Click here to read about cultured giant clam shells available for residents of Australia...these may be used in garden decoration, as footbaths, sinks or foodbowls.

Clam Shells

 

 

GIANT CLAM  BIOLOGY AND CULTURE

 

Giant Clams - a Solar Animal


Giant clams evolved over 65 million years ago in the Eocene, along with modern corals. Both clams and corals lead symbiotic lives along with millions of single cell dinoflagellate algae called 'zooxanthellae'.

Adult clams can obtain over 90% of their food requirements from the photosynthetic products of the algae, but when particulate organic matter (POM) is present an adult clam can obtain about 33% of its carbon requirements from POM. The third source of nutrition is dissolved organic matter (DOM) but it is probably more important in larval and post-larval stages than in sub-adult and adult clams. Clams also act as natural biofilters as they take up dissolved ammonia and nitrate from the surrounding seawater to supply their symbiotic zooxanthellae with nitrogen for growth.

Maybe in the near future giant clams will become accepted by aquarium enthusiasts as an ideal living biofilter. Someday giant clams may be found in aquaculture tanks in a Moon Colony to supply food, excess oxygen in sunlight, and as biofilters for fish and other aquacultured animals.

Until then, the aquaculture of giant clams for food and restocking of overharvested reefs is an example to follow for other endangered species.

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Giant Clam Species

 

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Tridacna gigas, the true giant clam.  It can reach to > 1.3m in shell length and hundreds of kilograms in weight, living to >50 years of age.   This is the largest species of bivalve mollusc to have ever lived in the fossil record of our planet.  It was being driven to extinction in some parts of its natural distribution (SE Asia, Australia, Micronesia) by poachers in the 1970s.  Natural high density populations are still present on the Great Barrier Reef of Australia.  This species is on the Threatened Species list of IUCN and CITES. 

 

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Tridacna derasa, the smooth giant clam.  Second largest species reaching > 55 cm shell length.   On the Great Barrier Reef (GBR) it is found only on mid-shelf and outer-shelf reefs where turbidity is lower than inner-shelf reefs.  The background pattern is of juvenile T. derasa reared by Dr. Braley at the Barrang Lompo Island giant clam hatchery, So. Sulawesi, Indonesia in the mid-90s.  The distribution is SE Asia, Micronesia to Tonga and Australia.

 

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Tridacna squamosa, the scaly or fluted giant clam held by Dr. Rick Braley at the Aitutaki atoll giant clam hatchery, Cook Islands, early 90s.  This species reaches 40 cm shell length and has light to moderate byssal attachment as adults.  It is widespread from E. Africa & Red Sea to French Polynesia. 

 

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Tridacna maxima, the elongated, rugose, or small giant clam.  This species grows up to 40 cm shell length and has great byssal attachment to the substrate.  The distribution is widespread and similar to T. squamosa.  The mantle is very colourful and the clam is sought after in the aquarium trade. 

 

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Tridacna crocea, the boring or crocus clam.  This is the smallest species of giant clam reaching to 15 cm shell length.  It has a wide byssal opening at the shell base and therefore burrows into the substrate so that only the extended colourful mantle touches the substrate surface.  Like T. maxima, it is a good species for the aquarium trade.   The distribution is SE Asia, Australia and western Pacific.  In places on the GBR densities can be >100/ sq m. 

 

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Tridacna tevoroa Lucas, Ledua & Braley, 1990, the tevoro  or 'devil' in Fiji language) clam.  This quite large (>50 cm shell length) clam is found in moderately deep water (14-35m depth).    The species is unusual in that it is the only species of the genus Tridacna which does not have the mantle overhang the edge of the shell.  It was first cultured at the  Tonga Fisheries hatchery at Sopu, Tongatapu around 1990-91.  There have been reports of findings off outer reefs of the GBR, so the natural distribution may be further extended. 

