Electron microscopical and histochemical observations on melanized sperm and spermatophores of pond-cultured shrimp, Penaeus vannamei

Black spermatophores were collected by manual ejaculation of pond-cultured Penaeus vannamei males and were prepared for light and electron microscopical study. Light microscopy revealed varying numbers of individual and aggregated yellow-brown to black polymorphic pigment droplets measuring 2 to 400 μm in diameter in spermatophore capsules. Some black spermatophores contained mostly degenerate sperm, while others contained mixtures of normal-appearing unistellate sperm, abnormal spikeless sperm, and degenerating sperm. Yellow-brown to black oval pigment droplets measuring 1 to 2.5 μm in diameter were encountered in the nuclei both of normal-appearing unistellate sperm and of abnormal spikeless sperm. Degenerating sperm exhibited no spikes and no pigment droplets in the remains of the sperm cell bodies. Oval pigment droplets occurred also among sperm in the extracellular matrix of spermatophores. Capsular, matrical, and intranuclear pigment droplets all gave histochemical reactions characteristic of melanin; i.e., pigment droplets were bleached by acidified permanganate, gave positive Schmorl and methenamine silver reactions, and adsorbed ferrous ions. Acid phosphatase activity was not detected in the sperm or spermatophores. Electron microscopy revealed oval, electron-dense droplets within sperm nuclei of both unistellate and spikeless sperm. These droplets corresponded in size to the pigment droplets seen in sperm by light microscopy. Intranuclear droplets were bounded by a single membrane, exhibited fairly homogeneous internal contents, and were frequently enmeshed in strands of intranuclear chromatin threads. Spikeless sperm, in addition to containing oval electron-dense intranuclear droplets, exhibited arrays of small, electron-dense particles packed in the region of the sperm where one would expect to have encountered the spike, suggesting that these particles play a role in sperm spike depolymerization. It is suggested that the melanized condition reduces fecundity in this species by promoting the depolymerization of sperm spikes and the degeneration of sperm. The mechanism by which sperm and spermatophores become melanized is unknown and remains to be determined. No bacteria and no hemocytes were encountered in either light or electron microscope preparations of melanized spermatophores.