While the female angler fish may measure three feet or more and weigh twenty or thirty pounds, the tiny males of some species have a maximum length of about four inches, and in some cases are so small that they can be seen only with difficulty.
A short time after hatching, the male attaches himself to the body of the female, and although the female continues to grow the male does not, to any extent. The tissues of the two fuse together, and the two blood systems become connected. After this happens, the male is absolutely dependent upon the female, and draws nourishment from her blood stream.
In 1936, one of these female anglers was caught in a net off the coast of Massachusetts at a depth of 600 feet. This fish was thirty-three and a half inches long and it weighed twenty pounds. Study revealed that a tiny male, only one and a half inches in length was attached to the skin of the female’s body just behind the gill covers.
It is not known why this peculiar association should have developed, but its advantages are apparent.
Most anglers of this type are deep sea fish and live in relative darkness most of their lives. Since the two sexes are attached almost from birth, when the breeding season arrives the males and females do not have to go blundering around in the darkness seeking a mate.
Comparable associations between male and female animals without backbones are known, but the angler fish are the only known vertebrates in which such a situation occurs.