Finally, a study that shines a light – albeit a dim-but-optimistic one – on the fundamental patterns in the evolution of animal life on this planet. Researchers have discovered that by living closer to the Earth’s poles, certain marine animals live longer. 

Bivalves such as oysters, clams, scallops and mussels –  marine molluscs that have laterally compressed bodies enclosed by a shell comprising of two hinged parts – live longer if they reside in the North or South Pole than if they live in the tropics. 

To find out why this is, and further understand the research, we caught up with the lead author of the study, David Moss, who is a Ph.D. student in the Department of Earth Sciences at New York’s Syracuse University.

“Our leading hypothesis right now is that the extremely seasonal food supply at high latitudes is promoting slow growth and long lifespans. For most bivalves, the primary food sources are phytoplankton that they filter from the water column. During high latitude winters there is essentially six months of darkness so there is very little food supply. Lots of research (done by others) has shown that caloric restriction promotes longer lifespans by decreasing metabolic rates.”

For the study, Moss and his team created a global database of more than 1,100 populations of marine bivalves, documenting their maximum reported lifespan and growth rate, along with body size.

“Most of the tropical bivalves in our database had maximum reported lifespans of less than five years,” Moss says. “The only tropical species with significant lifespans were the giant clams, Tridacna, which contain a photosymbiotic algae.” 

There was a wide range of growth rates reported at any latitude, but on average tropical bivalves were seen to grow two to three times faster than high latitude species.

The underwater world of the North Pole. © Wikimedia Commons

The underwater world of the North Pole. © Wikimedia Commons

So does this new study (which could well appeal to land-hugging vertebrates) suggest that humans balancing on the equator belt should trade in the sun and sand for ice floes and six-month blackouts? More data is needed to confirm whether the latitudinal patterns that exist in bivalves exist in other invertebrates, but Moss seems to believe that it is “very likely”.

“If we are correct and the pattern is driven by environmental factors like food availability then we would expect other invertebrates to show the same pattern as well.”

Sadly, any of this research directly linking to human lifespans and growth appears rather far-flung at this stage.

This could, however, give scientists a view of how evolution has shaped marine life over millennia. Many believe that polar ecosystems look and behave similar to ancient ecosystems, meaning the fauna could vaguely mirror the creatures of earlier times – creatures that once upon a time also opted for a slow-growth strategy.

“We set out to determine whether this [latitudinal] pattern exists [in other invertebrates] (anecdotal data suggested it did),” Moss states, “because we want to examine lifespans and growth rates of fossil bivalves through time. It is now well documented that the mean size of marine animals has increased over the past 500 million years, but the role that lifespan and growth rate play is not known. Bivalves offer a potential avenue to address this question because they record in their shells size at age.

“Since we now know that a latitudinal pattern does exist, we have to keep that in mind when heading to the fossil record and comparing lifespans/growth rates through time. Many people have suggested that animals today grow faster (have higher metabolic rates) than those in the past. If that is true, then we would expect that animals today might be living shorter than their ancient counterparts – an enticing statement when you know that at least nine modern bivalves can live for over 100 years!”

Oyster shells washed up in a marsh © Wikimedia Commons

Oyster shells washed up in a marsh © Wikimedia Commons

Moss’ research forces open the door to further explore the lifespans of marine organisms in much more detail. With an estimated 20-40,000 extant bivalve species, Moss and his team are looking to update their database of just under 300 bivalve species – currently the largest dataset of its kind.

“My efforts now will be focused more so on the fossil record. Bivalves have been on the planet for over 500 million years but so far only a handful of studies have examined their lifespans in the fossil record so the field is open for exploration. One of my next projects will be to examine lifespans of two closely related species of bivalves from Pliocene deposits on the Atlantic Coast of the US.”