Five arms, no heart and a global family: what DNA revealed about the weird deep-sea world of brittle stars

July 28th, 2025

You may have read that the deep sea is a very different environment from the land and shallow water. There is no light, it is very cold, and the pressure of all the water above is immense.

Plants can’t grow there, and the energy powering life mostly comes from organic matter sinking from the sunlit surface. These facts have been known for more than 150 years.

But I want to tell you something you probably don’t know about the deep sea: for animals on the seafloor, it is a very connected environment. There are few environmental barriers to stop animals slowly expanding their distribution to cover thousands of kilometres. Over a million years, deep-sea animals can spread from Iceland to Tasmania.

In a new study published today in Nature, we map the distribution and relatedness of a single group of marine animals across all ocean seafloors, from the coast down to the abyssal plains of the deep sea, from the equator to the pole.

Five arms, no brain, no eyes or heart

We sequenced the DNA of thousands of animal specimens stored in natural history collections of museums across the globe, deposited from hundreds of research voyages. For the first time, we have enough data to explore how marine life has evolved and dispersed across the oceans over the past 100 million years.

We studied a group of animals called brittle stars, strange spiny creatures with a disc-like body and five sinuous or branched arms. They have a central mouth and gut, but no brain, no eyes and no heart.