How do crustaceans keep time?
Digging into the shores, there's a group of creatures that live in sand and these creatures are extraordinary timekeepers; incredibly, they can predict the times of tides. Chris Smith spoke to David Wilcockson who studies this at the University of Aberystwyth. Chris was also joined by marine ecologist, Danni Green, who explained how light pollution can have a damaging affect on our sandy species.
Chris - So David, before you tell us about the timekeeping what are these creatures?
David - These creatures are a relative of the woodlouse, which most people are fairly familiar with I would imagine. They’ve got a scientific name which is Eurydice pulchra after the Greek mythology Orpheus and Eurydice, and we know them as the speckled sealouse.
Chris - What do they do - what’s their life cycle?
David - They’re a beautiful animals and they're a fascinating animal because they live buried in the sand (about 10 cm - usually less) and they emerge from the sand when the tide comes in. They swim around, they feed, they mate but, critically, they burrow back into the sand before the tide then retreats (as the tide goes out). The reason for this is because they want to maintain their preferred position on the shore. They don’t want to get washed out to sea and they also don’t want to get washed too far up the shore. So using this timing mechanism, which is based on a tidal or 12.4 hour cycle, they can maintain their position or station on the shore.
Chris - Because, of course, there are high tides every 12 hours, aren't there, so how do they do that? How are they keeping time on a 12 hour basis? Because, obviously, we’re familiar, you and me, and pretty much every living thing on Earth has a body clock and can keep time, but most body clocks we see keep 24 hour time not 12 hour time.
David - That’s the crux of a longstanding argument in biology about the nature of time keepers. You’re quite right, we have a circadian 24 hour clock, as do nearly all terrestrial organism. Marine biologists have been arguing about whether animals actually have a bonafide or a dedicated 12.4 hour clock, or whether it just some sort of modification of this 24 hour clock?
We have fairly recently discovered that whilst the speckled sealouse has a 12.4 hour clock, it does have a daily or a circadian clock as well. But we can actually separate them or disentangle the two clocks to show that they are separate entities. So they have a dedicated 12.4 hour clock and a daily clock that operates slightly different behaviours.
Chris - Is it working in their brain or nervous system in the same way that I have in my nervous system, a cluster of nerves cells which have a sort of genetic clock ticking, keeping time like a genetic domino effect so it ticks round taking 24 hours to do it? Do they have the same thing for 12 hours?
David - We believe so. We’re still working very hard to fully identify the cells in the brain that operate this, but we have a good idea that this is occurring in the brain alongside the 24 hour clock, yes.
Chris - The 24 hour clock: is that set by daylight in the same way that mine is? Because I get up in the morning, a nice deluge of bright blue light strongly activates my body clock and says it’s morning. Do these creatures have a light sensitive clock too?
David - Yes, it is light sensitive Chris. This has been important in our dissection of the 24 and the 12.4 hour clock in this animal. We can actually take Eurydice, or the speckled sealouse off of the shore and put them into autolight regimes, and we can change aspects of their 24 hour clock.
They have these beautiful cells, these chromatophores, these coloured pigment cells all over the back of the animal and they contract and expand on a 24 hour basis. What we can do is we can manipulate that using different light regimes. The other thing we can do, and what a lot of chrono or clock biologists use is constant light to disrupt rhythms. Organisms don’t like to be in constant light, it messes with their biological clock.
Chris - So that would include artificial light? If you deluge the shoreline in artificial light this could also disrupt this clock?
David - Absolutely. And what we did is we took animals from the shore, we put them into constant light, and we found the rhythm of the pigment cells was demolished. It was completely messed up, but the tidal rhythm, this 12.4 hour swimming was left intact. And that’s one of the clues that told us their two systems are operating independently.
Chris - I’d just like to bring Danni in here because, obviously, these are small crustaceans, but there are many other bigger creatures that could also be disrupted in the ocean by artificial light from humans?
Danni - Yeah, exactly. If you think about the fact that the majority of cities in the world are situated by the coast, there’s a lot of artificial along shorelines. There was a study recently by Bolton et. al. in Science of the Total Environment, 2017 and they found that artificial light made predators more active. This had cascading effects on the communities of invertebrates that were in the area because they just didn’t stop eating. They wouldn’t stop because it’s like they shouldn’t put a light in the fridge at night to help me with my diet. So basically they just eat all the time and this had huge effects on the communities.