The phylum mollusca has a Class named Pelecypoda, which means hatchet footed and refers to the shape of the foot in these mostly sessile bivalves. Clams and Oysters, Mussels and Pippies are all pelecypods Some of the most economically important shellfish are pelecypods, starting with the Pearl Oysters but including all manner of edible species, both cultivated and wild, that grace the tables of rich diners in big cities the world over. Some of the most economically destructive shellfish are also pelecypods, the so called “shipworms”, which are really bivalved molluscs that burrow, not only into ships, but wooden piers and wharves as well. How many sailors have been lost over the millenia because of the depredations of ancient shipworms? No one will ever know but it would have been a lot.
My favourite pelecypods are the Sydney Rock Oyster, because it gave me my first paid job as a zoologist, and the giant clams of the Barrier Reef, because of their luscious lips. Every year I get to go to the Barrier Reef with a swag of year 11 biology students and I spend most of my time cruising over the reefs with mask and snorkel or walking on sand tracks around the inner reef, where there are lots of big clams of the genus Tridacna. Not the really big ones and mostly young ones but they have the most beautifully colored and patterned lips, with the help of zooxanthellae. These are single celled symbiotic algae that also live in and color the corals.
I love to try and get photos of clam lips, some chocolate browns with creamy yellow markings, some bright blues or greens. But these clams are very precious about their lips and retract them into the shell at the slightest change in water currents. I try to sneak up on them but I have a lot of half-closed shell photos and very few full-lip photos and in fact the best ones were taken through glass in the big salt water aquaria that the Heron Island Research Station had before the big fire. I’m afraid I lost some good invertebrate friends in that fire: a couple of sea hares, some big blue and smaller orange starfish, two pairs of clown fish, “Nemos”, defending their anenomes, a wonderful red and white striped shrimp and a couple of nice clams that let me get a few truly luscious photos of these quietly beautiful animals.
For the most part, pelecypods do not produce shells as fancy as those of the gastropods. Clams are flattened from side to side and the two valves of the shell are fastened dorsally by an elastic ligament. There is an enlarged hump called an umbo at one end of the ligament. This is the oldest part of the shell, where the animal lived when it was small. As it grew and continues to grow, the mantle secretes successive layers of shell in concentric rings to give more living space.
The structure of the shell gives an insight into both the source of mother of pearl and pearls themselves. I will quote Buchsbaum here (p 209): “The shell consists of three layers. The dark, horny, outer layer (periostracum) forms the ligament and protects the calcareous shell from being dissolved by carbonic acid in the water. It is thin and is usually eroded from the older parts of the shell… The middle layer (prismatic layer) consists largely of crystals of calcium carbonate arranged perpendicularly to the surface of the shell. The innermost or pearly layer (nacreous layer) consists mostly of thin sheets of calcium carbonate laid down parallel to the surface of the shell. The first two layers are secreted only by the edge of the mantle, and hence show the concentric markings of discontinuous growth. The inner, pearly layer is laid down by the whole surface of the mantle and has a smooth, lustrous surface.” Pearls are formed in the same way around irritants such as grains of sand.
We live in the age of plastic but before that was the age of Pearl, when mother of pearl was used to make combs, buttons and jewelry and box lids and all sorts of other household and personal items. Pearls were rare back then and so very valuable. Now the pearls are cultured and the mother of pearl items have been replaced by plastic. Pity about that really, in some ways.
Oysters, clams and mussels are filter feeders, peeking out of their protective shells and then swirling the water with their cilia to draw in food particles. They can also draw in pollutants so one should be careful about eating raw oysters unless you know where they came from. As adults, bivalves are primarily concerned with only two activities; eating and reproducing, so their bodies consist mainly of digestive and reproductive organs. In the Sydney Rock Oyster, the best oysters are ‘fat’ which means they are full of eggs or sperm, a condition that may be what gives them the reputation of being aphrodisiacs. They are certainly packed with vitamins, minerals, proteins and fats and as such are a high quality food item, a fact not missed on hunter-gatherer cultures the world over. Aboriginal people in Australia for instance have left huge shell middens behind, meters deep, and mute testimony to thousands of years of shellfish consumption and enjoyment.
Pelecypods have separate sexes and shed their eggs and sperm together into the water. The larvae live in the plankton until big enough to settle and develop an adult shell and lifestyle. Most bivalves inhabit marine benthic communities or rocky intertidal ecosystems but some clams have successfully settled freshwater habitats too.
Reference: Buchsbaum. R. 1968. Animals without Backbones. Penguin Ed.
