The ultrastructural study of spermatozoa in parasitic platyhelminths provides a set of characters whose usefulness for interpretating phylogenetic relationships has been demonstrated repeatedly in monogeneans, digeneans, and cestodes. Currently, ultrastructural spermatological data for the family Opecoelidae are available for thirteen species across five subfamilies: three Hamacreadiinae, two Helicometrinae, three Opecoelinae, three Opistholebetinae, and two Plagioporinae. The present study provides the first ultrastructural data for the genus Opegaster (Opecoelinae).
Digeneans were collected from a brackish-water fish, giant mudskipper Periophthalmodon schlosseri (Pallas, 1770) (Gobiformes: Oxudercidae), in Chumphon Province, Thailand. Live worms were fixed in 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer (pH 7.4), rinsed in the same buffer, postfixed in 1% osmium tetroxide in the same buffer, rinsed in MilliQ water, dehydrated in an ethanol series and propylene oxide, embedded in Spurr’s resin, and finally polymerized at 60ºC. Ultrathin sections mounted on copper and gold grids were double-stained with uranyl acetate and lead citrate and examined using a JEOL 1010 transmission electron microscope.
Sperm cells of Opegaster are filiform and sharpened at both extremities. They display ultrastructural characteristics that are usually observed in most digeneans, including two axonemes of different length presenting the trepaxonematan 9+'1' pattern, parallel cortical microtubules, external ornamentation of the plasma membrane, spine-like bodies, mitochondria, and a nucleus; lateral expansion is absent.
The organization and distribution of these characters along the spermatozoon are generally consistent with those observed in other opecoelids. However, some discrepancies arise when comparing the available data among subfamilies. Within the Opecoelinae, the male gamete of Opegaster corresponds to the type III sperm pattern of Bakhoum et al., as also found in the three previously studied species: Labracetabulum gephyroberici, Opecoeloides furcatus, and Poracanthium furcatum.
