An ultrastructural examination of developing and mature euspermatozoa in pyrazus ebeninus (Mollusca,Gastropoda, Potamididae)

Summary Mature and developing euspermatozoa of the prosobranch gastropod Pyrazus ebeninus, have been examined using transmission electron microscopy and phase-contrast light microscopy. The head of the mature euspermatozoon consists of a conical acrosome capping a short, rod-shaped nucleus (laterally compressed posteriorly). A basal invagination in the nucleus contains the proximal portion of the axoneme and a dense attachment matrix. Four apparently non-helical mitochondrial elements (two large, two small) comprise the midpiece each being composed of curved, inclined cristal plates and a granular matrix. The structure and arrangement of the mitochondrial elements is thus distinguishable from the helical midpiece elements found in euspermatozoa of neogastropods and most mesogastropods and possibly is widespread in the Cerithiacea. A dense ring-like structure is found closely applied to the inside of the plasma membrane at the junction of midpiece and glycogen piece.Acrosome and midpiece formation and nuclear condensation have been studied in developing euspermatozoa. Acrosome development is divided into two phases: (1) a pre-attachment phase — during which a complex early acrosome is formed often at great distance from the nuclear apex, and (2) an attachment/post-attachment phase — during which the completed preattachment phase acrosome tilts into position at the nuclear apex and subsequently elongates. The nucleus passes through a recognizable sequence of condensation phases (reticular, fibrillar and lamellar phases). Microtubules surround both the nucleus and midpiece in the final phase of maturation. The four, elongate midpiece elements of the mature euspermatozoon are apparently derived from the four large, spherical mitochondria of the euspermatid.The potential usefulness of spermatozoal ultrastructure with regard to indicating affinities between groups of gastropod families is briefly discussed.Abbreviations a acrosome - ac euspermatozoon acrosomal cone - ar euspermatozoon axial rod - ax axoneme - bp basal plate - cy cytoplasmic droplet - cs cylindrical support structures of developing acrosome - dg dense granule of pre-attachment phase developing acrosome - dp dense plates of developing acrosomal cone - g glycogen granules - gp glycogen piece - G Golgi complex - j junction of midpiece and glycogen piece - l large midpiece element - m mitochondrion - M midpiece - mt microtubules - n nucleus - pm plasma membrane - sGv small Golgi vesicles - s small midpiece element     

Ultrastructure of the Spermatozoon of an Opisthobranch, Tornatina sp. (Mollusca, Gastropoda, Retusidae)

The spermatozoon of Tornatina sp. has been studied with phase-contrast light microscopy and transmission electron microscopy. The head of the spermatozoon consists of an elongate acrosome which caps the apex of an unusually complex, helical nucleus. This elaborate nuclear morphology has not been previously reported, but possibly is found in other opisthobranch gastropod spermatozoa. An axoneme is inserted deeply into the base of the nucleus whilst posterior from the nucleus, the axoneme is ensheathed successively by the mitochondrial derivative (midpiece) and 'glycogen' granules (glycogen piece). The midpiece exhibits fine structure similar to that observed in other euthyneuran spermatozoa (paracrystalline and matrix materials) and possesses a single helical compartment filled with what are probably glycogen granules. A dense ring structure occurs at the junction of the midpiece and glycogen piece. The spermatozoon of Tornatina and other gastropods (prosobranch and euthyneuran) are compared.     

Ultrastructure of spermiogenesis of Philippia (Psilaxis) oxytropis,with special reference to the taxonomic position of the architectonicidae (Gastropoda)

