Box jellyfish (Cubozoa) are famous for their symmetrical beauty (see Haeckel’s Kunstformen der Natur to the right) and infamous for there potent venom that they use to catch their prey (mainly small fish) and to defend themselves against predators. Yet, most intriguingly, they have a total of 24 eyes. These 24 eyes comprise four different types, the slit eye, pit eye, and the upper and lower lens eye. The lens eyes are actually similar to the chambered eyes of vertebrates, whereas the other eye types are simpler. However, it remained enigmatic what such a complex set of eyes may be good for. So far, the visually-induced behavior of box jelly fish seemed to be restricted to simple responses such as obstacle avoidance and phototaxis. Earlier this year, Anders Garm, Magnus Oskarsson, and Dan-Eric Nilsson published a very interesting paper, describing that box jellyfish are capable of a slightly more complicated visually-guided task. One species of box jellyfish, Tripedelia cystophora, actively navigates to their preferred habitats by exploiting terrestrial visual cues that are detected by the upper lens eye. The preferred habitats of this species are the edges of mangrove lagoons in the Caribbean (see photo below).
In this complex network of prop roots T. cystophora forage on copepods – small, planktonic crustaceans – that can be found in large aggregations wherever light penetrates the mangrove canopy. Obviously, there is a large motivation for box jellyfish to stay close to this cornucopia. Garm et al. argue that the upper lens eye is the navigation system that enables oriented movement towards the lagoon edge. The upper lens eye is suspended and points upwards at all times, independent of body rotation and position. The retinal visual field is surprisingly small (~95-105 degrees) but it matches Snell’s window. Snell’s window is the angle from which an underwater observer can actually see the entire terrestrial world above (180 degrees), because of the refraction of light at the air-water boundary. Hence, the retinal visual field of the box jellyfish seems appropriate to detect terrestrial cues. As it turns out, this may actually be the case. Garm et al. performed a behavioral test where they removed the box jellyfish from the lagoon edge and moved it away from their habitat. When taken away no further than 8m, they quickly swam back towards the nearest trees. This distance matched the theoretical prediction on the basis of an optical model of visual performance.
Now, what is the evolutionary significance of this finding? Garm et al. suggest that the structural peculiarities of the upper lens eye of T. cystophora and the navigational behavior are derived from a sun compass. This function has been postulated for the upper lens eyes in another box jellyfish species, yet this species (Chiropsella bronzie) is actually nested in a different clade within Cubozoa (Bentlage et al. 2010). It will be an interesting an important endeavor to analyze the evolutionary and adaptive significance of the function of the upper eye lens (and the other eyes and eye types, for that matter) of the Cubozoa in a phylogenetic, comparative framework.