This is a photo of two river otters from www.pgc.state.pa.us. River otters belong to the family Mustelidae which includes weasels and pine martens. The average weight for a river otter is 18 pounds. They are on average 40 inches long. Its eyes are adapted for underwater vision and it has short, powerful legs with webbed feet which enable it to swim well. They have a diet of fish mainly but sometimes also crabs and crayfish.