In Japan Pseudopythina tsurumaru is an up to 10.8 mm-long commensal of the burrowing sea cucumber Protankyra bidentata, whereas in Hong Kong the same species is smaller and associated with the crab Hexapus anfractus, itself a commensal of P. bidentata. Japanese P. tsurumaru is a hermaphrodite tending towards protogyny maturing to a female when ≥7 mm, and entering the hermaphroditic condition when ≥9 mm long. In addition to normal euspermatozoa, the species produces 30–32 μm long and 7 × 8 μm broad spindle-shaped paraspermatozoa provided with a conical acrosome, a nucleus, and a bundle of ∼15–16 flagella issuing from the head region. Paired pouch-formed seminal receptacles normally occur in bivalves ≥6 mm. Bulk sperm transfer presumably takes place by way of spermatozeugmata formed by the two types of sperm cells. Exogenous euspermatozoa attach to particular nonepithelial cells that occupy the interior of the receptacles. These cells, together with their associated sperm, are probably released as syncytial sperm-carrying bodies into the suprabranchial chamber, where the ova are fertilized. J. Morphol. 262:407–420, 2004. © 2004 Wiley-Liss, Inc.