Summary Spermiogenesis of the architectonicid Philippia (Psilaxis) oxytropis was studied using transmission electron microscopy. Both spermatids and mature sperm of Philippia show features comparable to sperm/spermatids of euthyneuran gastropods (opisthobranchs, pulmonates) and not mesogastropods (with which the Architectonicidae are commonly grouped). These features include: (1) Accumulation of dense material on the outer membrane of anterior of the early spermatid nucleus — this material probably incorporated into the acrosome; (2) Structure of the unattached and attached spermatid acrosome (apical vesicle, acrosomal pedestal) accompanied by curved (transient) support structures; (3) Formation of the midpiece by individual mitochondrial wrapping around the axonemal complex, and the subsequent fusion and metamorphosis of the mitochondria to form the midpiece; (4) Presence of periodically banded coarse fibres surrounding the axonemal doublets and intra-axonemal rows of granules. A glycogen piece occurs posterior to the midpiece but is a feature observed in both euspermatozoa of mesogastropods (and neogastropods) and in sperm of some euthyneurans.Despite the lack of paracrystalline material or glycogen helices within the midpiece (both usually associated with sperm of euthyneurans), the features of spermiogenesis and sperm listed indicate that the Architectonicidae may be more appropriately referable to the Euthyneura than the Prosobranchia.Abbreviations a acrosome - ap anterior region of acrosomal pedestal - as support structures of spermatid acrosome - av apical vesicle of acrosome (acrosomal vesicle of un-attached acrosome) - ax axoneme - b basal region of acrosomal pedestal - c centriole - cf coarse fibres - cr cristal derivative of midpiece - db intra-axonemal dense granules - drs dense ring structure - gg glycogen granules - gp glycogen piece - G Golgi complex - m mitochondrion - mt microtubules - n nucleus - pm plasma membrane - sGv small Golgi vesicles     

An Ultrastructural Study of Basommatophoran Spermatozoa (Mollusca, Gastropoda)

Spermatozoa of the basommatophoran pulmonate families Ellobiidae ( Ophicardelus ornatus Ferussac), Amphibolidae ( Salinator fragilis Lamarck, Salinator solida von Martens), Siphonariidae ( Siphonaria funiculata Reeve) and Lymnaeidae ( Lymnaea lessoni (Deshayes)) were studied using transmission electron microscopy. Spermatozoa of all species, like most euthyneuran spermatozoa, possess ( 1 ) an acrosome composed of an apical vesicle and acrosomal pedestal (many differences between families), (2) a helically keeled, posteriorly invaginated nucleus, ( 3 ) a midpiece composed of paracrystalline and matrix materials and a variable number of incorporated glycogen-filled helices (one in Salinator and Siphonaria , two or three in Lymnaea , three in Ophicardelus ) and (4) an axoneme associated with coarse fibres (periodically banded in "neck" region) and rows of intra-axonemal granules. The wide diversity of spermatozoon structure in the species studied, in particular the midpiece structure of Siphonaria (which resembles closely that of certain opisthobranchs) indicates that the Basommatophora may not represent a valid taxonomic unit. A comparison of basommatophoran sperm with other euthyneurans and with euspermatozoa of prosobranchs is given.     

Ultrastructure of paraspermatozoa,euspermatozoa and eusperm-like spermatozoa ofObtortio cf.fulva (Prosobranchia: Cerithiacea)

The cerithiaceanObtortio cf.fulva produces three distinct types of spermatozoa: (1) paraspermatozoa, (2) euspermatozoa and (3) eusperm-like spermatozoa. Like most mesogastropods, euspermatozoa ofObtortio are composed of a conical acrosome, short posteriorly invaginated nucleus, elongate midpiece and glycogen piece, and short terminal region. The midpiece, however, is distinctly cerithiacean in structure and is composed of four non-helical midpiece elements. Eusperm-like spermatozoa closely resemble euspermatozoa, but have a very short nucleus only one half to one third the length of the euspermatozoon nucleus. Paraspermatozoa of this species are composed of (1) head (mosaic sheath of dense blocks enveloping multiple axonemes which attach anteriorly to a long apical structure), (2) midpiece (multiple axonemes interspersed with elongate mitochondria), and (3) multiple tail tuft (axonemes each ensheathed by glycogen granules). The possible role of eusperm-like spermatozoa is briefly discussed together with the taxonomic implications of the structure of the three sperm types.     

Ultrastructure of euspermatozoa and paraspermatozoa in the volutid snail <Emphasis Type="Italic">Adelomelon ancilla</Emphasis> (Mollusca: Caenogastropoda)

