The larva of the box jellyfish Tripedalia cystophora is an uncomplicated creature: it consists of only five cell types. But studies of the animal and of one cell type in particular, the light-sensitive 'ocellus', have produced an intriguing set of observations. Most strikingly, the larvae seem not to have a nervous system of any kind to which the photoreceptors could be connected. In the words of Karin Nordström and colleagues, whose work it is, the photoreceptors are “self-contained sensory-motor entities” (Proc. R. Soc. Lond. B doi: 10.1098/rspb.2003.2504).
The life cycle of box jellyfish has three phases: a swimming larval stage; a stationary polyp; and the medusa, the familiar jellyfish form, again free-swimming, which in this case is square in cross-section (hence the name of the group). The larva is pear-shaped, but only about 200 µm in length. Tooled up with a transmission electron microscope, Nordström et al. set about looking at it in detail.
An individual larva has 10–15 ocelli, two of which are arrowed in this micrograph of the whole organism. Each consists of a single cell, and together they form an array around the rear-end of the animal relative to the direction of movement. The cell contains a 'cup' of screening pigment, which is filled with structures that Nordström et al. argue are the light-sensing devices. The cups point out at an angle towards the front of the animal. In swimming, the animal also rotates continually (at about two turns per second), so the photoreceptors constitute a scanning system of the light conditions ahead of it.
Another feature of each ocellus is a protruding hair-like cilium. Certain cilia can themselves act as photoreceptors. The authors think that that's not the case here: such a function requires heavy modification, not evident in the ocellar cilia of the Tripedalia larva. So could the cilium be functioning as an active motor? Again no, it seems. Another larval cell type bears cilia for propulsion; and the ocelli don't have the extra power packs, in the form of mitochondria, that would be expected in a system that has the dual functions of light-sensing and propulsion.
The explanation that Nordström et al. plump for is that the function of the ocellar cilia is that of rudders. They control a larva's swimming direction by flexing and stretching, steering it towards (or away from) desirable (or undesirable) conditions signalled by the light regime.
There remains the question of why an array for sensing light conditions ahead of an organism should be set at the back of the body, rather than at the front. Nordström et al. have an explanation for that too. It's because the sensor is also a rudder, and rudders work best at the stern.