New ‘barreleye’ fish pulled from the depths

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Barreleye fish, such as this specimen shot from above, have eyes that look directly upwards to scan for their prey’s silhouette.
Credit: DeAgostini / Getty Images

Two new fish species, each featuring a bizarre organ that reflects differently coloured light from inside their bellies, have been discovered by an international team of biologists.

Both new species, reported in the journal PLOS One, belong to the unique family known as Opisthoproctidae,or “barreleye” – so named for the cylinder-shaped eyes that extend from the head of these rare fish and point upwards, allowing barreleye fish to see their prey’s silhouette from deep below in the waters of the Atlantic, Pacific and Indian Oceans.

Some barreleye species also have luminous organs that emit light from within the body. These are complemented by another organ, called a sole, which reflects the interior light through pigmented scales, giving them the name “mirrorbelly tube-eyes”.

These beguiling fish are rare and fragile and haven’t been studied in much detail to date.

So when a research team began analysing specimens freshly caught near American Samoa and New Zealand on a recent research trip, they realised they had two new species belonging to the family.

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Different species of barreleyes – and their unique pigmentation.
Credit: Poulsen et al. (2016)

Jan Poulsen, a zoologist and lead author of the study, says the work has exposed the unknown biodiversity of the fish family.

The analysis of fresh specimens allowed the team – which included researchers from Greenland, Norway, Japan and the UK – to identify distinct differences among the soles of the four specimens that made up the study.

Specifically, the patterns of their pigmentation marked them as three distinct species, rather than just one. The researchers also observed dramatic changes in colouring between juvenile and adult fish.

The team’s paper suggests that barreleyes use light reflected from their soles to help them camouflage in sunlit waters – and perhaps also for communication.

Identification of the new species was supported by DNA analyses.

Why the fish weren’t identified as new species earlier may have been due to preservation methods. While older specimens were fixed in chemicals which tend to destroy pigmentation patterns, these new species were examined when they were fresh.

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