Fig. 1 Schematic diagram of the sagittal plane through rhopalia of a

Chiropsella bronzie and b Tripedalia cystophora. Box jellyfish have two different camera-type eyes, the upper lens-eye (ULE) and lower lens-eye (LLE), as well as two pairs of pigment pit eyes (not in the sagittal plane). Both the upper and lower lens-eyes contain the typical components of camera-type eyes: a cornea (C), a lens (L), and a retina consisting of a ciliary layer (CL), a pigment layer (PL) and neural layer (NL). In contrast to T. cystophora, the lenses of C. bronzie are

not spherical. Additionally, in C. bronzie, the pigment layer of the upper lens-eye does not meet the lens at the posterior side of the rhopalium. At the peripheral end of the rhopalia, there is a crystalline weight (statocyst, St). The gastric cavity (GC) extends into the rhopalium via the stalk (S) and a mass of large cells, referred to as ‘‘balloon cells’’ (Ba), is covering part of the aperture of the upper lens-eye, and extending towards the rhopalial stalk

eyes have poor spatial resolution because the retina is too close to the lens. This raises the question as to whether lens eyes in all box jellyfish are similarly built to remove fine image detail by under-focusing.

In this paper, we investigate the eyes of the species Chiropsella bronzie (Gershwin), concentrating on the structure and optics of its two camera-type eyes. C. bronzie is often found in knee-deep water along sandy beaches in North Queensland, Australia (Gordon et al. 2004), where they feed on shrimps (Acetes australis). The present anatomical description of their visual system adds to the descriptions of T. cystophora (Berger 1900; Laska and Hu¨ndgen 1982; Nilsson et al. 2005; Garm et al. 2008), Carybdea marsupialis (Claus 1878; Schewiakoff 1889; Berger 1898; Conant 1898; Berger 1900; Martin 2004) and Tamoya bursaria (Yamasu and Yoshida 1976). Our findings on the optics resemble the conditions in T. cystophora where the lenses are too weak to focus a sharp image on the retina. We also describe significant differences in the design and optics between C. bronzie and T. cystophora, suggesting differences in the role of vision between cubozoan species.

Materials and methods

Animals

Animals were collected by hand or by drag net along the beaches around Port Douglas, Australia. They were transported back to James Cook University, Cairns, kept in a large, indoor holding tank with a diameter approximately 2.5 m fitted with daylight lamps. All animals were processed within 2 days of capture.

Anatomical model

As a basis for our analyses of visual optics, a geometrically accurate model was made of the two lens eyes and their position in the rhopalium. The model was based on fresh rhopalia photographed from the front, the side and from above. The shape of excised fresh lenses, together with histological sections, was used to determine the position and dimensions of all optically relevant structures. The model was based on rhopalia from fully-grown medusae.