Starfish break the rules of animal distribution

Animals and plants around the world are not randomly distributed. They appear to follow trends and patterns.

But it’s often difficult to figure out if the patterns we see in the natural world actually hold true. To prove it, we need to study vast amounts of data that span huge geographical distances. For most groups of animals and plants, this data simply doesn’t exist and that makes it hard to say too much about where they live.

For starfish, however, that’s now changed. A new study has examined more than 200,000 records of starfish – a group also known as Asteroidea – mapping where in the oceans they live. This has allowed Dr Hugh Carter, our curator of starfish, and his colleagues, Dr Suzanne Williams and Dr Lupita Bribiesca-Contreras at the Natural History Museum, to produce an unprecedented look at the group.

The amount of data that Hugh and his colleagues were working with meant they were able to not only to map the diversity of starfish across the surface of the planet, but also at different ocean depths.

They found that from equator to poles and sea surface to the deep sea, starfish diversity patterns change. “What we showed is that in the shallow waters, there are more species in the tropics than at the poles,” explains Hugh. “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.”

This fascinating pattern of starfish diversity in the deep sea seemingly goes against one of the most well-studied phenomena in natural history. The study has recently been published in the journal Nature Ecology & Evolution.

Where do animals live?

Where animals live has long fascinated scientists because they appear to follow certain patterns.

One of the most well-known of these patterns is called the latitudinal diversity gradient. This trend finds that there are more species of plants and animals living along the equator and fewer as you move towards the poles.

If you think about the number of plants and animals living in the Arctic and Antarctic compared to those living in the Amazon rainforest, this trend might seem obvious. But these observations often need to be tested to make sure they hold true.

Hundreds of studies have proven that this is the case – as you move towards the equator, the number of species in particular groups does typically increase. This has even been shown to be true in the deep past. It’s tricky to unpick the reasons why this is, but one factor thought to contribute is temperature.

“The question we had was does that trend still exist in the deep sea?” asks Hugh. “One of the main correlates of the gradient and species is temperature, so we might expect that in shallow waters there is a very strong correlation of species diversity with warmth. But as we go deeper the environment becomes quite homogenous – the deep sea is roughly four degrees everywhere.”

Hugh and his colleagues were interested in whether there was an increase in the number of species around the equator, even in deep water where the temperature remains relatively stable.

Deep-sea diversity

By combining as many sources of starfish data as possible – from global databases such as GBIF and those in the collections we care for – the team were able to account for around 92% of all known species of starfish. This makes the study the most comprehensive ever carried out on Asteroidea.

The most interesting discovery was this shift in distribution towards the poles as you go deeper into the oceans – although this general trend wasn’t necessarily true for every family of starfish. For example, some families are only found in the shallows and are concentrated at the equator, while others only live in the deep sea and show this split diversity.

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

However, the discovery of this general shift in the diversity of the deep sea does raise some interesting questions. Hugh suspects that it might, in part at least, be caused by changes in nutrient availability.

Despite the impression that there’s not much living at the planet’s poles, they support hugely productive waters. This is why, for example, many species of whales will migrate to the colder waters to feed on the massive, seasonal krill blooms. This also means that large amounts of nutrients sink down to the deep sea.

So, the trend that’s been uncovered in deep-sea starfish diversity likely applies to other animals living at those depths. This in turn could have implications for how we decide which regions need protection and conservation.