AMER. ZOOI..,
14:457-465
(1974).
Comparative Ultrastructure of Cnidarian Sperm
GERTRUDE W. HINSCH
Institute for Molecular and Cellular Evolution, University of Miami,
Coral Gables, Florida 33134
SYNOPSIS. Marine Cnidarian sperm consist of a head containing the nucleus and several
electron dense vesicles. These vesicles aie anterior to the nucleus in the Hydrozoan and
Scyphozoan sperm and adjacent to the posterior legion of the nucleus in the Anthczoan
sperm. They are Golgi in origin and do not coalesce. They may be a primitive acrosome. Mitochondria may number four to five in the Hydrozoan and Scyphozoan sperm
while they form an aggregate in the Anthozoans. Both centrioles persist in the mature
spermatozoa. The distal centriole gives rise to the long flagellum and has associated
with it nine pericentriolar processes.
The small sperm of Cnidaria have been
studied by many investigators. Light microscopic studies classified cnidarian sperm
as a primitive type having a short or conical head, a midpiece with four to five
mitochondria and a long flagellum (see
Franzen, 1956, 1967, for reviews). Proper
fixation of cnidarian materials is and has
been difficult. Thus, electron microscopic
study of cnidarian sperm has been relatively recent but has included representatives of all classes. This paper will review
these works and present a "generalized"
description of the sperm of each class of
Cnidaria as is known to date. It is anticipated that these will be modified as more
knowledge is gained of other species.
CLASS HYDROZOA
(FIG. \A)
The nuclei of mature hydrozoan sperm
are small conical structures in which the
chromatin has condensed with the possible
formation of an intranuclear vacuole. The
Contribution No. 235 from the Institute for
Molecular and Cellular Evolution, University of
Miami. Work was supported in part by NIH grant
Tl-HD-2608, 09, and 10 to the Fertilization and
Gamete Physiology Training Program, Marine Biological Laboratory, Woods Hole, Mass.
The author is an NIH Career Development
Awardee.
The author wishes to thank Dr. M. G. O'Rand
for permitting her to use the photographs of
Campanularia and Gonolhyrea (Figs. 4 and 5) .
457
nucleus varies in size and shape among
the different species. The anterior and posterior regions of the nuclear membranes
are very closely applied to each other and
to the nucleus. Laterally the two layers
of the nuclear envelope may be quite loose.
The nuclear membranes and several additional membranes and cytoplasmic organelles are frequently found between the
nucleus and plasmalemma. The nucleus
frequently rests on the four to five
mitochondria.
Capping and lateral to the anterior end
of the nucleus are small vesicles filled with
an electron dense product (Hydractinia,
Tubularia: Hinsch and Clark, 1970a, 1973;
Tubularia: Afzelius, 1971; Campanularia,
Clava, Gonothyrea: O'Rand, 1972; Pennaria: Summers, 1970) (Figs. 2-5). These
vesicles are subject to great variations as
the result of fixation and embedding procedures. When fixed with paraformaldehyde-glutaraldehyde and postfixed in osmium (Hinsch and Clark, 1970a, 1973)
they are filled with an electron dense
material. In sperm fixed in osmium alone,
the vesicles appear hollow (Summers,
1970). The vesicles may appear as solid
circles, rings, or elongated strands. They
appear to arise in association with the
Golgi apparatus and thus may represent
proacrosomal granules (Hinsch and Clark,
1973). Similar structures are absent from
the freshwater hydrozoan Hydra spp.
sperm (Schincariol et al., 1967; Weissman
GERTRUDE W.
HINSCH
M
459
CNIDARIAN SPERM
et al., 1969; Stagni and Lucchi, 1970;
Moore and Dixon, 1972) although spermatids with vesicles of Golgi origin have
been reported (Weissman et al., 1969).
Summers (19726) has reported that the
marine hydroid Eudendrium ramosum
sperm lack such vesicles although his
Figure 6 shows structures very much like
those seen in other marine hydroid sperm.
With the possible exception of Eudendrium, the generalized marine hydroid
sperm pattern would be as presented in
Figure I A.
