CRESSEY ET AL.: COPEPODS AND SCOMBRID FISHES 



work (containing the central heat exchanger; Graham 

 1975, 1979) is present in Euthynnus and Kat- 

 suwonus, and is reduced (yellowfin tuna species 

 group) or absent (bluefin tuna group) in Thunnus. 

 Collette (1979) concluded that it was useful to utilize 

 subgenera in Thunnus to reflect the adaptive signi- 

 ficance of the difference in heat exchangers between 

 the two groups of species, the subgenus Thunnus for 

 the bluefin tuna group of species including T. obesus, 

 Neothunnus for the yellowfin tuna group. Copepod 

 fauna: 10 species in 5 genera. Three species oiCali- 

 gus and P. appendiculatus are widespread among 

 species of Thunnini. The lerneopodid Brachiella 

 thynni occurs, usually in the axil of the pectoral fin, on 

 a wide variety of hosts both scombrid and nonscom- 

 brid. In the Scombridae, it is most common onAcan- 

 thocybium and was also present on three species of 

 Thunnus (T obesus, 24%; T. albacares, 7%; T. thyn- 

 nus, 4%). Occurrence of the euryphorid Elytrophora 

 is of particular interest. Six species of Thunnus (all 

 butT. tonggot) share £. brachyptera with Allothunnus 

 fallal As noted under the discussion of the latter, this 

 indicates that the systematic position of Allothun- 

 nus within the tribe Sardini needs to be recon- 

 sidered. 



Subgenus Neothunnus Kishinouye 



This subgenus contains the three tropical species of 

 Thunnus which have central heat exchangers, as do 

 the three less advanced genera of Thunnini. Gibbs 

 and Collette (1967:99) found that these three species 

 were similar to each other inl5orl6ofl8 characters. 

 The three species are the blackfin tuna, Thunnus 

 atlanticus (Lesson), of the western Atlantic, Martha's 

 Vineyard, Mass., to Rio de Janeiro; the longtail tuna, 

 T tonggol (Bleeker), of the Indo- West Pacific, Japan 

 to Australia west through the Indo-Australian Archi- 

 pelago to Somalia and the Red Sea; and the yellowfin 

 tuna, T albacares (Bonnaterre), a pantropical spe- 

 cies. Differences between the species were treated in 

 detail by Gibbs and Collette (1967). 



Copepod fauna: 7 species in 4 genera. Differences 

 in copepod infestation in Thunnus appear to reflect 

 species differences rather than subgeneric differ- 

 ences. Caligus asymmetricus, a copepod common on 

 the three more primitive genera of Thunnini, was 

 found on T albacares, which tends to confirm closer 

 relationships between the three primitive genera and 

 Neothunnus than with Thunnus. However, the cope- 

 pod was found only on one specimen of T. albacares, 

 so this is only weak confirmatory evidence. We found 

 the most common copepods on T albacares world- 

 wide to be C. productus (46%), E. brachyptera (35%), 



C. coryphaenae (29%), P. appendiculatus (19%), and 

 B. thynni (7%). In an intensive study of 200 T alba- 

 cares from the Gulf of Guinea, Baudin Laurencin 

 (1971) found three of the five copepods in similar 

 rates of infestation: Caligus productus, 64%; P. ap- 

 pendiculatus, 27%; and B. thynni, 7%. He did not 

 report either E. brachyptera or C. coryphaenae, al- 

 though both occur in the eastern Atlantic, and we 

 have the latter from T albacares in the Gulf of 

 Guinea. 



Subgenus Thunnus South 



This subgenus contains the four larger species of 

 tunas which have invaded cooler waters owing to 

 their possession of effective lateral heat exchangers. 

 Gibbs and Collette (1967:99) showed that three spe- 

 cies of this group resembled each other in 1 4 -1 6 of 1 8 

 characters. Striations caused by blood vessels are 

 present on the ventral surface of the liver, and vas- 

 cular cones are associated with the dorsal surface of 

 the liver, indicating the presence of a visceral heat ex- 

 changer. Three species clearly belong to this sub- 

 genus: the Atlantic and Pacific bluefin tunas, Thunnus 

 thunnus thynnus (Linnaeus) and T t. orientalis (Tem- 

 minck and Schlegel); the southern bluefin tuna, T. 

 maccoyii (Castelnau); and the albacore, T alalunga 

 (Bonnaterre). The fourth species, the bigeye tuna, T 

 obesus (Lowe), is intermediate between the sub- 

 genera, sharing 12 characters with T maccoyii and 10 

 with T albacares (Gibbs and Collette 1967:99). 

 Because it has lost the central heat exchanger, Col- 

 lette (1979) believed that it belongs to the subgenus 

 Thunnus, although it is the most different of the four 

 species in the subgenus. The characters that dis- 

 tinguish the species of the subgenus Thunnus and the 

 distributions of the species are treated in detail by 

 Gibbs and Collette. All four species are found world- 

 wide. The bluefin tuna extend into temperate waters 

 of the North Atlantic (T. t. thynnus) and the North 

 Pacific (T. t. orientalis). The southern bluefin, T. mac- 

 coyii, has a distribution pattern similar to those of 

 Gasterochisma and Allothunnus in the Southern 

 Ocean. Thunnus alalunga is found from lat. 42°N to 

 32°S in the Atlantic, lat. 10°N to 30°S in the Indian 

 Ocean, and lat. 50°N to 45°S in the Pacific; however, 

 most of the albacore fisheries are concentrated in 

 temperate waters. Thunnus obesus has much the 

 same latitudinal distribution as T albacares, but it is 

 usually found in deeper and cooler waters than T 

 albacares. 



Copepod fauna: 8 species in 5 genera. Thunnus 

 obesus differs in infestation from the other three 

 species of the subgenus in lacking C. productus, in 



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