Alastair Sime, of Mosgiel, asks :-
I enjoy getting cockles from a sandy beach near Dunedin, cooking and eating them. But I have realised that I know nothing about their life cycle and would appreciate an outline from conception; including an explanation of how their shells grow, what age they are when the shell reaches about 4 cm across, and why the shell "clicks" open suddenly when heated towards boiling temperature
Roger Belton, of Southern Clams Ltd (nzclams.com) responded.
The 'cockle', or more correctly ‘clam’ you ask about (Austrovenus stutchburyi) like many bivalves in the Veneridae family has adapted to sandy inshore habitats. This accounts for most of the reproductive and growth characteristics of these shellfish. As they live burrowed into the sand, and have a relatively small foot and round shell profile they cannot move much. The NZ littleneck clam or 'cockle' generally lives buried just below the sand surface. Called 'littlenecks' in the USA, on account of their short feeding siphons, or 'necks', they live in relatively stable tidal sand banks or mud. They are not well adapted to mobile soft sands, as unlike tuatua or pipi they are not very good at moving up or down in the substrate. Also their shorter siphons mean they cannot feed when buried too deep.
'Conception' takes place with the fertilisation of ova in the surrounding sea water, which means there is no need for contact between males and females. When there is plenty of algal feed the gonad grows and swells with sperm or ova. This can be seen as a creamy, sometimes milky layer around and below the dark digestive gland. When 'ripe' under certain conditions (often with a cold snap in summer) the shellfish spawn. It appears that once initiated, there tends to be a mass spawning of sperm and ova into the water. Following fertilisation zygotes develop into free swimming veligers. These remain in the water for from two to three weeks, feeding on algae, presumably moving in and out of the inlets with the tides, and being swept with the currents. It’s a pretty-hit-and-miss process with the vast majority of the billions of veligers never finding a suitable home.
For those that happen to be in a suitable habitat when they 'set', the microscopic juveniles attach to course sand or shell fragments with a small byssal string. This is similar to a mussel 'beard', but does not last long. The newly set clam grows a shell quickly, but is still very vulnerable to predators. It has been reported that large female clam can produce up to half a million ova in a season, the vast majority of which never survive to become adults.
The shellfish feed by filtering plankton from the water. Shell growth uses the calcium and proteins in algal feed to form the various complex calcium/protein matrixes which form the shell. The new shell laid down on the outer edge of the valves in summer can be seen as a softer fawn colour layer, especially in fast growth areas. There is strong growth in periods of abundant feed, usually in spring and autumn. The exact mechanisms for shell growth are complex, and well beyond my understanding.
The "click" of the shell opening on cooking is readily explained by the way the shellfish are adapted to their habitat. In order to feed and respire the two 'valves' of the shellfish must be able to open. These two valves are held closed by the two adductor muscles inside the shell. Closing is easily and quickly achieved on muscle retraction.
But opening cannot be achieved using the same mechanism. So the two valves are in a sense 'spring loaded' at the hinge (or umbo) so that when the adductor muscles relax the valves spring open. This has to be quite a strong 'spring' for shellfish living in the sand. When you cook the clams the two adductor muscles detach from the shell, and the umbo spring (a dark brown firm elastic section at the hinge) pops them open. You can feel this when you take gaping cooked clam shells and close them by hand.
Notice also that mussels don't have the same strong umbo spring. They don't need it of course as they don't live in the sand.
The age to size relationship in clams is very variable. A 40mm littleneck may be anything from 3 to 25 years old, depending on the productivity of the site and availability of nutrients. Those clams near the channels get the first feed off incoming algal rich water, and grow much faster than those at the back of the inlet which have only the 'crumbs' of what is left.