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PACIFIC SCIENCE, Vol. XX, October 1966 
Fig. 1. Movement of the hyoid apparatus in 
Epinephelus spilotoceps ( a-c ) and Lutjanus vaigiensis 
(d-j). Diagrammatic side view with the mouth shut 
{a, d ) and the mouth open (b, e) , and top (or 
bottom) view with the mouth open ( c , f). Head of 
fish to the right, hy, Hyoid bar; and ur, urohyal. The 
distance xy indicates the amount of potential lateral 
expansion lost in Lutjanus by the downward pull of 
the urohyal. 
much longer jaws. In E. spilotoceps 140-150 
mm long the length of the upper jaw to the end 
of the maxillary is 32 mm and that of the lower 
jaw is 40 mm; these same dimensions for L. 
vaigiensis are only 21 and 23 mm. If the longer 
jaws of Epinephelus create the possibility of 
greater gape expansion they do not ensure it 
per se. (Fishes with the longest jaws quite fre- 
quently have a rather narrow gape, e.g., Lepi- 
sosteus, Belone .) In Epinephelus there are three 
ways in which gape expansion is actually ac- 
complished. First, the lower rims of the sus- 
pensoria may be swung out from the cranium 
by contraction of the levator hyomandibularis 
et arcus palatini (van Dobben, 1935:7, 8). 
Second, contraction of the sternohyoideus pulls 
the urohyal backward (Fig. 1) and in so doing 
forces the posterior portions of the epihyals out 
laterally (van Dobben, 1935:8). Finally, a 
twisting of the maxillary shaft rolls its lower 
border, and with it the lateral end of the pre- 
maxillary, somewhat outward away from the 
head in Epinephelus, as in Perea (van Dobben, 
1935:11). 
Epinephelus and Lutjanus show differences 
in all three of these processes. That having to 
do with the spreading of the suspensoria by way 
of contraction of the levator hyomandibularis et 
arcus palatini is merely one of degree: in 
Epinephelus this muscle appears to be much 
broader and more powerful than in Lutjanus. 
(The dilatator operculi, which is instrumental 
in expanding the gill cavity, is also much larger 
in Epinephelus than in Lutjanus. 
So far as the hyoid bar method of gape ex- 
pansion is concerned, there are differences both 
in degree and in the direction of the forces. In 
the first place, Lutjanus has hyoid bars that are 
only two-thirds as long as those of Epinephelus, 
so that the potentiality for lateral expansion is 
only two-thirds as great. But even allowing for 
this difference, the hyoid bars are not forced out 
to their maximum lateral expansion in Lutjanus 
when the mouth is opened. In Epinephelus 
(Fig. 1 a-c) contraction of the sternohyoideus 
pulls the urohyal (Fig. 1 , ur ) almost straight 
back, forcing the posterior ends of the hyoid 
bars directly outward. In Lutjanus (Fig. 1 d-f), 
however, as in most deep-bodied fishes, con- 
traction pulls the front of the hyoid bars back- 
ward and downward, and some of the potential 
lateral thrust is lost in the downward motion 
(the amount equal to xy in Fig. le). 
Though the hyoid bar factors just mentioned 
would seem to play the greatest quantitative 
role in the difference in lateral gape expansion 
between Epinephelus and Lutjanus, the feature 
showing the largest qualitative difference is the 
twisting or nontwisting of the maxillary shaft 
when the mouth is opened. In both Lutjanus 
and Epinephelus a ligament from the adductor 
mandibulae and the lower jaw runs to an at- 
tachment on the outer surface of the maxillary 
shaft anteriorly (Fig. 2, L). In narrow- 
mouthed fishes, when the adductor contracts 
this ligament helps to pull the maxillary up and 
back to the closed-mouth position. In wide- 
mouthed fishes it has a different function when 
the gape is widely opened: as the posterior end 
of the maxillary swings laterally with gape 
expansion, the ligament comes to pull across it, 
causing the maxillary to rotate on its shaft 
(Fig. 2). This rotation at the anterior end of 
the maxillary causes protrusion of the pre- 
maxillaries ; posteriorly it rolls the lower border 
of the maxillary outward, and with it to some 
extent the lateral ends of the premaxillary, thus 
