Memory in fishes has been shown to consist of more than the proverbial three seconds in recent studies.
In the current issue of the journal Ethology, Lucie Salwiczek and Redouan Bshary demonstrate that Cleaner wrasses (Labroides dimidiatus) are able to remember when they interacted with what after a single event for perhaps the first time documented in fishes.
The authors carried out a series of experiments to test whether wild-caught adult Cleaner wrasses were able to keep track of which food source (what or whom) they interacted with, and the temporal context of their interaction during an optimal foraging task.
The experiments used 12 Cleaner wrasses that were presented every 150 seconds with a choice between two of a total of four plates with different colours and patterns that contained food.
One plate was always accessible but contained a non-preferred food item while the other three contained a preferred food item, but allowed a next feeding event only after five, 10 or 15 minutes.
The experiment was conducted such that if for example, the 10-minute minimal time interval plate was presented five minutes after the wrasse had eaten off it and the fish approached it again, both the 10-minute interval plate and the alternative plate (with the non-preferred food item) were rapidly removed from the aquarium and the wrasse was not rewarded with food. If the fish approached the 10-minute interval plate after 10 or more minutes had elapsed, it was allowed to eat the food item on it.
The authors conducted a series of experiments in which various combinations of the plates were presented to each fish.
Thus, each wrasse had to remember the plate they had interacted with, as well as the last time they interacted with the plate in order to maximise food intake.
The authors found that six of the 12 fish studied were able to choose the 'correct' plate (i.e. one that allowed them to feed the most) when they were offered two plates with the preferred food.
In trials involving the always-accessible plate, 11 of the 12 fish were again able to choose correctly between the plates offered.
This indicated that the wrasse must have learned that the three high-quality plates offered food only after a minimal time interval had elapsed since the last cleaning interaction and that these time intervals consistently differed between client plates, while the low-quality plate was always accessible.
They also demonstrated the ability to recall when the last cleaning interaction with each high-quality plate had taken place in order.
This indicated that the fish were not only able to keep track of the 'what' (or possibly 'who') and but also the ‘when’ in order to maximise service quality (and food intake).
For more information, see the paper: Salwiczek, LH and R Bshary (2011) Cleaner wrasses keep track of the 'when' and 'what' in a foraging task. Ethology 117, pp. 939–948.
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