Hunting in Bioluminescent Light: Vision in the Nocturnal Box Jellyfish Copula sivickisi.

Anders Garm,Ronald Petie,Jan Bielecki,Dan-Eric Nilsson

doi:10.3389/fphys.2016.00099

Abstract

Cubomedusae all have a similar set of six eyes on each of their four rhopalia. Still, there is a great variation in activity patterns with some species being strictly day active while others are strictly night active. Here we have examined the visual ecology of the medusa of the night active Copula sivickisi from Okinawa using optics, morphology, electrophysiology, and behavioral experiments. We found the lenses of both the upper and the lower lens eyes to be image forming but under-focused, resulting in low spatial resolution in the order of 10–15°. The photoreceptor physiology is similar in the two lens eyes and they have a single opsin peaking around 460 nm and low temporal resolution with a flicker fusion frequency (fff) of 2.5 Hz indicating adaptions to vision in low light intensities. Further, the outer segments have fluid filled swellings, which may concentrate the light in the photoreceptor membrane by total internal reflections, and thus enhance the signal to noise ratio in the eyes. Finally our behavioral experiments confirmed that the animals use vision when hunting. When they are active at night they seek out high prey-concentration by visual attraction to areas with abundant bioluminescent flashes triggered by their prey.

Highlights

Within Cnidaria a small group, the cubozoans, have diverged to evolve an elaborate visual apparatus along with an according expansion of their nervous systems (Satterlie, 1979; Garm et al, 2007)
We focused on the lens eyes since they are the only image forming eyes and examined their optics and used electroretinograms (ERGs) to investigate their receptor physiology, including spectral sensitivity and temporal resolution
Along with the heavy crystal in the distal end, this results in the rhopalium always keeping the same vertical orientation with the upper lens eye pointing straight upwards (Figures 1C–F)

Summary

Introduction

Within Cnidaria a small group, the cubozoans, have diverged to evolve an elaborate visual apparatus along with an according expansion of their nervous systems (Satterlie, 1979; Garm et al, 2007). The optics have been investigated only in two species, Tripedalia cystophora and Chiropsella bronzie In these species, the lens eyes provide low spatial resolution in the order of 10◦ or worse (Nilsson et al, 2005; O’Connor et al, 2009). The lens eyes provide low spatial resolution in the order of 10◦ or worse (Nilsson et al, 2005; O’Connor et al, 2009) This does not allow visually guided hunting for prey. Instead they use vision to seek out habitats with high prey densities This is best understood for the Caribbean species, T. cystophora, which feed on copepods accumulating in light shafts between the mangrove prop roots (Buskey, 2003; Garm et al, 2011). Once in the right habitat they hunt passively with extended and trailing tentacles and the actual prey capture is no different from the typical scypho- and hydromedusa which rely on the prey accidentally contacting a tentacle

Methods

Results

Conclusion