Euspermatozoa and paraspermatozoa in the volutid gastropod Odontocymbiola magellanica,Patagonia, Argentina,Southwestern Atlantic Ocean

Giménez J. 2010. Euspermatozoa and paraspermatozoa in the volutid gastropod Odontocymbiola magellanica, Patagonia, Argentina, Southwestern Atlantic Ocean. —Acta Zoologica (Stockholm) 92 : 355–362. The ultrastructure of mature spermatozoa and paraspermatozoa of Odontocymbiola magellanica is investigated. Euspermatozoa consist of: (1) a tall, conical acrosomal vesicle (with a short basal invagination, constricted anteriorly); (2) a rod‐shaped, solid and highly electron‐dense nucleus; (3) an elongate midpiece consisting of the axoneme sheathed by helical mitochondrial elements each exhibiting a dense U‐shaped outer layer; (4) an elongate glycogen piece; (5) a dense annulus at the junction of the midpiece and glycogen piece; and (6) a short free‐tail region. Paraspermatozoa of O. magellanica are vermiform and dimorphic. First type contain approximately 14–17 axonemes (arranged peripherally and interspersed with microtubules) and numerous oblong dense vesicles, numerous less dense (round) vesicles, and scattered mitochondria; the second type contains 38–45 axonemes peripherally arranged and closer to the core region of the cell and occasional mitochondria. Most of the euspermatozoal features of O. magellanica are also observed in many neotaenioglossans and neogastropods. However, the U‐shaped outer layer of each mitochondrial element has only been previously reported in the Volutidae subfamily Zidoniinae. It is now reported here in the subfamily Odontocimbiolinae and may prove to be a diagnostic feature of the Volutidae family.     

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.     

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.     

AN ULTRASTRUCTURAL STUDY OF EUSPERMATOZOA, PARASPERMATOZOA AND NURSE CELLS OF THE COWRIE CYPRAEA ERRONES (GASTROPODA, PROSOBRANCHIA, CYPRAEIDAE)

The sperm duct of the cowrie Cypraea errones Linné containseuspermatozoa, paraspermatozoa and nurse celts, the latter bearingclumps of attached euspermatozoa. Nurse cell/euspermatozoa associationshave been described in certain littorinacean gastropods, butare previously unrecorded in the Cypraeacea. Euspermatozoa ofC. errones resemble those of many other mesogastropods and someneogastropods (for example members of the Strombidae, Epitoniidae,Naticidae, Volutidae; shared features include structure of theacrosome, nucleus, glycogen piece, and helically coiled midpieceelements). However in C. errones, radially arranged paracrystallinefibres partly occupy the space between adjacent midpiece elements.Paracrystalline material is only rarely observed in spermatozoaof prosobranch gastropods and in all cases (including C. errones),is readily discernible form the complex paracrystalline layerspresent in the mitochondria) derivative of opisthobranch andpulmonate spermatozoa. Paraspermatozoa of C. errones are elongate,vermiform cells containing: multiple axonemes (bunched anteriorly,peripherally distributed posteriorly); granulated deposits (anteriorly);numerous dense vesicles (posteriorly) and scattered mitochondria(elongate, with parallel cristae). The axonemal attachment complexesfuse apically to form a sharp, conical structure. Comparisonwith available electron microscopic and light microscopic accountsof prosobranch euspermatozoa and paraspermatozoa suggests closeties between the higher Mesogastropoda (with which the Cypraeidaeshould be included) and the Neogastropoda. Further researchwill be required to determine whether the paracrystalline fibresobserved in euspermatozoa of C. errones are characteristic ofthe Cypraeidae or perhaps shared with the Ovulidae or Triviidae. (Received 26 November 1985;     

An ultrastructural examination of developing and mature paraspermatozoa in Pyrazus ebeninus (Mollusca,Gastropoda, Potamididae)

