Collette and Gillis: Osteological differences between two species of Orammatorcynus 



35 



is present. This process joins with the fourth epibran- 

 chial, which is larger than the third and is interposed 

 between the fourth ceratobranchial and pharyngobran- 

 chial. It is a curved bone with the angle formed by the 

 lateral and medial arms being much more acute in 

 Grammatorcynus than in Acanthocybimn and Scom- 

 beromorus. A dorsal process arises from the middle of 

 the bone and attaches to the third epibranchial. 



Pharyngobranchials There are four pharyngo- 

 branchials attached basally to the epibranchial of their 

 respective gill arch. The first is long and slender, 

 articulates dorsally with the prootic, and is frequently 

 called the suspensory pharyngeal (Iwai and Nakamura 

 1964). The elongate second pharyngobranchial bears 

 a patch of teeth. The third is the largest element in the 

 series; it has a broad patch of teeth on its ventral sur- 

 face, a broad posterior end, and tapers to a narrow 

 anterior end. In Grammatorcynus and Acanthocybium. 

 the third pharyngobranchial is shorter than in Scom- 

 beromorus. The fourth pharyngobranchial also bears 

 a ventral tooth plate, has a rounded posterior end, and 

 has an elongate strut (pharyngobranchial stay) mesially 

 which overlaps the third pharyngobranchial. This stay 

 is much shorter in Grammatorcynus and Acantho- 

 cybium than in Scomberomorus. 



Gill rakers The hypobranchial, ceratobranchial, 

 and epibranchial of the first gill arch support a series 

 of slender, rigid gill rakers. The longest gill raker is 

 at or near the junction of the upper and lower arches, 

 between the ceratobranchial and epibranchial. Magnu- 

 son and Heitz (1971) have clearly shown that there is 

 a correlation between numbers of gill rakers, gap 

 between gill rakers, and size of food items in a number 

 of species of Scombridae. 



The number of gill rakers is easily countable and is 

 an especially useful taxonomic character in differen- 

 tiating between the two species of Grammatorcynus: 

 G. bilineatus has more gill rakers (18-24) than 

 G. bicarinatus (12-15) (Table 3). Acanthocybium dif- 

 fers from all other genera of Scombridae in complete- 

 ly lacking gill rakers. Three species of Scomberomorus 

 have greatly reduced numbers of gill rakers: S. multi- 

 radiatus (1-4 gill rakers), S. commerson (1-8), and 

 S. queenslandicus (3-9). Scomberomorus concolor 



stands out in having the most gill rakers (21-27). Other 

 species of Scomberomorus fall between these extremes. 

 There is a correlation between number of gill rakers 

 and number of jaw teeth in Grammatorcynus and 

 Scomberomorus. Species with the fewest gill rakers, 

 G. bicarinatus and S. multiradiatus, also have the 

 fewest jaw teeth, and species with the most gill rakers, 

 G. bilineatus and S. concolor, have the most teeth. 



Axial skeleton This section is divided into four parts: 

 vertebral number, vertebral column, ribs and inter- 

 muscular bones, and caudal complex. 



Vertebral number Vertebrae may be divided into 

 precaudal (abdominal) and caudal. The first caudal 

 vertebra is defined as the first vertebra that bears a 

 notably elongate haemal spine and lacks pleural ribs. 

 Vertebral counts include the urostyle which bears the 

 hypural plate. Grammatorcynus has 31 vertebrae, 

 which is less than Scomberomorus (41-56 vertebrae), 

 which in turn is less than Acanthocybium (62-64). The 

 same situation also applies to precaudal and caudal 

 vertebrae. Both species of Grammatorcynus have 12 

 precaudal and 19 caudal (except for one specimen of 

 G. bicarinatus with 1 1 plus 20 caudal). Scomberomorus 

 has 16-23 precaudal and 20-36 caudal, and Acantho- 

 cybium has 31-33 precaudal and 31-33 caudal. The 

 presence of only 31 vertebrae in Grammatorcynus is 

 a primitive condition agreeing with Scomber and 

 Rastrelliger, the most primitive members of the Scom- 

 brinae. The increased number of vertebrae in Acantho- 

 cybium is clearly a specialization. 



Vertebral column The neural arches and spines 

 are stout and compressed on the first to the fourth 

 vertebra (especially the first 3) in Grammatorcynus. 

 They extend farther back, to the fifth or sixth verte- 

 brae, in most species of Scomberomorus, and extend 

 farthest, to the seventh vertebra, in Acanthocybium 

 and S. commerson. Posteriorly, toward the caudal 

 peduncular vertebrae and caudal complex, the neural 

 spines bend abruptly backward and cover most of the 

 neural groove; caudally they merge into the caudal 

 complex as in Thunnus (Kishinouye 1923, Gibbs and 

 Collette 1967) and the bonitos (Collette and Chao 1975). 



