Abstract Cubomedusae have a total of 24 eyes of four morphologically different types. Two of these eye types are camera-type eyes (upper and lower lens-eye), while the other two eye types are simpler pigment pit eyes (pit and slit eye). Here, we give a description of the visual system of the box jellyfish species Chiropsella bronzie and the optics of the lens eyes in this species. One aim of this study is to distinguish between general cubozoan features and speciesspecific features in the layout and optics of the eyes. We find that both types of lens eyes are more severely underfocused in C. bronzie than those in the previously investigated species Tripedalia cystophora. In the lower lens-eye of C. bronzie, blur circles subtend 20 and 52L for closed and open pupil, respectively, effectively removing all but the coarsest structures of the image. Histology reveals that the retina of the lower lens-eye, in addition to pigmented photoreceptors, also contains long pigment-cells, with both dark and white pigment, where the dark pigment migrates on light/dark adaptation. Unlike the upper lens-eye lens of T.cystophora, the same eye in C.bronzie did not display any significant optical power.

Keywords Box jellyfish Visual systems Photoreceptors Eyes Visual optics

M. O’Connor (&) D.-E. Nilsson Department of Cell and Organism Biology, Lund University, Lund, Sweden

e-mail: Megan.OConnor@cob.lu.se; megsoc@gmail.com

A. Garm

Department of Comparative Zoology,

University of Copenhagen, Copenhagen, Denmark

Introduction

Box jellyfish belong to the phylum Cnidaria, arguably the most basal phylum to possess true vision. Their phylogenetic position, simple nervous system and elaborate set of eyes (Nilsson et al. 2005; Garm et al. 2008) make their visual system of key importance for understanding the early evolution of vision, and also for understanding the biology of box jellyfish. The eyes of box jellyfish share many features with those of vertebrates. Morphologically, they are similar by way of their ciliary photoreceptors (Eakin and Westfall 1962; Singla 1974; Yamasu and Yoshida 1976; Laska and Hu¨ndgen 1982), and recently, some molecular components characterised by Kozmik et al. (2008) suggested that the box jellyfish visual system is more closely related to vertebrate than to invertebrate visual systems. However, recent sequencing has revealed that box jellyfish express a unique class of opsins (Koyanagi et al. 2008).

Box jellyfish have a surprisingly complex visual system consisting in total of 24 eyes of four morphologically different types. Two of these eye types are camera-type eyes (upper and lower lens-eye), while the other two eye types are simpler pigment cup eyes (pit and slit eye) (Figs. 1, 2a). All species of box jellyfish share the overall features of this elaborate visual system. However, little is known about variations in the performance or role of eyes in different cubozoan species.

A previous anatomical and optical investigation of the visual system of the box jellyfish species Tripedalia cystophora revealed that the lenses of the upper and lower lens-eye are spherical and able to focus light remarkably well, with the lens of the upper eye bringing light to a near perfect focus (Nilsson et al. 2005). Despite this, both lens