Phylogenetic analysis of Mexican cave scorpions suggests adaptation to
caves is reversible
New evidence that specialized adaptations are not evolutionary dead ends
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Typhochactas mitchelli is among the smallest known scorpions and part of
the Typhlochactidae family of cave scorpions, endemic to Mexico. Like all
scorpions, it fluoresces in long-wave ultraviolet light as...
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(http://www.eurekalert.org/multimedia/pub/20859.php?from=156122)
Blind scorpions that live in the stygian depths of caves are throwing light
on a long-held assumption that specialized adaptations are irreversible
evolutionary dead-ends. According to a new phylogenetic analysis of the
family Typhlochactidae, scorpions currently living closer to the surface
(under
stones and in leaf litter) evolved independently on more than one occasion
from ancestors adapted to life further below the surface (in caves). The
research, currently available in an early online edition, will be published
in the April issue of Cladistics.
"Our research shows that the evolution of troglobites, or animals adapted
for life in caves, is reversible," says Lorenzo Prendini, Associate Curator
in the Division of Invertebrate Zoology at the American Museum of Natural
History. "Three more generalized scorpion species living closer to the
surface evolved from specialized ancestors living in caves deep below the
surface."
Scorpions are predatory, venomous, nocturnal arachnids that are related to
spiders, mites, and other arthropods. About 2,000 species are distributed
throughout the world, but only 23 species found in ten different families
are adapted to a permanent life in caves. These are the specialized
troglobites.
This study concentrates on the family Typhlochactidae that includes nine
species of scorpions endemic to the karstic regions of eastern Mexico. These
species were initially grouped together by Robert Mitchell in 1971 but were
elevated to the rank of family for the first time last year, based on
morphological data published by Prendini and Valerio Vignoli of the Department
of Evolutionary Biology, University of Siena, Italy, in the Bulletin of the
American Museum of Natural History. Prendini, Vignoli, and Oscar F.
Francke of the Departmento de Zoologia, Instituto de Biología at the
Universidad
Nacional Autónoma de México, Mexico City, also created a new genus,
Stygochactas, for one species in the family and described a new surface-living
species, Typhlochactas sissomi, in a separate American Museum Novitates
paper. All species in the family have adapted to the dark with features such
as
loss of eyes and reduced pigmentation. The family contains the most
specialized troglobite scorpion, Sotanochactas elliotti, one of the world's
smallest scorpions, Typhlochactas mitchelli, and the scorpion found at the
greatest depth (nearly 1 km below the surface), Alacran tartarus. Three of the
species (including T. mitchelli) live closer to the surface and are more
generalized morphologically than the other six, making this family an
excellent
model with which to test and falsify Cope's Law of the unspecialized
(novel evolutionary traits tend to originate from a generalized member of an
ancestral taxon) and Dollo's Law of evolutionary irreversibility (specialized
evolutionary traits are unlikely to reverse).
For the current research paper, Prendini and colleagues gathered data for
195 morphological characteristics, including a detailed mapping of the
positions of all trichobothria (sensory setae) on the pedipalps, among the
species of Typhlochactidae. The resulting phylogenetic tree shows that
adaptation to life in caves has reversed among this group of scorpions: two of
the
less specialized, surface-living species, T. mitchelli and T. sylvestris,
share a common ancestor with a much more cave-adapted species, and a similar
pattern was found for the third less specialized, surface-living species,
T. sissomi.
"Scorpions have been around for 450 million years, and their biology is
obviously flexible," says Prendini. "This unique group of eyeless Mexican
scorpions may have started re-colonizing niches closer to the surface from the
deep caves of Mexico after their surface-living ancestors were wiped out by
the nearby Chicxuluxb impact along with non-avian dinosaurs, ammonites,
and other species."
The research was funded by the National Science Foundation, the Theodore
Roosevelt Memorial Fund, and a SYNTHESYS grant.
_http://www.eurekalert.org/pub_releases/2010-03/amon-pao031210.php_
(http://www.eurekalert.org/pub_releases/2010-03/amon-pao031210.php)
