So the megamouth is presumed to be a vertical migrator on a diel cycle, spending the daytime in deep waters and ascending to midwater depths at night. Rep. NMFS 90 Body stout, tapering posteriorly. Gill slits moderate long but not reaching dorsal surface of head. Mouth broad, terminal, corner extending behind eyes. Caudal peduncle without keels or ridges.
Pectoral fins shorter than head length in adults.
Caudal fin asymmetrical, with pronounced ventral lobe. Coloration Dorsal surface of body, pectoral and pelvic fins, dorsal fins, center of anal fin and caudal fin are blackish brown. Ventral surface of body, below level of pectoral end pelvic fins, tips and posterior margins of pectoral and pelvic fins, abruptly white, as the posterior margins of dorsal and anal fins and postventral caudal margins. Dorsal side of head is blackish brown, except areas between nostril and eye and between eye and spiracle, which are paler than the rest of dorsal area of head. Lateral side of head behind mouth corner is darker.