One of the most fascinating creatures of the reef were the giant clams (Tridacna sp., Family Tridacnidae), which could be found both as large "free-standing" individuals or as smaller clams embedded in the coral. Amazingly, it used to be thought that divers could get themselves "trapped" by these clams, but when put to the test, I found that they closed slowly, and never completely.
Tridacna gigas, the classic Giant Clam, at Michaelmas cove, off of Cairns.
Tridacna gigas again, this time with the mantle retracted. Interestingly, these clams get about 70% of their energy from photosynthesis, due to symbiotic algae in the mantle.
This one is Tridacna crocea, a smaller species (this individual is about 10cm long) almost always found embedded in coral up to the margins. These were quite common in large coral heads, but easily overlooked if you weren't keeping an eye open.
It was fascinating to swim over these clams. The mantles were often a bright bluish purple which was almost luminescent. You could easily get a fist in each siphon (although probably not out again)
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