Box 171 Chemical Defence

Sponges are among the most toxic of animals, with toxins derived from one or a combination of the sponge's own metabolic products, sequestered chemicals excreted by other reef organisms and filtered by sponges, and symbioses with a variety of microbial in-floras. These toxins are thought to have a variety of biological functionality including repelling predators, out-competing other sessile reef organisms for space, controlling parasites and microbes, chemical recognition between host and resident in-faunas, and bioerosion of the substrate. Many of these chemicals have also demonstrated toxicity against human pathogens, cancers, parasites and other therapeutic properties of interest to the pharmaceutical industry, and over the past few decades the majority of new chemical structures and new classes of chemical compounds have been isolated from sponges, including species from the GBR.

Chaetetids
Figure 17.2 'Flabellazole A', a new P2X7 antagonist, isolated from Stylissa flabellata from the GBR. P2X7 antagonists may provide new treatment for inflammatory diseases. (Photo: J. Hooper; molecular structure T. Carroll, Natural Products Discovery Griffith University.)

Reef-builders, caves and crevices

In prehistoric reefs many hard-bodied sponges, like 'lithistids', 'stromatoporoids', 'chaetetids' and 'sphinc-tozoans' (all now considered grades of construction in body plan, and not taxonomic clades), were the major structural components of reefs. In modern day reefs hard-bodied sponges have only a minor structural role, although they are still considered to be of some importance in accreting coral skeletons in deeper waters where light is limiting and coral growth is rudimentary. Nevertheless, living representatives of these 'reef-building sponges' are still found on coral reefs and other deeper reefs, with their ancient body plans persisting over many millions of years. Some of these sponges (such as Astrosclera willeyana) have a solid cal-citic skeleton (contributing to reef-building, analogous

Fibrous Tissue Corals

Figure 17.3 Sponges of the reef flat and shallow lagoon, including the cryptic, coral rubble, and under-boulders communities: A, Myrmekioderma granulate; B, Leucetta microraphis; C, nudibranchs feeding on Clathria (Microciona) aceratoob-tusa (photo: B. Rudman); D, Stelletta sp.; E, Haliclona nematifera; F, Aplysinella rhax; G, Chelonaplysilla sp.; H, Gelliodes fibulatus. (Photos: J. Hooper, except where noted.)

Figure 17.3 Sponges of the reef flat and shallow lagoon, including the cryptic, coral rubble, and under-boulders communities: A, Myrmekioderma granulate; B, Leucetta microraphis; C, nudibranchs feeding on Clathria (Microciona) aceratoob-tusa (photo: B. Rudman); D, Stelletta sp.; E, Haliclona nematifera; F, Aplysinella rhax; G, Chelonaplysilla sp.; H, Gelliodes fibulatus. (Photos: J. Hooper, except where noted.)

Acanthella Sea Fan Sponge

Figure 17.4 A-B, sponges of the reef flat and shallow lagoon, including the cryptic, coral rubble, and under-boulder communities: A, Hyrtios erecta; B, Suberea ianthelliformis. C-H, reef-builders and cave faunas: C, Leucetta chagosensis; D, Levinella prolifera; E, Ulosa spongia; F, Soleniscus radovani; G, Astrosclera willeyana; H, Petrosia (Strongylophora) strongylata. (Photos: J. Hooper.)

Figure 17.4 A-B, sponges of the reef flat and shallow lagoon, including the cryptic, coral rubble, and under-boulder communities: A, Hyrtios erecta; B, Suberea ianthelliformis. C-H, reef-builders and cave faunas: C, Leucetta chagosensis; D, Levinella prolifera; E, Ulosa spongia; F, Soleniscus radovani; G, Astrosclera willeyana; H, Petrosia (Strongylophora) strongylata. (Photos: J. Hooper.)

