Magnetic Island visitors are the luckiest snorkelers in the region following the introduction of 14 giant clams to the Snorkel trails on the island. 30+ year old aquacultured giant clams (~100 kg ea.) of the largest species of mollusc the planet has ever seen (Tridacna gigas), are now easy to see by snorkelers using the self-guided snorkel trails.

 

Dr. Rick Braley, one of the worlds’ authorities on giant clams had cultured these clams from fertilized eggs during his PhD thesis.  The clams have been held since 1990 at the opposite end of the island to the snorkel trails. The clams were moved into the trails and set in groups of five and three at Nelly Bay and four and two at Geoffrey Bay. 

 

The clusters of these clams are like ‘mimics’ of high density natural populations mapped on reefs of the northern GBR during Dr. Braley’s thesis field work. They are all tagged on the outside of their shells and can be observed and photographed by the public. They are one of the emblems of the GBR.

 

When Captain Cook first saw these massive clams he quipped “there is a cockle so large, two men can not eat one at a sitting”. However, poaching by foreign vessels of giant clams for their adductor muscle in the late 1960s-70s resulted in reduced populations or local extinction in some Asia – Pacific countries.

 

While poaching also occurred in GBR waters, the size of the reef still contained natural untouched populations. Following the poaching, this giant clam species has been on the CITES  Appendix 2 list of endangered species. 

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Clam Death in Geoffrey Bay Snorkel Trail in January 2021 

Clam number 1 near surface float 2 at the Geoffrey Bay Snorkel Trail was first observed dead on 7 January 2021. 

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This giant clam shell was aquacultured from eggs by Dr. Rick Braley in January 1986 at Seafarm Pty. Ltd., Flying Fish Point, near Innisfail.  This species (Tridacna gigas Linne’ 1758 ) is the largest species of bivalve mollusc that has ever existed in the fossil record. This batch of aquacultured clams had a tumultuous start as they survived Cyclone Winifred (1 February 1986) in an outdoor tank with a lowered salinity for over a week. About 15,000 seed were translocated to James Cook University’s Orpheus Island Research Station in June 1986 and used in experiments along with other batches reared there.  In 1990-1992 some specimens of this unique batch were permitted to be translocated to Magnetic Island whilst some were left to remain with all other batches reared at Orpheus. 

 

In 2013 the Marine Park Authority permitted the translocation of 14 clams from the source clams on Magnetic Island to the two established Tourism Magnetic Island Snorkel Trails (6 at Geoffrey Bay and 8 at Nelly Bay). In February/March 2020 serious bleaching of corals and some clams occurred in the Snorkel Trails, and it took 2 – 2.5 months for the recovery of the symbiotic algal cells (zooxanthellae). 

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Boring sponge tubes in one shell valve reached the clam mantle tissue, being a factor in the death of this 35 yr. old clam. Dr. Merrick Ekins of the Queensland Museum (Brisbane) identified the sponge as Siphonodictyon mucosum Bergquist, 1965.  There is sparse literature on the chemicals they produce and how this may have affected this clam. The following measurements of the growth of the shell since translocation to Geoffrey Bay were: 13.6.13 – 62 cm shell length (SL); 6.5.15 – 65.5 cm SL, 26.12.15 – 66.5 cm SL; 10.6.17 – 72.2 cm SL; 31.1.21 – 76 cm SL. Weight 91 kg on 4.2.21.

Pleasure Divers has assisted Dr. Braley with regular cleaning of the snorkel trail floats, rope and annual maintenance checks. We are proud to display this shell at the Pleasure Divers  location, Arcadia, for the education of the public about giant clams, a threatened species worldwide.

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