Finnerty and Block: Evolution of cytochrome b in the Scombroidei 



79 



B 



Sphyraena 



Lepidocybium 



Gempylinae 



Trichiurinae 



Grammatorcynus 



Scombrini 



Sardini + Thunnini 



Scomberomorus 



Acanthocybium 



XIPHHDAE 



ISTIOPHORIDAE 



TRICHIURIDAE 

 GEMPYLIDAE 



1 GEMPYLIDAE 

 > TRICHIURIDAE 



SCOMBRIDAE 



} 



- SCOMBRIDAE 



XIPHHDAE 

 ISTIOPHORIDAE 



Scombrini 

 Gasterochisma 

 Grammatorcynus 

 Scomberomorus + Acanthocybium 



V Sardini 

 Thunnini 



Figure 1 



Two phylogenetic hypotheses for the scombroid fishes based on morphological evidence. The 

 studies of (A) Johnson (1986) and (B) Collette et al. (1984) are examples of the scombrid- 

 subgroup and the scombrid-sister group hypotheses of billfish (Istiophoridae and Xiphiidae) 

 relationships. Johnson ( 1986) considered billfishes a subgroup of the family Scombridae most 

 closely related to the wahoo, Acanthocybium solandri. Collette et al. ( 1984 ) placed the billfishes 

 as a sister group to the Scombridae. Both hypotheses propose that billfishes and scombrids 

 share a common ancestor to the exclusion of other scombroids. An alternative hypothesis for 

 billfish relationships is that billfishes are not scombroids. This hypothesis has never been 

 depicted explicitly in the form of a cladogram (Gosline, 1968; Nakamura, 1983; Potthoff et al., 

 1980; Potthoff et al., 1986). 



al. (1984) study and several additional characters 

 (Fig. 1). Like Collette et al., Johnson proposed that 

 billfishes and scombrids compose a monophyletic 

 group, but he regarded billfishes as a subgroup of 

 the Scombridae. A critical piece of evidence support- 

 ing this hypothesis that billfishes are a derived group 

 within scombrids is the presence of cartilaginous in- 

 terconnections between gill filaments in billfishes 

 and the scombrid Acanthocybium solandri. Based 

 largely on this proposed synapomorphy, Johnson 

 placed Istiophoridae and Xiphiidae as derived 

 scombrids and Acanthocybium as their sister group. 

 This association has been suggested by others 

 (Lutken, 1880; Fraser-Brunner, 1950). However the 

 position of billfishes in Johnson's study was only 

 weakly supported because of homoplasy. For ex- 

 ample, five of the ten character-state transitions that 

 support billfish monophyly on Johnson's cladogram 

 are reversals. We will refer to the Johnson hypoth- 

 esis as the scombrid subgroup hypothesis. 



Other workers have proposed that billfishes are 

 not scombroids. In 1986, Potthoff et al. published a 

 study of bone development in scombroids in which 

 they discussed scombroid phylogeny They concluded 

 that billfishes are not scombroids because of their 

 lack of resemblance to other scombroids in vertebral 

 number and osteological development. They sug- 

 gested that these characters indicate billfish affini- 

 ties to the percoids. This hypothesis has been sug- 

 gested in previous studies (Potthoff et al., 1980; 

 Nakamura, 1983). We will refer to this hypothesis 

 as the nonscombroid hypothesis. 



It is evident from the morphological studies that 

 there has been a great deal of homoplasious mor- 

 phological evolution in billfishes. Therefore, it is dif- 

 ficult to reconstruct the evolutionary relationships 

 of this group based on morphology alone. In an at- 

 tempt to derive additional, independent data on 

 scombroid intrarelationships and, in particular, to 

 address the position of billfishes, we compiled a mo- 



