Morphological stasis and phylogenetic relationships in tadpole shrimps, Triops (Crustacea: Notostraca)

The tadpole shrimp (Triops) is a well-known 'living fossil' whose fundamental morphology has been unchanged for over 170 million years. Thus, tadpole shrimps are suitable subjects for the study of morphological stasis. We were able to obtain samples of three species of Triops (T. granarius, T. longicaudatus and T. cancriformis) from four regions in Japan. Taxonomic species were identified by diagnostic morphology. We inferred phylogenetic relationships between individual samples using mitochondrial 16S rRNA. Carapace shapes were compared among populations using shape coordinate methods. The phylogeny inferred from mtDNA shows that T. granarius is phylogenetically more similar to T. longicaudatus than to T. cancriformis. mtDNA sequences did not differ among the populations of T. granarius. However, there were two distinct phylogenetic species within T. longicaudatus. In spite of the similarity in fundamental morphological characteristics among Triops species, mtDNA sequences of Triops showed marked differences among the four phylogenetic species. Among the populations of T. granarius, the carapace shape of the Fukuoka population was significantly different from those of other populations of T. granarius. The carapace shape of the Kagawa population of T. longicaudatus was more similar to those of the Shizuoka and Kagawa populations of T. granarius. The shape of the carapace of T. cancriformis was significantly different from those of T. granarius and T. longicaudatus. Thus, taxonomic species, phylogenetic species and populations with similar carapace morphology did not correspond with each other. The present results indicate that most of the morphological change did not occur at the time of speciation (lineage separation) and that morphological stases are important evolutionary patterns, but they are not species-level properties.