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The bivalve Pseudopythina macrophthalmensis (Galeommatoidea) is a commensal with the crab Macrophthalmus convexus (Ocypodidae) in Okinawa Prefecture, Japan. It is a protandric hermaphrodite which incubates the 65-µm large ova in the suprabranchial cavity. The species produces two types of sperm, which were studied with the electron microscope. The euspermatozoon has an elongate 2.8-µm-long, pointed acrosome, a slender 12-to 13-µm-long nucleus and a middlepiece containing several closely packed mitochondria arranged as a 5.5-to 6.0-µm-long sheath around the basis of the flagellum. The paraspermatozoon is vermiform, 220-µm-long and up to 5-µm-broad. Anteriorly there is a ca 7-µm-long bullet-shaped acrosome followed by a subcylindrical 3.0-to 4.7-µm-long nucleus. Adjacent to the nucleus occurs a bundle of 26-42 40-µm-long flagella. The cytoplasm is packed with spherical lipid droplets and ovoid granules of unknown composition. Sperm of both types aggregate to form spermatozeugmata, which were found in the posterior mantle cavity or in paired seminal receptacles. Within the receptacles the euspermatozoa dissociate themselves from the spermatozeugma and become attached to the epithelial lining of the receptacle whereas the paraspermatozoa presumably disintegrate. The possible significance of the two types of sperm is discussed in the light of their presumed functions in gastropods.

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The courtship behaviour and cycles of male courtship activity and colouration of Ilyoplax orientalis, I. delsmani and I. gangetica were studied in the field in Malaysia and Thailand. Each species had a distinctive chela waving or beckoning display. Depending on species, the chelipeds, carapaces, or both of waving males blanched to white in contrast to the cryptic colour of nonwaving males and females. All three of these tropical Ilyoplax exhibited semilunar cycles in male waving activity at the colony level. It was confirmed for I. orientalis that individual males cycled each semilunar period between waving and non-waving phases and exhibited different behaviour toward females during these two behavioural phases.