Summary The mesogastropod Pyrazus ebeninus, produces true spermatozoa (here termed euspermatozoa) and multi-flagellate, mobile cells (here termed paraspermatozoa). The mature paraspermatozoon consists of an elongateconical head (6.5–8.5 m in length), constructed of an electron-dense mosaic sheath surrounding a similarly dense, rod-shaped nuclear core (which runs almost the full length of the head). An acrosome-like structure forms the apex of the head. Five to eight axonemes are fixed to the posterior extremity of the nuclear core, each by means of an attachment complex (dense attachment rod, centriolar cap and centriole). A short (3–4 m) midpiece zone follows the head and consists of the multiple axonemes interspersed with very elongate mitochondria. A tuft of short (20 m) tails (termed minor tails) emerges from the midpiece in addition to one very long tail (termed the major tail) ensheathed in dense granules which resemble glycogen granules. A single membrane surrounds head, midpiece and tails whilst the nuclear core retains the original double nuclear membrane.Developmentally, the multiple axonemes arise from one of a pair of wheel-shaped arrangements of centrioles and attach to posterior indentations in the nucleus prior to its transformation into the nuclear core. Dense vesicles, derived apparently from the endoplasmic reticulum, accumulate along and around the developing nuclear core and (in the presence of microtubules) condense into the mosaic head sheath. Cytoplasmic mitochondria elongate and collect at the posterior axis of the cell, where, together with the axonemes, they form the midpiece.Features not previously reported in other ultrastructural studies of paraspermatozoa include the acrosome-like structure of the head, the structure of the midpiece zone, the glycogen sheath of the major tail, the dense annular structure at the junction of the midpiece and major tail and the presence of microtubules in the final phase of head and midpiece maturation. Some features of the euspermatozoon are also described and the comparative ultrastructure of mature and developing paraspermatozoa and their possible functions in the Gastropoda, are reviewed.Abbreviations ac euspermatozoon acrosomal cone - ar euspermatozoon axial rod - ax axoneme - b dense block of mosaic sheath - c centriole - cc centriolar cap - co cone of acrosome-like structure - dr dense attachment rod - dv dense vesicle - g glycogen granules - G Golgi complex - GER granular endoplasmic reticulum - H head of paraspermatozoon - m mitochondrion - M midpiece (euspermatozoon, paraspermatozoon) - maj major tail - min minor tails - mt microtubules - n nucleus - nc nuclear core - p dense plug of acrosome-like structure - pm plasma membrane - sGv small Golgi vesicles - Z transition of centriole to centriolar cap of attachment complex     

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.     

Euspermatozoa and paraspermatozoa of the relict cerithiacean gastropod,Campanile symbolicum (prosobranchia,mesogastropoda)

Euspermatozoa and paraspermatozoa ofCampanile symbolicum Iredale, 1917 — a large, relict cerithiacean from Western Australia — have been examined using transmission electron microscopy and phase-contrast light microscopy. The euspermatozoa resemble those of many other mesogastropods with the important exception that the midpiece region exhibits unusual and possibly unique features. These include possession of seven or eight straight, periaxonemal elements (each containing scattered cristae) and a closely associated sheath composed of electrondense segments which are semicylindrical in shape and longitudinally aligned. This sheath — here termed the accessory midpiece sheath-surrounds only one half of the periaxonemal midpiece elements and lies outside the mitochondrial membrane (but nevertheless within the plasma membrane). Two types of paraspermatozoa occur inCampanile: (1) those with a nuclear core within the mosaic sheath of the head (nucleate paraspermatozoa) and (2) those lacking a nuclear core (dense blocks of mosaic sheath surrounding one to three axonemes — anucleate paraspermatozoa). An acrosome-like structure forms the apex of the head in both types of paraspermatozoa, while beyond the head region, electron-dense glycogen deposits are associated with each of the multiple tails. While the form ofCampanile paraspermatozoa suggests links with families such as the Cerithiidae, Potamididae and Turritellidae, the highly unusual morphology of the euspermatozoan midpiece indicates that the Campanilidae should occupy an isolated position within the superfamily Cerithiacea.     

