Each of the trunk segments of the polychaete Eusyllis blomstrandi is equipped with paired epidermal luminescent domains. They luminesce upon mechanical or electrical stimulation. Light emission can be rapidly turned on and off, appears intracellular and is highly coordinated among the trunk segments. Luminescent light is typically emitted in series of flashes. Light emission in a flash starts locally in a group of segments and recruits adjacent segments at a rate as fast as 1 ms/segment. The collapse of light emission at the end of a flash is almost simultaneous in all of the segments involved. In the intact worm, the luminescent reaction usually involves only a posterior group of segments. Facilitation becomes manifest as the consecutive flashes in a series increase in brightness and duration and recruit additional anterior segments that were not active in earlier flashes. The flash series stops abruptly instead of decreasing asymptotically in brightness. In posterior fragments, all the segments participate in flashing luminescence, indicating the loss of an inhibitory effect exerted by the anterior end in the case of whole animals. Posterior fragments survive and are still capable of luminescence weeks after fragmentation although they do not regenerate a head. Immediately upon fragmentation of the worm, the posterior fragment luminesces continuously for some seconds while the anterior part quickly stops light emission. This suggests a decoy and/or a predator-alerting function of prolonged, strong luminescence by the moribund posterior fragment to the benefit of the survival of the anterior fragment. |
