Bending of 9 + “1” axonemes of flatworm spermatozoa in hypotonic media: An experimental study

Spermatozoa of the monogenan Amphibdella paronaperugiae are fundamentally straight-shaped and made up of an anterior centriole followed by a 9 + “1” axoneme, associated with a filiform mitochondrion and nucleus. Observations were made of individuals in male phase, parasitic in the heart cavity of the torpedo-ray Torpedo marmorata, the blood of which is known to display a high osmolarity (900–1200 mOsm) primarily due to the presence of urea and sodium chloride. Spermatozoa were studied in ultrathin sections of whole parasites. When submitted to a low osmotic pressure, the spermatozoa of Amphibdella folded or coiled on themselves. Experiments were performed to induce these folds with media of varying osmolarity containing urea, sodium chloride, or both. Media of osmolarity higher than 600 mOsm did not induce folding; media of osmolarity about 300 mOsm induced folds. It is hypothesized that the folds of the spermatozoa are produced by folds of the 9 + “1” axoneme, related to water permeability of the sperm cell membrane. Several fixatives (glutaraldehyde in sodium cacodylate buffer, with addition of varying proportions of sucrose, sodium chloride, and urea) were tested; the osmolarity of the fixative was found to have no influence on sperm folds, probably because these folds are physiological phenomena induced only in living sperm cells, not in cells killed by the fixative.