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     

Spermatozoon structure and spermiogenesis in Nassarius kraussianus (Gastropoda,Prosobranchia, Nassariinae)

Summary

The testis of Nassarius kraussianus (Nassariinae) produces two types of spermatozoa, a motile euspermatozoon and a non-motile paraspermatozoon. The euspermatozoon is filiform and about 95/μm long. The elongated head (40 μm long) is comprised of a slender nucleus (about 0.5 μm diameter) which is penetrated throughout by an intranuclear canal housing the anterior portion of the axoneme. A short (about 2 μm long) conical acrosome surmounts the nucleus anteriorly. The mid-piece (23 μm in length) consists of six to seven modified mitochondria which are helically arranged around the axoneme. Posterior to the mid-piece the tail is composed of a short glycogen piece and an end piece. The paraspermatozoon is spindle-shaped (about 50 μm long) and contains multiple (16–20) axonemes the basal bodies of which fuse anteriorly. Posteriorly, numerous small mitochondria and electron-dense bodies lie between the axonemes. Structural changes during eu- and paraspermiogenesis mirror those described for other species of gastropod mollusc with dimorphic spermatozoa. However unlike other molluscs, the cytoplasmic bridges which connect developing spermatids contain well developed stacks of endoplasmic reticulum which form a continuum with that in the cytoplasm of the spermatids. These structures may in some way facilitate the synchronous development of the spermatozoa.     

Ultrastructure of Euspermiogenesis in the Mesogastropod Stenothyra sp. (Prosobranchia, Rissoacea, Stenothyridae)

The structure of mature and developing euspermatozoa of the rissoacean gastropod Stenothyra sp. has been studied using transmission electron microscopy. During cuspermiogenesis nuclei pass through fibrillar and lamellar phases of condensation. A Golgi-derived acrosome attaches to the nucleus during the fibrillar phase. Spherical mitochondria of early euspermatids fuse to form the mitochondrial sheath which undergoes metamorphosis to form helical midpiece elements, paracrystalline material and helical midpiece compartments. Mature euspermatozoa consist of a flat acrosome (acrosomal cone, axial rod, basal plate), short curved nucleus (2.5–2.8 μm) and elongate midpiece and glycogen piece. Coarse fibres associated with the axoneme emerge from a posterior invagination of the nucleus and continue into the initial portion of the midpiece. In the proximal portion of the midpiece, two helical compartments (filled with membranous material) are present—only one of which persists further posteriorly. No compartments occur in the distal region of the midpiece. Posterior to the midpiece, the axoneme is surrounded by tightly-packed (glycogen) granules and terminates within this region. The distal end of the euspermatozoon consists solely of glycogen granules surrounded by the plasma membrane. Although coarse fibres (associated with the axoneme), midpiece paracrystalline material and helical compartments are commonly reported in sperm of euthyneuran gastropods, this represents the first report of all three features in any prosobranch euspermatozoon.     

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     

COMPARATIVE SPERM MORPHOLOGY OF THE PULMONATE LIMPETS TRIMUSCULUS COSTATUS, T. RETICULATUS (TRIMUSCULIDAE) AND BURNUPIA STENOCHORIAS AND ANCYLUS FLUVIATILIS (ANCYLIDAE)

Sperm ultrastructure is described for the first time in representativesof the pulmonate ‘limpet’ families Trimusculidae(Trimusculus costatus, T. reticulatus: marine) and Ancylidae(Burnupia stenochorias, Ancylus fluviatilis: freshwater). Allshow characteristic heterobranch sperm features (a spheroidalacrosomal vesicle supported by an acrosomal pedestal; a helicallykeeled nucleus and a complex, very elongate midpiece featuringparacrystalline and matrix layers sheathing the axoneme, coarsefibers and one or more glycogen helices). Posterior to the midpiece,a glycogen piece (axoneme sheathed by glycogen granules) andannulus are also present in all species. Taxonomically usefuldifferences in the shape and dimensions of the acrosome, nucleusand midpiece occur between the species. Results support thedecision of recent workers to transfer the Trimusculidae fromthe Siphonarioidea to a separate superfamily Trimusculoidea(characteristic sperm features including: narrow acrosomal pedestaloverlapping with nuclear apex; heavily keeled nucleus; midpiecewith strongly projecting secondary and glycogen helices). Therelationship of the Trimusculoidea to other pulmonates, as indicatedby sperm ultrastructure, remains uncertain largely because comparativedata for several important groups are unavailable. Spermatozoaof the two ancylids most closely resemble those of other investigatedplanor-boideans and to a lesser extent, those of the Lym-naeoidea.However, differences between Burnupia stenochorias (unique(?)accessory structure on the acrosomal pedestal; glycogen wedgeswithin the nuclear fossa; other features similar to planorbids)and Ancylus fluviatilis (all sperm features very similar toplanorbids) suggests that these patelliform ancylids are notclosely related. (Received 20 November 1997; accepted 23 January 1998)     

