A new chiton species, Leptochiton myeikensis n. sp. is described from the southern Myeik (Mergui) Islands, Andaman Sea, Myanmar. This is the fifth species of the genus Leptochiton from tropical shallow waters. The new species is distinguishable from other congeners by the sculptures of tegmentum and girdle scales, and shape of the radula teeth.

Of the marine crab specimens collected by the RV Hakuhō Maru from East and Southeast Asia in 1972, the specimens from the Sulu Sea and the Sibutu Passage are identified and recorded in this first part. They comprise 17 species from 14 genera in 9 families, including three new species, Cymonomus suluensis (Cymonomidae), Homolodromia hakuhoae (Homolodromiidae), and Lysirude goekei (Lyreididae). Cymonomus suluensis is the 45^th in the genus and the 32^nd from the Indo-West Pacific, H. hakuhoae is the 7^th in the genus and the 4^th from the Indo-West Pacific, and L. goekei is the 4^th in the genus and the 3^rd from the Indo-West Pacific. Both of C. suluensis and H. hakuhoae are represented each only by one specimen obtained from the depth of 2,030 m, which is the deepest record among the known species of both genera.

Based on rearing experiments in central Honshu, Japan, the previously unknown life history of Allomorpha hirasana Takeuchi, 1929, is outlined and the larva is briefly described and illustrated for the first time. This is a univoltine species with adult emergence in late April in rearing conditions. The egg period is 10–14 days and the larval period is about 28 days. The larva is a solitary external leaf feeder. The host plant association with Symplocos may support validity of the genus Allomorpha.

As the result of the third survey of our co-operative inventory project by FRI, Myanmar and NMNS, Japan conducted in Tanintharyi Region in Feb. 2020, 23 pselaphine species of 20 genera were recognized. Fourteen species of which are first recorded in the research project. A list of pselaphine species recognized in the three surveys (2017, 2018, 2020) in this project are also given.