to the skeletons of modern hermatypic corals), as well as discrete siliceous spicules within the soft tissues, virtually identical to those seen in their fossil ancestors from the Lower Cretaceous (160 My) and Triassic (250 My) respectively. Others have only a solid skeleton composed of aragonitic crystals (a form of calcium carbonate) with no free spicules (e.g. Vaceletia crypta, with a continuous fossil record from the Middle Triassic, 245 My, to present). Others have a solid skeleton of either linked or rigid calcareous spicules (e.g. Plectroninia hin-dei, with a body plan known from the Mid Miocene, 23 My) or a rigid basal mass of calcite, and some (the so-called 'lithistids') have only siliceous skeletons composed of special spicules called desmas, ranging from entirely fused and forming a rock hard skeleton to loosely articulated rendering the body more flexible (e.g. Theonella swinhoei, with ancestors recorded from the Tertiary, 65 My). These so-called 'living fossil' sponges are usually found in shaded or dark habitats, such as in crevices, deep caves or under coral rubble, rarely in full light, and due to their hard coralline texture may be confused with corals by the novice. Similarly, caves and dark crevices are also home to a variety of soft-bodied demosponges, sometimes lacking pigments (e.g. Petrosia (Stongyl-ophora) strongylata), sometimes colourful (e.g. Ulosa spongia), and especially the multitude of frequently brightly coloured calcareous sponges living on the reef (e.g. Leucetta chagosensis, Lev-inella prolifera and Soleniscus radovani) (Fig. 17.4C-H).

Reef slope

Between the reef crest and the base of coral reefs, across the shelf of the GBR, occur large, predominantly hetero-trophic sponges that feed on food particles and waste products filtering down from the coral reef above. On some reefs, particularly those closer to the coast, these faunas number several hundred species, which in some instances occur as large populations. Their morphologies are as diverse as the conditions they live under, such as flexible whips, fingers and fans adapted for coping with high currents (Ianthella basta, Axosflabelliformis, Clathria (Thalysias) cervicornis); soft tubes, vases and other shapes that predominate in silty, turbid water where inhalant and exhalant pores are located on different surfaces to prevent smothering (e.g. Echinochalina

(Protophlitaspongia) isaaci and Fascaplysinopsis reticulata), and a number of ubiquitous, amorphous, bulbous, massive, spherical (and other shaped) forms that appear nearly anywhere they can settle and survive (e.g. Cin-achyrella schulzei, Stylissa massa, Stylissa carteri, Acanthella cavernosa, Amphimedon terpenensis, Pericharax heterorha-phis, and Agelas axifera) (Figs 17.5A-H, 17.6A-H).

Deeper lagoon and inter-reef

Sponges living on the seabed in between the reefs are generally very different to those found on the coral reefs, living in high current, low light, turbid waters where they burrow into soft sandy and muddy sediments (e.g. Oceanapia renieroides and Disyringia dissimilis), in seagrass and Halimeda beds (e.g. Oceanapia sagittaria), or attached to hard objects on the sea floor (e.g. Xestospongia testudinaria, Melophlus sarassinorum and Liosina paradoxa) (Fig. 17.7A, C, G, H). Growth forms include elongate species with root-like tufts or bulbs for anchoring in soft sediments and long tubes to prevent smothering, flexible fans and whips in high current areas, and massive barrels and volcanoes attached to rock and coral outcrops. This fauna comprises a significant proportion of the 'benthos', providing important habitat for other marine species, ranging from aggregating fishes to numerous crustacean in-faunas. In a recent major survey across the length and breadth of the GBR, conducted under the auspices of the Cooperative Research Centre for the GBR (CRC Reef), approximately 1300 morphospecies of sponges were discovered from the inter-reef region, which is particularly susceptible to human impacts such as trawling (see also Chapter 6). Whereas a number of species living on the coral reef flats may be found on both sides of the Australian continent, these deeper water GBR lagoonal and inter-reef species appear to differ significantly in composition from the fauna found at similar latitudes on the west coast of the continent.

