SCOMBROID PHYLOGENY: AN ALTERNATIVE HYPOTHESIS

A new hypothesis of the cladistic intrarelationships of the perciform suborder Scombroidei and immediate outgroups is proposed, based on 49 characters, most of them osteological. This hypothesis differs from the one proposed recently by Collette et al. (1984) in several respects. An hypothesis of precise outgroup relationships is presented. Outgroup taxa are, in phyletic sequence: Dinolestes plus several genera previously placed in the Acropomatidae; Scombrops; Scombrolabrax; and Pomatomus (first outgroup). Sphyraena is included in the Scombroidei as the sister group of all other scombroids. Tbe Gempylidae, rediagnosed, is the sister group of the scombrids plus bill fishes; within the Gempylidae, Lepidocybium is the sister group of the Gempylinae plus Trichiurinae. The billfishes (Istiophoridae plus Xiphias) are the sister group of Acanthocybium and, in a cladistic classification, must be treated as a subgroup of the Scombridae. The precise affinities of Gasterochisma remain unresolved; data on larval morphology and developmental osteology, currently unavailable, should help resolve the placement of this enigmatic genus, which exhibits a perplexing combination of primitive and derived character states. The suborder Scombroidei is among the most extensively studied and best known taxonomically of the perciform suborders. Despite continuing disagreement over its limits, the generally accepted concept of the Scombroidei is essentially that of Regan (1909), who included six families: Gempylidae, Trichiuridae, Scombridae, Istiophoridae, Xiphiidae and Luvaridae. Recently Tyler et al. (MS)I have shown that the monotypic Luvaridae is not a scombroid but an acanthuroid. Placement of the billfishes, Istiophoridae and Xiphiidae, remains controversial. Although most classifications (Regan, 1909; Berg, 1940; Greenwood et a1., 1966; Collette et a1., 1984) have included the billfishes in the Scombroidei, some authors (Gosline, 1968; Nakamura, 1983; Potthoff et a1., 1980) have disagreed and suggested that billfishes probably originated within the percoids. The extensive classificatory history of the Scombridae was reviewed briefly by Cressey et al. (1983), that ofgempylids by Russo (1983) and of bill fishes by Nakamura (1983) and will not be recounted here. At the 1979 meetings of the American Society of Ichthyologists and Herpetologists, I presented a paper in which I described several distinctive specializations of the gill arches and upper jaw development of scombroids. Certain of these features have been cited subsequently (Potthoff et aI., 1980; Potthoff and Kelley, 1982; Cressey et aI., 1983; Russo, 1983; Collette et aI., 1984), but, until now, none have been illustrated or fully described. A primary purpose of this paper is to describe and illustrate these specializations and emphasize their significance. I concluded, in 1979, that these specializations indicate not only that billfishes are scombroids but that their relationships lie within the Scombridae, specifically with the genus Acanthocybium. Recently, Collette et al. (1984) incorporated most of these characters, along with many others, into a phylogenetic hypothesis for the Scombroidei (initially prepared for and presented at the 1981 meetings of the ASIH by Collette and I J. C. Tyler, G. D. Johnson, I. Nakamura and B. B. Collette. Osteology and relationships of the oceanic fish Luvarus imperia/is (luvaridne): an oceanic acamhuroid. Manuscript. 2 BULLETIN OF MARINE SCIENCE, VOL. 39, NO. I, 1986 Russo). Collette et al. (1984) agreed that the billfishes are scombroids, but rejected the Acanthocybium connection and placed the billfishes as the sister group of the Scombridae. Their analysis, published in the Ahlstrom Memorial Volume, Ontogeny and Systematics of Fishes, was necessarily abbreviated and consisted of only a cladogram (their fig. 312) generated by the Wagner 78 computer program, and an appendix legend listing states of the" 40 characters considered significant in assessing scombroid relationships. " A full description of characters and discussion of character analysis would undoubtedly have identified and allowed resolution of some of the shortcomings that seem to have resulted from the abbreviated treatment. Problems that I encountered with the Collette et al. (1984) analysis are noted below: 1. Information provided in the appendix legend about symbols and the way in which they apply to reversals and independent acquisitions on the cladogram is insufficient, particularly for multistate characters. Directionality for multistate characters is not clearly indicated, and it is thus impossible to identify from the symbols which state is present at a particular node. 2. Polarity designation, based on Scombrolabrax as the outgroup, is confusing and, for some characters, wrong. In the appendix legend, polarity is not indicated by the 0 or 1 code given after each state, as implied at the beginning ofthe legend, but apparently by the order in which the states are listed. 3. For a few characters, states were not known for some taxa (particularly Gasterochisma and the bill fishes), but this was not indicated in reference to their distribution as indicated on the cladogram. Thus, as they note in the text, the larva of Gasterochisma is unknown, but character states shown on the cladogram imply that Gasterochisma lacks the beak of most scombrid larvae and shares with them a precocious spinous dorsal. The confusion results from the fact that character states for larvae were assumed so that developmental characters could be used in the analysis (Collette, pers. comm.). 4. The distributions of a number of known character states (e.g., vertebral number, sequence of dorsal fin development, several caudal fusions) are incorrectly indicated on the cladogram, although in some cases correctly reported in one of the tables. Specific errors are discussed in the character analysis below. 5. Homology of several character states (e.g., articulation and expansion of infraorbitals, subocular shelf, compressed teeth) seems questionable and, in at least one instance (gill filament cross-connections), was incorrectly interpreted. 6. Treatment of the Gempylidae is confusing. Although the question of its monophyly is not specifically addressed, the family is characterized in the discussion, and a separate cladogram of its generic phylogeny (taken from Russo, 1983) implies that it is monophyletic. Nonetheless, the scombroid cladogram indicates paraphyly for the Gempylidae, and one of several unspecified lineages is shown as the sister group of all other scombroids. Additional confusion results from a single listing of all character states that occur at the nodes of the various gempylid clades and another of all homoplasies occurring at various levels within the Gempylidae. Also perplexing is the placement of the highly specialized Trichiuridae as the sister group of all other scombroids; the affinities oftrichiurid genera clearly lie with the gempylids. 7. Characters of the caudal skeleton may have been overemphasized. For example, fusions ofhypurals 1-2 and 3-4 are treated as two independent characters, even though, as in most perciforms, they never occur independently. Sixteen of 40 characters (II reductive) involve morphology of the caudal peduncle or skeleton related to increased rigidity (hypurostegy, fusions) and hydrodynamic efficiency (caudal keels). None are unique to scombroids but are seen repeatedly in JOHNSON: SCOMBROID PHYLOGENY