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.