Predators sniff out the quality of their prey

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Predatory reef fishes can not only detect their prey by smelling them, they can also tell the size and condition of their prey based on smell alone, according to a study to be published in a forthcoming issue of the journal Oecologia.

Studying the predatory Brown dottyback (Pseudochromis fuscus) and their most common prey item (the Ambon damselfish, Pomacentrus amboinensis), Oona Lonnstedt, Mark McCormick and Douglas Chivers conducted a series of experiments to test whether predators were differentially attracted to damage released chemical cues from prey that differed in body condition and size.

The Brown dottyback is a common cryptic predator found throughout the Indo-Pacific, where it exhibits a strong preference for newly settled damselfish in the summer months.

It is a gape-limited predator, meaning that it is capable of eating fishes that can only fit inside its mouth.

The authors carried out the first two series of experiments in a y-shaped aquarium (y-maze). A dottyback was acclimated for 6–12 hours at the base of the y-maze and separated from the arms by a transparent divider that allowed water to flow through.  

At the start of each experiment, 15 ml of each treatment stimulus (extracts from damaged skin of euthanised prey fish) was simultaneously injected into the two arms of the y-maze, followed by 60 ml of tank water to flush through. The stimuli were re-injected at 4-minute intervals.

Observations were made for 20 minutes immediately after the injection of the two stimuli, during which the observer recorded which compartment the predator was oriented in front of as well as the total time spent inside each compartment.

The first series of 15 experiments tested the preference for quality of the prey, using skin extracts from well-fed damselfish (good quality) and poorly-fed damselfish (poor quality), while the second series of 21 experiments tested the preference for prey size, using skin extracts from adult damselfishes and newly-settled damselfishes.

The authors next investigated whether the Brown dottyback was attracted to and could differentiate size of prey fish based on skin extracts in their natural environment.

Carrying out an in situ experiment in Lizard Island (in the Great Barrier Reef), the authors placed a plastic tube inside the home range of the dottybacks to be studied and tested the response of the fish to the introduction of three treatments through the tube: (1) extract from damaged skin of newly-settled damselfish; (2) extract from damaged skin of adult damselfish and (3) seawater. The behaviour of the dottyback was recorded for three minutes before and three minutes after the addition of the stimulus.

In the first series of experiments, the authors found that the dottybacks spent more than twice as much time in front or inside the chambers in which skin extract from well-fed damselfish were released while in the second series, the dottybacks also spent more than twice as much time in front of, or near the chamber in which skin extract from newly-settled damselfish were released.

In the in-situ experiment, the dottybacks showed significantly greater foraging behaviour when presented with skin extracts from newly-settled damselfishes.

The results of the experiments indicated that dottybacks showed a preference for good quality, smaller prey (that the dottybacks could comfortably ingest) and that the odour signatures of the damselfishes could convey information about the owner’s condition and size to the dottybacks.

The findings suggest a new level of sophistication in the use of chemicals in hunting whereby predators track down wounded prey of a particular size and condition.

For more information, see the paper: Lonnstedt, OM, MI McCormick and DP Chivers (2011) Well-informed foraging: damage-released chemical cues of injured prey signal quality and size to predators. Oecologia doi:10.1007/s00442-011-2116-8

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