Marine Biology (2+1)

Donax cuneatus

Family : Donacidae

Common Name : Wedge clam, Beach clam

Habit : Surf beaches

Size : 46 mm length

Shell medium sized triangular and compressed. Colour of the shell extremely variable, ranging from dull white to yellowish brown or grayish brown, commonly with dark brown or mauve rays radiating from umbo to ventral margin. Anterior margin is more rounded; ventral margin slightly convex and almost straight. Posterior dorsal margin short and steeply sloped. A short ligament close to umbo on the postero dorsal margin. Posterior margin is truncated. Sculpture with fine concentric striae, slightly and conspicuoulsly ridged on the posterior end. Radial striae fine, thread like except on posterior lobe. Lunule present on both the valves. Anterior adductor muscle scar oval shaped and posterior adductor to muscle scar rounded. The pallial sinus is exceptionally large and broader. The ventral margin of the pallial sinus is parallelly joining the pallial line. The cardinal teeth in D. cuneatus are distinct. The strong external ligament keeps the two valves intact when the animal dies and strewn onto the beach the two shells appear like “butterfly wings”.

This is the primary species of donacids of the East coast of India. Clean, sandy beaches are characterized by Donax cuneatus and these beaches can be referred to as ‘Donax beaches’. Beaches that are clean and generally of white quartz sand and highly populated with Donax are firm and compact underfoot. They do not retain standing water, and are always perceptibly sloped, but the gradient is always gentle, seldom above 5◦,the profile steepening only at the berm or where coarse sand gives place to gravel.

Donax cuneatus with its short siphons exists in huge numbers just beneath the sand surface at mid to low tide level. The streamlined shells can be pulled almost instantly into an upright position, and drawn down rapidly into the sand. In many places the Donax species have been shown to migrate with the rise and fall of the tides. The sand burrowing mole crab, Emerita asiatica, regularly accompanies Donax cuneatus. Spawning is initiated around December-January and extends up to June.

Recently it has been reported that the colour variations in D. cuneatus are pseudopolymorphic.