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Islands of the Three-Eyed

Listening to a group of cruise ship tourists recently, I
overheard this comment, “They sure have a lot of armadillos here and
they’re all bright green”. Bright green
armadillos??? This same woman then screeched and ran behind her
companion as a Green Iguana sauntered out of the bush. Ah, my next subject for All At Sea…the unappreciated lizards of
the Caribbean!

How many
species are there in not only the Caribbean
but also the world? No one knows for certain since new species are discovered
on a fairly regular basis. More interestingly, known species demonstrate the
ability to rapidly evolve based on habitat, leading to the creation of even
more new species whose appearance is distinctively different but who share
quite similar DNA.

Biologists consider the Caribbean
Islands a “hot
spot” not only for studying currently known lizard species but also a
fertile ground for discovering new species and observing rapid evolutionary
changes as new species emerge.

 In 2001, a lizard touted as the
world’s smallest – adults less than ¾” long and fitting on a
US Dime- was discovered on a small island off the coast of the
Dominican Republic.
In 1965, another species of tiny lizard was discovered in the British
Virgin Islands. Just this year, a new species of dwarf gecko,
brightly colored and patterned in reds, yellows, and browns, was discovered by
an amateur naturalist on a small island in
St. Vincent and the Grenadines.
People who saw it in the past assumed it was an insect and passed it by, due to
its small size and bright coloration and the fact that it lives
in leaf litter.

While the
diversity of lizards in the Caribbean is almost overwhelming and includes dwarf
geckos, anoles, iguanas, skinks, curly tails, and the legless glass lizards
(distinguished from snakes by their eyelids and external ear openings), many of
them have a very interesting common characteristic…a third eye!

Known as
a parietal eye, it is found on top of the lizard’s head, sometimes
visible as a grayish spot as in the case of iguanas, or it may be invisible,
appearing as another color blended opaque scale. But, it’s there and for
those insistent on keeping lizards as pets it should never be mistaken for an
injury because it serves a critical function in the life of lizards.

The
parietal eye does not see shapes as do the ‘regular’ eyes that, in
most species, rotate and move independently of the other, giving the lizards excellent visionary coverage of their surroundings.
Instead, the parietal, pineal, or third eye senses only light and dark with its
limited retina and nerves connecting directly to the pineal area of the
lizard’s brain.

This
third eye provides protection for the lizards from flying predators and even
human hands as one attempts to swoop in from above and
snatch the lizard. The parietal eye’s ability to discern a sudden change
from light to dark has saved many a lizard from becoming prey; even when it
sleeps, since the parietal eye is always “open”. The third eye also
assists in triggering reproductive hormone secretions; signaling to the
lizard’s brain that it’s time to mate as daylight changes with the
seasons.

Perhaps
the most important function of the parietal eye is its ability to send
thermoregulatory messages, based on changing light, to the lizard’s
brain. Since lizards are reptiles rather than mammals, they are
“cold-blooded” meaning they must thermo-regulate by moving from sun
to shade, from cooling ground to sun warmed rocks and often pavement where they
absorb the stored heat.

In
addition to its thermoregulatory messaging, the parietal eye is believed by
many scientists to contain a circadian clock, a timer so to speak, that tells
the lizard it’s time to sleep, wake, and feed; much like our own
circadian clocks located in the parietal area of the human brain.

Islands
of the Three-Eyed
is a work in progress since future articles will present
individual species and some of the characteristics unique to them.

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