The ultrastructure of the euspermatozoa and the paraspermatozoa is investigated in Adelomelon ancilla, through histological section observed by transmission electron microscopy. Euspermatozoa of A. ancilla consists of: (1) a conical acrosomal vesicle (with a short basal invagination, constricted anteriorly) which is flattened at the apex and associated with an axial rod, a centrally perforated basal plate and a short accessory membrane, (2) a rod-shaped, solid and highly electron-dense nucleus (with a short basal fossa containing a centriolar complex and a initial portion of a 9 + 2 axoneme), (3) an elongate midpiece consisting of the axoneme sheathed by 5–6 helical mitochondrial elements each exhibiting a dense U-shaped outer layer, (4) an elongate glycogen piece (where the axoneme is sheathed by nine tracts of glycogen granules), (5) a dense annulus at the junction of the midpiece and glycogen piece, and (6) a short free tail region (where the axoneme is surrounded only by plasma membrane). We observed a parasperm in A. ancilla. This is vermiform in shape and is composed of multiple axonemes and extensive cytoplasm with numerous vesicles, and mitochondria are scattered inside the axonemes. Sperm of A. ancilla is characterized by the euspermatozoa type 2 and the paraspermatozoa morphology belongs to type 5. The U shaped electrodense mitochondrial element in the midpiece of the eusperm and the constriction in the acrosomal vesicle present in A. ancilla are exclusive. We suggest that these characteristics could have taxonomic importance, because these was observed in other volutids and have not been observed in the rest of caenogastropods studies. We consider that the morphology of paraspermatozoa in A. ancilla corresponds to the “lancet” type.     

EUSPERMATOZOAN ULTRASTRUCTURE IN BEMBICIUM AURATUM (GASTROPODA): COMPARISON WITH OTHER CAENOGASTROPODS ESPECIALLY OTHER LITTORINIDAE

Euspermatozoa of Bembicium auratum Quoy & Gaimard are examinedultrastructurally and compared with euspermatozoa of other caeno-gastropods,especially other species of Littonnidae The acrosomal vesicleis conical, deeply invaginated (accommodating an axial rod)and exhibits radial plates and a weakly developed apical bleb.Unlike euspermatozoa of the Littonmnae which have a long tubularnucleus (sheathing a significant portion of the axoneme), theeusperm nucleus of B. auratum is short, rod-shaped and solidwith the exception of a shallow centriolar fossa posteriorlyAvailable evidence suggests this is also the case for otherspecies of Lacuninae and for the Laevilitonninae The euspermmidpiece of B auratum consists of the axoneme and 7–9helically arranged mitochondria (containing short, randomlyarranged cnstae) Immediately posterior to the annulus, the axonemeis surrounded by nine tracts of glycogen granules to form theglycogen piece. The euspermatozoon terminates in a short endpiece, in which the 9+2 axoneme degenerates into isolated microtubules,only two of which survive to the posterior extremity of thecell Paraspermatozoa have not been observed in any species ofBembicium or in fact any other species of the Lacuninae, suggestingthat absence of paraspermatozoa is characteristic of the subfamily(contrasting with well developed round paraspermatozoa of theLittonninae) (Received 16 June 1994; accepted 26 July 1995)     

ULTRASTRUCTURE OF THE MATURE SPERMATOZOA OF THE LAND SNAIL EPIPHRAGMOPHORA TUCUMANENSIS (DOERING, 1874) (GASTROPODA: HELICOIDEA)

The ultrastructure of the sperm of Epiphragmophora tucumanensis(Doering), a pulmonate land snail belonging to the family Xanthonychidae,was examined. The results showed that this spermatozoon presents a similarmorphology to the other advanced stylommatophoran sperm described.The most peculiar characteristic is the form of the nucleus,which is straight instead of helically coiled. An acrosome ispresent, composed of an apical vesicle and an acrosomal pedestal.The connecting piece or neck region is formed by a basal bodyplus a centriolar derivative. It is also the point of originof coarse fibres that accompany the axoneme. Thus, the axonemalcomplex exhibits a 9+9+2 pattern. The midpiece is an elongatedregion composed of paracrystalline and matrix materials (collectively,the mitochondrial derivative) enclosing a single glycogen helixand the axoneme. The mitochondrial derivative extends to theposterior tip of the spermatozoon. As observed in other stylommatophorans,both an annulus and a glycogen piece are absent. (Received 25 October 1993; accepted 4 February 1994)     

Ultrastructure and Phylogenetic Significance of Notaspidean Spermatozoa (Mollusca, Gastropoda, Opisthobranchia)

