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Though circatidal rhythms were first observed in the early 20th century in the Roscoff worm and studied in detail in crabs, mussels and other marine species since the 1950s, the molecular and genetic underpinnings of the circatidal clock, as well as its relationship to the circadian clock, have remained elusive, in part due to lack of an animal model amendable to complete and specific genetic knockouts to determine gene function.

“The lack of an animal model amendable to genetic knockdown and transgenic manipulations has prevented scientists from definitely investigating the molecular origins of the circatidal clock and its relation to circadian clock genes,” said Emery. “Only a handful of studies about circatidal genetics exist and these are unable to either rule in or rule out a role for circadian clock genes in circatidal behaviors in animals.”

Setting Up Tides in the Lab

Erica Kwiatkoski, an MD/PhD student in the Emery lab at UMass Chan Medical School, in collaboration with Rosenthal’s lab at the Marine Biological Laboratory, identified the small amphipod crustacean P. hawaiensis as a promising model to study the genetic underpinnings of circatidal behavior. To simulate P. hawaiensis’ natural environment, the researchers developed an artificial tidal habitat in the lab for the one-centimeter-long animal using artificial seawater that was pumped in and out of an aquarium every 12.4-hours.

Kwiatkowski and colleagues exposed the amphipods to 10 cycles (the equivalent of five days) of an artificial tidal environment. Once acclimated to these conditions, the researchers removed P. hawaiensis from the artificial tidal environment they had created into a habitat with a constant water level. Placed inindividual test tubes, the animals’ swimming activity was recorded using infrared beams.

Strikingly, every 12.4 hours, the majority of the animals (80 percent) increased their swimming activity in anticipation of high tides and then reduced activity in anticipation of low tide, even though they no longer experienced changes in water levels. This demonstrated the existence of a circatidal clock controlling locomotor behavior in Parhyale hawaiensis.

Making Parhyale Amenable to Genetic Study

“MBL Director������𱹱���DZ�����P. hawaiensis��as a model organism for the studying the genes controlling many different aspects of embryo development, including limb patterning,” Rosenthal said. “Over the years, the Patel lab created valuable resources in this organism, such as a sequenced genome and methods to knock-out genes using CRISPR. Although the original intention wasn't to use Parhyale for the study of circatidal rhythms, it has turned out to be excellent for this purpose. We predict that this organism will catalyze a lot of future research in this area."

Having established the strong presence of a circatidal rhythm in P. hawaiensis, Kwiatkowski and colleagues then used CRISPR/Cas9-guided gene knockdown to hunt for genes connected to the circatidal behavior. By knocking down individual genes, scientists can observe the effect that a lost gene has on a biological process.

Using the genes that control circadian rhythms in mammals as a guide for looking for circatidal genes, Kwiatkowski and colleagues found that knocking down the circadian gene Bmal1 changed P. hawaiensis behavior. When Bmal1 was knocked down in P. hawaiensis, the animal no longer exhibited circatidal swimming behaviors. Instead, the animals exhibited arhythmic behavior unconnected to tidal flows.

“Bmal1 is a critical component for the maintenance of circatidal behavior in P. hawaiensis,” said Kwiatkowski. “This is the first evidence that a gene involved in circadian rhythms is also involved in circatidal rhythms. This establishes a molecular link between the two systems.”

The next step for Emery and colleagues is to investigate the exact role Bmal1 plays in driving circatidal behavior and which other genes may be involved.

Citation:

Erica R. Kwiatkowski, et al. (2023) Behavioral circatidal rhythms require Bmal1 in Parhyale hawaiensis. Current Biology, DOI:

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The Marine Biological Laboratory (MBL) is dedicated to scientific discovery – exploring fundamental biology, understanding marine biodiversity and the environment, and informing the human condition through research and education. Founded in Woods Hole, Massachusetts in 1888, the MBL is a private, nonprofit institution and an affiliate of the .