Sperm Attachment on the Egg of Malaysian King Crab, Carcinoscorpius rotundicauda

Abstract

The structures of mature gametes of Carcinoscorpius rotundicauda were investigated using scanning electron microscopy (SEM). In general, morphology of the sperm was found to be similar to the other three horseshoe crab species, Limulus polyphemus, Tachypleus tridentatus, and Tachypleus gigas. The sperm consists of a head, 4.6 µm, containing the nucleus capped with an acrosomal vesicle, and a long flagellum, 33 µm. The spherical egg (2.25–2.58 mm in diameter) consists of two main parts: yolk and chorion. The chorion is composed of two layers: the thin and hard outer layer, or basement lamina (3.03 µm), which has irregular-sized pores on its outer surface, and the thick inner layer, vitelline envelope (43.6 µm). The egg membrane uniformly surrounds the yolk and shows no evidence of a micropyle. Sperm attachment to egg envelope includes three successive steps: initial attachment, acrosomal reaction, and sperm penetration. The initial attachment is observed to occur while spermatozoa are motile. In this step, large numbers of sperm attach to the egg surface. This is followed by the acrosome adhesion, whereby the acrosomal vesicle opens and releases its contents to adhere to the egg material. This is indicated by the flattened appearance of the sperm acrosome. Most of the attached undergo this step and pass through the inner layer of egg. In the last step, the sperm head penetrates through the egg envelope.