The animals are either male or female. During reproduction eggs and sperm are released into the ocean, where fertilization takes place. The young clams then free themselves and burrow into mud or sand. Some clams are very long-lived. Sturgeons and paddlefishes, family Polyodontidae, are the only surviving members of the Chonodrostei, a group that includes many extinct fish families.
Sturgeons have an elongated body and head and a sharklike upper lobe of the tail fin.
The mouth lies far below the head and has four fleshy barbels in front of it. The head itself, sometimes flattened, is covered by large interlocking skull bones. Sturgeons are scaleless except for five rows of large, pointed, platelike scales running along the top and sides of the body. Their skeleton is part bone and part cartilage, placing them midway between sharks and bony fishes. They are confined to Northern Hemisphere waters.