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.     

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)     

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.     

Ultrastructural aspects of spermatogenesis in Calyptogena pacifica Dall 1891 (Vesicomyidae; Bivalvia)

The process of spermatogenesis and spermatozoon morphology was characterized from a deep‐sea bivalve, Calyptogena pacifica (Vesicomyidae, Pliocardiinae), a member of the superfamily Glossoidea, using light and electron microscopy. Spermatogenesis in C. pacifica is generally similar to that in shallow‐water bivalves but, the development of spermatogenic cells in this species has also some distinguishing features. First proacrosomal vesicles are observed in early spermatocytes I. Although, early appearance of proacrosomal vesicles is well known for bivalves, in C. pacifica, these vesicles are associated with electron‐dense material, which is located outside the limiting membrane of the proacrosomal vesicles and disappears in late spermatids. Another feature of spermatogenesis in C. pacifica is the localization of the axoneme and flagellum development. Early spermatogenic cells lack typical flagellum, while in spermatogonia, spermatocytes, and early spermatids, the axoneme is observed in the cytoplasm. In late spermatids, the axoneme is located along the nucleus, and the flagellum is oriented anteriorly. During sperm maturation, the bent flagellum is transformed into the typical posteriorly oriented tail. Spermatozoa of C. pacifica are of ect‐aqua sperm type with a bullet‐like head of about 5.8 μm in length and 1.8 μm in width, consisting of a well‐developed dome‐shaped acrosomal complex, an elongated barrel‐shaped nucleus filled with granular chromatin, and a midpiece with mainly four rounded mitochondria. A comparative analysis has shown a number of common traits in C. pacifica and Neotrapezium sublaevigatum.     

TRANSFER OF THE GASTROPOD FAMILY PLESIOTROCHIDAE TO THE CAMPANILOIDEA BASED ON SPERM ULTRASTRUCTURAL EVIDENCE

Using sperm ultrastructure the systematic placement and affinitiesof the caenogastropod family Plesiotrochidae are re-examined.The simultaneous hermaphrodite, Plesiotrochus crinitus Thiele,1930, produces both euspermatozoa (uniflagellate, fertile sperm)and paraspermatozoa (bi- or triflagellate, infertile sperm).Features of each type of sperm clearly indicate that the Plesiotrochidaeare closely related to the Campanilidae (Campaniloidea) andare not, as previously believed, referable to the superfamilyCerithioidea. Significant sperm synapomorphies of Plesiotrochus(Plesiotrochidae) and Campanile (Campanilidae) include the morphologyof the eusperm midpiece (seven to nine straight mitochondriasurrounded by a segmented, accessory sheath of membrane-boundvesicles) and morphology of the anucleate parasperm head (axialcore of mitochondria surrounded by a bilaterally symmetricalarrangement of axonemes and dense vesicles). The characteristicsubstructure of the cerithioidean eusperm midpiece (four straightmitochondria each containing parallel, cristal plates) is notobserved in Plesiotrochus or Campanile. Euspermatozoa of Plesiotrochusdiffer from Campanile principally in details of the acrosomalcomplex (Plesiotrochus with apical bleb, probable absence ofan accessory membrane; Campanile without apical bleb, accessorymembrane well developed), the transverse profile of all midpiecemitochondria (thin in Plesiotrochus; thick in Campanile), andmorphology of the annulus (double ring in Plesiotrochus; singlering in Campanile). In addition, all observed paraspermatozoaof Plesiotrochus are anucleate, whereas in Campanile anucleateand nucleate paraspermatozoa are present. On the basis of spermsynapomorphies of Plesiotrochus and Campanile, the Plesiotrochidaeare transferred from the Cerithioidea to the Campaniloidea. (Received 3 August 1992; accepted 18 September 1992)     