Spermatozoa of five notaspidean opisthobranchs [Berthellina citrina, Berthella ornata, Pleuro-branchus peroni, Pleurobranchaea maculata, Umbruculum sinicum] were examined using TEM. In all five species, the acrosome (sensu lato) consists of an apical vesicle (the acrosomal vesicle) and acrosomal pedestal. The acrosomal pedestal overlaps the nuclear apex, and in P. peroni (and possibly B. ornata) is periodically banded—-the first reported incidence of this type of substructure in any euthyneuran acrosome. Although sperm nuclei of P. peroni, B. ornata and B. citrina differ in length and also the number of keels present (nucleus 7 μm long with four/five keels present in Pleurobranchus; 17 μm long with one keel in Berthella; 15 μm long with a very weak keel in Berthellina), the basal invagination to which the centriolar derivative, axoneme and coarse fibres are attached is always poorly developed, and very little overlap between nucleus and midpiece occurs. In P. maculata and U. sinicum, the nucleus forms a helical cord around the axoneme and mitochondrial derivative such that it is not possible to recognize exclusively ‘nuclear’ and ‘midpiece’ regions of the spermatozoon. In all notaspideans investigated, (1) the axoneme, coarse fibres and glycogen helix are enclosed by the paracrystalline and matrix components of the mitochondrial derivative and (2) a dense ring structure (attached to the plasma membrane) and glycogen piece are observed. While the glycogen piece is very short (0.85–1.43 μm) with a very degenerate axoneme in B. citrina, B. ornata and P. peroni, this region of the spermatozoan is well developed (30–35 μm long) in U. sinicum and exhibits a fully intact 9 + 2 axoneme. The ‘glycogen piece’(or its presumed homologue) in P. maculata spermatozoa is very short (0.65 μm), devoid of any axonemal remnant and constructed of a hollow, internal cylinder attached to an outer (incomplete) shell, and contains scattered (glycogen) granules. Spermatozoal structure supports a close relationship between the genera Berthellina, Berthella and Pleurobranchus. These three genera have more distant links with Pleurobranchaea, while Umbraculum maintains an isolated, specialized position within the Notaspidea.     

The ultrastructure of spermatozoa and spermiogenesis in pyramidellid gastropods,and its systematic importance

Ultrastructural observations on spermiogenesis and spermatozoa of selected pyramidellid gastropods (species ofTurbonilla, Pyrgulina, Cingulina andHinemoa) are presented. During spermatid developement, the condensing nucleus becomes initially anterio-posteriorly compressed or sometimes cup-shaped. Concurrently, the acrosomal complex attaches to an electrondense layer at the presumptive anterior pole of the nucleus, while at the opposite (posterior) pole of the nucleus a shallow invagination is formed to accommodate the centriolar derivative. Midpiece formation begins soon after these events have taken place, and involves the following processes: (1) the wrapping of individual mitochondria around the axoneme/coarse fibre complex; (2) later internal metamorphosis resulting in replacement of cristae by paracrystalline layers which envelope the matrix material; and (3) formation of a glycogen-filled helix within the mitochondrial derivative (via a secondary wrapping of mitochondria). Advanced stages of nuclear condensation (elongation, transformation of fibres into lamellae, subsequent compaction) and midpiece formation proceed within a microtubular sheath (‘manchette’). Pyramidellid spermatozoa consist of an acrosomal complex (round to ovoid apical vesicle; column-shaped acrosomal pedestal), helically-keeled nucleus (short, 7–10 μm long, shallow basal invagination for axoneme/coarse fibre attachment), elongate helical midpiece (composed of axoneme, coarse fibres, paracrystalline and matrix materials, glycogen-filled helix), glycogen piece (length variable, preceeded by a dense ring structure at junction with midpiece). The features of developing and mature spermatozoa observed in the Pyramidellidae are as observed in opisthobranch and pulmonate gastropods indicating that the Pyramidelloidea should be placed within the Euthyneura/Heterobranchia, most appropriately as a member group of the Opisthobranchia.     

