Cluttered mangrove swamps may not seem to be the best locations to set up home if you're a delicate jellyfish, but this is exactly where you'll find tiny Tripedalia cystophora box jellyfish. Guided by their visual system of 24 simple eyes, the animals successfully avoid contact with damaging mangrove roots and assemble in shafts of light where their favourite copepod meals congregate. Ronald Petie from Dan Nilsson's group in Lund University, Sweden, explains that Nilsson's team has studied the jellyfish's vision extensively, but little was known about how they use vision to control their movements. Petie says, `I thought of combining vision and biomechanics to see how these eyes control the animal's steering'. Explaining that box jellyfish control the direction of their propulsive jet by asymmetrically contracting the bell and a membrane ring around the bell's lower edge, known as the velarium, Petie, Nilsson and Anders Garm decided to find out how different lighting patterns affect the way the jellyfish contracts its bell (p. 2809).
Explaining that the jellyfish's eyes are arranged in four clusters called rhopalia, Petie says, `I wanted to be able to film the jellies from underneath because the rhopalia are quite dark and I could use automatic tracking of the rhopalia to analyse their movements'. Designing a temperature-controlled box where he could tether the jellies by the top of their bell, Petie placed four blue–green LED panels around the box to produce different light stimuli. Then he gently placed a jellyfish in the box, so that each rhopalium looked square onto one of the green LED panels and allowed the jellyfish to become acquainted with its setting. Finally, he turned off one of the panels – to simulate the jellyfish approaching a dark object – and filmed the animal's responses in infrared light.
After switching off each panel in turn and analysing the movements of the dark rhopalia that track each side's contraction, Petie realised that the contraction in the side of the jellyfish closest to the dark panel was delayed relative to that in the other three sides, which continued contracting in synch. And when he looked at the velarium, he realised that the side closest to the dark panel remained relaxed while the other three sides contracted, probably directing the pulsatile jet towards the dark panel to propel a free jellyfish toward the light.
So how do the jellyfish's eyes control this behaviour? Petie explains that instead of looking outward, each of the jelly's four rhopalia are directed inward, looking through the animal's transparent body. This means that the rhopalium closest to the dark panel is facing toward the illuminated panels, while the other three rhopalia are directed – to a greater to lesser degree – toward the dark panel. He also explains that each rhopalium houses a visually controlled pacemaker; they interact together to control contraction of the jellyfish's entire bell and motion. Somehow the contraction due to the pacemaker adjacent to the light becomes delayed relative to the other three, allowing the jelly to orient its propulsive jet to swim away from looming dark objects and home in on light beams packed with tasty copepods.