Tidal flats or mud flats are located in quiet sheltered areas on open shorelines where mud and sand are deposited. They are frequently part of a salt marsh system. The muddy substrate has a high concentration of organic detritus populated by large colonies of decomposing bacteria. Dissolved oxygen readings are low particularly in the deeper sediments. The area is exposed at low tide and the substrate may contain several burrowing species as well as a rich meiofauna.
Walking along a mud flat at low tide can reveal the presence of many living things. Some of the common mollusks living in this habitat include the soft shell clam (Mya arenaria) which may squirt water from small round holes in the mud. One of the fastest diggers is the razor clam (Ensis directus). It uses its foot to dig into the substrate very quickly whenever it needs to hide. The razor clam shell is about five times longer than it is wide and is brown along the edges. These two mollusks are bivalves and filter feeders in which they take in water through a siphon to trap and select food particles.
Other animals in this habitat are deposit feeders and include several polychaete worms. These animals obtain their nutrition by consuming the organic material in the sediments. The most common is the clam or sand worm (Nereis virens) which can grow over a foot in length. This brightly colored species can bite so caution should be exercised when handling them. Another common worm is the lug worm (Arenicola marina) that lives in a U-shaped burrow. Though this animal is inconspicuous along the shore, its castings are commonly found.
Other animals that can be seen include small dark snails that may be feeding on the carcass of a fish or a crab. These scavengers are the eastern mud snails (Ilyanassa obsoleta) that move in large numbers to a food source. Several crab species in this area include the blue crab (Callinectes sapida), characterized by its blue carapace, saddle-shaped hind legs, and sharp spines on the edges of its shell. Also the spider crab (Libinia emarginata) has a brown spiny carapace that camouflages it against the muddy bottom.
Mud flats are interesting habitats and best observed at low tide, particularly at locations such as First Encounter Beach in Eastham, Massachusetts.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.