Even the early investigators at the end
of the 19th century have noted that
most deep-sea fish have strikingly
large eyes. Demersal (deep bottom liv-
ing) species are relatively large (up to
>1m), and the general body shape is
not affected by big eyes. However, fish
living in the water column between
500 and 1,000m (called mesopelagic)
are considerably smaller (5-35cm)
and their large eyes and consequently
large heads may conflict with a more
delicate body shape and their ability to
swim agilely. Tubular eyes are found in
many mesopelagic fish species (Fig. 1)
and may be considered as a mecha-
nism to overcome this dilemma. They
may be interpreted as cylindrical cen-
tral segments of the ordinary hemi-
spherical eyes. This design conserves
the large pupillary opening, and the
lens projects a focused image onto
the well-developed retina at the bot-
tom of the cylindrical eye. The vertical
walls of the tubular eye are lined with
an “accessory“ retina characterised by
thinned layers receiving unfocussed
light. The advantage of saving volume
comes at a serious drawback, how-
ever, being that Tubular
eyes have a much-reduced
visual field. In most spe-
cies, the optical axes are
directed dorsally making
them optimally suited to
detect silhouettes of ani-
mals above that block the
residual sunlight. On the
other hand, this eye design
would make their bearers
vulnerable to attacks from
other directions.
as details, the diversity of evolution-
ary adaptations in eye design found
in deep-sea fishes with their different
optical environment, have fascinated
researchers for ages. However, the
functions of some structural modifi-
cations, though known for decades,
have not been understood. On one
of my previous deep-sea expeditions
with the German research vessel “FS
Sonne,” we were lucky to make some
progress in this respect.
In the area of the Tonga trench, in a
trawl from between 600 and 800m,
we discovered a f