In some sperm all mitochondria are of
the same size, while in others an asymmetry to the sperm structure is established
by a single mitochondrion which is larger
than all the others. The cristae of the
mitochondria are flattened plates with circular ends. Afzelius (1971) reports that
the cristae lie parallel to the long axis
of the sperm in Tubularia although our
micrographs show various orientations
(Hinsch and Clark, 1973). There is often
some membrane modification between adjacent mitochondria, which is perhaps
associated with the pericentriolar processes.
Between and beneath the mitochondria
are the proximal centriole and the distal
centriole with its oericentriolar processes.
The pericentriolar processes will be discussed later. The flagellum arises from the
A and B tubules of the distal centriole and
presents the expected "9 + 2" flagellar
pattern. Small spokes radiate between the
9 doublets and the central pair of tubules
as well as to the plasmalemma. Neither A
nor B tubules are filled with an electron
dense material.
As indicated earlier, the vesicles anterior
to the nucleus may represent a primitive
acrosome. O'Rand (1972) has reported
their loss from Campanularia sperm introduced into Campanularia gonangia. The
vesicles in sperm of Gonothyrea or Clava
do not break down when they are placed
into gonangia of Campanularia (O'Rand,
FIG. 1. Diagrammatic representations of spermatozoa of Cnidaria. A, Class Hydrozoa sperm. B,
Class Scyphozoa sperm. C, Class Anthozoa sperm.
V, vesicles; DS, dense strata; N, nucleus; M, mito-
1972). The reverse experiments have not
been done. The role, if any, of the other
spermatozoan cytoplasmic structures in
sperm motility, chemotactic response, sperm
penetration, and contribution to the zygote
following fertilization needs further investigation, particularly in light of the work
of Miller (1966^6, 1970) and Miller and
Brokow (1970) demonstrating the species
specificity of chemotaxis in hydrozoans and
O'Rand (1972) demonstrating the specificity of vesicle loss.
CLASS SCYPHOZOA
(FIG.
Studies of Nausiihoe
IB)
(Afzelius and
Franzen, 1971), Aurelia and Daclylometra
(Hinsch and Clark, 1970a, 1973) and
Casseopia (Hinsch, unpublished) indicate
some major differences between scyphyzoan
and hydrozoan sperm. Many of these
sperm are very pointed and elongated (i.e.,
AiiraJia, Pclagia, Casseopia). Early workers
attributed the conical point to an acrosome
(see Franzen, 1967, for review). However,
ultrasiriictiiral studies indicate that this
area is not an acrosome in the sense of the
acrosome of other invertebrate or mammalian sperm. Located beneath the plasmalemma of the conical point and anterior
to the nucleus may be several membranebound vesicles filled with an electron dense
matrix (Figs. 6, 7). Some appear to arise
in association with the Golgi apparatus
(Hinsch and Clark, 1973). They may be
circular or rod-shaped while others may
be ring or cup-shaped. Many microtubules
accompany these structures in some sperms
depending on the species. The microtubules
may act to support the long tip anterior
to the nucleus. Behind the vesicles but
anterior to the nucleus is an accumulation
of electron dense non-membrane bound
material. If the vesicles are indeed comparable to the proacrosomal granules then
this material is perhaps comparable to the
subacrosomal material of mammalian
sperms.
chondria; PC, proximal centriole; DC, distal centriole; PCP, pericentriolar processes; F, flagellum;
MT, microtubules; LB, lipid bodies.
GERTRUDE W.
HINSCH
:i^2h3&* *^<yy
7
CNIDARIAN SPERM
The nucleus of scyphozoan sperm is an
elongate cone. It rests on the four to five
mitochondria as in the hydrozoans. The
distal centriole is posterior to the mitochondria and has associated with it the
pericentriolar processes (Hinsch and Clark,
1973) or the anchoring fiber apparatus of
Afzelius and Franzen (1971). Microtubules, membranes, and cytoplasmic organelles appear in the area between the
plasma membrane and nucleus or mitochondria. Adjacent mitochondrial membranes are fused (Fig. 9G).
