KONSTANTINIDIS, Peter*; GEMBALLA , Sven; University of Tuebingen, Germany; University of Tuebingen, Germany: Derived trunk morphology in a thunniform swimmer: The musculotendinous system of Euthynnus alletteratus

Tunas are apex pelagic predators being well adapted to fast and efficient swimming: They have a fusiform body shape with the muscle mass being concentrated anteriorly; their body tapers to a narrow caudal peduncle that gives rise to a lunate caudal fin. Given this highly derived external body design in tunas we also expect internal structures of their locomotory system, such as the segmented musculotendinous system, to be derived and well adapted to fast and efficient swimming. For the first time, we examine the musculotendinous system of a tuna (Euthynnus alletteratus) and compare it to that of locomotor generalists. Results show several morphological specializations of the musculotendinous system in Euthynnus: The longitudinally arranged myoseptal tendons (myorhabdoid tendons and lateral tendons) are prominent and elongated when compared to other teleost fishes. Caudally, the myorhabdoid tendons and the lateral tendons of subsequent segments merge, and form the medial and great caudal tendons of the caudal peduncle. Red muscles are internalized and associated with lateral tendons. This association indicates a novel pathway of transmission of red muscle forces through the series of lateral tendons, and via caudal tendons to the caudal fin.