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ABSTRACT Aflagellate sperm of the crayfishes Cherax tenuimanus and Cherax albidus are composed of an acrosome, subacrosomal region, collar, and nucleus. The acrosomes are helmet-shaped and PAS positive. In C. tenuimanus, they can be subdivided into a granular inner region and a homogeneous outer region. A series of electron-translucent canals runs through the acrosomes of both species. In C. albidus, these canals are arranged in whorling patterns, while in C. tenuimanus they are present near the base of the acrosome and contain a small electron-dense component. The subacrosomal regions are metachromatic when stained with methylene blue and contain a network of fibrils and granules. The collars are composed of granular material, which is positioned immediately subjacent to the subacrosomal region, and of highly convoluted membrane lamellae. Mitochondria and microtubules are associated with the membrane lamellae. The nuclei contain uncondensed chromatin and are bounded by a complete nuclear envelope. Sperm are surrounded by a moderately electron-dense matrix containing granules and filaments. Sperm of Cherax tenuimanus have large deposits of an electron-dense material on their surface. In contrast, sperm of C. albidus have long extracellular lamellae associated with the extracellular surface of their plasma membrane, and these lamellae are also found in the extracellular matrix surrounding the sperm of C. albidus. In both species, the sperm and extracellular materials are surrounded by a moderately PAS-positive primary spermatophore layer which is in turn surrounded by a secondary spermatophore layer secreted from the wall of the vas deferens. This layer includes dense granules which are supported in a matrix. In C. tenuimanus, this matrix is homogeneous, while in C. albidus, this matrix contains light and dark staining areas. The darker regions often have a fibrous appearance. A comparison is made between sperm of C. tenuimanus and C. albidus and the sperm of other pleocyematan species.


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