The evolution of thunniform locomotion and heat conservation in scombrid fishes: New insights based on the morphology of Allothunnus fallai

Two significant functional differences-a more anterior and internally positioned red myotomal muscle mass and modification of the red-muscle vascular supply to form countercurrent heat exchangers-distinguish the tunas (tribe Thunnini) from other species in the teleost family Scombridae.Neither of these characteristics is found in the bonitos (tribe Sardini), the sister group to the Thunnini.The most recent scombrid classification places the slender tuna, Allothunnus fallai, in the tribe Sardini, but some earlier studies suggested that this species should be a member of the Thunnini.Allothunnus fullai does not possess the lateral subcutaneous arteries and veins or the lateral heat-exchanging retia typical of tunas.However, we have found that this species has a highly modified central circulation (dorsal aorta, post cardinal vein, and associated branch vessels) similar to the central heat-exchanging retia of certain tunas, an enlarged haemal arch to accommodate this structure, and the anterior, internal placement of red muscle characteristic of tunas.With these new characters, phylogenetic reconstructions based on parsimony place A. fallai as the sister taxon to the tunas, establish that it is the most basal tuna species, and support the hypothesis that the derivation of tunas from a bonito-like ancestor occurred through selection for an integrated set of characteristics affecting locomotion and endothermy.The major features of this hypothesis are as follows.(1) Selection for continuous, steady, and efficient swimming resulted in changes in body shape (the result of enlargement of the anterior myotomes, the anterior and internal shift of red muscle, and a narrowing of the caudal peduncle) which increased streamlining and led to the adoption of the thunniform swimming mode unique to the tunas.