Consider the rockfish, a family of ray-finned sea perch that belongs to the greater fish family of scorpion fish. The range of rockfish coloration and shape is stunning and so, it turns out, is the little beast’s range of life span.
Sebastes minor lives only up to 11 years, write Sree Rohit Raj Kolora and Peter Sudmant of the University of California, Berkeley, and colleagues. Some salmon types live about that long, though a trout will max out at about 7. But the rougheye rockfish (Sebastes aleutianius) can soldier on for more than 200 years.
The oldest known rockfish was 205 years old, Sudmant tells Haaretz. As we are unlikely to have encountered the one that lived longest, the inference is they can live longer than that. The question is how they do that.
A fish’s age shouldn’t be estimated by size, which is affected by nutrition and environmental circumstances. Fish age is better estimated by analyzing its otoliths – its ear bones. Otoliths feature seasonal growth rings similarly to some trees. (Not all trees do that.)
Expert fish observers note that counting otolith rings is an art in itself, and some species are extremely challenging to age. Case in point: In 2013 a 39-pound shortracker rockfish was caught off the Alaskan coast 900 feet down. That monster, horror-movie worthy, was estimated to be 200 years old, though later analysis showed that it was actually a youngling of 64 – in the long-lived rockfish types, the females may only reach maturity at age 20 to 27. More on that later.
Anyway, given that humans are obsessed with living long, an international team of researchers sought insight into rockfish DNA. Is senescence truly “theoretically inevitable,” as theoretical work has been concluding for decades? Why does a pygmy goby flash in and out this dark void in about five weeks while some rockfish live a couple of centuries and the colossal Greenland shark may pass 400 and possibly 500 years?
The shark also matures very slowly. What, the team inquired, are the genetic underpinnings of extreme life span?
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So the team sequenced the genomes of 102 fish from 88 rockfish species hoping for a glimpse at the genetic adaptations that influence lifespan in vertebrates. And they did find.
From here to forever, mole rat style
One key adaptation for extreme longevity is DNA repair. In all living beings, DNA is under constant attack. It gets degraded. But we living beings have enzymes that can fix it.
Note that this isn’t about mutation – changes when DNA is replicated – this is mishaps, which can be repaired by cellular processes of identifying the damage and correcting it. Radiation, for example, damages DNA by causing the molecule to break; free radicals also harm DNA by causing breaks and other damage.
“What’s really cool about rockfish is that there are many different long-lived species. We found that while all of these long-lived rockfish exhibit signatures of positive selection in genes encoding for DNA repair enzymes, different genes are under selection in different long-lived species,” Sudmant enthuses, adding, “This seems to be a case of convergent evolution.”
In other words, various long-lived rockfish species have enhanced DNA repair mechanisms, but not necessarily the same ones. That indicates they didn’t inherit the trait from a common ancestor but the different rockfish developed the adaptation on their own.
Yes, their DNA is more robust than yours. And guess who else has this trait – our friend the naked mole rat, denizen of the dirt. These tiny bare-skin burrowing rodents, a few of which could fit in the palm of your hand, can live for decades, and they also have superb DNA repair.
“Better DNA repair can also lead to not getting cancer,” Sudmant says, and yes, cancer is all but unknown in the naked mole rat. Respect.
Another contribution seems to be genes called butyrophilins that help fight chronic inflammation, which has been emerging as a key therapeutic target in humans. The team believes these genes are related to the extreme longevity of the relevant rockfish.
The team also listed genetic adaptations that enable extreme longevity by influencing insulin signaling, and others that influence size and adaptations to depth.
Mutations in the insulin signaling pathway are often found in long-lived species, the scientists note, and there they are in the rockfish. Insulin is a hormone that regulates the metabolism of the carbohydrates, fats and protein we eat – it’s complicated. Insulin also activates in-house cleaning of the cell from protein aggregates; nobody wants those cluttering up one’s cells.
How exactly does all this connect to extreme longevity? “There is much we still don’t know about how this regulates life span though, to be honest,” Sudmant says.
The researchers also observed that long-lived rockfish species are less genetically diverse than short-lived species.
If it’s huge, have respect and throw it back
One area where the rockfish, and mole rat, defy the odds is size. Across the tree of life we already know some interesting things about life span, Sudmant says. Generally, the bigger the vertebrate, the longer it lives. Elephants outlive mice.
This is far from being a rule – by and large Chihuahuas outlive Great Danes, and so do cats, and rockfish aren’t particularly big. But the researchers did identify genes that enable the rockfish to grow larger and tolerate extreme cold in the ocean depths.
It has been separately observed that slowing down your metabolism by being cold, for example, allows you to live longer. This has birthed a whole genre of sci-fi wishful thinking.
While influencing insulin signaling can directly influence life span, cold-and-depth-tolerance genes indirectly enable the fish to live longer, Sudmant explains.
It bears adding that the rockfish would have likely evolved these cold-and-depth-tolerance traits for the sake of survival per se, not necessarily survival over centuries. Hence such genes are called pleiotropic drivers in the argot because they have multiple effects.
Also, the long-lived rockfish obey the command to be fruitful and multiply relatively late in life; as they get older the ladies produce more, and more robust, youngsters. These fish just get better with age, the Washington-based National Oceanic and Atmospheric Administration observes, explaining: The bigger the female, the better the chance it has the energy reserves to sustain production during difficult times. Also, a long reproductive life span increases the chance that some young will be born during good years.
Okay. DNA repair aside, can any other parallels be drawn with the naked mole rat? That little guy, to the amazement of scientists, evinces no signs of aging during its decades of burrowing until it flops over and dies.
“I think there are many possible parallels,” Sudmant says. “Perhaps the biggest parallel is the scenario in which extreme life span evolved. Once a rockfish reaches maturity, it has few predators. This is similar to the subterranean mole rats. It seems that the scenarios for extreme life span to evolve are those in which species are able to avoid predation.”
Let’s merely add that while the state of speech in the fish is unknown, and one might add, unsuspected, mole rats do have speech of a kind, if they deign. The fact is, the bizarre, eternally youthful, xenophobic little animals live in colonies, but each colony has its own dialect and won’t even acknowledge members of a different colony.
Separately, it was also demonstrated that their life in burrows, under conditions of low oxygen, high carbon dioxide and asexuality (only the queen breeds), also keeps their metabolic rates low and extends their life span.
The rockfish may be a branch in the greater family of scorpion fish, some of which are venomous. Extremely venomous. Step on one and you may die. But they don’t actually look for us to sting us, and usually live no more than 15 years, which is a comfort for snorkelers.
So now you know why the rockfish can live for centuries and you can’t.