Fig. 1.

Functional anatomy of the cichlid and zebrafish head. A dissected and alizarin red stained head of a representative cichlid, Tropheops sp. “red cheek”, is depicted at left, and a zebrafish is shown at right. Craniofacial bones are red, and muscles are white. The lever mechanism that defines the mechanical advantage of jaw closing is illustrated for each species, whereby the jaw joint acts as the fulcrum (F), jaw length is the out-lever (OL), and a dorsally projecting bony process, on which the second subunit of the adductor mandibulae (A2) inserts, acts as the in-lever (IL). In cichlids, the in-lever is the ascending arm of the articular (AP), whereas in zebrafish it is the coronoid process (CP). Thus, in each species, this functional system is comprised of nonhomologous bony processes. Scale bar equals 1 cm in the cichlid image (left), and 1 mm in the zebrafish image (right).