Date Published: October 14, 2013
Publisher: Public Library of Science
Author(s): C. Elisa Schaum, Robert Batty, Kim S. Last, Roberto Pronzato.
The harbour ragworm, Nereis (Hediste) diversicolor is a common intertidal marine polychaete that lives in burrows from which it has to partially emerge in order to forage. In doing so, it is exposed to a variety of predators. One way in which predation risk can be minimised is through chemical detection from within the relative safety of the burrows. Using CCTV and motion capture software, we show that H. diversicolor is able to detect chemical cues associated with the presence of juvenile flounder (Platichthys flesus). Number of emergences, emergence duration and distance from burrow entrance are all significantly reduced during exposure to flounder conditioned seawater and flounder mucous spiked seawater above a threshold with no evidence of behavioural habituation. Mucous from bottom-dwelling juvenile plaice (Pleuronectes platessa) and pelagic adult herring (Clupea harengus) elicit similar responses, suggesting that the behavioural reactions are species independent. The data implies that H. diversicolor must have well developed chemosensory mechanisms for predator detection and is consequently able to effectively minimize risk.
Predation is an important force that affects evolution, behaviour, life history and phenotypic traits in prey animals ,  and animals in both the aquatic and the terrestrial environment have evolved predator evasion and avoidance mechanisms, which are not the result of direct attack. Many marine polychaete worms are involved in particularly interesting predator-prey interactions as they have to expose themselves to a relatively high level of risk every time they emerge from their sea-bed burrows . At such times the energetic costs of not emerging to feed are outweighed by the costs of not being eaten by a predator . Reliance on purely sound, mechanical disturbance and shadowing for predator detection would be of limited value due to the frequency of abiotic non-predatory stimuli in the marine environment . Such abiotic cues are known to habituate nereidid polychaete worms and an argument has been made by  that the worms are likely to be in a state of almost permanent habituation to these stimuli.
Here we have shown that the marine polychaete worm H. diversicolor is behaviourally responsive to a waterborne chemical cue(s), released by demersal as well as pelagic fish. All behavioural responses investigated, i.e. number, duration and distance of emergences from the burrow were reduced in the presence of predator conditioned or predator mucous spiked seawater above certain thresholds. Out-of-burrow food searching, which naturally exposes the polychaetes to predation risk , is therefore reduced and it is hypothesised that an unknown substance naturally occurring in fish mucous, or cocktail of substances , is acting as an alarm cue(s) to these worms. It is likely, that not all of the observed reduction in activity was a direct response to an alarm cue, but that a fraction also resulted from a positive feed-back loop: We assume that some worms will pick up the signal, reduce their activity, and release more cues themselves, which might in turn affect the behaviour of other worms in the same tank. As we have treated worms within one tank as not independent from each other in our analysis, it is likely that any such details are lost in the strong overall signal. More detailed research is needed as to how much – if any – information H. diversicolor can gather from alarm cues that do not originate from a directly attacked conspecific.