Maria Byrne with
[Photo: S. Uthicke]
Stichopus horrens from One Tree Island, Southern Great Barrier Reef, Queensland.
[Photos: M. Byrne]
Report from an ABRS grantee
Species composition of Australia’s tropical bêche-de-mer sea cucumbers (Echinodermata: Holothuroidea) in the Stichopus complex
Sea cucumbers in the Order Aspidochirotida are harvested for the bêche-de-mer fishery, with most activity in the tropical waters of the Indo-Pacific. Aspidochirotid holothurians are deposit feeders, directly ingesting sediment to feed on microalgae and bacteria attached to sediment particles. They provide important ecosystem services in nutrient cycling and oxygenation of coral reef sediments. Tropical sea cucumbers are often the dominant organisms on the sea floor particularly on reef flats and sandy areas. Taxonomically, aspidochirotids have long been a challenge, with numerous species that appear to be species complexes.
For the bêche-de-mer fishery whole animals are gutted, boiled and dried to produce a body wall product highly prized in many Asian countries. This fishery can provide a valuable source of income for remote communities in northern Australia and elsewhere in the region because the processing and storage of product is simple and does not require costly equipment.
Historically, fishers from the island communities to the north of Australia harvested bêche-de-mer in the Torres Strait and other Australian waters. The major markets are China and South-east Asia. Bêche-de-mer can be seen in shops in the Chinatown district of most major Australian cities in hessian sacs filled with the black, sausage-shaped dried product. The species with the thickest body wall are the most valuable. Dried sea cucumber body wall is used in soups and other Asian dishes.
Bêche-de-mer species are fished under a range of names with the teatfishes [Holothuria (Microthele) whitmaei and H. (M.) fuscogilva)] being the most valuable. These species are so-called because of the teat-like extensions along their body. The teatfishes were among the first holothuroids to be over-harvested throughout the Indo-Pacific, resulting in concerns for their conservation. The Australian fishery for the black teatfish collapsed in short order, repeating the pattern seen elsewhere. 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. Sea cucumbers are particularly susceptible to over-fishing because of their limited mobility, poor recruitment and density-dependent propagation. Local areas are quickly stripped of valuable species.
Fishery names of other bêche-de-mer species include prickly red fish, surf fish, lolly fish, pink fish, curry fish, passionfruit fish and peanut fish, to name a few. It is clear that many of these fish comprise more than one species. Among these are the focus of this project, the Stichopus fishery. Stichopus species (curry fish, passionfruit fish, peanut fish) are harvested in tropical Australia and elsewhere in the Indo-West-Pacific.
The Stichopus horrens fishery is particularly problematic. This fishery extends from the Galapagos to the Pacific islands, north to the Philippines and south to Australia. It is fished in Australia under the name peanut fish. Examination of freshly harvested specimens revealed that S. horrens is not the same in all places. To assist with sorting out the taxonomy of what fisheries managers were calling S. horrens, many colleagues provided samples from local catches and these included a striking array of species.
Fortunately, Dr Frank Rowe from the Australian Museum in Sydney established a comprehensive suite of voucher specimens for a large number of the Stichopus species from tropical Australia and elsewhere. He also produced a set of body wall skeleton preparations to support his species diagnoses.
Aspidochirotid sea cucumbers have tiny pieces of skeleton called spicules in their body wall that can be isolated with careful use of household bleach. Microscopic examination of these spicules greatly assists with species identification. Assisted by traditional taxonomy, the challenge was to see if we could match the ancient DNA of these museum specimens with DNA from the ones being fished. Fortunately we were able to generate sequence data for a good number of the Australian Museum specimens to address the peanut fish horrens challenge. We were not so lucky with the Stichopus specimens provided by the Natural History Museum, London. None-the-less the question as to which one was the real horrens could be addressed.
Comparison of DNA from species being fished with the DNA from new voucher specimens collected from the Great Barrier Reef and from the Australian Museum specimens revealed that at least three species are currently being fished under the name Stichopus horrens. The real S. horrens however is readily identified by its distinct body wall spicules. It was discovered that several cryptic species are being fished. Cryptic species are new species recognised from within an otherwise widely distributed single species. So far, all the species of uncertain status fall into the S. monotuberculatus species complex. Sorting out the S. monotuberculatus species complex will be a major challenge for future work. The starting point will be the molecular phylogeny established for Stichopus.
It is difficult to manage a fishery without knowing what species are being harvested. The results of this research and ongoing investigations will determine the identity of species currently being harvested in Australia and elsewhere in the Indo-Pacific. Hopefully, this will contribute to the conservation and sustainable use of our bêche-de-mer resources.