Elsevier, N.Y. 

 Thomas, W. H., A. N. Dodson, and C. A. Linden. 



1973. Optimum light and temperature requirements for Gym- 

 nodinium splendens, a larval fish food organism. Fish. Bull., U.S. 

 71:599-601. 



Richard B. Forward, Jr. 



Duke University Marine Laboratory 



Beaufort, NC 28516-9721 



and 



Zoology Department, Duke University 



Durham, NC 27706 



Blanca Rojas de Mendiola 



Instituto del Mar del Peru 

 P.O. Box 22, Callao, Peru 



Richard T. Barber 



Duke University Marine Laboratory 

 Beaufort, NC 28516-9721 



MORPHOLOGY AND 



POSSIBLE SWIMMING MODE OF A 



YELLOWFIN TUNA, THUNNUS ALBACARES, 



LACKING ONE PECTORAL FIN 



In September of 1982, the Mexican bait boat, Paesa, 

 fishing off Baja California, captured a 36.5 cm fork 

 length (861.2 g wet weight) yellowfin tuna, Thunnus 

 albacares, that lacked a left pectoral fin (Fig. 1). The 

 fish was frozen and was brought to the Inter-Ameri- 

 can Tropical Tuna Commission, La Jolla, CA, for 

 study by W. H. Bayliff. 



Pectoral fins provide virtually all hydrodynamic lift 

 in scombrids and are essential for stable and effi- 

 cient swimming at sustained speeds (Magnuson 

 1973, 1978). A specimen with only one pectoral fin 

 raises questions on what ways the fish might have 

 compensated for an asymmetrical decrease in hydro- 

 dynamic lift and how the presence of only one pec- 

 toral fin might have affected its locomotion. We ex- 

 amined the fish to determine what may have caused 

 fin loss and whether morphology was noticeably 

 altered in a manner suggesting some compensation. 



Skin in the area where the left pectoral fin should 

 have been was thin, smooth, and silvery in appear- 

 ance (Fig. 1). There was neither a trace of pectoral 

 fin remnants nor a skin groove for it, suggesting the 

 fin had never formed. On the other hand, the ap- 

 pearance of the skin and the presence of variably 

 sized scales in the area around the normal fin posi- 

 tion is compatible with a healed wound, and we thus 



could not rule out the possibility that the fin had been 

 bitten off cleanly. 



Methods 



The specimen was X-rayed and maximum body 

 height and width measured. We measured and 

 traced its median fins, caudal keel, pectoral fin, and 

 both pelvic fins, and estimated their surface areas 

 with a planimeter. The same body and fin measure- 

 ments were made on similarly sized, preserved 

 yellowfin tuna in the Scripps Institution of Ocean- 

 ography Fish Collection (SIO). Morphometric data 

 were compared with values derived from the litera- 

 ture (Gibbs and Collette 1967; Fierstine and Walters 

 1968; Magnuson 1973, 1978; Magnuson and Wein- 

 inger 1978, app. II). Although some of the specimen's 

 caudal rays were bent (Fig. 1), all rays were present, 

 and the fin was extended to a more natural position 

 before its span was measured and area (which was 

 well defined) traced. Also, to avoid measurement er- 

 rors noted by Fierstine and Walters (1968) and 

 Magnuson (1978), care was taken not to overextend 

 caudal fins during span measurement. 



Density of the thawed fish was determined by 

 water displacement (density = wet weight/displace- 

 ment volume). The right and left pectoral girdles 

 were then removed and the gas bladder was in- 

 spected. Transverse sections were cut (see Graham 

 et al. 1983), concentric myotomal rings on the right 

 and left sides were counted, and red and white 

 muscle were weighed for each section. 



Results 



The abundance of comparative morphometric and 

 anatomical data for the yellowfin tuna permits a 

 nearly complete assessment of the morphologic and 

 hydrodynamic status of the one-finned specimen. 

 The length (L; 36.5 cm)/weight (861.2 g) relationship 

 and the density (1.080 g-mL -1 ) agree with values 

 published for yellowfin tuna by Magnuson (1973, 

 tables 1, 4). Also, the maximum thickness value (i.e., 

 max. height + max. width/2 = 21.6% L) is within 

 the range (20.5-23.0% L) measured for four SIO 

 specimens (L from 28.5 to 42.5 cm) and near the 

 value given by Magnuson (1973, table 7, 22.3% L). 

 Finally, the point of maximum body thickness in the 

 study fish (39.7% L) and that of SIO fish (36-40% 

 L) are near Magnuson's value of 41.2% L (for fish 

 from 28 to 45 cm L). 



The dorsal fin of this fish is normal in shape, with 

 13 spinous rays, a maximum height of 3.5 cm and 

 a surface area of 9.5 cm 2 . The second dorsal fin is 



FISHERY BULLETIN: VOL. 84, NO. 2, 1986. 



463 