The ultrastructure of the spermatozoon of Haplotaxis ornamentus (Annelida,Oligochaeta, Haplotaxidae) and its phylogenetic significance

Summary The spermatozoon of Haplotaxis ornamentus has characteristics common to all oligochaete sperm: filiform; primary acrosome vesicle carried on an acrosome tube and containing an axial rod (perforatorium) in an invagination (subvesicular space or secondary acrosomal invagination); an elongate, highly condensed cylindrical nucleus followed by a cylindrical midpiece of radially adpressed mitochondria not penetrated by the axoneme; a single (distal) centriole persistent, though modified, at maturity; axoneme with 9 doublets, each with two outer glycogen granules, and centrally two singlets accompanied by two solid fibres. A peculiar haplotaxid combination of characters (none unique) is slight withdrawal of the primary vesicle into the acrosome tube with a strongly emergent capitulate axial rod and moderately short midpiece. This ultrastructure is consistent with location of the Haplotaxidae at the base of the Haplotaxida (Haplotaxina — Alluroidina — Moniligastrina — Lumbricina). Tubificida sperm, although also plesiomorph for the Oligochaeta, have the autapomorphy elongate periaxial sheath (secondary tube), excepting the Phreodrilidae whose sperm show convergent resemblances to the Lumbricina. The term annuloid has been introduced for annulus-like structures of varied origins.     

Vergleichende Ultrastrukturuntersuchungen der Eu- und Paraspermien von 13Protodrilus-Arten (Polychaeta,Annelida) und ihre taxonomische und phylogenetische Bedeutung

The morphology of the slender, filiform spermatozoa of 13Protodrilus species of 22 different populations is investigated by light and transmission electron microscopy. All species have two types of spermatoza: fertile euspermatozoa, and paraspermatozoa, which are probably infertile and may comprise up to 20% of the total number of mature gametes. This is the first record of sperm dimorphism in polychaetes. The general construction pattern of the euspermatozoa is very complex. It shows a longish tapering acrosomal vesicle with an internal acrosomal rod, a rod-like conical nucleus, and a midpiece with numerous very complex supporting elements and two thin mitochondrial derivatives. Further, it has a ‘peribasal body’ surrounding the basal body of the axoneme, an anulus region with an ‘anchoring apparatus’ and an anulus cuff. Posteriorly, the tail region proper contains in some species 2 to 9 supporting rods. In several species the euspermatozoon shows very distinct and species-specific alternations of this ‘general pattern’ relating to e.g. size of sperm elements, structure of acrosome and nucleus, presence or absence of axial rod, and number, shape and size of supporting elements in midpiece and tail. In a number of species some sections of the euspermatozoon overlap with each other more or less strongly. The paraspermatozoon has a comparatively simple construction pattern and possesses no supporting structures in midpiece and tail region. The midpiece is very short and, in some species, entirely surrounded by its two thin and elongate mitochondrial derivatives. An axial rod is often missing or reduced; different sperm sections never overlap each other. In contrast to the euspermatozoa, the paraspermatozoa of the different species have a very similar ultrastructure. Their possible function in spermatophore transfer and histolytical opening of the female epidermis is discussed. A comparison of the different forms of euspermatozoa inProtodrilus elucidates possible plesiomorphous and apomorphous sperm traits. Very likely, the hypothetical plesiomorphous type of spermatozoa inProtodrilus has a very similar morphology to that of the paraspermatozoa, which for this reason are considered to be a sort of persisting representatives of the ancientProtodrilus sperm type. InProtodrilus, the different traits of the euspermatozoa represent excellent taxonomic characters for distinguishing species (e.g. ‘sibling species’). They can also be used well for phylogenetics within the genus, whereas the relations ofProtodrilus to other polychaete groups cannot be clarified solely on the basis of sperm characters, since in all groups the sperm structure is primarily an adaptation to a specific mode of reproduction. Generally, the value of sperm characters in phylogenetic considerations at higher taxonomic levels seems to be very limited due to the surprisingly wide range of different sperm structures within a single genus as is demonstrated in the present paper.      

