GRUBER und COMPAGNO: TAXONOMIC STATUS AND BIOLOGY OF BIGEYE THRESHER 



Springer 1943; Cadenat 1956; etc.). Since the 

 gestation period is probably 12 mo (Holden 1974), 

 the reproductive capacity of this shark may be 

 said to be relatively low. 



Guitart Manday ( 1975) reported that most large 

 females throughout the year contained embryos. 

 If the reproductive pattern is similar to that of the 

 common thresher (Gubanov 1972, 1979), then 

 mating occurs throughout the year. Not enough 

 data are available for the bigeye thresher to 

 demonstrate seasonality. However, most of the 

 large females examined have been pregnant. 



FOOD 



Stomach contents of bigeye threshers have been 

 reported in only three studies: Fitch and Craig 

 (1964) obtained some 5 kg of Pacific whiting, 

 Merluccius productus, a benthic teleost, from 

 the stomach of their specimen; Bass et al. (1975) 

 reported that a bigeye thresher captured in the 

 protective shark nets along the beach at Durban 

 (hardly deep water) had recently eaten another 

 elasmobranch, perhaps also fouled in the net; 

 Stillwell and Casey (1976) examined the stomachs 

 of 35 bigeye threshers and found over 507c to have 

 food remains — squid was the most common food, 

 composing some 66% of the stomach contents. 

 Other prey included remains of pelagic teleosts, 

 such as scombrids, alepisaurids, clupeids, and 

 istiophorids. 



Stomach contents recovered from one of 

 our specimens (SHG-A2) consisted of several eye 

 lenses and two pairs of squid beaks. These were 

 identified by Gilbert Voss^^ as ommastrephid 

 remains, probably from the genus Illex. Voss 

 mentioned that Illex made up 75-80% of the 

 cephalopod diet of the swordfish, Xiphias gladius, 

 caught in the Florida Current. 



The food of the bigeye thresher thus consists of 

 small to moderate benthic and pelagic teleost fish, 

 crustaceans, and cephalopds, and as presently 

 known, is restricted to a few species. 



PREY CATCHING 



According to Springer (1961) the upper caudal 

 lobe of A lop ias (along with the armed rostrum of 

 the pristiophorids Pristiophorus and Pliotrema) 



"Gilbert Voss, Professor of Biology and Living Resources, 

 RSMAS, University of Miami, Miami, FL 33149, pers. commun. 

 to S. H. Gruber, December 1979. 



are the only structures of modern sharks func- 

 tioning specifically for killing prey (jaws and teeth 

 being used for other purposes in addition to 

 feeding). However, it has not been universally 

 accepted that the tail of thresher sharks is ac- 

 tually used in feeding. In an interesting discussion 

 of this controversy, Lineweaver and Backus (1969) 

 noted that the ichthyologists J. T Nichols and C. 

 M. Breder doubted that the tail was sufficiently 

 rigid or muscular to kill prey. 



Grossly overdeveloped appendages such as the 

 claw of the male fiddler crab, Uca sp., often evolve 

 along with elaborate courtship signals, and it is 

 possible that the elongated tail of Alopias evolved 

 in the context of a social or species recogni- 

 tion signal. However, field observations support 

 Springer's (1961) concept of the thresher's tail as 

 an offensive weapon for prey capture. One of the 

 first such observations is that of Blake-Knox 

 (1866), who claimed that a common thresher, 

 A. vulpinus, used its caudal fin to kill a loon and 

 then consumed the bird. Coles (1915) reported 

 common threshers as feeding in shallow water by 

 throwing fish into their mouth with their caudal 

 fins. Allen (1923) gave a similar detailed descrip- 

 tion of the feeding behavior of a common thresher. 

 Grey (1928) observed common threshers following 

 baits trolled from a sport fishing boat and striking 

 at the bait with their tails. 



Recently, indirect but compelling observations 

 from longline fisheries confirm that threshers use 

 their tail in feeding. Gubanov ( 1972 ) reported that 

 97% of all three thresher species captured were 

 foul-hooked in the upper caudal. This agrees with 

 Stillwell and Casey ( 1976), who noted that several 

 bigeye threshers were tail-hooked and that two or 

 more baits were often recovered from a captured 

 bigeye thresher's stomach. This suggested to Still- 

 well and Casey as well as to Gubanov that the live 

 baits were dislodged from the hooks probably by 

 blows from the thresher's caudal fin. 



EXPERIMENTAL STUDIES 



The bigeye thresher has occasionally been the 

 subject of study unrelated to fishery, natural 

 history, or taxonomic observation. Carey et al. 

 (1971) measured the muscle temperature of a 

 number of freshly captured sharks and teleosts 

 and concluded that, among others, the bigeye 

 thresher is warm-bodied. They described a single 

 vascular heat exchanger which probably makes 

 the storage of heat possible in this species. 



635 



