étude ostéo-morphométrique des tortues marines de ... - Seaturtle.org

DISCUSSION
Over the last centuries, several scientists studied the genus Chelonia and defined several
species or subspecies like Chelonia mydas japonica (Thunberg, 1787) (Japan), Chelonia
agassizii Bocourt, 1868 (Pacific coast of Guatemala) and Chelonia mydas carrinegra
Caldwell, 1962 (Gulf of California). Today, these names are no more in use, except for the
black turtle from East Pacific whose species status is still debated.
In 1986, Carr wrote that if one made a comparative statistical analysis of features of every
important nesting population, differences could probably be found to distinguish each of
them. But if he also stated that, although zoologists will probably one day describe them as
species, he personally recognized only three species, Chelonia mydas, Chelonia agassizii and
Chelonia depressa. The latter species was a little later separated in a separate genus, Natator
(Zug et al., 1988).
In this study, we documented the geographic variation of skull morphology using powerful
statistical tools: geometric morphometrics. Our sample was not as consequent as the 145 sea
turtles of the genus Chelonia used by Kamezaki and Matsui (1995); however these authors
did not use the skull geometry, but independent measurements. They also used a statistical
procedure to reduce the size effect that is nowadays widely criticized (Adams et al., 2004). So
our method took into account information about the spatial relationships among the measured
variables and had great statistical power (Rohlf, 2002). It should be more accurate than past
studies, even with such little samples.
Skull of turtles from OI was more different than we expected. In plot of PCA they appeared
more distinct from AO turtles than PE turtles were. This means that the strongest skull
geometry variation observed in our sample was between AO and OI turtles. But this could not
be interpreted as phylogenetic differences but just morphometrical differences. As stressed by
many authors (Rohlf, 1998, 2002; Bookstein, 2002; Adams et al., 2004), there is no reason
why continuous characters should be correlated with genetic distances.
So this observation could result from two reasons. First we were just interested in the skull
and other studies that described genus Chelonia using many other characters like coloration’s
patterns, scute’s patterns (Pritchard, 1967), carapace’s shapes, etc. may have found more
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