Prey capture and accommodation in the sandlance, Limnichthyes fasciatus (Creediidae; Teleostei)

The eyes of the sandlance, Limnichthyes fasciatus (Creediidae. Teleostei) m ove independently and possess a refractive cornea, a convexiclivate fovea a nd a non-spherical lens giving rise to a wide separation of the nodal point from the axis of rotation of the eye much like that of a chameleon. To inv estigate this apparent convergence of the visual optics in these phylogenet ically disparate species, we examine feeding behaviour and accommodation in the sandlance with special reference to the possibility that sandlances us e accommodation as a depth cue to judge strike length. Frame-by-frame analy sis of over 2000 strikes show a 100% success rate. Explosive strikes are co mpleted in 50 ms over prey distances of four body lengths. Close-up video c onfirms that successful strikes can be initiated monocularly (both normally and after monocular occlusion) showing that binocular cues are not necessa ry to judge the length of a strike. Additional means of judging prey distan ce may also be derived from parallax information generated by rotation of t he eye as suggested for chameleons. Using photorefraction on anaesthetised sandlances, accommodative changes were induced with acetylcholine and found to range between 120 D and 180 D at a speed of 600-720 D s(-1). The large range of accommodation (25% of the total power) is also thought to be media ted by corneal accommodation where the contraction of a unique cornealis mu scle acts to change the corneal curvatures.