The development and structure of the supports of the unpaired fins and of the vertebral column are presented from a graded size series of 51 blackfin tuna (Thunnus atlanticus, 5.1-85.0 mm SL, 504 mm SL). This descriptive study deals with the first appearance of cartilaginous
structures and their subsequent ossification. Details are shown of the partial fusion of the caudal complex, the development of the dorsal- and anal-fin pterygiophores, and of the centra with their neural and haemal arches and spines. The bones that support the caudal fin are the same number
for all scombrids but fuse differently in the various genera. Thus, the supporting bones of the caudal fin in adult Thunnus have four centra, three haemal spines, one parhypural, a fused hypural plate consisting of an urostyle with five hypurals and one uroneural, one free epural, one
captured epural, and two neural spines. The pterygiophores of the first dorsal, second dorsal, and anal fins have two radials. Pterygiophores of the dorsal and anal finlets have three radials. The posteriormost (terminal) pterygiophores of the dorsal and anal finlets have four radials. The
individual finrays and finlets of all the fins each form a series with their respective distal to proximal radials. Each fin ray and finlet also articulates non-serially with the adjacent posterior proximal radial. The first anterior pterygiophore of the anal fin represents a fusion of two
cartilages, which ossify after their fusion. The vertebral column of T. atlanticus has 19 precaudal and 20 caudal vertebrae. Ossification of the column occurs in two directions. For most vertebrae (1 through 35) ossification proceeds from the anteriormost centrum, progressing posteriad;
whereas the centra that support the caudal fin ossify from the posteriormost centrum (urostyle) anteriorly to the 36th.
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