Ultrastructure of the spermatozoon of the American Alligator,Alligator mississippiensis (Reptilia: Alligatoridae)

This study details the ultrastructure of the spermatozoa of the American Alligator, Alligator mississippiensis. American Alligator spermatozoa are filiform and slightly curved. The acrosome is tapered at its anterior end and surrounded by the acrosome vesicle and an underlying subacrosomal cone, which rests just cephalic to the nuclear rostrum. One endonuclear canal extends from the subacrosomal cone through the rostral nucleus and deep into the nuclear body. The neck region separates the nucleus and midpiece and houses the proximal centriole and pericentriolar material. The distal centriole extends through the midpiece and has 9 × 3 sets of peripheral microtubules with a central doublet pair within the axoneme that is surrounded by a dense sheath. The midpiece is composed of seven to nine rings of mitochondria, which have combinations of concentrically and septate cristae. The principal piece has a dense fibrous sheath that surrounds an axoneme with a 9 + 2 microtubule arrangement. The sheath becomes significantly reduced in size caudally within the principal piece and is completely missing from the endpiece. Dense peripheral fibers, especially those associated with microtubule doublets 3 and 8, penetrate into the anterior portion of the principal piece axoneme. The data reported here hypothesize that sperm morphology is highly conserved in Crocodylia; however, specific morphological differences can exist between species. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Ultrastructure of spermiogenesis in the American alligator,Alligator mississippiensis (Reptilia,Crocodylia, Alligatoridae)

Testicular samples were collected to describe the ultrastructure of spermiogenisis in Alligator mississipiensis (American Alligator). Spermiogenesis commences with an acrosome vesicle forming from Golgi transport vesicles. An acrosome granule forms during vesicle contact with the nucleus, and remains posterior until mid to late elongation when it diffuses uniformly throughout the acrosomal lumen. The nucleus has uniform diffuse chromatin with small indices of heterochromatin, and the condensation of DNA is granular. The subacrosome space develops early, enlarges during elongation, and accumulates a thick layer of dark staining granules. Once the acrosome has completed its development, the nucleus of the early elongating spermatid becomes associated with the cell membrane flattening the acrosome vesicle on the apical surface of the nucleus, which aids in the migration of the acrosomal shoulders laterally. One endonuclear canal is present where the perforatorium resides. A prominent longitudinal manchette is associated with the nuclei of late elongating spermatids, and less numerous circular microtubules are observed close to the acrosome complex. The microtubule doublets of the midpiece axoneme are surrounded by a layer of dense staining granular material. The mitochondria of the midpiece abut the proximal centriole resulting in a very short neck region, and possess tubular cristae internally and concentric layers of cristae superficially. A fibrous sheath surrounds only the axoneme of the principal piece. Characters not previously described during spermiogenesis in any other amniote are observed and include (1) an endoplasmic reticulum cap during early acrosome development, (2) a concentric ring of endoplasmic reticulum around the nucleus of early to middle elongating spermatids, (3) a band of endoplasmic reticulum around the acrosome complex of late developing elongate spermatids, and (4) midpiece mitochondria that have both tubular and concentric layers of cristae. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

蓝尾石龙子精子的超微结构

蓝尾石龙子(Eumeces elegans)附睾以2．5％戊二醛和1％锇酸双重固定,按常规制作超薄切片,用H-600透射电镜研究观察精子的超微结构。精子由头部和尾组成,头部由顶体复合体和核组成,尾由颈段、中段、主段和末段组成。头部的顶体囊前部扁平,分为皮质和髓质,顶体下锥由类结晶状的顶体下物质组成,穿孔器顶端尖,、穿孔器基板塞子状,细胞核延长,核内小管缺,核伸展部前端具一电子透明区,核肩圆,核陷窝锥形。颈段具片层结构,近端中心粒和远端中心粒的长轴呈直角,9束外周致密纤维与远端中心粒相应的9束三联微管相联,向后与轴丝相应的9束双联微管相联,中央纤维与2个中央单微管相联。中段短,含有线状嵴的柱状线粒体,由连续的规则小卵状或小梯形致密体组成线粒体间的环状结构,纤维鞘伸入中段,终环紧贴于细胞膜的内表面。线粒体与环状结构的模式为：rs1／mi1,rs2／mi2,rs3／mi3,rs4／mi4,横切面上每圈线粒体数目为10个。主段前面部分具薄的细胞质颗粒区。纤维3和8至主段前端消失。轴丝复合体呈“9 2”型。蓝尾石龙子精子超微结构与已描述的石龙子科种类比较发现,与蜓蜥群和胎生群的石龙子相似;但没有发现石龙子科精子的独征。     

Spermiogenesis in the Oligochaetoid Polychaete Questa (Annelida, Questidae)

