Researchers study fish with transparent head

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Researchers from the USA have discovered how a bizarre fish with a transparent head uses specially adapted tubular eyes to focus on what it is eating.

The barreleye, Macropinna microstoma, was first described by Chapman in 1939 and marine biologists knew that its tubular eyes were very good at collecting light to enable it to see better in the dark ocean waters in which it lives.

The eyes were believed to be fixed so it was assumed that this gave the barreleye a very narrow "tunnel-vision" field of view covering only objects above the fish's head.

However, new research by Bruce Robinson and Kim Reisenbichler from the Monterey Bay Aquarium Research Institute has provided evidence to suggest that the fish can rotate their eyes within a transparent shield which covers their head, allowing them to look upwards at potential prey above, or point their eyes forward to see what is ahead.

Transparent heads
Robinson and Reisenbichler observed five barreleyes in the deep waters off the coast of Central California at depths of 600-800m/2000-2600', using Remotely Operated Vehicles (ROVs) and spotted a previously unknown feature of the species - its tubular eyes sit within a transparent, fluid-filled shield that covers the head, giving the appearance that the fish has a see-through head.

Previous descriptions of the species did not show this feature, and the authors believe this is because the fragile structure would have been destroyed when the fish were brought to the surface in nets.

However, not only did Robinson and Reisenbichler manage to film the species, they also managed to catch two of the fish and observe them for several hours in an aquarium on board their ship, allowing them to watch the tubular eyes being rotated as the fish rotated its body.

The authors said: "The shield is a tough, flexible integument that attaches to dorsal and medial scales behind the head, and to broad, transparent subocular bones that protect the eyes laterally. This fragile structure is typically lost or collapsed during capture by nets, and it has not been previously described or figured.

"Beneath the shield is a fluid-filled chamber that surrounds or protects the eyes. Scales are present just behind the eyes at the nape of the back, within this chamber. Separating the eyes is a thin, bony septum that expands posteriorly to enclose the brain. In living specimens, eye lenses are a vivid green colour."

Behaviour
The authors believe that Macropinna microstoma uses green pigments in its eyes to filter out sunlight coming from the sea surface, which they think may help it spot jellyfish which have a bioluminescent glow.

Most of the time the fish hangs largely motionless in the water, but is thought to rotate its eyes forward and swim upwards when it spots jellyfish drifting above over its head.

Last month scientists from London discovered that another barreleye species, Dolichopteryx longipes, used special "mirrors" in its eyes to help focus light into the retina. (See Deep-sea spookfish has mirrors in eyes).

The fish observed range in size from 3.6cm to around 11cm and were filmed at depths of 616-770m.

For more information see: BH Robison and KR Reisenbichler (2008): Macropinna microstoma and the paradox of its tubular eyes. Copeia. 2008, No. 4, December 18, 2008.