Barreleye

The Barreleye is a fish introduced in that appears in the Ocean. It can be found between 9 pm to 4 am all year. It quite possibly may have taken the spot of the rarest fish in the game, surpassing the coelacanth.

Donating to the museum
Like all fish and insects in Animal Crossing, the Barreleye can be donated to the museum by talking to Blathers, the curator. He will tell the player some information about it when the player donates it.

In
When asked about the fish, Blathers will say the following:

"The barreleye is a deep-sea fish with protuberant eyes and a clear head, so some of its organs are... visible. At first blush, a transparent head seems absurd, but there is a reason: so it can see things directly above! But I must confess - the more I learn about deep-sea fish, the happier I am to live on dry land, wot."

When donated, the Barreleye can be found in the north-eastern-most room of the fish exhibit, in the "deep sea" tank which is near the submersible and diving suit. It shares this tank with the Coelacanth, Football Fish, and Oarfish.

Further Information
Macropinna microstoma is the only species of fish in the genus Macropinna, belonging to the Opisthoproctidae, the barreleye family. It is recognized for a highly unusual transparent, fluid-filled dome on its head, through which the lenses of its eyes can be seen. The eyes have a barrel shape and can be rotated to point either forward or straight up, looking through the fish's transparent dome. M. microstoma has a tiny mouth and most of its body is covered with large scales. The fish normally hangs nearly motionless in the water, at a depth of about 600 metres (2,000 ft) to 800 metres (2,600 ft), using its large fins for stability and with its eyes directed upward.

M. microstoma has been known to science since 1939, but is not known to have been photographed alive until 2004. Old drawings do not show the transparent dome, as it is usually destroyed when brought up from the depths.