Both mussels and steamers have no backbone and live in the water. They have shells that protect their bodies. Mussels and steamers seem similar at first, but they have various differentiating features.
Both mussels and steamers have the same elongated shape to their hinged shells. The steamer or soft-shelled clam has a pair of thin, white shells, while the mussel has darker, thicker shells. The steamer also has a siphon that juts out of the shell.
A steamer usually measures 5 to 7.5 inches when harvested, while an average mussel measures about 3 inches. As such, recipes sometimes call for substituting up to twice the number of mussels for a certain number of steamers.
Steamers live in medium-soft sand, compact clay, coarse gravel or mud. People often harvest steamers from sandbars. Mussels attach themselves to rocks, piers, ropes and even other shellfish.