An. Hodgson et J. Heller, Spermatozoon structure and spermiogenesis in four species of Melanopsis (Gastropoda, Prosobranchia, Cerithioidea) from Israel, INVERTEBR R, 37(3), 2000, pp. 185-200
The structure of the spermatozoa of four species of Melanopsis (melanopsida
e) is described by light and transmission electron microscopy. All species
produce two types of spermatozoa, euspermatozoa (fertilizing) and parasperm
atozoa (non-fertilizing). The eusperm of all species are about 42-44 mu m l
ong and have a morphology similar to that of other cerithioideans. The head
is composed of a laterally flattened nucleus and conical acrosome of appro
ximately equal size. The nucleus has a shallow posterior fossa which houses
a centriolar derivative from which the axoneme emerges. The four equal-siz
ed mitochondria of the midpiece are non-helically arranged around the axone
me. Each mitochondrion has an anterior flange and the cristae are plate-lik
e. This midpiece arrangement suggests that the Melanopsidae are more closel
y related to the Modulidae, Potamididae and possibly Pachychilidae than to
other cerithiacean taxa. Posterior to the midpiece is a 10-12 mu m-long gly
cogen piece in which the glycogen granules encircle the axoneme as inner an
d outer rings. Although the structure of the euspermatozoa is very similar
in all four species, there are significant differences in the lengths of th
e nucleus, acrosome and midpiece between species. These dimensional differe
nces suggest that sperm morphology could be of value in the separation and
identification of species within the genus Melanopsis. The oligopyrenic par
asperm are about 65 mu m long and consist of a head and tail region of mult
iple flagella. The axonemes of the flagella are positioned centrally within
the head and run for its length. Spermiogenesis (both eu- and paraspermiog
enesis) is as described for other caenogastropods. During sperm development
, spermatids are closely associated with Sertoli cells. Unlike previous rep
orts for some caenogastropods, the Sertoli cells do not completely surround
the early spermatids. Instead, long tubular cytoplasmic processes from the
Sertoli cells encircle the developing spermatozoa. These processes contain
microfilaments which are thought to be composed of actin.