Presence of F-actin in sperm head of Armadillidium peraccae (Isopoda, Oniscidea)

Sperm of Armadillidium peraccae have been examined with cytochemical and immunocytochemical methods for fluorescence and electron microscopic visualization of cytoskeleton components. Sperm incubation in an antibody anti-β-tubulin shows only the presence of two centrioles located in the cytoplasmic region above the nucleus; no other microtubules are present in the sperm head. Instead, fluorescence microscopy of sperm incubated in FITC-phalloidin allowed to detect the presence of a large amount of F-actin in the apical region of the sperm head. The incubation of ultrathin sections of sperm embedded in Lowicryl K4M with a phalloidin-gold complex allowed a more precise localization of F-actin in the amorphous part of the acrosome and in the cytoplasmic region between acrosome and nucleus; F-actin is also present in the thin cytoplasmic layer between plasma membrane and nuclear envelope at the apical portion of the nucleus. Although the sperm was always found completely devoid of motility, the discovery of the presence of an actin cytoskeleton leads us to hypothesize a possible acquisition of motility by the sperm at the time of its interaction with the female gamete. Such a hypothesis is supported by what is known for ostracods whose aflagellate sperm implement a type of amoeboid movement only at the time of their interaction with the female gamete.