Atlantic jackknife clam

The Atlantic jackknife clam is a species of razor clams known by the scientific name Ensis directus.

Anatomy
The razor clam has a slender, lengthy, slightly curved shell. This shell has an outer tissue-like layer called the periostracum that changes color as the bicuspid grows. When the clam is very young this layer is brown. In adolescence and adult life it is a brownish-yellow, and brown again with old age. The periostracum in large and older razorclams are usually eroded. The inner part of the shell is normally a glossy white. Occasionally, purple areas may be showing through this layer. A prominent rib extending from the upper part of the shell to the shell edge is also evident. This shell is created by the mantle. The mantle has two parts and each part of the mantle secretes a shell. These two layers are connected by an elastic ligament that allows the shell to open and close. The clam will grow about 6 cm by it's first winter and can grow up to 17 cm.

Reproduction
Most jackknife clams will be sexually mature within the third and seventh growing season. Mating can occur between May and September and is associated greatly with the water temperature. 55°F is believed to be the perfect temperature for mating. There are separate genders for the razor clam. In the species Ensis directus, the male will release sperm into the water which will enter the female through openings and are implanted in the gill with the eggs. These fertilized eggs will remain in the gills until they are larva. They are released into the surrounding water as first stage larva. The first-stage larva are free-swimming, pear-shaped, translucent, and ciliated. The first stage is considered the trocophore stage. They also go through a second larval stage called the veliger stage, which is also a free-swimming stage. This long pelagic stage allows the larvae to be scattered. Once it is done with this 5 to 16 week larval stage it will settle in the sand or mud and start to develop. The body, as well as the mantle, will develop creating the shell. 

Ecology
Razor clams live on sand beaches in low to sub-tidal zones. They live at depths of 1-20 meters down in the sand. It is normally found on the East Coast of North America and Canada, but can be found in The Northern coasts of Belgium, the Netherlands, Germany, Britain, and Denmark. 

If the clam ever feels threatened, it can use its foot to dig into the sand, using it like a wedge. It is compressed and extended to create the hole. Some of these clams have been known to dig and disappear within 15 seconds. Once the razor clam has made it down far enough it relaxes its abductor muscle and pushes the shell open compacting the sand around it to make it hard. The foot is then made into an anchor by forcing blood to enter the foot. When the foot is enlarged, it contracts the longitudinal muscles in its foot sending the clam downwards. If the animal wishes to return to the surface, it uses its foot to push itself upward. The Razor Clam only surfaces at high tide. The species Ensis directus will allow its two siphons are exposed. These siphons are used to filter food and water. The razor clam can also swim through the water by expelling water through the shell and drawing in its foot. This action is repeated to continue the action of swimming. 

Razor clams are filter feeders that draw its food from the water around it. When feeding, the razor clam will stay close to the surface and expose its siphons. Cilia covering the gills pulls food into the shell. As the food passes along the gills (ctenidia) they combine with mucous and become trapped. The cilia along the gills then move the food to the digestive tract. They mainly feed upon small plants and plankton (animal). 

The clam has a positive influence on humans because it can be sold and eaten. However, razor clams can damage trawls and other fishing nets. They may also leave deep cuts if stepped on.