The trunk lateral line system and its innervation in Mugil cephalus (Mugilidae: Mugiliformes)

The trunk lateral line system and its innervation were examined in Mugil cephalus. Almost every scale had an open longitudinal groove that housed a superficial neuromast (SN), the total number of SNs being 550 (including 55 on the caudal fin). Grooved lateral line scales constituted 14 longitudinal rows (14 lateral lines), although each groove was independent, not forming a continuous common groove along adjacent scales. The lateral ramus of the posterior lateral line nerve gave off four characteristic ramules: the anteriormost two dorsal ramules extended strongly backward, innervating SNs on the dorsal surface of the trunk; the anteriormost ventral ramule was extensively ramified to innervate SNs on the anterior flank, passing ventrally in front of the pectoral fin base; and the fifth ventral ramule extended posteriorly close to the anal fin origin, innervating SNs on the ventral half of the middle flank. The remaining SNs were innervated by 29 dorsal, 33 lateral and 25 ventral ramules, excepting those on the caudal fin. The innervation pattern revealed that the sixth lateral line (from uppermost) was homologous with that typically found in teleosts, the others being considered as additional. The sixth lateral line was anteriorly regressive, suggesting that the high insertion of the pectoral fin on the trunk is a secondary condition.