Bluefin tuna, Thunnus thynnus. (Credit: © lunamarina / Fotolia) Enlarge

Sep. 13, 2013 — Some of the strangest fish in the sea are closely related to dinner table favourites the tunas and mackerels, an international team including Oxford University scientists has found.

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.

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