The flagellum arises from the distal centriole. At its proximal end it may or may
not be surrounded by a cytoplasmic sheath.
In this region Y-fibers connect the doublets
to the plasma membrane. The 9 + 2 pattern of the flagellum is as in the hydrozoans.
461
nucleus in hydrozoan and scyphozoan
spermatozoa, this area is lacking in such
structures in the sperm of anthozoa (Dewel
and Clark, 1972; Hinsch and Clark, 1973).
However, structures of similar appearance
and origin (Golgi) are found in the cytoplasm along the posterior lateral regions
of the nucleus. The proximal and distal
renrrioles are present as in. the hydrozoan
and scyphozoan sperms. Slightly modified
pericentriolar processes appear associated
with the distal centriole. The flagellum
is long and arises from the distal centriole
and has the 9 + 2 tubule pattern.
THE CENTRIOLES AND PERICENTRIOLAR
PROCESSES
To date only two species of anthozoan,
Metridium and Bunodosoma, have been
studied. The head and midpiece vary
greatly in shape. The latter configuration
is due to a variation in mitochondrial
number. In Metridium, the mitochondrial
mass forms a sleeve around the proximal
end of the flagellum (Hinsch and Clark,
1973). It is formed by the fusion of many
mitochondria. In Bunodosoma cavernata
(Dewel and Clark, 1972) fusion of several
mitochondria has resulted in a single asymmetrical mitochondrial mass. Several lipidlike inclusions are located in close association with the mitochondria of Metridium
and Bunodosoma. Their suggested function
may be one of energy storage and utilization (Dewel and Clark, 1972).
Although vesicular bodies derived from
the Golgi complex appear anterior to the
The proximal centriole in cnidarian
sperms has an intricate pattern of fibers
and spokes with the triplets. It has no
associated structures. The distal centriole,
however, may possess satellites as well as
pericentriolar processes. The structures
with the distal centriole display a striated
appearance. Their structure was first reported in the hydrozoan Phialidium by
Szollosi (1964). He described nine "satellites" arising from the matrix of the distal
centriole and extending to the plasmalemma. More recently such "satellites"
were reported in Tubularia, Hydractinia,
Metridium, Cyanea, and Amelia (Hinsch
and Clark, 1970a,b, 1973), Pennaria (Summers, 1970), Campanularia, Clava, and
Gonothyraea (O'Rand, 1972) and Eudendrium (Hanisch, 1966; Summers, 19726).
Similar structures were referred to as
"spikes" in Campanularia by Lunger
(1971), as the fiber anchoring apparatus
in Nausithoe (Afzelius and Franze-n, 1971)
and in Tubularia (Afzelius, 1971), or as
FIG. 2. Hydractinia sperm. Note the way the nucleus (NT) abuts on the mitochondria. Apical vesicles (V) , ends of pericentriolar processes (arrows).
FIG. 3. Anterior region of Tubularia sperm. Apical
vesicles (V) are rounded.
FIG. 4. Oblique section of Campanularia sperm
showing that the apical vesicles (V) can also be
elongate in Hydrozoa.
FIG. 5. Apical vesicles (V) here in Conothyrea are
clustered anterior to the nucleus much as they are
in Pennaria (Summers, 1970, Figs. 3, 14) .
FIG. 6. Apical region of Amelia spermatids. Vesicles
(V) and microtubules are found here. Between
vesicles and nucleus is a dense granular material
(arrows) . Insert. Cross-section through the region
of the dense material (arrow) .
FIG. 7. In the Casseopia sperm a large dense vesicle
(V) is found anterior to the nucleus (N) . Along
one side of the sperm is a layer of increased electron density (arrow) .
CLASS ANTHOZOA (FIG. 1C)
462
GERTRUDE W.
HINSCH
CNIDARIAN SPERM
463
basal bodies in Hydra littoralis (Weissman et al., 1969). All have a common
origin in the electron dense matrix surrounding the triplets of the distal centriole.
