SCHAAD, E. W.*; GOBALET, K. W.; California State University, Bakersfield; California State University, Bakersfield: What is the function of the caudal opening of the posterior myodome of scombrid fishes?

The posterior myodome of actinopterygian fishes is a prominent recess of the neurocranium ventral to the braincase. Its anterior opening is continuous with the orbit and it is defined ventrally by the parasphenoid and laterally and dorsally by the basioccipital and prootic. Dry neurocrania illustrate that many actinopterygians have caudal openings of the posterior myodome of varying sizes. Some scombrids have a dramatic ventrally convex posterior opening of the myodome of unknown function.Little attention has been given to the myodome itself or to the distinctive caudal opening in scombrids. A reasonable explanation for the function of the myodome is that it servers to house one or more of the four recti muscles. It may also serve as a tube through which elongate recti muscles pass and that originate caudal to it. The purpose of this study has been to explore the function of the myodome in scombrids bearing the prominent enlarged and arched posterior opening and to establish a context through comparison with other actinoperygians. Through dissections we found that a thick membrane closes the caudal end of the myodome and forms a separation as effective as a bony partition. In scombrids the transverse membrane is deep (rostral) to the arched caudal opening and the area caudal to the membrane is continuous with the region between the neurocranium, branchial apparatus, and head kidney. Because of the presence of the transverse membrane, the recti do not extend beyond the caudal opening of the posterior myodome in any fishes studied. In scombrids, only the lateral rectus is found within the myodome while the remaining three insert anterior to the myodome. Muscles do not attach to the arched caudal myodome opening in scombrids. The function of the dramatic bony arch of the posterior opening remains enigmatic.