 

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Hippopus hippopus, the horse's hoof clam, the bear paw clam, the rolling clam, or the strawberry clam.  This species reaches 50 cm shell length and is often found on sandy areas and on seagrass beds.   The mantle does not extend over the shell edge.  It is widespread in SE Asia, Australia to Vanuatu and parts of Micronesia.  It was recently extinct from Fiji and Tonga. 

 

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Hippopus porcellanus, the china or porcelain clam.  This species has a smoother shell than H. hippopus and generally reaches <50 cm shell length.  The natural distribution of the species is eastern Indonesia, southern Philipines, Palau, and Papua New Guinea.  It has been cultured in Palau, Philippines and Indonesia, but continues to be a rare species.    Dr. Braley took this photo of broodstock collected from reefs around Barrang Lompo Island, So. Sulawesi, Indonesia in the mid-90s.

 

This is a possible 9th species of giant clam which was named by Sirenko and Scarlato(1991) based on a shell specimen only.  The authors have called this clam Tridacna rosewateri.  It is from the Indian Ocean (Mauritius area).  It is said to be similar to T. squamosa, as the scutes in this picture show.  

 

Tridacna costata sp., nov. (2008)
[found in the Red Sea and described by Richter et al., Collapse of a New Living Species of Giant Clam in the Red Sea, Current Biology
(2008), doi:10.1016/j.cub.2008.07.060 ]. This is a 10th species and may well be last new species to be described.

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Giant Clam Life Cycle

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After H.P. Calumpong, ed. 1992.  The Giant Clam: an Ocean Culture Manual.  ACIAR Monograph No. 16, 68 p.

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To see giant clam culture video clips on YouTube taken by Gerald Heslinga (currently of Indo-Pacific Seafarms, Kona, Hawaii) in the late 1980s at Orpheus Island, North Queensland, Australia and also in Palau in the mid-1990s click on the urls below:

1) Making an intertidal clam nursery (Orpheus Island, Australia)
http://www.youtube.com/watch?v=jCUvcA1aZWc&feature=related

2) Dr. John Lucas (James Cook University)
http://www.youtube.com/watch?v=gMR45mTAX4w&feature=related

3) Orpheus Island clam nursery (Australia)
http://www.youtube.com/watch?v=o-_TR4lOHAA&feature=related

4) Clams to Cash (Palau)
http://www.youtube.com/watch?v=Y9rXAAuIKbs&feature=more_related


Front Cover Photo by Dr. Rick Braley in the June 2003 issue of the Global Aquaculture Advocate [vol. 6, issue 3].  James Cook University (ACIAR-funded Giant Clam project) Research Assistants Jamie Whitford and Peter Lee move 5-year old Tridacna gigas in the Palm Island group.  Photo taken in 1991.

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Giant clam ocean nursery exclosure at low tide, Pioneer Bay, Orpheus Island Research Station, late-1980s.  The exclosure net floats to the tidal level, keeping potential clam predators out. 

 

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Ocean intertidal growout of Tridacna gigas at Orpheus Island, ~1989.  After 3 years of age the clams can be left on the substrate without protective cages or netting.  Research on production rates indicated 29 tonnes/ha/yr could be produced by these 'Solar Animals'. 

 

Translocations of Giant Clams

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OPERATION CLAMSAVER (named by the Great Barrier Reef Marine Park Authority [GBRMPA]Movement of cultured T. gigas from Orpheus Island Research Station (OIRS) reef flat to reefs in the GBR with the Australian Naval Vessel M.V. Tarakan.  Dr. Rick Braley was consulting for James Cook University to oversee the translocations, while the GBRMPA had an assisting officer / observer aboard.  When the operation first began there was a big media coverage.   All the major television channels had interviewers and camera crew fly in by float plane to get this on the national news.  The plan worked out by GBRMPA, JCU, and AIMS [Australian Institute of Marine Science]  was to move clams to reefs which were considered Source Reefs.  Currents travelling past these reefs during austral summer [same time as clam spawning season] theoretically would carry the larvae resulting from spawning on the source reef to other reefs designated Sink Reefs.   Given that we attempted to move clams from a single cohort to each source reef, it may be possible in future to confirm the Source / Sink theory by finding juvenile clams on sink reefs which would be genetically identical to the artificially-placed cultured populations translocated to these source reefs from OIRS.  Without giving exact details of reef names, the details of numbers moved, dates, and general area are shown below [note:  the trips which Dr. Braley was present at, representing JCU show an asterisk *]:

