Japanese Journal of Ichthyology 魚 類 学 雑 誌 Vol.34, No.1 1987 34巻1号1987年 Jaw Structures and Movements in Higher Teleostean Fishes William A. Gosline (Received May 9, 1986) Abstract All of the diverse jaw structures in higher teleosts appear to be modifications of a single basal type and are treated as such. Only some of the principal variants are discussed. Though the two jaws act as a coordinated unit during feeding, their movements are different. The upper and lower jaws are discussed separately. In the upper jaw the principal concern is with the various types of premaxillary protrusion and with the secondary development in some groups of a rocking premaxilla. For the lower jaw most of the account is devoted to the repeated differentiation of movements in its anterior and posterior sections. The paper concludes with comments on the jaw apparatus as a functional unit and its evolution in higher teleosts. Higher teleosts show a great diversity in what seem to be modifications of a type associated with they eat and how they feed. What they eat is a particular (acanthopteran) system of premaxil- most directly reflected in dentition, but how they lary protrusion (Alexander, 1967a; Gosline, 1981). eat is at least in large part associated with jaw The diagnostic feature of this protrusion system, construction and jaw movements. Any com- which provides a firm bite with protruded pre- parative study of jaw structures and movements is maxillae, is an inner maxillary process that moves complicated by a number of factors. The first forward as a wedge between the extended pre- is that many, perhaps most, higher teleosts move maxillae and the skull. This maxillary blocking their jaws in at least qualitatively different fashion system is present in nearly all higher teleosts , depending on the nature and position of the items even some of those, e.g., Scomber, that have they are eating (Liem, 1979). Second, the effect secondarily lost the ability to protrude the pre- of contracting certain muscles with jaw attach- maxillae. ments remains questionable. This is most notably The nature of the movements in the toothed true of the "geniohyoideus" (Osse, 1969; the pro- limb of the upper jaw almost always depend on tractor hyoidei of Winterbottom, 1974a) and the the relationship between the premaxillae and the A, section of the adductor mandibulae. Third, skull. The medial part of the toothed limb usual- there is the great diversity of jaw structures to ly has a posterodorsal projection, the ascending cope with. Fortunately, all of the variants process (Fig. 1), that via an underlying rostral appear to represent modifications from a type of cartilage rides over or abuts against the front of mouth that is still approximately represented in the skull. Movements of the premaxillae are such modern fishes as the percoids Doederleinia basically of two types with intermediate condi- (see Gosline, 1986) and Perca (see Osse, 1969) tions. If the ascending processes are short, the and the scorpaeniform genera Sebastes (see premaxillae generally rock around or over the Alexander, 1967a) and Helicolenus (see de la Hoz front of the skull as the mouth opens. If the and Dyer, 1984). This mouth type provides a ascending processes are long (Figs. 1,2) they slide base with which the various modifications can be forward over the skull providing a more or less compared. unidirectional protrusion. The acanthopteran protrusion system apparently The upper jaw evolved in some fish with an upper jaw construc- tion like that of Aulopus in which the premaxillae In higher teleosts the two bones of the upper are still primarily of the original rocking type jaw-the toothless maxilla behind and the ge- (Gosline, 1980). In the majority of percoids and nerally toothed premaxilla in front-are movably higher teleosts protrusion is more or less well associated with the skull and rarely united to one developed, and those forms without protrusion another. The different kinds of upper jaw all seem to have secondarily lost it. •\ 21•\ 魚 類 学 雑 誌Japan. J. Ichthyol. 34 (1), 1987 C A B D E Fig. 1. Rhomboplites (Lutjanidae): A, side view of head with the mouth open; B, right upper jaw bones with the mouth open and C, with the mouth closed. Orthopristis (Haemulidae): D, side view of head with the mouth open; E, medial view of right upper jaw bones. ap, articular process of the premaxilla; as, ascending process of the premaxilla; co, condyle for the articulation between the maxilla and the skull; em, ethmoid-maxillary ligament; li, ligament from the inner surface of the distal end of the maxilla to the outer surface of the mandible; mp, anterior maxillary-premaxillary ligament; Mx, maxilla; Pa, palatine; pp, palatine-premaxillary ligament; Px, premaxilla; rc, rostral cartilage; te, tendon from the A1 section of the adductor mandibulae muscle. The basal type of protrusion in these higher evolved in, and mostly occurs