Starfish aren’t just scattered randomly across the ocean floor. Like most animals, they follow patterns – showing up in some places more than others.

Some of those patterns make sense. Others don’t quite fit the rules that scientists thought applied across the planet.

Until now, figuring out where starfish actually live – and why – was tough. That’s because researchers need massive amounts of data that cover wide distances and depths. For most animals, that kind of data just doesn’t exist. But for starfish, it finally does.

Starfish diversity changes with depth

The researchers analyzed more than 200,000 records of starfish, also known as Asteroidea, from oceans all over the world. With that much data, they were able to map not just where starfish live, but how their diversity shifts with depth – from shallow tropical waters to the deep sea.

“What we showed is that in the shallow waters, there are more species in the tropics than at the poles,” explained Dr. Hugh Carter. “But as we go deeper, the diversity shifts. We start losing tropical diversity and it starts to increase as we move to the temperate zones.”

Dr. Carter, a curator of starfish at the Natural History Museum, led the study with colleagues Dr. Suzanne Williams and Dr. Lupita Bribiesca-Contreras.

The usual pattern, flipped

One of the oldest known biological trends is that more species live near the equator. It’s called the latitudinal diversity gradient. Think of the difference in life between the Arctic and the Amazon, and the trend becomes clear.

This pattern has been seen in countless studies – and even in ancient fossils. A big factor behind it is temperature: warmer regions tend to support more species.

But in the deep sea, temperature doesn’t vary much. It stays close to 39°F, whether you’re near the equator or the poles. That raised an important question: if temperature is steady down there, does the same biodiversity rule still apply?

Some starfish break the rules entirely

To find out, the team pulled together global databases and museum collections to build the most complete map of starfish life ever made. In total, they covered about 92% of all known starfish species.

As they looked deeper into the ocean, they found that some families of starfish follow the usual pattern – more species in the tropics. But others do the opposite. These deep-dwelling families tend to be more diverse in temperate zones.

“The difference between families is one of the bits that gets missed a lot in these big scale patterns,” said Dr. Carter. “They don’t really work at smaller scales, where other aspects have more of a stronger effect on patterns of diversity.”

One possible reason for this shift is food. Polar waters, despite seeming empty, are extremely productive. Whales head there to feed on krill blooms, and when those nutrients sink, they help feed the deep sea below.

That extra food might support more species in colder regions – at least in places where it’s not too cold.

Fewer species in the deep sea

But even with nutrients, there’s a limit. The researchers found that starfish diversity drops sharply when sea-floor temperatures fall below about 35°F (1.5°C). Below that, even high nutrient levels can’t support many species.

This temperature threshold showed up across all oceans, acting like a hard cutoff. No matter how much food was available, the coldest parts of the ocean floor remained low in starfish diversity.

This could help explain why some deep regions have fewer species, even if they seem similar to more diverse zones.

What this means for ocean science

The findings don’t just teach us more about starfish. They offer a fresh look at how life works in the deep sea – and raise new questions about what drives biodiversity below the surface.

If hotspots of deep-sea life aren’t always near the equator, that could change how conservation efforts are planned. It also means scientists need to look deeper – and colder – to fully understand life on Earth’s ocean floor.

The full study was published in the journal Nature Ecology & Evolution.

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