Reef bioeroders (sponges as 'parasites')

A special group of sponges are responsible for significant carbonate recycling on the reef, collectively termed excavating ('boring' or bioeroding) sponges (see also Chapter 8). They are responsible for extensive damage

Phosphorus Symbiont Sponge
Figure 17.5 Reef slope faunas: A, Axos flabelliformis; B, Clathria (Thalysias) cervicornis; C, Ianthella basta; D, Echinochal-ina (Protophlitaspongia) isaaci; E, Fascaplysinopsis reticulata; F, Cinachyrella schulzei; G, Stylissa massa; H, Amphimedon terpenensis. (Photos: J. Hooper.)
Reniochalina
Figure 17.6 Reef slope faunas: A, Pericharax heterorhaphis; B, Agelas axifera; C, Acanthella cavernosa; D, Reniochalina sp. (photo: Chris Ireland); E, Phycopsis fusiformis; F, Callyspongia aerazusa; G, Pipestela candelabra; H, Stylissa carteri. (Photos: A-C, E-H, J. Hooper.)
Stylissa Carteri

Figure 17.7 Deeper lagoon and inter-reef faunas, reef bioeroders and soft sediment faunas: A, Liosina paradoxa; B, Sphe-ciospongia vagabunda; C, Melophlus sarissinorum; D, Oceanapia sagittaria; E, Cliona sp.; F, Coelocarteria singaporensis; G, Xestospongia testudinaria; H, Oceanapia renieroides. (Photos: J. Hooper.)

Figure 17.7 Deeper lagoon and inter-reef faunas, reef bioeroders and soft sediment faunas: A, Liosina paradoxa; B, Sphe-ciospongia vagabunda; C, Melophlus sarissinorum; D, Oceanapia sagittaria; E, Cliona sp.; F, Coelocarteria singaporensis; G, Xestospongia testudinaria; H, Oceanapia renieroides. (Photos: J. Hooper.)

to hard and soft corals, as well as shellfish and other molluscs, and are among the most destructive internal bioeroding organisms of coral reefs both in terms of effects (such as weakening coral platforms and producing dead coral rubble). The rates of destruction by these organisms range up to 15 kg m2 per year. Much of the damage caused to corals during storms has been attributed to weakening of basal structures by bioerosion (see also Chapter 8). An excavating mode of existence has been independently acquired by several sponge orders. Some of these (e.g. Terpios) simply overgrow coral at rapid rates, periodically resulting in extensive tracts of coral bleaching and the destruction of large tracts of coral. Others burrow into dead coral, eventually occupying the entire original coral head, with breathing tubes (fistules) protruding (e.g. Coelocarteria singaporen-sis and Aka sp.). The most significant of these are the Hadromerida ('clionaids') belonging to the families Clionaidae, Alectonidae and Spirastrellidae (e.g. Cliona sp., Spheciospongia vagabunda and Cliona montiformis) (Fig. 17.7B, E, F). Clionaids excavate chambers within the coral skeleton using a cellular process undertaken by special etching cells secreting acid phosphatase and lysosomal enzymes that dissolve organic matter and produce limestone chips that are physically liberated into the sea water via the sponge exhalant canal. Etching initially produces a cavity with sponge papillae protruding outside the coral (alpha stage), after which the external papillae fuse to produce a continuous sponge crust covering the coral (beta stage), eventually becoming massive and consuming the entire coral (gamma stage). Although clionaid sponges have the ability to invade living coral tissue and to survive direct contact with coral polyps, their ecological success may be largely due to their ability to undermine and erode the coral skeletal base, thus avoiding contact with the coral polyp defensive mucus and nematocysts.

Sponge body plans and classification

The Phylum Porifera is defined by their unique possession of chambers lined by a single layer of flagellated cells (choanocytes or collar cells) that actively beat to produce a unidirectional water current through the body, connected to the external water column by a system of differentiated inhalant and exhalant canals with

Figure 17.8 Diagrammatic sponge morphology: Arc, totipotent phagocytotic cells (archaeocytes); Bas, basipinacocytes lining internal aquiferous system; Cho, choanocytres or collar cells; ChoCh, choanocyte chamber (lined by choanocyte cells); Exo, exopinacocytes (lining exterior surfaces); Fla, flagellum on choanocytes; Ost, inhalant pores (ostia); Osc, exhalant pores (oscula); Spi, spicules (siliceous or calcitic depending on class). Red arrows (inhalant water current with food particles etc.); blue arrows (exhalant water current with waste products). (Modified from UCMP Berkeley.)

