Tuna closely related to some of the strangest fish in the sea

Deep sea fish such as the black swallower, with an extendable stomach that enables it to eat fish larger than itself, and manefishes, some sporting spiky fins like a Mohican haircut, are close cousins to mackerels and tuna despite having completely different body shapes and lifestyles.

The team, led by Dr Masaki Miya at Chiba Natural History Museum in Japan, suggests that this extended family of fishes might owe its success today to the devastating extinction that marked the demise of dinosaurs and many other creatures 66 million years ago.

The researchers report in the journal PLOS ONE this week how they combined DNA analysis of over 5,000 modern fish species with fossil evidence to solve the mystery of which species were closest to tunas and mackerels in the fish family tree.

'What was immediately clear from our result is that the extended family of tunas and mackerels is made up of fishes that all look very different from one another, with different ways of life, but which share one key trait: they all dwell in the open ocean,' said Dr Miya of Chiba Natural History Museum. 'This had been suggested before, but we were able to show that many additional groups of fishes inhabiting the open ocean -- called the pelagic realm -- were closely related to one another and to tunas.'

Reflecting this preference for the open ocean the team has called the extended tuna family tree: 'Pelagia'. Although they share a preference for open-ocean habitats, members of Pelagia show radically different ways of life ranging from deep-sea fishes that live inside sac-like invertebrates to speedy, shallow-water predators such as the tuna.

'Discovering that such radically different fish species are related is a bit like finding that a seal is more closely related to a cat than it is to a walrus!' said Dr Matt Friedman of Oxford University's Department of Earth Sciences, a co-author of the PLOS ONE paper. 'By comparing genetic data with fossil evidence we were able to show that the origins of all these disparate groups lie in a period of rapid evolution that occurred around 65 million years ago. This is significant because this is when the Cretaceous extinction event that wiped out the dinosaurs also killed off many groups of large fishes inhabiting the open ocean.

'It's likely that the common ancestor of this family lived in the deep ocean, helping it to survive this ancient extinction. It then emerged from its refuge to diversify and colonise the shallower waters to produce the profusion of related, but very different, species we see today.'

According to the team the new findings suggest a different way of thinking about past extinctions.

'We tend to think of extinction events as damaging diversity but in fact they always offer opportunities for other species -- for example, we mammals famously took advantage when the dinosaurs died out,' said Dr Friedman. 'What our study shows is that while extinctions sweep away old diversity they also see a new kind of diversity rapidly, at least on an evolutionary timescale, flooding in.'