FISHERY BULLETIN: VOL. 81, NO.2 



having the highest infestation by B. thynni (24%), 

 and in having a second species of Elytrophora, E. in- 

 dica, which was found only on Indo-Pacific speci- 

 mens of T. obesus. Elytrophora indica frequently 

 occurs with E. brachyptera, but we lack data on pos- 

 sible microhabitat differences between the two 

 copepods. 



HOST SPECIFICITY AND 



TAXONOMIC RELATIONSHIPS OF 



CAI1GUS PARASITIC ON SCOMBRJDS 



The genus Caligus, with over 200 recognized spe- 

 cies, has been reported from species of marine fishes 

 of several diverse higher taxa. Most species do not 

 exhibit strict host specificity; those which have been 

 commonly reported are known from more than one 

 host species. Adults of Caligus species are occasion- 

 ally found in plankton samples, indicating that spe- 

 cies of Caligus may easily transfer from one host 

 individual to another. Many, however, seem to be re- 

 stricted to a genus or family of fishes. Furthermore, if 

 one analyzes the data from comprehensive collec- 

 tions, it becomes clear that although a parasite may 

 be present on several host species, it is consistently 

 more common on some than others, which we inter- 

 pret as a trend toward specificity. The first author 

 has never found an equal rate of infestation of any 

 Caligus species among its hosts in any large collec- 

 tions examined. There have always been one or two 

 host species with significantly higher infestation 

 rates, when as many as 10 host species are involved 

 (unpubl. data). We have analyzed the data for 

 Caligus most common on scombrids, and the results 

 are consistent with this concept. 



Ten most common of the 16 species of Caligus 

 reported by Cressey and Cressey (1980) were chosen 

 for study. These 1 Caligus species can be divided in- 

 to 5 subgroups, based on the segmentation and num- 

 ber of setae on the fourth leg exopod, the presence or 

 absence of a posterior process on the base of the sec- 

 ond antenna, and the presence or absence of the 

 postantennal spine. These groups are 1) productus, 

 asymmetricus; 2) bonito, omissus, mutabilis; 3) infes- 

 tans; 4)pelamydis, cybii; and 5) coryphaenae, regalis. 

 All of the 10 species are found on more than one 

 species of host (scombrid or otherwise). Frequency 

 of their occurrences, however, indicates definite 

 host preferences. 



The distribution of infestation rates and host spec- 

 ificity indices (based on Rohde 1980) for C. produc- 

 tus and C. asymmetricus are given in Table 9. Neither 

 of these two species are found on species of Scom- 



246 



brini. Caligus productus is most common on the close- 

 ly related genera Katsuwonus and Thunnus and to a 

 lesser extent on Scomberomorus, Acanthocybium, 

 and Gymnosarda. Five of the six records of C. produc- 

 tus on species of Scomberomorus are from the Atlan- 

 tic. Caligus asymmetricus complements C. productus 

 in host distribution. It is common on hosts where C. 

 productus is absent or rare, and uncommon or absent 

 on those where C. productus is most common. The on- 

 ly genera of the three tribes infested, which so far are 

 negative for either of these two copepods, are Or- 

 cynopsis andAllothunnus. This is due probably to the 

 few specimens (seven) of each that we have ex- 

 amined. 



Two species of Caligus (cybii and infestans) are ap- 

 parently specific to Indo-West Pacific Scom- 

 beromorus, whereas there is apparently no Caligus 

 species-specific to Atlantic Scomberomorus. 



The next group of Caligus species are bonito, muta- 

 bilis, and omissus. Caligus bonito is circumglobal where- 

 as C. mutabilis is restricted to the western Atlantic 

 and C. omissus is, so far, only known from the eastern 

 Pacific. The latter two species are very similar. In 

 1960, Causey reported C. mutabilis from several spe- 

 cies of fishes, including Scomberomorus sierra from 

 the Gulf of California and the Pacific coast of Mexico. 

 The material from S. sierra was undoubtedly C. 

 omissus, and it is likely that the rest was also. Wilson 

 (1937) also reported C. mutabilis from S. maculatus 

 (presumably sierra) from Pacific Mexico, which was 

 probably C. omissus. None of these collections are 

 available for verification, but we feel that these Paci- 

 fic records of C. mutabilis should be discounted. The 

 first author has collected C. mutabilis from two spe- 

 cies of Lutjanus from the west coast of Florida, and it 

 is apparent from the literature that all three of these 



TABLE 9. — Infestation rates and host specificity indices 

 of Caligus productus and C. asymmetricus on genera of 

 Scombrinae (specificity indices in parentheses). 