They are distinct from centriolar satellites
which are transient structures. Thus, we
have adopted the term pericentriolar
processes (Hinsch and Clark, 1973) to
describe these structures which persist into
mature sperm and are found in the fertilized egg.
The pericentriolar processes extend laterally and beneath the mitochondria in
the hydrozoan sperm (Figs. 8A-G). As reported earlier (Hinsch and Clark, 19706),
these processes exhibit a definite periodicity. At a distance from the centriole,
enlargements of the primary processes
divide into three parts (secondary processes) . The two side members fuse with
the secondary processes of the adjacent
pericentriolar processes. Many thin strands
(tertiary processes) extend between adjacent pericentriolar processes apparently
serving as additional stabilization of these
structures. At the base of the centrioles,
the evidence suggests that the processes
form the Y-fibers first reported in the
Hydroides sperm by Colwin and Colwin
(1961).
In the scyphozoan sperm, the centriolar
complexes are very similar to those seen
in the hydrozoan sperms (Fig. 9A-G).
However, a few major differences do exist.
Each pericentriolar process seems to end
in a band or ring-like structure. Likewise,
a "rootlet"-like structure projects out from
the centriole (Hinsch and Clark, 1970b)
and to a point beyond the terminal ring
previously mentioned. In Nausitho'e, this
structure was referred to as a "spur" by
Afzelius and Franzen (1971). It may be
transitory and exist only in spermatids.
The pericentriolar apparatus in the
anthozoan sperm is much reduced in size
when compared with that of the hydrozoan and scyphozoan sperms. There is insufficient evidence from Metridium (Hinsch
and Clark, 1973) to enable us to construct
a model. However, the structure in Bunodosoma (Dewel and Clark, 1972) suggests
that here too the structures arise from
matrix between the triplets. Enrh primary
process widens to split into secondary processes which end blindly in the cytoplasm.
Although a model of the structure of
the pericentriolar processes in sperm was
reported by Summers (1972a), a more
complete model can be proposed based on
the evidence summarized here. The structure varies somewhat in each class of
Cnidaria and therefore can be assumed to
do so in the sperm of different phyla.
Proximally, the processes arise from a
dense matrix between the centriolar triplets and, as reported by Szollosi (1964) and
Hinsch and Clark (1970b), the enlarged
ends of the primary processes divide.
Summers (1972a) has not included the
matrix nor shown the division of the primary processes in his model. Further the
apparent fusion of adjacent secondary
processes distal to the centriole is not
included.
The degree of complexity of the pericentriolar processes may be determined by
the location of the centrioles in the mature
sperm. In those forms where no nuclear
fossa appears to be formed (i.e., hydrozoan, scyphozoan), they are much more
elaborate. In those having a nuclear fossa
(i.e., anthozoans) their structure is reduced. In light of this, the role of the
pericentriolar processes may be, as suggested by Szollosi (1964), that of providing
support (structural) to the centriolar and
flagellar structures. In the scyphozoans
where the centrioles are behind the mito-
FIG. 8 A. Model of the centriole and pericentriolar
processes as seen in the hydrozoan sperm, c, centriole; m, matrix; p, primary process; s, secondary
process; t, tertiary process; pin, plasiiialcinnia.
It-C, Eleilion micrographs Llnoiigh various levels
of the distal centriole and associated processes. B,
Centriole with 9 sets of triplet tubules embedded
in an electron dense matrix (arrow) . C, Slightly
lower level showing the 9 primary processes (arrows) and beginnings of secondary processes. D,
Oblique section showing primary and secondary
(arrow) pnxrSM'.s. E, Itianched scumdary pime.sses
(ariows) . Side inciiiliris fuse with adjacent processes. F, Near base of centriole and beginning of
flagcllum. G, Longitudinal section of centriole (C)
and primary process (arrow).
464
GERTRUDE W.
HINSCH
CNIDARIAN SPERM
465
Afzelius, B. 1971. Fine structure of the spermatozoon of Tubularia larynx (Hydiozoa, Coelenterata) . J. Ultrastruct. Res. 37:679-689.
