barreleye1 350 Truth is stranger than fiction – especially in the deep

No, this fish is not animated by Pixar – it is a very real fish created by Mother Nature deep down in the ocean. Its name is Macropinna microstoma and it has puzzled ichthyologists since it was first described by Chapman in 1939.

Macropinna microstoma, also known as the Barreleye fish, has a fluid-filled dome on its head through which the lenses of its barrel shaped eyes can be clearly seen. The fish lives at a dept of 600-800 metres where it spends most of its time hanging almost completely still in the water.

Even though the Barreleye was described by science in the late 1930s, the transparent dome is a fairly new discovered since it is normally destroyed when the fish is brought up from the deep. Old drawings of the fish do not show the see-through part of the head and the species was not photographed alive until 2004.

Thanks to new technology, it is now possible for researchers to explore the deep sea much more efficiently than ever before and we are therefore learning more and more about the weird and wonderful creatures that inhabit these baffling parts of the planet. It has long been known that the tubular eyes of the Barreleye are good at collecting light; an adaptation to a life deep down in the ocean where light is scarce. The eyes were however presumed to be fixed and the fish was therefore believed to have a very narrow upwards-facing tunnel-vision. Researchers Bruce Robinson and Kim Reisenbichler from the Monterey Bay Aquarium Research Institute (MBARI) has now changed this notion completely by providing evidence suggesting that this fish can rotate its eyes within the transparent dome in order to see both upwards and straight forward. Robinson and Reisenbichler observed that when suitable prey, e.g. a jellyfish, is spotted, the fish will rotate its eyes to face forward as it turns its body from a horizontal to a vertical position to feed.

Robinson and Reisenbichler were able to get close to five living Barreleyes using Remotely Operated Vehicles (ROVs) at a depth of 600-800 meters off the coast of Central California. In addition to observing and filming the fish in its native habitat, the researchers also captured two specimens and placed them in an aquarium for a few hours in order to study them more closely.

Live specimens of Macropinna microstoma turned out to have beautifully coloured green eyes; probably in order to filter out sunlight from the surface of the ocean since this would make it easier for the fish to spot bioluminescent jellyfish. Robinson also suggests that Macropinna microstoma might be using its supreme eye sight to steal food from siphonophores[1].

If you want to know more about the intriguing Barreleye fish, check out the paper BH Robison and KR Reisenbichler (2008) – Macropinna microstoma and the paradox of its tubular eyes. Copeia[2]. 2008, No. 4, December 18, 2008.


[1] Siphonophores are a class of marine invertebrates belonging to the phylum Cnidaria. They are colonial and a colony can look almost like a jellyfish. The most well-known siphonophore is the dangerous Portuguese Man o’ War (Physalia physalis).

[2] Copeia, the official publication of the American Society of Ichthyologists and Herpetologists, can be found on www.asih.org.