorganisms clearly influences the competitive advantage that a particular sponge may have, and where it can effectively grow and survive. Habitat availability Sponges are generally not evenly distributed within all habitats. Some sponges are Widely distributed throughout adjacent habitats and are called opportunistic or generalist species, whereas the greatest diversity of species appear to be 'specialists', having very strict ecological requirements (e.g. only found in HaJimeda beds; on shallow-water beach rock; burrOWing into mud; bioeroding dead coral; reef-bUilding coralline species, etc.). Within any coral reef system, like New Caledonia, there are many different habitats, and thus patterns of species distributions are generally heterogeneous or patchy, largely determined by the presence or absence of these 67 Sponge diversity, distribution ond biogeography
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Claude Levi Ed itor Pierre Laboute,
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8 Cel ouurage est le resu/lat d'une
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This work is the result of a long s
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Author profiles Chris Battershill i
Page 17: Clive WiIkinson is Principal Resear
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Page 25 and 26: Sponges are fascinating. Each spong
Page 29 and 30: a What IS sponge .- Cll0anocytes in
Page 31 and 32: Sponge structure s ponge adults are
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Page 35 and 36: whiCh are always present irrespecti
Page 37 and 38: How con you s identi fy sponge? o p
Page 39 and 40: Soft parts of the sponge (photo J.
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Page 51 and 52: Anchorate isochela (pl,oto P.R. Ber
Page 57 and 58: The role of sponges In carol reefs
Page 59 and 60: the production of much coral rubble
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Page 63 and 64: The fascinating world of sponge che
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Page 67: Sponge diversity, distribution and
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Page 73: Actual proportions of endemic speci
Page 86: Order Astrophorida 84 I SteJlerro (
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Further reading Brien (P.), Levi (C
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186 Sponges of the New Caledonian L
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Glossary Acantho- Addition of spine
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incorrectly interchanged with the t
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Clathrate Interwoven tubular constr
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Ectosome Peripheral region of the s
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List of general illustrations provi
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115 Mycale (Zygomycale) parishi (BO
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