Abstract
Physiological and behavioral systems exist to reduce the stress that the intertidal fauna may face during the unsuitable tidal phase. Cerithidea decollata is a common western Indian Ocean mangrove gastropod. It feeds on the ground at low tide, and climbs the trees two-three hours before the water arrival to avoid submersion. Moreover, it regularly settles on the trunk roughly 40 centimeters above the level that the water will reach, in spite of the irregular East African tidal pattern. Migration usually takes place about twice a day unless at Neap Tide, when snails may remain on the dry ground. Biological clock cannot account for water level foreseeing while direct visual cues or chemical information from the water itself or from previous migrations have already been experimentally discarded. Indirect cues could be hypothesize related to the effect of the oceanic wave reaching the coast and the barrier reef (seismic noise), or alternatively related to changes in ground resistivity (self potential) caused by the sea water moving close. To verify these hypotheses a seismic noise and self potential survey was carried out at Mida Creek (Kenya). This paper presents the first results of the seismic noise measurements. A significant correlation between the time evolution (mean value) of the low frequency seismic signal, tides, and snails movements has been identified.
Highlights
Intertidal fauna faces several problems caused by the periodic adverse conditions created by the continuous variation of the water-air border
We examined the existence of possible cues caused by variations of microseismic intensity correlated with the tidal gravity variation [20] [21] that can be felt by the C. decollata
Assuming that the marine transgressions and regressions, due to the tide events, generate pressure fluctuations on the ground that are locally transformed into microseismic waves at the seafloor that propagate inland [14], the main goal of this preliminary work is to evaluate a possible correlation between the seismic signals and the snails’ movements, in terms of decision to climb up or not
Summary
Intertidal fauna faces several problems caused by the periodic adverse conditions created by the continuous variation of the water-air border. Behavioural responses mostly consist in hiding in appropriate refuges or else migrating across the intertidal belt to reach the suitable intertidal level [2] [3] Such responses are usually told to be triggered by an internal biological clock (12.4 h periodicity), able to anticipate the actual respiratory phase change, and not by direct cues such as the water presence/absence or sudden temperature variation [4] [5]. It has been recently questioned whether a regular 12.4 h clock can cope with the variable pattern of water level due the several tide irregularities within the synodic month [6].
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