Notostomum cyclostoma have direct development within cocoons deposited on the carapaces of various species of crab, including the Red King Crab, the Golden King Crab, and Tanner Crab.  The relationship between the crab and the leech is virtually unknown other than the carapaces provide a harder surface for cocoon deposition than is provided by the soft undersides of the leech host.  It is thought that the leeches do not feed on the crab, though they are often found in their gill chambers.

Although the leeches are hermaphroditic, meaning they contain reproductive organs of both sexes, they are also protandric, meaning they only produce either female or male gametes at any one time.  Because of this, leeches cannot reproduce asexually, and they do not regenerate body parts like other annelids; instead, they reproduce through sexual copulation.  Sperm are transferred from the clitellar region of the male into the clitellar region of the female through a penis, and fertilization occurs internally.  After impregnation the clitellum of the female, consisting of three of the body segments, secretes a cocoon that is shed off over the head, picking up the fertilized egg as it passes over the female gonopore. 

 The small egg cocoons are brown in color and cylindrical in shape, and contain one developing larva per cocoon.  The immature leach then hatches out of the egg and swims through the ocean until it finds a compatible host in which to feed off of.  Most leaches have one or two breeding periods within their one to two year life cycles.