Animals of the intertidal zone tolerate substantial environmental fluctuations. Survival under such unstable conditions requires specific adaptations. Several intertidal species have evolved endogenous mechanisms that follow tidal rhythms permitting behavioural alignment with periodic inundation. For example, aggregation behaviour in the springtail Anurida maritima (Guérin-Méneville, 1836) (Collembola: Neanuridae) is controlled by a free-running clock with a period of ~ 12.4 h. This cosmopolitan species is found in the upper intertidal zone where it forages during low tide. Before high tide, specimens aggregate in cracks in the substrate or under rocks to survive inundation. Here we report that the closely related intertidal species Anurida bisetosa Bagnall, 1949, displays a similar endogenously controlled circatidal behaviour. To obtain a minimum age estimate for this shared derived trait, we sequenced the full mitochondrial genome of A. bisetosa, which was then used for phylogenetic inference and molecular dating. The mitochondrial genomes of A. maritima and A. bisetosa are highly divergent. This divergence extends throughout the whole mitochondrial genome and is mirrored by a similar pattern in the nuclear internal transcribed spacer 2 (ITS2) region. Dating analyses suggest the species potentially split more than 40 million years ago. Under the assumption that the endogenously-controlled rhythmic behaviour evolved once in an ancestor of the two species, the trait must be of older age. Use of a single genetic marker and a limited number of fossil calibration points constrains accuracy of the age estimate, but nevertheless, it offers a glimpse at otherwise ‘intangible’ palaeoecological and palaeoethological attributes. |
