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Chief predators on even relatively large tunas are the temperate and warm temperate xiphiid Swordfish, Xiphiasgladius, and the more tropical istiophorid sailfishes, spearfishes, and marlins (Istiophorus, Tetrapturus, Kejikig, and Makaira). The suborder includes some of the fastest and largest predators in the sea. The bill in both groups consists of an expanded premaxillary bone that is depressed and smooth in the swordfish and more rounded and prickly inmarlins and their relatives. Other differences include nopelvic fins, a single caudal keel, and relatively stiff, sharklikepectoral, dorsal, and anal fins in the swordfish. The istiophoridsin contrast have long pelvic filaments, flexible pectorals,double keels, and a long, depressible spiny dorsal that reaches its extreme expression in the sail of the sailfish,a structure of debated function. Controversy has also ragedover how and whether billfishes utilize their bill for feeding,but recent observations indicate it can serve as a spear oras a cutlass or billy. Billfishes have independently evolved cranial endothermy (see Heterothermicfishes; The open sea). Swordfish attain sizes of 530 kg, whereas both Blue and Black marlin grow to 900 kg.
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