There are 12 species of fresh water mussels. Eleven of them occur in the mainstem of the Connecticut; the brook floater is found only in small streams and rivers. Species diversity is higher in the southern part of the watershed (Connecticut and Massachusetts) than in the northern part (Vermont and New Hampshire), largely due to differences in stream gradient and substrate. Eight of the 12 species in the watershed are listed as endangered, threatened, or of special concern in one or more of the states in the watershed.