Marshall Islands Giant Clams
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There is no way mere words can describe the incredible colors, patterns or variability of these truly magnificent creatures. The numerous photos on the subsequent pages offer only a hint of the color and variety. It seems that like fingerprints, no two are alike. On some reefs, they are rather sparsely distributed, but on others you cannot look in any direction without seeing many clams.
The Tridacninae (members of the family Cardiidae) in the Marshall Islands consists of four species in two genera.
Tridacna gigas are the real giants, certainly the most massive bivalve mollusks in the world. While there have been stories of larger specimens, the largest reliable record I've seen in the literature measured about 137cm, or about 54 inches. One of these days I need to bring a measuring tape along with me to verify, but using a rather inexact measurement (holding out my arms and measuring later) of one of the large T. gigas we visit occasionally, it appeared to be about 52 inches. Large individuals rest loose on the substrate, usually upright, but sometimes we find them rolled over. There have been a fair number we have pushed or propped upright--sometimes with great effort, since large ones can weigh many hundreds of pounds. Tridacna gigas is the least abundant of the Tridacninae here; it is downright rare or absent near inhabited islands but is still reasonably common in some parts of the atoll.
By far the most abundant of the Tridacninae in the Marshalls is Tridacna maxima. The name maxima must refer to their numbers, since these are actually the smallest clams of the family found here. Maximum size is probably not much more than 30cm (about 12 inches) but most are smaller. They are found from the intertidal to depths of 30m or more, although they prefer shallow reeftops. On some reefs you can see hundreds of variably and sometimes brilliantly colored animals in a short time. T. maxima usually bores a bit into the reef as it grows, becoming embedded in the substrate and attached to the underlying reef with a fibrous byssus. Still, they seem to be a favorite prey for both octopus and nurse sharks (as well as some of the local islanders). We have seen nurse sharks at night twisting their bodies around to pull a clam out of the reef.
Tridacna squamosa is one of our favorites. The shell usually bears distinct flutes easily visible in many of the photos. These are medium-sized clams, maxing out at maybe about 50cm, although most are smaller. These clams do not embed themselves in the reef as do T. maxima, but they are usually attached to the substrate with a byssus. Sometimes the larger specimens are found loose on the bottom. Because of this, we often find ourselves uprighting shells that have rolled over. The color and pattern of these animals are extremely variable; we are excited when we find one of the rarer blue or bright green color forms.
The final Tridacnid known from the Marshalls is Hippopus hippopus. This one is usually intermediate in size between T. gigas and T. maxima and usually sits loose on sandy or rocky bottoms. Less variable in color than the three species of Tridacna, it nonetheless has an interesting pattern of close-set lines over backgrounds of light yellow to dark gray, often ornamented with tinges of iridescent blue or green.
The Tridacninae can flourish on these reefs in part because of the symbiotic relationship they have with single-celled algae called zooxanthellae. As in reef corals, large numbers of these microscopic plants live within the tissue of the giant clams. Whenever there's sunlight the algae converts carbon dioxide produced by the clam into oxygen and food that the clam can use. The relationship is advantageous to both clam and algae. But it does requires light. Consequently, the clams do not live very deep. We have seen T. gigas as deep as 30m (about 100 feet). Recently on the seaward slope we saw an apparently healthy T. squamosa at 38m (125 feet), but it could have rolled down from above.
In some Pacific islands, the Tridacninae, particularly the large T. gigas and to a lesser extent, T. squamosa, have become very rare or even locally extinct from fishing pressure. Kwajalein Atoll in the Marshalls is partially leased as a testing range by the US military and living on the central atoll islands is restricted. While the large T. gigas have nearly been fished out from the southern part of the atoll where most of the islanders live, there are still reasonably healthy populations in some less accessible parts of the atoll, although fishermen do make trips into those areas.
One of us spent several years working at the Mid-Pacific Research Lab on Enewetak Atoll, where nuclear testing in the 1940s and 1950s required the relocation of the atoll's original inhabitants to other atolls for more than 30 years. During that time, despite the detonation of 43 nuclear "devices" from the late 40s to late 50s, the lack of fishing pressure allowed the Tridacnid clam population to bloom, an unintended positive result of displacing a local population, blowing up some islands and contaminating a bunch more with radiation. I remember snorkeling some shallow interisland channels over numerous large and magnificent T. gigas as late as 1981. But the islanders were finally repatriated to the atoll in 1980, and by the time I left Enewetak in 1983, that one channel at least held nothing but empty shells.
To a photographer and naturalist, it seems tragic to lose these magnificent animals, which some have suggested could have lived as long as 100 years, for the sake of a meal or two. But conditions in the outer atolls of the Marshalls are not always conducive to survival. The people living in these atolls learned to do what they had to do to live, particularly during lean years. Unfortunately, the clams cannot cope with modern outboard motors and dive equipment. Populations are at risk.
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