Clam shrimp (formerly all grouped in order Conchostraca) are actually crustaceans and are not related to clams at all. Their carapace takes the form of two brownish, hinged, dish-shaped shells, which they clamp shut when disturbed, so they look a lot like clams. However, the animal inside has jointed, shrimplike legs (up to 32 pairs) and bristly, feathery antennae, which they extend outside the shell and use for swimming. Like clams, they have a strong muscle that clamps the two shells together when the animal is disturbed. These fascinating animals are related to fairy shrimp, water fleas, and tadpole shrimp.
Shell length: usually less than ½ inch long.
Habitat and Conservation
Clam shrimp may live in a variety of aquatic habitats. Different species prefer different types.
Organic detritus or microscopic animals (zooplankton). The sweeping motions of the legs carry food particles to the mouth.
Clam shrimps used to all be grouped together into a single order — Conchostraca — in class Branchiopoda, along with sister orders such as the fairy shrimps (Anostraca), tadpole shrimps (Notostraca), and water fleas (Cladocera). But it turns out that the clam shrimps actually comprise three unrelated groups, with separate lineages, so the order Conchostraca has been erased, and three new orders have been created: Cyclestherida, Laevicaudata, and Spinicaudata. Meanwhile, as an alternative to this system, some scientists consider the three groups to be suborders within order Diplostraca (which also includes the water fleas/cladocerans).
Reproduction in clam shrimp varies according to the family. For example, some groups are hermaphroditic (where each individual functions as both male and female), while others have individuals with separate genders.
Eggs are produced and, at least in some species, females carry eggs along the top part of their body, inside the shell. With the shells often being transparent, you may be able to see these through the shell. Eggs are deposited into suitable habitats but have tough shells and can withstand dry or freezing conditions; in some cases, they have been known to hatch after 7 years. They are often one of the aquatic invertebrates found in ephemeral, or vernal pools, which fill with water in springtime but then dry up for the rest of summer and winter.
Like many other crustaceans, the newly hatched clam shrimp are nauplius (NAH-plee-us) larvae, which have appendages on the head and only a single eye. In the next stage, called a metanauplius, they have their first small shell.
Going on field trips to study pond life is a favorite part of many children’s science education. In addition to the fun of being out in nature, it also teaches kids how stream ecologists can gauge a stream’s health by what kinds of invertebrates are present. Kids use dip nets and shallow white bowls to fish out, sort through, and identify the many strange invertebrates that live in the shallow water of a pond edge. Clam shrimp can be one of the discoveries. What a surprise to find these small, nifty creatures!
Clam shrimp occur in the fossil record as early as the Devonian period, about 400 million years ago. The Devonian is often called the Age of Fishes because during that time ancient, armored placoderm fishes dominated aquatic habitats and early cartilaginous and bony fishes also diversified and flourished. During the Triassic, many clam shrimp species lived in saltwater, though today, all clam shrimp live in freshwater. At times in geologic history, clam shrimp used to be more abundant and diverse. Around 300 extinct species are known from fossils, but only about 150 species live today.