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PHYLOGENETIC
SIGNIFICANCE OF RADULA IN CHITONS: CRYPTOPLACOIDEA AS AN EXAMPLE
The
confusing and yet unstable state of the phylogenetic system of
chitons led Boris Sirenko to claim that the chiton taxonomy is
in the "crisis situation". To assess the phylogeny of
chitons, he explored new characters with a review on traditional
characters, such as gill arrangement, egg hull morphology,
articulamentum development among others. For establishing better
phylogenic hypothesis, further useful morpho-logical characters
should be explored besides the recent molecular approaches.
I convince that the radula is one of such characters that should
be re-evaluated through a thorough review. Because of its
complexity of structure and conservativeness, the radulae have
not yet been fully employed for chiton taxonomy to date. The
radular characters are still promising tool for approaching
phylogeny of chitons, but not a mere morphological characters of
individual species. Radular characters are useful even for
higher taxonomic level, for example, Thiele used presence of
petaloid process in the major lateral tooth for definition of
the families Ischnochitonidae and Chitonidae. This may support
the monophyly of the Suborder Chitonina in the latest system
proposed by Shirenko in 1997. If the family Callochitonidae
lacks this process in the major lateral, it should be removed
therefrom. The most effective use of radular characters may be
for assessment of the phylogenetic relationship among a genus
and/or a family. The present study demonstrates the
characteristics of radulae of the following cryptoplacoid genera
in detail by scanning electron microscopy:
Craspedoplax, Craspedochiton, Spongio-chiton, Notoplax,
Cryptoconchus, Leptoplax, Bassethullia, Pseudotonicia,
Acanthochitona, Choneplax, Cryptoplax, Hemiarthrum and
Cryptochiton. The result of morphological comparison suggest:
close relationships between Notoplax and Crypto-conchus,
between Acanthochitona
and Choneplax;
paraphyly of Cryptoconchus;
presence of some species groups in Acanthochitona.
Problems to use radular characters in chiton taxonomy are also
discussed. |
