Outgroup Patterns Fishes

The monophyletic clades of extant fishes that form out-group taxa to tetrapods and amniotes are the sharks and relatives (Elasmobranchiomorpha), ray-finned fishes (Actinopterygii), coelacanths (Actinistia), and lungfishes (Dipnoi). The feeding mechanisms of members of all of these taxa have been studied in some form or other during recent years, and a comparative analysis of feeding morphology and function in these clades provides the basis for our subsequent consideration of tetrapod feeding systems.

Initial Prey Capture

Despite the diversity of skull morphology represented by taxa as phylogenetically divergent as sharks, bass, and lungfishes, many common fundamental features of the process of initial prey capture have been observed. Most important is the observation that many taxa capture prey by suction feeding (Grobecker and Pietsch, 1979; Lauder, 1985a; Liem, 1970; Norton and Brainerd, 1993; Nyberg, 1971; Westneat and Wainwright, 1989).

The process of suction feeding involves creating a pressure within the oral cavity that is less than ambient. As shown in figure 1, expansion of oral volume occurs by lateral movement of the suspensoria, elevation of the neurocranium, depression of the lower jaw, and ventral movement of the hyoid region. The result of these movements is a reduction in oral cavity pressure that draws water into the mouth anteriorly carrying the prey toward the gape. The strike may be unsuccessful, in which case the prey escapes; the strike may result in prey being caught between the upper and lower jaws as the mouth closes (as in Fig. 1); or the prey may be completely drawn into the oral cavity. During the time that the mouth is opening, bones covering the gills laterally prevent water influx from the area posterior and lateral to the head and allow an essentially unidirectional flow of water through the mouth from anterior to posterior. Water flows first into the oral cavity, then between and around gill bars and filaments to exit finally in an expanding gap between opercular elements and the side of the head (Fig. 1). In the absence of an appropriate morphological design, the reduction in oral cavity pressure would be expected to draw in water from both posterior and anterior to the head, reducing the effectiveness of suction directed toward the prey.

Direct measurement of pressure changes simultaneously at several sites within the mouth cavity of ray-finned fishes using suction feeding shows that the branchial apparatus may have a significant influence on the function of the feeding mechanism. Figure 2 illustrates the comparative pressures measured at three sites in the oral cavity of a ray-finned fish during suction feeding. Note that, first, negative pressures may be quite large, reaching nearly 600 cm H20 below ambient. Second, pressures measured anteriorly and posteriorly within the oral cavity are essentially equivalent in magnitude. Third, posterior to the gill bars in the opercular cavity the pressure drop is only about one-fifth that in the oral cavity. Experimental studies have shown that this reduced negative pressure is caused by the gill bars themselves, which are adducted to form a high resistance to flow at the posterior limit of the oral cavity as the mouth opens (Lauder, 1983c). The gill bars are then abducted to allow water to pass posteriorly as the mouth closes.

Although many taxa do not generate large negative pressures during suction feeding (Norton and Brainerd, 1993), fishes as phylogenetically divergent as sharks (Frazetta, 1994; Moss, 1977; Motta et al., 1991), lungfishes (Bemis, 1987; Bemis and Lauder, 1986), and coelacanths (inferred by Lauder; 1980b) are capable of using suction during feeding.

A typical pattern of jaw muscle activity used during suction feeding is illustrated in figure 3. The time from the onset of mouth opening to peak gape is called the expansive phase, and muscles active at the start of this phase include the levator operculi, sternohyoideus (rectus cervicis), and epaxial muscles (Fig. 3). These muscles act to depress the lower jaw and hyoid, and to elevate the neurocranium. Muscles connecting the hyoid to the lower jaw (such as the geniohyoideus) and the adductor mandibulae muscles may also be active during this time. In such cases, there is considerable overlap between the activity of mouth closing and opening muscles. As the mouth closes (the compressive phase), activity continues in the

Figure 2. Diagram of the pattern of pressure change in the oral cavity of a percomorph fish during prey capture based on the experimental data from Lauder (1980c; 1983c). Suction feeding is produced by intraoral pressure changes. Note that the negative pressure posterior to the gill bars is greatly reduced compared to both the anterior and posterior sites within the oral cavity (after Lauder 1985c).

Figure 2. Diagram of the pattern of pressure change in the oral cavity of a percomorph fish during prey capture based on the experimental data from Lauder (1980c; 1983c). Suction feeding is produced by intraoral pressure changes. Note that the negative pressure posterior to the gill bars is greatly reduced compared to both the anterior and posterior sites within the oral cavity (after Lauder 1985c).

adductor mandibulae and geniohyoideus muscles. One consistent kinematic pattern found in almost all teleost fishes studied to date is the peak in hyoid excursion during the compressive phase. This maximal hyoid excursion occurs later than peak gape (Fig. 3) and yet prior to maximal opercular expansion; there is thus an anterior to posterior sequence of peak gape, peak hyoid, and maximum opercular excursion. The recovery phase (defined as the time

Figure 3. Schematic diagram of kinematic and motor patterns common to initial prey capture events in many ray-finned fishes. The names of phases associated with kinematic events are indicated at the top. Note that phase names differ in the fish and tetrapod literature. For example, in tetrapods the compressive phase is referred to as the closing (or fast closing) phase. The preparatory phase has only been observed in a few taxa to date. Black bars indicate times when muscles are consistently active whereas gray bars indicate activity that is only intermittently present. Modified from Lauder and Reilly (1994).

Figure 3. Schematic diagram of kinematic and motor patterns common to initial prey capture events in many ray-finned fishes. The names of phases associated with kinematic events are indicated at the top. Note that phase names differ in the fish and tetrapod literature. For example, in tetrapods the compressive phase is referred to as the closing (or fast closing) phase. The preparatory phase has only been observed in a few taxa to date. Black bars indicate times when muscles are consistently active whereas gray bars indicate activity that is only intermittently present. Modified from Lauder and Reilly (1994).

from jaw closure to the return of hyoid, suspensorial, and opercularelements to their initial positions) typically involves activity in the jaw, hyoid, and suspensorial adductor muscles. Finally, in some ray-finned fishes, a preparatory phase occurs prior to mouth opening in which the volume inside the mouth cavity is reduced by activity of jaw and hyoid adductors. This phase has primarily been observed in percomorph ray-finned fishes and has not been found in plesiomorphic taxa (Lauder, 1980a).

Intraoral Prey Transport

The process of moving prey from the jaws to the esophagus is referred to as prey transport. In many fishes, the process of transport involves two discrete components: hydraulic transport and pharyngeal

0 0

Post a comment