Ultrastructure of spermatogenesis in Cerastoderma glaucum (Cardiacea) and Spisula subtruncata (Mactracea)

Summary

In Cerastoderma glaucum, Sertoli cells are rich in lipids, glycogen and lysosomes, and premeiotic cells exhibited nuage, a prominent Golgi complex and endoplasmic reticulum cisternae encircling the nucleus. The Golgi complex gives rise to proacrosomal vesicles during mid-spermiogenesis, and the round acrosomal vesicle, with a dense fibrillar core, migrates laterally while linked to the plasma membrane as it develops the subacrosomal material. In its final position, the vesicle becomes cap-shaped (0.6 μm) and differentiates into apical light and basal dense regions. The elongated and helicoidal nucleus (8–9.9 μm) has a thin tip (0.3 μm) that invades the subacrosomal space, and in the midpiece (0.8 μm) two of the four mitochondria extend laterally to the nucleus (1.5–2.1 μm). In Spisula subtruncata, Sertoli cells are rich in lipids, glycogen and phagocytosed sperm. Premeiotic cells exhibit nuage, a prominent Golgi complex that gives rise to proacrosomal vesicles from the leptotene stage and a flagellimi that is extruded at zygotene. The acrosomal vesicle forms during the round spermatid stage and differentiates into a large and dense basal region and an apical light region. It then migrates while linked to the plasma membrane by its apical pole. Development of the subacrosomal perforatorium is associated with nuage materials and endoplasmic reticulum vesicles. The mature cap-shaped (0.6 μm) acrosomal vesicle exhibits a large apical and irregular region with floccular contents and a basal dense region. The round nucleus becomes barrel-shaped (1.5 μm) and the midpiece (0.8 μm), with four mitochondria, contains a few glycogen particles.     

Euspermatozoa,Paraspermatozoa and Spermatozeugmata of Littoraria (Palustorina) articulata (Prosobranchia: Caenogastropoda) with Special Reference to the Pseudotrich

Abstract Paraspermatozoa and euspermatozoa of the littorinid gastropod Littoraria (Palustorina) articulata are examined using transmission electron microscopy and light microscopy. In the seminal vesicle, both sperm types occur, either as free cells or organized into spermatozeugmata. It is shown that the elongate (120–140 μm), flagellum-like component of the paraspermatozoon is in fact a tubular extension of the plasma membrane which encloses granular material but no axonemes or microtubules. This structure, here termed the pseudotrich, shows no evidence of motility and its function remains obscure. The main body region of the paraspermatozoon (length 32–36 μm) contains numerous spherical vesicles, scattered mitochondria, one or two large, rod-shaped bodies (length 20–24 μm) and a fusiform, granular body (containing DNA; probably a modified nucleus). The rod-shaped bodies, granular body and surrounding matrix are contained by a common membrane, and are therefore separated from other contents of the paraspermatozoon. In each spermatozeugma, euspermatozoa are attached via the tips of their acrosomes to the paraspermatozoan body at the opposite end to the pseudotrich. Euspermatozoa exhibit a conical acrosomal complex (with axial rod and basal plate), a tubular nucleus sheathing the axoneme, a midpiece (5–6 helical mitochondrial elements sheathing the axoneme), an annulus (with two rings), a glycogen piece and an end piece (total sperm length 268–272 μm). The euspermatozoa of L. articulata are similar to those of most littorinids and many other caenogastropods. The presence of a pseudotrich in the paraspermatozoon appears to be restricted to the subgenus Palustorina.