Holothuroidea

The Holothuroidea (Figs 26.2F—I, 26.3-26.5) are diverse and common all along the GBR and there are three major orders: (1) the Aspidochirotida, the common deposit feeders that graze on the surface of the sediment; (2) the Dendrochirotida, suspension feeders with an array of branching tentacles and; (3) the Apodida, which are also deposit feeders. Several surveys of holothuroid diversity on the GBR have been undertaken (see Additional reading).

Aspidochirotids are by far the most abundant and

BOX 26.2 BÊCHE-DE-MER SPECIES

Approximately 15 species comprise the bêche-de-mer fisheries across northern Australia and elsewhere in the Indo-Pacific. Several Australian fisheries for the high value species collapsed in short order, repeating the pattern seen elsewhere in the Indo-Pacific. As illustrated by the teatfish (Holothuria [Microthele] whitmaei and H. [M.] fuscogilva) complex, there are serious conservation concerns for populations of commercial species that have been on the decline for some time or have completely disappeared from some localities (see Additional Reading). Holothuroids are particularly susceptible to overfishing because of their limited mobility, slow growth, late maturity, poor recruitment and density-dependent propagation. Local areas are quickly stripped of valuable species. Currently, fishers in Australia are moving on to less valuable or less accessible (remote areas, deeper waters) stock, repeating the 'fishing-down' pattern seen in other jurisdictions.

One of the first species targeted by fishers on the GBR was the high value species, the black teatfish (BTF) Holothuria whitmaei. This species is still listed as H. nobilis in some guides, but this is an Indian Ocean species. The GBR fishery for the BTF was short-lived as the stock of this shallow water species was quickly depleted. Unfished reefs and green zones support high densities of bêche-de-mer species. The large populations of H. whitmaei at Raine Reef and other reserves are crucial for conservation of the species.

The general lack of juveniles in sea cucumber populations indicates that recruitment will be slow. The first principle of reproduction, gamete contact, is likely to be compromised by the current low densities of spawning individuals on some reefs.

conspicuous holothuroids on the GBR (Figs 26.3-26.5). They occur in a variety of habitats from the reef dwelling Actinopyga (surf red fish, Fig. 26.5F), to the intertidal and deep water aspidochirotids Actinopyga, Bohadschia, Holothuria, and Stichopus species. The most abundant species on the GBR are capable of asexual and sexual reproduction. These include H. atra (Fig. 26.3A), H. difficilis (Fig. 26.3G), H. edulis (Fig. 26.3D), H. hilla (Fig. 26.4C) and S. chloronotus (Fig. 26.5A). They are fissiparous, splitting in half and subsequently regenerating the anterior and posterior halves to make two complete individuals. Although these species also reproduce sexually, spawning gametes and have dispersive larvae, it appears that fission is important in maintaining the populations. Many species provide habitat to commensal crabs and scale worms. These are commonly seen on the body wall of H. atra (Fig. 26.3B, C). Parasitic eulimid snails are also commonly seen on the body surface of some holothuroids (Fig. 26.4F). These snails extend their proboscis through the body wall and into the coelom where they utilise nutrients in their host body fluids.

Aspidochirotids are deposit feeders and are prominent members of the biota of soft sediment environments. They feed with their spatula-like tentacles (Fig. 26.5G) scooping up sand, algal films and detritus. Aspidochirotids provide important ecosystem services by enhancing local productivity through their biotur-bation and feeding/digestive activity. Burrowing species are particularly important in bioturbation of the nutrient poor carbonate sediments that dominate much of the GBR. Taxonomically, these sea cucumbers have long been a challenge, with some aspidochirotids well characterised, but others being intractable to traditional taxonomy. Several commercial species of Stichopus and Actinopyga appear to be morphospecies complexes and the identity of species currently being fished needs to be assessed (Box 26.2). Synaptids (Fig. 26.2G-!) are less familiar because many are small and some are noctur

Figure 26.4 Holothuroidea. A, Holothuria impatiens (x 0.23); B, Holothuria lessoni (x 0.20) (photo: H Eriksson); C, Holothuria hilla (x 0.14); D, Holothuria arenicola (x 0.25); E, Holothuria isuga (x 0.12); F, Holothuria isuga with parasitic snails (x 1.00); G, Bohadschia argus (x 0.17) (photo: H. Eriksson); H, Labidodemas semperianum (x 0.40); /, Personothuria graeffei (x 0.16). (Photos: M. Byrne, unless noted.)

Figure 26.4 Holothuroidea. A, Holothuria impatiens (x 0.23); B, Holothuria lessoni (x 0.20) (photo: H Eriksson); C, Holothuria hilla (x 0.14); D, Holothuria arenicola (x 0.25); E, Holothuria isuga (x 0.12); F, Holothuria isuga with parasitic snails (x 1.00); G, Bohadschia argus (x 0.17) (photo: H. Eriksson); H, Labidodemas semperianum (x 0.40); /, Personothuria graeffei (x 0.16). (Photos: M. Byrne, unless noted.)

nal. The suspension feeding dendrochirotids are less common. Afrocucumis africana (Fig. 26.2F), a small, dark purple dendrochirotid is often seen under boulders and rubble in the intertidal zone. Other dendrochirot-ids burrow in sediment or live in the reef infrastructure or algal turfs and so are rarely seen. Their tree-like dendritic tentacles that they use for filter feeding can be seen emerging from the substrate.

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