Figure 17.8 Diagrammatic sponge morphology: Arc, totipotent phagocytotic cells (archaeocytes); Bas, basipinacocytes lining internal aquiferous system; Cho, choanocytres or collar cells; ChoCh, choanocyte chamber (lined by choanocyte cells); Exo, exopinacocytes (lining exterior surfaces); Fla, flagellum on choanocytes; Ost, inhalant pores (ostia); Osc, exhalant pores (oscula); Spi, spicules (siliceous or calcitic depending on class). Red arrows (inhalant water current with food particles etc.); blue arrows (exhalant water current with waste products). (Modified from UCMP Berkeley.)

external pores (ostia and oscula, respectively), together forming a highly efficient aquiferous system that maintains basic metabolism and contributes significantly to reef filtration (Fig. 17.8). Sponges have a cellular grade of construction without true tissues, with their highly mobile populations of cells capable of differentiating into other cell types (totipotency), thus conferring a plasticity to growth form. The outer and inner layers of the sponge individuals are formed by special cells (ex-opinacocytes and basipinacocytes) that lack a basement membrane (except in some members of one group, the homoscleromorphs). The middle layer (or mesohyl) is variable among the orders of sponges but always includes motile cells and usually some skeletal material. Sponge skeletons are essentially divided into the ecto-some ('skin') and choanosome (body containing the choanocyte chambers). Adult sponges are generally sessile, attached to the seabed or other substrate for most of their lives (although some are capable of slow movement), and most have motile larvae that swim or crawl away from their parent. Body plans range from simple (asconoid and syconoid, found in a few calcar-ean sponges) through to complex (leuconoid, occurring in most sponges), produced by varying degrees of Infolding of the body wall and complexity of water canals throughout the sponge. Adults are asymmetrical or radially symmetrical, and have evolved an amazing range of growth forms best described as highly irregular and sometimes completely plastic, frequently altered by prevailing external conditions (currents, turbidity, salinity etc.). Sponges also have evolved an amazing array of colours, some linked to dietary caro-tenoid proteins and others with a photoprotection functionality.

The current classification of the Porifera is based primarily on features of the organic (collagen fibres and filaments) and inorganic skeletons (discrete and/or fused spicules composed of calcium carbonate or silicon dioxide), with some species also having a hypercal-cified basal skeleton of solid limestone. The taxonomic scheme is primarily morphologically-based, and as complex as the diversity of sponges—the study of sponge taxonomy is not for the faint-hearted. Applying taxonomic principles to sponges is made even more difficult by the occurrence of frequent character losses, modifications and apparently convergent features reappearing within the classification. No attempt is made here to provide more than a very basic summary, with a list of further reading provided. There are three distinct classes of living sponges (plus a fourth extinct one): Calcarea, having calcific spicules with three or four rays; Hexactinellida, with discrete and/or fused siliceous spicules, the larger ones three or six rayed; and Demospongiae, with siliceous spicules in many (but not all) species, and/or a fibrous skeleton, and spicules with one, two or four rays divided into megas-clere and microsclere categories. Only Calcarea and Demospongiae have so far been recorded from the GBR, although Hexactinellida live in deeper waters on the continental slope and shelf adjacent to the GBR. An overview of the phylum, including a taxonomic revision and identification keys for approximately 25 orders, 127 families and 700 genera, has recently been undertaken but species-level identifications remain appallingly difficult, with few easily accessible taxonomic publications that would be useful to a non-specialist audience. Further useful reading is listed below, including general reading on sponge biology, sponge cell biology, a web checklist of the published Australian sponge fauna (including Queensland species) with keys to genera, a web list of all published sponge species worldwide, and sponge higher classification. The recent escalation of the molecular study of sponges will certainly have a major impact on our current ideas of the phylogeny and classification of Porifera, and to this end a Sponge Barcoding Project (based on a systematic use of molecular tools) has commenced and is also available on the web.

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