When as a child I first saw blind cave
tetras (Astyanax mexicanus) inhabiting the
caves and underground watercourses of
Mexico, I was astounded by how a fish
without eyes could nevertheless navigate
the water very well. The cave forms of A.
mexicanus can have atrophied sight, no sight
at all, or even lack eyes altogether. For these
fish, Mother Nature must have decided that
sight is useless in such a dark environment,
and so, in the course of evolution, it became
a “use it or lose it” adaptation.
Nevertheless, the energy required to
maintain sightedness and/or eyes was utilized
elsewhere: the navigational skills of A.
mexicanus are made possible by their lateral
line system being very sensitive to water
pressure, and they also have an excellent sense
of smell. Their lack of sight is compensated
for by a greater number of receptors, both
olfactory and those along the lateral line,
enabling the fish to create a detailed sensory
map of their surroundings. With this internal
information, they are efficient swimmers that,
though sightless, can find their way among
rocks and locate food.
For nature lovers, these animals are really
interesting aquarium specimens that are
likely to gain the attention of other aquarists,
friends, and family, who will want to see
with their own eyes the amazingly adaptive
fish without eyes!
Toxotes spp., the archerfishes, stand apart
for their unique method of obtaining food.
Targeting insects that fly by or alight upon
a leaf, an archerfish will shoot a narrow jet
of water at its prey, assessing the distance
required based on the refraction of light and
the ballistic curve of the water jet. All this
usually happens in less than one second, and
it is truly a remarkable skill to witness.
The strength of the water jet is due to
the unique design of the archerfish’s buccal
cavity. Unlike A. mexicanus, nature also
gifted this fish with specially designed eyes
that make it easy for them to identify prey
both in the water and on land, as well as at
great heights above the waterline.
A blind Mexican cave tetra
Cortez swordtails (Xiphophorus cortezi ), one of the livebearing species of the family Poeciliidae.