Date General Location Number of clams translocated
21 May 1992* Reefs in the vicinity of Townsville 1,305
23 May 1992* Reefs in the vicinity of Townsville 1,354
29 May 1992* Reefs in the vicinity of Cairns 1,020
30 March 1993 Reefs in the vicinity of Innisfail 1.400
1 April 1993 Reefs in the vicinity of Innisfail 1,166

There were also translocations of one cohort of T. gigas made by the Great Barrier Reef Aquarium (now called Reef HQ) from OIRS to 7 (seven) reefs in the vicinity of Townsville.  Each reef had 60 clams placed there (total 420 clams). 

Only in 2003 has there been a resurgence of interest by JCU and other organisations in these translocated clams.  Some genetic work has recently been done on the now-grown-up clams by JCU.  Aquasearch would like to have some future involvement with any studies which are carried out on these high density artificially placed populations. 

 

First generation (F1) cultured T. gigas in display tanks at Aquasearch Aquarium, Magnetic Island.

Note the bleached area down the centre of the mantle. Small bleached areas may last a long time, but in some clam individuals zooxanthellae can rejuvenate in bleached areas in less than a year.

Clown anemonefish treat this T. gigas as a host, something that would never be seen in nature due to competition from tougher damselfish.

 

Tridacna gigas (cultured) at White Lady Bay, Magnetic Island with Dr. Rick Braley (below):

 

Photo of Rick Braley holding up a Tridacna gigas at Orpheus Island, in an action photo of water squirting from both siphons. This photo and a story entitled "Giant clams like cabbages in an ocean garden" written by Sybil Nolan appeared on the front page of the Weekend Australian 9-10 May 1987 (below):

 

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My first sighting of the true giant clam, Tridacna gigas,  in December 1973, Tonumea Island, southern Ha’apai group, Tonga Islands.

I was working as a Smithsonian / Peace Corps Environmental program volunteer with Fisheries, Tonga.  I was on a turtle nesting survey and seabird survey in uninhabited islands of the southern Ha’apai group of Tonga.  The photos below are of 4 islands in this group with Tonumea being the northernmost.  Note the two channels cutting out through the fringing reef on the southern side of Tonumea.  This is where I saw my first true giant clam, Tridacna gigas.  Joseph Rosewater in Indo-Pacific Mollusca, vol. 1, no.6 (April 30, 1965) does not show the distribution going eastward beyond Fiji.  While I do not have a photo of that clam it is indelibly printed on my mind.  I was used to diving in Tonga to collect T. derasa, T. squamosa,or  T. maxima but the size of this clam took my breath away.  It was humbling to look at how massive a bivalve mollusc could grow.  As it turns out, this uninhabited island was possibly one of the last refugias for this species in Tonga.  Selective collection for food by the Polynesian people was the cause of it local extinction.

 

Tonumea Is. northernmost, Nuku 1 and tiny Nuku 2 south of Tonumea, and Kelefesia Is. southernmost.

Tonumea Is. and fringing reef.

Close-up view of channels on the SSW side of Tonumea is. where I saw my first Tridacna gigas.

Giant clams moved into the snorkel trails on Magnetic Island which Dr. Braley designed and setup for TOBMI (Tourism Operators and Businesses Magnetic Island). The move was made on 13 June 2013 and the following link shows the evening news on Channel 7 (Townsville) on 19 June 6pm 2013:

http://www.youtube.com/watch?v=0jRxgkwjy5U&feature=youtube_gdata_player


Click here to read papers by Dr. Braley on Giant Clams

 

 

 

 

 

Click here to read about the idea of giant clams being used on a space station in the future:

Clams in Space

 

 

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Updated18 September 2014; Copyright Aquasearch