among, broad- teleosts appears to be one in which moderately headed fishes that eat larger, free-swimming well-developed ascending processes are separate animals. In such fishes a moderate amount of from slightly more lateral articular processes premaxillary protrusion is accompanied by a (Fig. 1B, C). The mechanism of protrusion in highly-developed suction into the oral-opercular such fishes has received much attention and need cavities caused by rapid lateral as well as vertical not be described again in detail (Van Dobben, 1935; expansion of these cavities (Osse, 1969). The Alexander, 1967a; Gosline, 1981; Dutta and Chen, lateral expansion is also a cause, via expansion of 1983; De la Hoz and Dyer, 1984). Two separate the distal ends of the maxillae, of premaxillary premaxillary processes, indicating this basal protrusion and the insertion of the maxillary block system, are present in a wide range of higher behind the premaxillae (Alexander, 1967a). How- teleostean taxa: various lower percoids, e.g., ever, in large-mouthed fishes, e.g., the scorpaenids Rhomboplites (Fig. 1A-C), the northern blennioid Sebastes (see Alexander, 1967a) and Helicolenus Ronquilus, the southern blennioid Tripterygion, (see De la Hoz and Dyer, 1984), forward movement the gobioid Eleotris, and in various Scorpaeni- of the distal end of the maxilla as the mandible is formes (Sebastes), Batrachiformes (Batrachoides) lowered can also bring about protrusion. It and Lophiiformes (Lophius). seems probable that forward movement of the This basal protrusion system seems to have distal end of the maxilla becomes at least the •\ 22•\ Gosline: Jaw Structures in Higher Teleosts principal cause of protrusion not only in the more of the articular process relative to the ascending narrow-headed of the fishes with separate ascend- process as the distal end of the premaxilla is manip- ing and articular premaxillary processes, e.g., ulated forward. Tripterygion, but also in all those forms with pre- The change from a premaxilla in which the maxillary protrusion that have the two premaxil- ascending and articular processes are separate to lary processes united. one in which they are firmly united has occurred The great majority of the other fishes to be dis- many times in higher teleosts. The shift from one cussed here have the ascending and articular pre- type to another and its effect on upper jaw move- maxillary processes united into a single compound ments can be exemplified by a comparison between structure. As compared with these, the forms the lutjanoid fishes (processes separate except in with the two processes separate have certain Aphareus and probably Randallichthys) and the limitations. First, they seem capable of only a haemuloids (processes united). moderate amount of protrusion. Second, they In lutjanoids the amount of premaxillary pro- appear to have a rather imprecise occlusion trusion is never more than moderate and is com- (Gosline, 1981). On the other hand they have pletely suppressed in Aphareus and Randallichthys developed one adaptation that does not occur in (see Johnson, 1980). Aside from the forms with- fishes in which the articular process is united to a out protrusion, the species investigated have either well-developed ascending process. This has to a flexible area at the base of the ascending pre- do with the combination of moderate protrusion maxillary process (Fig. 1B, C) or, in the Caesioni- and the formation of a rounded mouth opening. dae, a definite hinge in that area (Johnson, 1980). A rounded gape is advantageous not only for Most of the haemuloids have long ascending elimination of lateral escape hatches for prey but premaxillary processes with the articular processes more notably in the development of suction into forming shoulders along their sides (Fig. 1E). the oral cavity (Alexander, 1967b; Liem and Osse, However, the range in the length of the ascending 1975). In higher teleosts without well-developed processes and in the amount of premaxillary pro- ascending premaxillary processes, a rounded gape trusion is very great. At one extreme, in Xenocys, can be approximated by simply swinging the the ascending premaxillary processes are short, maxilla and toothed limb of the premaxilla protrusion is minimal, and the toothed limb of forward over the side of the lower jaw as in lower the premaxilla simply rocks forward to provide teleosts. Here, the proximal end of the premaxilla about the same sort of rounded mouth opening as simply rocks over the anterior end of the skull. in the lutjanid Rhomboplites. In haemuloids with However, a forward plane of protrusion by a well- longer ascending processes there is no premaxillary developed ascending process is never combined rocking, and, as noted, the toothed limb of the with a forward swinging of its distal end if the two premaxilla remains at a fixed angle to the ascending premaxillary processes are united (as in Fig.