Sonic motor pathways in teleost fishes: a comparative HRP study

Among marine teleost fishes, one neuroeffector pathway for sonic communication consists of two components: a peripheral effector organ that consists of a swimbladder with associated 'drum' muscles, and a swimbladder or 'sonic' motor nucleus (SMN) located at the junction of the spinal cord and the medulla oblongata. Here, the organization of the SMN is compared in two unrelated groups of teleosts, the midshipmen, Porichthys notatus and P. myriaster, and the sea robin, Prionotus carolinus. Horseradish peroxidase (HRP), used as a retrograde tracer, revealed the position of the SMN in each species. While the SMN is a fused midline structure in midshipmen, it is bilateral in sea robins. The functional significance of these two contrasting patterns of organization remains to be explored. A third study group included mormyrid freshwater electric fish, which are also sonic. Mormyrids were included in part because an earlier study identified androgen-binding cells at a brain level comparable to that of the SMN of marine fishes. Using HRP methods, a swimbladder motor nucleus was identified at the caudal pole of the vagal motor column. However, the nucleus in mormyrids lies dorsal to the fourth ventricle and central canal, not ventral as it does in midshipmen and sea robins. Its position corresponds to the steroid-concentrating cells identified in a previous study.