External morphology of eyes and Nebenaugen of caridean decapods-ecological and systematic considerations.

Magnus L Johnson,Nicola Dobson,Sammy De Grave

doi:10.7717/peerj.1176

Abstract

Most caridean decapods have compound eyes of the reflecting superposition kind, and additionally some possess an accessory eye-like organ of unknown function, also referred to as the nebenauge. We examined 308 caridean genera to assess the general morphology of the eye, rostrum length, eye diameter and the presence or absence and, when present, the diameter of the nebenauge. We have attempted to relate these data to ecological and taxonomic considerations. We consider there to be 6 distinct eye types based on the margin between the eyestalk and cornea. The presence of nebenaugen appears to be generally linked to an active lifestyle, as evidenced by the fact that species that have nebenaugen tend to have larger eyes and are more likely to have a distinct rostrum. We suggest that the inconsistencies in its presence/absence under both systematic and ecological lenses may indicate that when present it has various roles relating to behavioural and physiological rhythms.

Highlights

Decapod eyes The laws of physics impose strict regulation on the optical and physical design of eyes and their limitations are manifest in their physical dimensions
Eye diameter was measured as the maximum anterio-posterior length across the eye and nebenauge diameter was taken as the measurement at the widest point
Morphological variations We found that eyes could generally be classed into 6 major types according to the shape of the margin between the eyestalk and the cornea (Fig. 2), general shape and further classified by the presence or absence of the nebenauge

Summary

Introduction

Decapod eyes The laws of physics impose strict regulation on the optical and physical design of eyes and their limitations are manifest in their physical dimensions. Larger eyes generally have greater sensitivity both spatially and with regard to light intensity (Laughlin, 2001). In species such as long-bodied decapods that generally have super-position compound eyes, physiological evolution would in theory encourage the development of large, perfectly formed hemispheres on long eyestalks that would give the bearer highly sensitive all round vision and sufficient resolution to enable accurate triangulation.

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