New Worlds: Marine mollusks got rhythm from the tides

Organisms that live in shallow waters are presumably influenced by the lunar and tidal cycles more than the solar ones and therefore mostly exhibit “circatidal,” rather than circadian rhythms.

A FISH SWIMS along Tamar, an artificial reef created by Ben-Gurion University, in Eilat. (photo credit: KEREN LEVY)
A FISH SWIMS along Tamar, an artificial reef created by Ben-Gurion University, in Eilat.
(photo credit: KEREN LEVY)
Just as humans and other mammals have an internal biological clock that synchronizes their behaviors with their environment, so too does a marine mollusk with a shallow conical shell and a broad muscular foot clinging tightly to rocks, according to new research at Bar-Ilan University. But unlike man and higher animals that have a circadian (24-hour) clock, the limpets have a rhythm that corresponds with the tides.
A comprehensive study on the rhythmicity of limpets – mobile mollusks known to scientists as Cellana rota – has just been published in the prestigious journal Scientific Reports by Yisrael Schnytzer of BIU’s Mina and Everard Goodman Faculty of Life Sciences who conducted the research as part of his doctoral dissertation under the supervision of Prof. Yair Achituv and Prof. Oren Levy.
Life on Earth began its journey at sea, presumably influenced to a great extent by the tidal cycle, which perhaps went on to evolve into a 24-hour cycle. So to understand how timing works, it is important to first understand how it works in the sea, particularly in its shallow waters.
The research team used field and lab observations, as well assembled a clock-oriented transcriptome (the sum total of all the messenger RNA molecules expressed from the genes of the organism). Schnytzer showed that in the same way these sea creatures behave with a tidal rhythm, so too are a majority of their genes expressed in a tidal (and not circadian) rhythm, even in the absence of light fluctuations.
Endogenous biological clocks follow the major cyclical rhythms: the solar-influenced 24-hour transition of day and night; the tidal 12.4-hour rising and falling of the tides that is governed by the lunar cycle; and annual seasonal changes. Organisms that live in shallow waters are presumably influenced by the lunar and tidal cycles to a greater extent than the solar ones and therefore mostly exhibit “circatidal,” rather than circadian rhythms.
As its name implies, the circa (about) tidal rhythmicity is a 12.4-hour rhythm following the tidal oscillation of high and low tides that alternate approximately every six hours. Consequently, intertidal organisms are typically covered in water for part of the day and exposed the rest of the time. Having evolved in this type of environment has presumably been a very strong driving force for them to adapt an endogenous tidal clock.
Countless studies conducted over the past century have helped establish a comprehensive understanding of how the circadian clock works, and importantly, which genes are involved in its ticking. These studies have been predominantly conducted in the usual biological model organisms but have expanded in recent years into further “real world” species, such as tropical corals, arboreal monkeys and many others from wide-ranging habitats.
In a first-of-its-kind transcriptomic study conducted over four years, researchers at Bar-Ilan University set out to broadly examine the rhythmicity of the intertidal limpet. The aim of this study was to use C. rota to understand the temporal landscape of a world manifested by two strong exogenous rhythms, circadian and tidal cycles, and their impact on the biological clocks of the organism. To date, very few studies have been conducted in this realm and still little to nothing is known about the molecular basis of tidal rhythmicity.
Throughout the course of the study, the researchers collected hundreds of thousands of images through a camera deployed on the sea shore of Eilat in southern Israel. The camera setup monitored a limpet population for several years, day and night. Few studies have conducted such a long-term, high-resolution sampling of animal behavior in such a challenging environment as the tidal zone. Portions of this data were quantified, showing that these limpets have a robust tidal rhythmicity, and curiously only exhibiting a circadian component to their behavior only at one particular time of the year.
The researchers then removed the limpets from their natural habitat of high and low tides and brought them into the lab, where they were held under constant conditions – with no tidal or circadian cues – to establish that they indeed possess an internal clock and are not just behaving in tune with the rising and falling of the tides. The researchers correctly predicted that if the organism has an internal clock it would continue to behave in the tidal manner in the lab, and so they did.
The limpets were then kept under these constant conditions for longer periods of time in order to desynchronize their rhythmicity. The researchers then set up an aquarium with a novel mechanism that sprayed them with water every 12.4 hours, mimicking the tidal cycle.
“We established that limpets have a tidal rhythm under laboratory conditions, they didn’t take the day and night cycle into account at all,” noted Schnytzer,
Quantum tech lab opens in Beersheba
Ben-Gurion University of the Negev has dedicated a quantum technology lab by holding a day-long symposium attended by Nobel Prize laureates. French Nobel Laureate Prof. Serge Haroche of the Collège de France offered his thoughts on “How Blue Sky Research and Technology Nurture Each Other: the Case of Quantum Physics.” Prof. Rainer Blatt of the University of Innsbruck and the Austrian Academy of Sciences addressed “Advanced Quantum Technologies with Trapped Atoms and Ions.” Representatives from the Defense Ministry, as well as industry, also offered their views.
“The variety of potential applications is enormous,” said Prof. Ron Folman, founder and head of the new lab. “These range from much-more-exact clocks that will synchronize fast communication to navigation systems without GPS that are much more precise than what we are familiar with. These will make it possible to navigate in space or deep underwater, as well as gravitational sensors to detect oil and water and lead to much faster computers or unbreakable communication lines to safeguard the global banking system and our privacy.”
Additional potential applications include quantum simulators for planning new types of chemicals that would form the basis for new medicines, or a magnetic detector so sensitive it could map the brain, replacing current scanners which have various limitations.