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Res. 37:186-199.
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(Annelida) with special reference to the acrosomal region. J. Biophys. Biochem. Cytol. 10:211230.
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the mature .sperm in the anthozoan liunodosoma
cavernata (Cnidaria) . J. Ultrastruct. Res. 43:
417-431.
Franzcn, A. 1956. On spermiogenesis, morphology
of the spermatozoon, and biology of fertilization
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Franzen, A. 1967. Remarks on spermiogenesis and
morphology of the spermatozoon among the
lower Meta/oa. Aik. Zool. 19:335-342.
Hanisch. J. 1966. Spcrmienentwicklung von Eudendrium ramosum. Naturwissenschaften 53:587.
Hinsch, G. VV., and W. H. Clark, Jr. I97()n. Comparative studies of Coelenterate sperms. J. Cell
Biol. 47:88a.
Hinsch, G. W., and W. H. Clark, Jr. 1970/J. Centriolar satellites. Amer. Zool. 10:523. (Abstr.)
Hinsch, G. \V., and W. H. Clark, Jr. 1973. Comparative fine structure of Cnidaria spermatozoa.
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Lunger, P. D. 1971. Early stages of spermatozoan
development in the colonial hydroid Campanularia fiexuosa. Z. Zellforsch. 116:37-51.
Miller, R. L. 1966<v. Chemotaxis during fertilization
in the hydroids Tubularia and Conotliyrea.
Amer. Zool. 6:509-510.
Miller, R. L. 19666. Chemotaxis during fertilization
in the hydroid Campanularia. j . Exp. Zool. 102:
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Miller, R. L. 1970. Sperm migration during fertilization in the hydroid Gonothyrea loveni. J. Exp.
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Miller, R. L., and C. J. Brokaw. 1970. Chemotactic
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Moore, G. P. M., and K. E. Dixon. 1972. A light
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483-502.
O'Rand, M. G. 1972. Feitilization in the hydroid
Campanularia fiexuosa (Hinks) : an IN vivo and
in vitro study. Ph.D. Diss., Temple Univ., Philadelphia, Pa.
Schincariol, A. L., J. E. J. Habowsky, and G. Winner. 1967. Cytology and ultrastructure of differentiating interstitial cells in spermatogenesis in
Hydra jusca. Can. J. Zool. 45:590-594.
Stagni, A., and M. L. Lucchi. 1970. Ultrastructural
observations on the spermatogenesis in Hydra
attenuate, p. 357-363. In B. Bacetti [eel.], Comparative spermatology. Academic Press, New
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Summers, R. G. 1970. The fine structure of the
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Pliialidiuin gregarium. J. Cell Biol. 21:465-479.
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Fine structural observations on nuclear maturation during spermiogenesis in Hydra litloralis.
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FIG. 9 A. Diagrammatic model of centriole and
pericentriolar processes from a scyphozoan sperm,
r, Rootlet; tr, terminal ring; other labels as in Fig.
8 A. li-G, Micrographs of scyphozoan sperm, li,
Centriole. C, Centriole plus 9 primary pericentriolar processes (arrows) and secondary processes
which end on the terminal ring (tr) . D, Oblique
section showing pericentriolar processes and an-
terior end of the flagellum. E, Cross-section of
pericentriolar processes and rootlet (r) which extends beyond the terminal ring (tr) . F, Oblique
longitudinal section showing rootlet (r) and a primary pericentriolar process (p) . Note dense layer
between mitochondria. G, The outer mitochondrial
membranes appear united by an electron dense
layer of substance (arrows).
chondria, the rootlet-like structure may be
an additional supportive structure. Afzelius
(1971) suggests that the pericentriolar
processes determine the position of the
mitochondria and Summers (1970) suggests that they may serve some locomotor
function.
It is to be expected that the "generalized" models presented here might be
modified as attention is directed to more
cnidarian sperm. It will be particularly
interesting to determine whether major
variations in structure will be found
within classes or orders.
REFERENCES