The spermatids are connected to a central cytophore by cytoplasmic bridges and are polarized in the sequence: "empty cytoplasm"; uncondensed nucleus; mitochondria which surround the distal region of the nucleus and the centrioles; axoneme; posterolateral to the base of the axoneme, the Golgi apparatus and (when secreted) the acrosomal rudiment. The dome-shaped acrosome vesicle elongates progressively as it migrates to the tip of the elongating and condensing nucleus; subacrosomal material gives rise to an almost equally long, tubular, thick-walled perforatorium. After the acrosome has greatly elongated, the mitochondria are reduced to two, which lose their rounded form and invest the growing axoneme to give a very elongate midpiece. Transfer of materials from nucleus to mitochondria is discussed. Microtubules surrounding the acrosome and nucleus disappear by maturity, but those internal to the mitochrondria apparently persist as the accessory microtubules, unique in the Annelida, which surround the 9 + 2 axoneme. Microvilli of the egg envelope, which have tetrads of terminal branches (epivitelline projections) resembling epicuticular projections, are less than 1 μm long, whereas the mature acrosome exceeds 5 μm. This suggests that the correlation seen in oligochaetes is absent.     

Ultrastructure of euspermatozoa and paraspermatozoa in the marine gastropod Adelomelon beckii (Caenogastropoda, Volutidae)

The sperm morphology of Adelomelon beckii is described by optical and transmission electron microscopy. Both euspermatozoa and paraspermatozoa were found in the specimens studied. Euspermatozoa are filiform and have an elongate nucleus capped by an acrosome. A small basal plate lies between the base of the acrosome and the nucleus. The mid-piece consists of U-shaped mitochondria wrapped helically around the central axoneme. A dense annulus at the junction of the mid-piece and glycogen piece is found, ending in a short end-piece, composed of the axoneme surrounded by a plasma membrane. Two types of paraspermatozoa are found, both vermiform but differing internally with respect to the disposition and number of axonemes, as well as to the types of secretory vesicles. We suggest the use of paraspermatozoa as a systematic character to reveal phylogenetic relationships in this family.     

Ultrastructure and potential taxonomic importance of euspermatozoa and paraspermatozoa in the volutid gastropods Zidona dufresnei and Provocator mirabilis (Caenogastropoda, Mollusca)

The ultrastructure of mature spermatozoa is investigated for the first time in the Volutidae, based on the commercially significant South American species Zidona dufresnei (Donovan, 1823) (fresh material) and supplemented with observations on testicular (museum) material of the deep sea New Zealand species Provocator mirabilis (Finlay, 1926). Euspermatozoa of Z. dufresnei (ex sperm duct) consist of: (1) a tall-conical acrosomal vesicle (with short basal invagination, constricted anteriorly) which is flattened anteriorly and associated with an axial rod, centrally perforate basal plate and short accessory membrane; (2) a rod-shaped, solid and highly electron-dense nucleus (with short basal fossa containing centriolar complex and initial portion of a 9 + 2 axoneme); (3) an elongate midpiece consisting of the axoneme sheathed by 5–6 helical mitochondrial elements, each exhibiting a dense U-shaped outer layer; (4) an elongate glycogen piece (axoneme sheathed by nine tracts of putative glycogen granules); (5) a dense annulus at the junction of the midpiece and glycogen piece and (6) a short free tail region (axoneme surrounded only by plasma membrane). Paraspermatozoa of Z. dufresnei are vermiform and dimorphic: the first type contains approximately 14–20 axonemes (arranged peripherally and interspersed with microtubules) and numerous oblong dense vesicles, numerous less dense (round) vesicles, occasional, large lipid-like vesicles, and scattered mitochondria; the second type contains 25–31 axonemes (peripherally arranged, interspersed with microtubules), occasional mitochondria and extensive cytoplasm. Results obtained for P. mirabilis from testis material are essentially as observed in Z. dufresnei, although the euspermatozoan acrosome still has to achieve its compressed transverse profile. Observations on paraspermatozoa were limited by fixation quality of available (testis) tissues, but these cells are similar to the first type of Zidona paraspermatozoa. Although most of the euspermatozoal features are also observed in many neotaenioglossans and neogastropods, the U-shaped outer layer of each mitochondrial element has not previously been reported and may prove a diagnostic feature of the Volutidae, the subfamily Zidoniinae or possibly only the Zidonini (in which Z. dufresnei and P. mirabilis are currently placed).