Tudge, C.C. and Jamieson, B.G.M. (1991). Ultrastructure of the mature spermatozoon of the Coconut Crab, Birgus latro (Coenobitidae, Paguroidea, Decapoda). Marine Biology 108, 395-402.
The spermatozoon of Birgus latro (Linnaeus, 1767) is approx 14 .mu.m in length. It is composed of a large multi-layered oblong-ovoid acrosome which is capped by a conical operculum and lies anterior to a small ring of cytoplasm and an amorphous nucleus which is drawn out into a series of arms or extensions. Originating from the cytoplasmic area are three further long microtubular arms. The sperm of B. latro is very similar to the sperm of the only other genus in the Coenobitidae, Coenobita, of which C. clypeatus is a representative species. They share a suite of ultrastructural characters including: a long, cylindrical, capsule-bound acrosome containing an inner acrosome core, a large acrosome ray zone and a thin outer acrosome zone; an apical operculum anterior to a subopercular zone divided into two areas of differing density; an invaginated perforatorial zone with a bipartite granular matrix; microvillus-like extensions of the inner acrosome core projecting into the perforatorial invagination; a ring of cytoplasm, around the base of the acrosome, containing numerous mitochondria, extensive lamellar systems and the bases of three microtubular arms; granular nuclear material forming irregular arms; and at the posterior portion of the cell membrane a combination of nuclear and plasma membranes. Some ultrastructural characters which separate the two genera are: a domed operculum in C. clypeatus as opposed to a conical one in B. latro; in B. latro there is some residual cytoplasm external to the operculum and centrioles are absent from the mature spermatozoon; in C. clypeatus the inner acrosome core does not appear to invest the perforatorium and a series of dense "rods" are found subjacent and internal to the operculum. Similarities between the two coenobitids are greater than those shared with the pagurid Eupagurus bernhardus. In this latter species, the acrosome is more ovoid than cylindrical and the acrosome zones are less conspicuous; the operculum is absent from the mature sperm (although present during spermiohistogenesis); no subopercular zone is present; the perforatorium contains longitudinally arranged microtubules and extends the full length of the acrosome; and the nuclear material does not form separate nuclear arms. Sperm ultrastructure supports monophyly of the Paguroidea, while distinguishing coenobitids from pagurids within this superfamily.