Corals have no brain. Instead of a centralized nervous system, they have nerve nets, but no central control unit. Yet this primordial animal, which emerged in the Cambrian over half a billion years ago, needs its beauty sleep just like we do, according to a study published Wednesday in the prestigious journal Cell Host & Microbe.
That in and of itself indicates that sleep is a necessity going back to the furthest reaches of animal evolution.
Even better, as the coral slumbers at night just like we, the algae inside it remain active, performing their microbely duties, Bradley Weiler, Javier del Campo and colleagues report in the journal Cell Host & Microbe. The coral, which is an animal, sleeps while the algae, which is a plant, does not, though its activities change.
If we assumed that sleep was the fief of the brained among us, we were wrong. The research suggests that sleep is a deeply ancient mechanism existing from the dawn of Animalia, before the evolution of a central nervous system. ALso it apparently arose from a need shared by cells – to clean up.
Corals look like stone plants, but they are cnidarians, a phylum with more than 11,000 known members, including anemones and our friend the jellyfish. They are not single-celled; they are complex multi-cellular animals called “polyps” that form colonies.
The earliest coral-like fossils such as Cothonion appeared about 540 million years ago. They were not sessile colonists like today’s types. They were solitary animals, and note that they were far from being the earliest animals. That honor may go to proto-sponges as much as 900 million years ago. By the time we get to the Ediacaran epoch over 600 million years ago, the oceans were crawling with animals, and corals were soon to come.
Maybe Ediacarans had sleep cycles too, going by the revelation in coral. How was this discovered?
By diving every six hours over three days at a reef off the island of Curaçao in the Caribbean, home to the misleadingly named brain coral, Pseudodiploria strigose, with its symbiotic alga Breviolum inside. The divers went down about five meters and for the first time, they compared the activity of coral and its internal algae – its version of a microbiome – during the day and night.
The coral doesn’t snore or twitch in their sleep like dogs. To test their state of activity, every six hours, the team analyzed the gene expression of the coral and the algae. The results were striking.
The coral P. strigosa sleeps for a third of the day, like humans. It has a biological clock regulated like ours by a circadian rhythm, explains first author Bradley Weiler.
Why do they sleep? We are still not sure why we do, let alone oceanic animals from which our line split at least hundreds of millions of years ago, but the researchers posit a similarity. While the corals rest and metabolisms power down, their bodies clean up, recovering from the oxidative stress caused during the day by the algae as they photosynthesize and release oxygen.
What sort of damage can oxidative stress cause? Like in us, it can damage genetic material, DNA and soft tissues. We know sleep is critical to animal survival, and it seems to have to do with the coral cells cleaning up “debris,” internal toxic waste that built up during its busy day being sessile and trying to eat. In short, while we sleep our body (and brain) seems to be repaired at the cellular level and coral is doing the same, the team suggests.
So: As the sun rises and we curse our lives, the coral’s gene expression revs up, producing RNA that lead to the production of new proteins, while “bad” proteins are broken down.
Mid-day gene expression emphasizes the metabolic pathways of the coral breathing, eating and excreting, and dusk brings about transitional fat and amino acid metabolism. Midnight shows evidence of metabolic stresses; “used mRNA” is broken down, for instance.
In short the coral has a strict schedule and sleeps at night despite having no brain, while the algae does not sleep per se, though there are differences between what it does at night and during the day.
Note that the relationship is symbiotic. During the day, the inner algae photosynthesize – producing sugars that nourish itself and the coral – and also generate oxygen. The coral protects the plant from being eaten.
Summing up: “At night, the corals repair DNA damaged by their symbionts,” says del Campo of the University of Miami. At night, the algae stop photosynthesizing, but continue to engage in other activities until the sun comes up.
Actually both can go it alone, but corals that lose their algae are not happy campers. That is exactly what happens in the phenomenon called coral bleaching – due to unfortunate conditions, the algae flee the coral, the coral loses its color and a key source of nutrition. They can limp along but will potentially die if the conditions don’t improve and the algae don’t come back.
In other words, their houseplants can enable corals to survive in nutrient-poor environments until bad things happen, like global warming, that makes the water uncomfortably hot.
Do corals dream of electric algae?
How did this mutually beneficial relationship arise? Maybe the same way we think eukaryotic cells gained mitochondria and other internal organelles: by engulfing bacteria, or archaea or some other microbe that wound up not being digested, but surviving inside. There are differences: The coral and its algae can survive on their own, albeit not as well, while our mitochondria are nothing without us and vice versa.
“It is possible that these microorganisms became trapped inside coral cells billions of years ago, and that this contact ended up giving rise to a mutually beneficial relationship. However, these relationships are not intentional,” coauthor del Campo points out in a statement, adding, “If they are maintained, it is thanks to certain commitments.”
So what have we? A strong indication that sleep is deeply ancestral. Its function was to enable the earliest animals to repair damage their cells suffer during the active hours. In the coral, the algae is causing some of the damage, but the bigger picture is that its presence is beneficial to the coral.
Note that it isn’t necessary to sleep at night as opposed to during the day unless one is hosting a vegetal symbiont. Bats will continue to do fine.
Also, the team may have shown that sleep played a role in the development of a primordial relationship between species that co-evolved, such as coral and the algae. But whether or not the primitive and plausibly unicellular animal that engulfed microbes, resulting in mitochondria, enjoyed beauty sleep, we do not know, nor whether corals dream.
Separate work has shown activity/rest cycles analogous to sleep in jellyfish, hydra and even sponges, those earliest of beasts. Far up the evolutionary ladder, more separate work has demonstrated dreaming in the bearded lizard.
Lacking a brain, maybe the coral doesn’t dream. They don’t have eyes, we note, though they do have light-sensitive cells. But meanwhile, the conclusion is that the basal animals from which all reptiles, birds and mammals arose needed sleep and at some point, the night visions of dreaming began too. Some people claim they never dream, but that seems to be more a function of their memory, because if a tegu or tuatara is escaping a dinosaur in a dream, how likely is it that our bed partner isn’t.
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