| Research Abstract |
During the present project, we have obtained samples mainly by SCUBA-diver-operated hand dredges from Kuroshima Is., Melbourne and Port Curtis (Australia), Negros Is. (Philippines), Abu Dhabi (United Arabu Emirates), Dar es Salaam (Tanzania), Oahu Is. (Hawaii), New Caledonia, and Sarawak (Malaysia), including Asymmetron lucayanum species group (composed of two or more species), Epigonichthys maldivensis、 E. cultellus、 E. australis、 Branchiostoma moretonense、 B. arabiae, B.belcheri (from the type locality). And we have analyzed the whole mitochondrial genomes of many samples, and got significant results as follows:
1. "Asymmetron lucayanum", inhabiting circumtropical coral reefs, has been found to contain three cryptic species in terms of mitochondrial DNA sequences. Molecular analyses have revealed that the 4th species of the genus (A. inferum), which is the firstly known deep-water lancelet and very similar morphologically to A. lucayanum complex, is the sister group to the complex.
2. Recently, researchers in Xiamen University revealed from molecular examination that the Xiamen area is inhabited by the two species, B. japonicum (Willey, 1897) living also in Japanese waters and Quindao, and "B. belcheri". Our molecular comparison of all these populations, together with the specimens from the type locality of B. belcheri (Sarawak), has shown that "B. belcheri" in their sense may well be referred to B. belcheri. Further, our molecular analyses has revealed that there may be two cryptic species of B. belcheri complex in Negros Island and Australia.
3. So far as the living species are concerned, our molecular-based estimation of their divergent time can be dated as old as the Jurassic. On the other hand, all the living lancelets are similar to each other in terms of morphology very much. Therefore, a kind of morphological stasis is possible in the lancelet evolution.
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