Visual Tracking of Box Jellyfish: A Real-Time Motion Tracking System

Introduction

Box jellyfish, also known as cubozoans, are a small group of some 50 species of jellyfish that have a somewhat cube-like body with tetra-radial symmetry and tentacles attached to the four lower corners of the bell (Daly et al. 2007, Bentlage et al. 2010). Jellyfish in general have the ability to detect light, but in box jellyfish this has evolved into an elaborate visual system with 24 eyes of four different types, controlling advanced visually guided behaviours (Nilsson et al. 2005, Berger 1898, Claus 1878). Box jellyfish are primarily known for their highly potent venom, resulting in extremely painful stings. Some species from the indo-pacific tropical region are among the most venomous animals known, and cause severe illness or even death within a few minutes after contact with the stinging tentacles (Fenner & Williamson 1996). The different species of box jellyfish mostly inhabit shallow coastal waters, where they feed from small crustaceans or fish that accidentally get in contact with the stinging tentacles (Bentlage et al. 2010, Berger 1898, Buskey 2003). Their bodies and tentacles are largely transparent, making them hard to spot. This facilitates both their passive feeding strategy and reduces attacks from larger animals. The adult body size may be less than 10 mm in the smallest species, whereas the largest species can grow to almost 20 cm, with tentacles that are several meters long (Daly et al. 2007). The most studies species, Tripedalia cyctophora, is one of the few completely harmless species. It only grows to little more than 10 mm in diameter and its tentacles may reach about 10 cm in length (Buskey 2003).

One of the distinguishing features of box jellyfish is their bizarre visual system with a total of 24 eyes grouped on four sensory structures called rhopalia (Nilsson et al. 2005, Pearse & Pearse 1978, Laska & Ündgen 1982, Piatigorsky & Kozmik 2004, Kozmik et al. 2008, Conant 1898). The four rhopalia are suspended on flexible stalks between the tentacle-bearing corners of the bell. Each rhopalium carries 6 eyes. Two of these 6 eyes are elaborate with corneas, lenses and retinas and resemble vertebrate and cephalopod eyes (Nilsson et al. 2005, Martin 2004, Koyanagi et al. 2008). One of the two lens eyes of each rhopalium is typically a little larger than the other, but they are still minute (100-700µm in diameter, depending on species (Garm et al. 2007). Because the rhopalia are equipped with a heavy crystal in the most peripheral end, opposite to where they are attached by the flexible stalk, gravity maintains them constantly in same vertical orientation irrespective of the orientation of the jellyfish (Garm et al. 2011). This has the consequence that the smaller lens eye is always looking straight up through the water surface, and the large lens eye is always aimed obliquely down to monitor the under-water surroundings. For this reason, the two eyes are termed the upper and lower lens eyes. Even though the lens eyes use graded refractive index optics and has retinas with about 1000 light-sensitive cells, they are under-focused and provide only low spatial resolution of about 10° (Nilsson et al. 2005).

Figure 1.

Left: The box jellyfish Tripedalia cystophora is only a couple of mm large and almost completely transparent. Right: A close-up of the rhopalia from one frame recorded in the experimental setup.