On an Additional Diagnostic Character for 



the Identification of Billfish Larvae with 



Some Notes on the Variations in Pigmentation 



SHOJI UEYANAGI 



ABSTRACT 



The larvae of five species of billfishes (Istiophoridae) occurring in the Indian and Pacific Oceans 

 — sailfish, Istiophorus platypterus; short hill spearfish, Tetrapturus angustirostris; striped marl in. T. audax; 

 blue marlin, Makaira mazara; and black marlin, M. indica — have now been identified. The identification of 

 these larvae has depended on such characters as the shape of the pectoral fin, pigmentation of the 

 branchiostegal membrane, pigmentation of the lower jaw membrane, and head profile. 



Some problems in identification remain, however, as for example in the differentiation between very 

 small larvae (under 7 mm) of striped marlin and blue marlin. Recent studies have resulted in additional 

 diagnostic characters which differentiate between these two species, namely the differences in the pterotic 

 and preopercular spines. 



The larvae of sailfish generally have pigment on the posterior half of the lower jaw, and this 

 pigmentation is recognized to be species specific. There exist, however, some larvae of this species which 

 lack this characteristic pigmentation, and the occurrence of these larvae seems to vary geographically from 

 the more typical sailfish larvae. 



One of the problems related to the identification of 

 billfish larvae concerns the identification of the lar- 

 vae of striped marlin, Tetrapturus audax. The head 

 profile ("the tip of the snout and the midpoint of the 

 eye are on a nearly equal level") has been regarded 

 as a diagnostic character for this species. However, 

 unlike the pigmentation pattern, this character is 

 rather difficult to use, and there is a possibility of 

 error depending on the physical condition of the 

 specimens examined. For example, sailfish, Is- 

 tiophorus platypterus , larvae have been erroneously 

 identified as striped marlin due to the occasional 

 close resemblance in this particular character 

 (Ueyanagi, 1959: Figs. 4 and 5; Ueyanagi, 1963). 

 Furthermore, in very small specimens of striped 

 marlin and blue marlin, Makaira mazara, where the 

 snout has not yet lengthened, discrimination be- 

 tween the two species is very difficult. 



Because of these problems in identification, 

 further studies were conducted to locate additional 

 diagnostic characters. As a result, it was found that 

 the pterotic and preopercular spines are effective 



'Far Seas Fisheries Research Laboratory. Shimizu, Japan. 



characters particularly in differentiating the larvae of 

 striped marlin from those of the other species. 



GENERAL DESCRIPTION OF 



THE PTEROTIC AND PREOPERCULAR 



SPINES 



Spination on the head is a prominent characteristic 

 of the larval stages of billfishes, and chief among the 

 spines are the pterotic spine and the main preopercu- 

 lar spine (the latter is hereafter referred to simply as 

 preopercular spine). Although there are some varia- 

 tions among species in the length of the spines, the 

 development of the spines appears to progress uni- 

 formly in all species. For this reason, the following 

 general description of the development of these 

 spines is restricted to that of blue marlin. 



The pterotic and preopercular spines are absent in 

 larvae under 3 mm in total length. After about 3 mm. 

 the spines appear. They lengthen rapidly with 

 growth of the larvae and are markedly developed 

 when the larvae are about 6-7 mm long. At this size, 

 the preopercular spine reaches slightly posterior of 

 the anus when pressed against the side of the body. 



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