Abstract. — The suborder 

 Scombroidei (Teleostei) has an ex- 

 tensive taxonomic history which 

 traces its beginnings to at least 

 1832 (Cuvier and Valenciennes). 

 However, a single well-corrobo- 

 rated phylogenetic hypothesis for 

 the scombroid fishes does not ex- 

 ist. To date, efforts to define this 

 suborder and determine the inter- 

 relationships of its constituent taxa 

 have utilized morphological data 

 almost exclusively. In this paper we 

 present a molecular data set for 

 addressing scombroid relation- 

 ships: DNA sequences from the mi- 

 tochondrial gene cytochrome b. 

 These data provide valuable in- 

 sights into scombroid relation- 

 ships, especially regarding the 

 long-standing debate over the 

 placement of the billfishes (Istio- 

 phoridae and Xiphiidae). The cyto- 

 chrome b data strongly refute a 

 close relationship between Scom- 

 bridae and billfishes and also sup- 

 port separation of the billfishes 

 from the Scombroidei. In addition, 

 these data suggest a new hypoth- 

 esis on the evolutionary relation- 

 ships among istiophorid billfishes. 



Evolution of cytochrome b in the 

 Scombroidei (Teleostei): molecular 

 insights into billfish (Istiophoridae 

 and Xiphiidae) relationships 



John R. Finnerty 

 Barbara A. Block* 



The University of Chicago 



Department of Organismal Biology and Anatomy 



and Committee on Evolutionary Biology 

 1025 East 57 th Street, Chicago, Illinois 60637 



Manuscript accepted 29 August 1994. 

 Fishery Bulletin 93:78-96 ( 1995). 



The suborder Scombroidei is a well 

 studied assemblage of more than 

 100 marine teleosts. A consensus on 

 the taxonomic limits and intra- 

 relationships of this group remains 

 elusive despite more than 150 years 

 of study (Cuvier and Valenciennes, 

 1832; Regan, 1909; Gregory, 1933 

 Berg, 1940; Fraser-Brunner, 1950 

 Collette et al., 1984; Johnson, 1986 

 Potthoff et al., 1986; Block et al., 

 1993). In 1832, Cuvier and Valen- 

 ciennes proposed the Scombroidei 

 as a natural group containing the 

 oilfishes and snake mackerels (fam- 

 ily: Gempylidae), the cutlassfishes 

 (Trichiuridae), and the "tunnies" 

 (Scombridae). Regan (1909) ex- 

 panded the Scombroidei by adding 

 three families: Istiophoridae (mar- 

 lins, sailfish, and spearfishes); Xi- 

 phiidae (the swordfish); and Luvar- 

 idae (the louvar). Most subsequent 

 classifications agree that the mono- 

 typic Luvaridae is not a scombroid 

 but an acanthuroid (Leis and 

 Richards, 1984; Tyler et al., 1989). 

 However, there is substantial dis- 

 agreement over the relationships of 

 the families Istiophoridae and Xiphi- 

 idae, collectively known as billfishes. 

 In the last ten years three mor- 

 phological studies have proposed 

 three different hypotheses on the 

 relationships of billfishes. In 1984, 

 Collette et al. published a scombroid 

 phylogeny based on 40 morphologi- 

 cal characters (Fig. 1). They pro- 



posed that billfishes are the sister 

 group of the Scombridae. Their cla- 

 dogram suggested that several 

 synapomorphies unite billfishes and 

 scombrids, including a pharyngeal 

 toothplate stay, a pair of lateral 

 keels on the caudal peduncle, and 

 the extension of the caudal-fin rays 

 to cover the hypural plate. However, 

 the position of billfishes was not 

 strongly defined in the Collette et 

 al. study because of homoplasious 

 character evolution. Of the twelve 

 character-state transitions that oc- 

 curred within the billfish lineage on 

 their cladogram, five were reversals 

 to the primitive state, and six oc- 

 curred independently in other lin- 

 eages. Collette et al. (1984) consid- 

 ered the placement of billfishes 

 within the suborder Scombroidei to 

 be uncertain and conditional upon 

 additional evidence. They cited lar- 

 val evidence (Potthoff et al., 1986) 

 which indicates that the scombroid 

 families Scombridae, Gempylidae, 

 and Trichiuridae are closely related 

 to each other and are distantly re- 

 lated to billfishes. We will refer to 

 the Collette et al. hypothesis as the 

 scombrid sister group hypothesis. 



In 1986, Johnson published a 

 scombroid phylogeny using many of 

 the characters from the Collette et 



* Present address: Stanford University, 

 Hopkins Marine Station, Department of 

 Biological Sciences, Pacific Grove, Califor- 

 nia 93923. 



78 



