A. Jespersen et al., Sperm dimorphism and spermatozeugmata in the commensal bivalve Pseudopythina macrophthalmensis (Galeommatoidea, Kelliidae), ZOOMORPHOL, 120(3), 2001, pp. 177-189
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-mum large ov
a in the suprabranchial cavity. The species produces two types of sperm, wh
ich were studied with the electron microscope. The euspermatozoon has an el
ongate 2.8-mum-long, pointed acrosome, a slender 12- to 13-mum-long nucleus
and a middlepiece containing several closely packed mitochondria arranged
as a 5.5- to 6.0-mum-long sheath around the basis of the flagellum. The par
aspermatozoon is vermiform, 220-mum-long and up to 5-mum-broad. Anteriorly
there is a ca 7-mum-long bullet-shaped acrosome followed by a subcylindrica
l 3.0- to 4.7-mum-long nucleus. Adjacent to the nucleus occurs a bundle of
26-42 40-mum-long flagella. The cytoplasm is packed with spherical lipid dr
oplets and ovoid granules of unknown composition. Sperm of both types aggre
gate to form spermatozeugmata, which were found in the posterior mantle cav
ity or in paired seminal receptacles. Within the receptacles the euspermato
zoa dissociate themselves from the spermatozeugma and become attached to th
e epithelial lining of the receptacle whereas the paraspermatozoa presumabl
y disintegrate. The possible significance of the two types of sperm is disc
ussed in the light of their presumed functions in gastropods.