Idiosepius biserialis and Idiosepius thailandicus have been previously described as separate species although the difference in morphological characters is only the arrangement of pegs in tentacular-club suckers. The former species inhabits sea grass beds in the Andaman Sea of Thailand. The latter species inhabits mangroves in the eastern Gulf of Thailand. The present study of reproductive behaviour, mating, and spawning of the two species in captivity demonstrated that they are closely related. Copulation was performed using tentacles by the male for fixation of spermatophores at the buccal region of the female. Behavioural patterns were distinguished based on the hovering and adhering positions of each sex. Cross-mating between two species was initiated and observed, yielding fertilized eggs which developed to the organogenesis stage. Reproductive behaviour of crossed pairs was similar to those of individual species. This study revealed that the two ‘species’ are not reproductively isolated.
