Biological rhythms are essential to life on Earth, allowing organisms to regulate their physiology in anticipation of different times of day to optimize fitness. The most well-known example is the circadian clock. However, in addition to the day/night cycle, marine intertidal organisms are also faced with the rhythm of the tidal cycle. The intertidal zone alternates between submergence by the ocean and aerial exposure. During low tide, intertidal organisms face potential desiccation and heat stresses, and inability to feed; during high tide, organisms experience ample water and oxygen, cool temperatures, and opportunity to feed. Limpets are intertidal herbivorous gastropods. They forage during high tide, stopping before low tide. Circatidal (~12.4 hour) limpet movement has been shown in a few studies to be endogenous, rather than a reaction to high tide (Gray & Hodgson, 1999; Santina & Naylor, 1994), but it has not been studied in Lottia, nor has the mechanism underlying these movements been studied in any limpet genus. Using video analysis, we investigated if circatidal movement is endogenous in Lottia paradigitalis, entraining them to 12.4 or 9-hour tidal cycles before releasing them into free-running conditions. Furthermore, we produced the first transcriptome associated with free-running circatidal movement in limpet.
Results/Conclusions
Our results reveal that circatidal movement is endogenous in L. paradigitalis, persisting for at least two days, and with a period of 12.6 hours, in the absence of environmental cues. Interestingly, limpets entrained to a 9-hour tidal cycle reverted to circatidal movement patterns, with a period of 12.8 hours, when released into free-running. Preliminary transcriptomic results indicate that circatidal rhythms are important gene expression rhythms in L. paradigitalis, with over 500 of mapped transcripts exhibiting circatidal expression patterns. Combined, these results suggest that L. paradigitalis have an endogenous circatidal time-keeping mechanism that allows them to anticipate the environmental changes associated with the changing of the tides. This hypothetical circatidal time-keeper represents an important aspect of the fundamental biology of intertidal organisms. Just as greater understanding of the circadian clock has shed new light on the physiology and pathology of terrestrial organisms, elucidating the circatidal clock will lead to greater understanding of marine intertidal organisms and how they can deal with the challenges they face, particularly with respect to global climate change and ocean acidification.