Ventilation rate and behavioural responses of two species of intertidal goby (Pisces : Gobiidae) at extremes of environmental temperature

We investigated the behavioural responses of two gobiid fish species to temperature to determine if differences in behaviour and ventilation rate might explain any apparent vertical zonation. A survey of the shore at Manly, Moreton Bay revealed Favonigobius exquisitus to dominate the lower shore and Pseudogobius sp. 4 the upper shore. These species were exposed to a range of temperatures (15-40 degreesC) in aquaria for up to 6 h. At 20 degreesC F. exquisitus exhibited a mean gill ventilation rate of 26 +/- 1.4 bpm (beats per minute) differing significantly from Pseudogobius, which ventilated at a fivefold greater rate of 143 +/- 6 bpm. The ventilation rate in F. exquisitus underwent a fivefold increase from normal local water temperature (20 degreesC) to high temperature (35 degreesC) conditions, whereas that of Pseudogobius did not even double, suggesting that Pseudogobius sp. is a better thermal regulator than F. exquisitus. While both species emerged from the water at high temperatures (>30 degreesC) the behaviours they exhibited while immersed at high temperature were quite different. F. exquisitus undertook vertical displacement movements we interpret as an avoidance response, whereas Pseudogobius sp. appeared to use a coping strategy involving movements that might renew the water mass adjacent to its body. The thermal tolerances and behaviours of F. exquisitus and Pseudogobius sp. are in broad agreement with their vertical distribution on the shore.