Mudskippers scream at each other when they are out of the water, according to a study published in a recent issue of the online journal PLoS ONE.
Mudskipper expert Gianluca Polgar and coauthors made this discovery while studying the mudskipper Periophthalmodon septemradiatus, a species sometimes encountered in the aquarium trade.
The authors observed sound production when male mudskippers were competing for food when held in a community tank, making preliminary recordings using a hydrophone (underwater microphone) inserted into the wet mud.
They then observed the fish under more controlled conditions, placing a single male in a tank with only one shelter made of slate or terracotta.
A single hydrophone was inserted in front of the shelter opening, and a digital video camera was set up to record the behaviour of the mudskipper.
A second male mudskipper of comparable size placed within a cylindrical mesh was then introduced and the encounters between the residents and intruders recorded.
The authors found that the mudskippers made both pulsed and tonal sounds of low frequency during each encounter.
Immediately before the emission of each pulse, the head was slightly lifted, and during pulse emission the fish made a short, rapid and downwardly directed vertical movement of the lower jaw (when the mouth was open), or of the whole head (when the mouth was closed). No movements were ever observed during tonal sounds. The head was never in contact with the substrate during vocalisations.
The method of sound production remains a mystery. The mudskippers studied lack a swimbladder, which is the organ used by many fishes in sound production.
Although mudskippers frequently gulp air and could be using the air bubble held in their gill chambers to produce sounds, the authors note that sound continued to be produced even when the fishes apparently did not hold an air bubble.
They also ruled out stridulatory mechanisms, ie. sound produced by two hard structures (e.g. bone) rubbing against each other, as sounds produced in this manner are of a higher frequency than that recorded.
The most likely mechanism they hypothesise is that the fish use muscles to produce the sound, utilising some part of their body as a transducer.
For more information, see the paper: Polgar G, S Malavasi, G Cipolato, V Georgalas, JA Clack and P Torricelli (2011) Acoustic communication at the water's edge: evolutionary insights from a mudskipper. PLoS ONE 6,: e21434. doi:10.1371/journal.pone.0021434
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