Suspension and optical properties of the crystalline lens in the eyes of basal vertebrates

Abstract

We have investigated the apparatus suspending the crystalline lens in the eyes of basal vertebrates. Data are presented for Holocephali (Chondrichthyes) and the actinopterygians Polypteriformes, Polyodontidae (Acipenseriformes), Lepisosteiformes, Amiiformes, and one teleost species, the banded archerfish (Toxotes jaculatrix). We also studied the optical properties of the lens in Polypteriformes, Lepisosteiformes, and the archerfish. Together with previously published results, our findings show that there are three basic types of lens suspension in vertebrates. These are i) a rotationally symmetric suspension (Petromyzontida, lampreys; Ceratodontiformes, lungfishes; Tetrapoda), ii) a suspension with a dorso-ventral axis of symmetry and a ventral papilla (all Chondrichthyes and Acipenseriformes), and iii) an asymmetric suspension with a ventral muscle and a varying number of ligaments (all Actinopterygii except for Acipenseriformes). Large eyes with presumably high spatial resolution have evolved in all groups. Multifocal lenses creating well-focused color images are also present in all groups studied. Stable and exact positioning of the lens, in many cases in combination with accommodative changes in lens position or shape, is achieved by all three types of lens suspension. It is somewhat surprising that lens suspensions are strikingly similar in Chondrichthyes and Acipenseriformes (Actinopterygii), while the suspension apparatus in Polypteriformes, usually being regarded as an actinopterygian group more basal than Acipenseriformes, are considerably more teleostean-like. This study completes a series of investigations on lens suspensions in nontetrapod vertebrates, covering all major groups except for the rare and highly derived coelacanths.

A lambda‐shaped retractor lentis muscle in the yellowfin goby Acanthogobius flavimanus

“…Previously, Kröger, Gustafsson, and Tuminaite () described that the gray bichir Polypterus senegalus seneguls (Polypteriformes), has the retractor lentis muscle divided into two shanks in the eye. The shape of the muscle of the gray bichir can be seen as lambdoid, however, a more detailed functional study was not done.…”

Aerial and aquatic visual acuity of the grey bichir Polypterus senegalus, as estimated by optokinetic response

“…Polypterus senegalus was expected to be able to see in air, as absence of visual input had been shown to affect the kinematics of terrestrial locomotion (K. R. Znotinas & E. M. Standen, unpublished data). However, poor spatial acuity was also expected, since the P. senegalus eye is anatomically similar to that of typical teleosts (Rochon‐Duvingneaud, , in Kröger et al ., ; Pfeiffer, ) and does not possess common adaptations that improve aerial spatial acuity in fishes with an amphibious lifestyle (Sayer, ).…”

“…Vision may therefore be a significant source of information for P. senegalus , but both temporal and spatial acuity were expected to be poor in comparison to acuity of diurnal species inhabiting clearer waters. As P. senegalus are not known to possess any adaptations for aerial vision (Kröger et al ., ; Pfeiffer, ; Rochon‐Duvingneaud, ), they were expected to demonstrate decreased spatial and temporal acuity in air relative to water, but not a complete loss of visual ability.…”