The functional morphology of the retractor lentis muscle of a teleost fish, Astronotus ocellatus

Visual teleost species such as the oscar, Astronotus ocellatus, accommodate by moving the lens within the eye. This movement is controlled by a smooth muscle, the retractor lentis. We observed two components in the accommodative lens movement: a nasotemporal and a mediolateral component. In the present study, the anatomical basis of these two components was investigated. Photographs of freshly dissected specimens were used to determine the position and relative size of the retractor lentis muscle in situ. Light microscopy sections reveal two fibre orientations within the muscle. Scanning electron micrographs illustrate that the muscle ligament fans out in two primary directions where it inserts into the lens capsule. The observation that two independent lens movements occur during accommodation is supported anatomically by the orientation of the fibres within the muscle and by the nature of the insertion of the ligament into the lens capsule.