Fine structure of an unusual spermatozoon of a brooding sea cucumber, Cucumaria lubrica

Abstract

The sperm of Cucumaria lubrica consists of an elongated head, 1.5 microns (n) in diameter and 5.2 μ in length; a mitochondrial midpiece of 1.9 μ in length; and a tail of 65 μ. The acrosome contains an irregularly circular granule which is located in an anterior nuclear depression. The granule is surrounded by a complete limiting membrane in most of the sperm examined. Occasional sperm contain granules surrounded by incomplete membranes. The acrosomal granule consists of a homogeneous material except for an electron-dense sphere displaced anteriorly. A subacrosomal layer consisting of a homogeneous reticular material encircles the granule. This layer contains several vesicular structures and a fibrous component immediately posterior to the granule.The nucleus measures 6.8 μ in length and 1.4 μ at its greatest diameter, but tapers to a diameter of 0.5 μ at the posterior end. A single mitochondrial mass surrounds the posterior 1.6 μ of the nucleus. The proximal and distal centrioles, lying perpendicular to each other, are found posterior to the nucleus. Microtubules are observed in close association with the satellite of the distal centriole.The adaptive significance of this unusual sperm is discussed.