The environmental cues that influence clock control of tidal movement in the New Zealand eagle ray (Myliobatis tenuicaudatus)

Abstract

Animals that live in shallow marine environments are not only subjected to daily changes in light but also regular, repeated and substantial changes to water levels, and current direction, through the rise and fall of the tide. For estuarine animals it is important to anticipate the tide since this allows organisms to adjust their behaviour and physiology to exploit intertidal resources while avoiding stranding. Batoid elasmobranchs have a well-known capability of moving into estuaries to forage at high tide and retreat back into deeper water before low tide, to avoid stranding. This research investigates the cues employed by the New Zealand eagle ray Myliobatis tenuicaudatus to undertake this intertidal foraging yet avoid stranding. One hypothesis is that M. tenuicaudatus use negative rheotaxis to follow the current flow in and out of estuaries and that they have an internal circatidal clock which allows them to predict when tidal changes will occur. The presence of tidal rhythmicity was assessed by using SwisTrack 3.0™ to analyse locomotor activity patterns when rays (n=4) were exposed cues of current flow, food and light dark cycles. To asses if M. tenuicaudatus use rheotaxis when exposed to current reversal cycles, the orientation of individuals was recorded using SwisTrack 3.0™ and Matlab®. A series of experiments where the sequential removal of 12.4h current flow and 12.4h timed feeding regime was successful at generating tidally rhythmic behaviour in three out of four and two out of three rays respectively. 12:12 light dark cycles were successful at generating circadian rhythmicity in rays when tidal cues were present and also once they had been taken away. Individual variation did exist, however the tidal cues of current flow and timed feeding were effective at generating tidally rhythmic behaviour in combination with each other or separately. All rays (n =4) demonstrated rheotaxis when exposed to current reversal cycles. Two rays of these rays displayed negative rheotaxis whereas the remaining two rays showed positive rheotaxis. M. tenuicaudatus showed tidally modulated rhythms when exposed to current reversal and/or timed feeding regime. M. tenuicaudatus shows rheotaxis when subjected to current reversal cycles. Therefore these tidal cues may be candidate zeitgebers for the clock control of estuarine movement in M. tenuicaudatus.