Anatomy and Sexual Dimorphism of the Swim Bladder and Vertebral Column in Ophidion Holbrooki (Pisces: Ophidiidae)

The Family Ophidiidae exhibits marked sexual dimorphism among a number of its species. An example of this is found in Ophidion holbrooki (Putnam, 1874) in which the vertebral column and swim bladder of the female and male differ markedly. Both sexes have the dorsal area of the first vertebra modified to form a winglike process. In the male the third to the sixth vertebrae are modified to form a supporting mechanism in which a free “bone,” referred to as the rocker, is seated. In the female there is no supporting mechanism or rocker. However, the third vertebra has a modified transverse process which attaches to the swim bladder.

The physoclistous swim bladder of the male and that of the female are similar except for the anterior portion. The anterior end in the male swim bladder is associated with the rocker and in the female the anterior end has two round hollowed protuberances.

In both sexes three similar sets of muscles are found in association with the swim bladder, the vertebral column, and the skull. One set of muscles differs in that in the male the set attaches to and rotates the rocker while in the female the place of attachment is directly to the swim bladder.

A description is given of the functional morphology of the muscles, of the innervation of the associated musculature and the swim bladder by the vagus nerve, and of the blood supply of the swim bladder.

The swim bladder probably functions as a sound producing organ in both sexes. No live material was studied and therefore no evidence is available to support or reject this theory.