Abstract

The motion behaviour of three species of intertidal foraminifera, Ammonia tepida, Cribroelphidium excavatum and Haynesina germanica, was investigated continuously in the laboratory. We first infer the presence of geotactic and phototactic responses. Significant geotactic responses were observed for all three species; A. tepida was found to be negatively geotactic while C. excavatum and H. germanica showed positive geotaxis. In contrast, no response to light was ever observed. The detailed nature of motility, investigated in terms of both geometric and stochastic complexity of their motion behaviour, was consistently characterised by a strong inter-specific, inter-individual and intra-individual variability. Specifically, A. tepida and H. germanica were characterised by an intensive search behaviour (they explore their environment slowly with straighter trajectories), while C. excavatum adopted an extensive search strategy (it moves more rapidly with more convoluted trajectories). These observations are discussed in the specific context of the ecology of these species. We see their responses as adaptive advantages in spatially and temporally complex environments prone to a range of both predictable and unpredictable rapid stressors. From a methodological point of view, we showed that previous methods used to infer foraminiferal motion behaviour are likely to have systematically underestimated their speed and distance travelled by a factor ranging from 2 to 80.

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