Ultrastructure of spermiogenesis in the gastropodHeliacus variegatus (Architectonicidae), with description of a banded periaxonemal helix

Spermiogenesis and spermatozoa of the marine gastropodHeliacus variegatus Gmelin were examined using transmission electron microscopy (TEM). Mature spermatozoa are composed of an acrosomal vesicle, helical nucleus, elongate midpiece, annulus, glycogen piece and short end piece. The midpiece consists of a 9+2 axoneme, nine coarse fibres, and a banded helix, all enclosed by a continuous mitochondrial sheath (with multiple, helically coiled grooves). Anterior extensions of the mitochondrial sheath and banded helix form a double sheath around the basal half of the nucleus—an arrangement possibly unique in the Mollusca. During spermiogenesis, dense plaques delineating the anterior and posterior poles of the spermatid nucleus become attachment sites for the acrosomal vesicle and the axial complex (respectively). As the nucleus condenses and elongates, midpiece formation involves fusion of numerous, oblong mitochondria along the length of the axoneme. The coarse fibres and banded helix of the midpiece probably are derived through centriolar activity. Results of the study support inclusion of the Architectonicidae within the Heterobranchia, but in view of midpiece specializations, do not clarify the precise relationship of the family within this subclass.     

Fine structure of spermatids and sperm of Dolioletta gegenbauri and Doliolum nationalis (Tunicata: Thaliacea): implications for tunicate phylogeny

In Dolioletta gegenbauri and Doliolum nationalis, collected in 1987 in the bay of Villefranche-sur-mer (French Mediterranean Sea), spermiogenesis is essentially the same. Early spermatids have a round head, a flagellum arising from a single centriole with short microtubules at 45° to its base, several mitochondria, and an acrosome 50 nm thick and 250 nm long with its long axis parallel to the plasma membrane. The acrosomal contents are dense, with a central denser plate. The nuclear envelope next to the acrosome is thickened and concave. In elongating nuclei, strands of chromatin become oriented parallel to the length of the nucleus and then twist helically. Although the mitochondria surround the nucleus, they remain relatively short and do not fuse into a single mitochondrion as in sperm of other tunicates. In very late doliolid spermatids, the acrosome undergoes exocytosis, and exposes fibrous material that stays associated with the tip of the sperm; no acrosomal tubule forms. Exocytosis at this stage may be triggered by fixation. If so, exocytosis probably occurs naturally at some time before fusion of sperm and egg. Sperm have elongate heads (1 m×10 m), with the anterior two-thirds of the nucleus surrounded by mitochondria. Spermiogenesis in doliolids, compared to that in other tunicates, is most like that in solitary members of the class Ascidiacea, except that in the latter the sperm mitochondria fuse and the acrosome appears incapable of exocytosis. In contrast, previous work has shown that salps (class Thaliacea) and colonial didemnid ascidians have an acrosomeless sperm with a spiral mitochondrion, while the class Appendicularia has a sperm with a midpiece, a compact head and an acrosome capable of exocytosis and acrosomal tubule formation. By outgroup comparison with echinoderms and acraniates, appendicularian sperm are plesiomorphic within the Tunicata. Thus, gamete morphology indicates that (1) solitary ascidians and doliolids had a common ancestor, (2) the popular idea that doliolids gave rise to appendicularians is incorrect, and (3) the Thaliacea are polyphyletic, doliolids having arisen very early from the ascidian lineage and salps having arisen later.     

Acrosome differentiation and the acrosome reaction in ascidian spermatozoa:<Emphasis Type="Italic"> Ascidiella aspersa</Emphasis> and<Emphasis Type="Italic"> Ascidia mentula</Emphasis> with some implications for tunicate phylogeny

The spermatozoa of both Ascidiella aspersa and Ascidia mentula have architectural features characteristic of ascidian spermatozoa that have previously been described. They have an elongated head (7 µm long for A. aspersa and 4 µm long for A. mentula), a single mitochondrion that is applied laterally to the nucleus and lacks a midpiece. The acrosome of A. aspersa spermatozoa is a flattened vesicle, about 200 nm×100 nm×40 nm (length, width and height). The acrosome of A. mentula spermatozoa consists of multiple vesicles; they are about 50 nm×50 nm×40 nm (length, width and height). During spermiogenesis in both species, several proacrosomal vesicles (50–70 nm in diameter) appear in a blister at the future apex of the spermatid. In A. aspersa, these vesicles fuse with each other to form a single acrosomal vesicle, while in A. mentula these vesicles do not fuse with each other, and form multiple acrosomal vesicles. In A. aspersa spermatozoa, calcium ionophore A23187 induces the acrosome reaction in which membrane fusion between the acrosomal apical membrane and the overlying sperm plasma membrane occurs along the peripheral margin of the acrosome, resulting in the release of a hybrid, membrane-bound, small vesicle. In A. mentula, multiple acrosomal vesicles disappear by releasing small vesicles after treatment with the calcium ionophore A23187; this also appears to be an acrosome reaction. This paper discusses the way in which acrosome structure and function may have changed during the evolution of the Tunicata.Communicated by T. Ikeda, Hakodate     

Spermatozoon structure of some North American prosobranchs from the families Lottiidae (Patellogastropoda) and Fissurellidae (Archaeogastropoda)

The spermatozoa of four species of the patellogastropod family Lottiidae (Lottia pelta, L. digitalis, L. strigatella, Tectura scutum) and one species of the archaeogastropod family Fissurellidae (Diodora aspera) were examined in 1990 using transmission electron microscopy. All have primitive or ect-aquasperm, typical of invertebrates using external fertilization. Sperm of the lottiid limpets are characterized by a 5 to 9 m-long head composed of a conical acrosome which constitutes >50% of the head length, and a cylindrical nucleus. The acrosome of all species of lottiids is differentiated internally, and has a posterior invagination 0.9 to 1 m in depth, into which an elongate acrosomal lobe protrudes. Between the posterior acrosomal lobe and the nucleus, the subacrosomal material is aggregated as a fibrous column. The midpiece of the sperm has a ring of 4 to 5 spherical mitochondria of 0.6 m diam, posterior to which is a collar of cytoplasm 1 m long, which sheaths the anterior portion of the axoneme. The size and morphology of the acrosome and large cytoplasmic collar clearly distinguish the spermatozoa of the Lottiidae from other families of Patellogastropoda. The sperm of D. aspera (Fissurellidae) is typical of the family of archaeogastropod; the head has a length to breadth ratio of 4:1, and the cylindrical nucleus is capped by a small acrosome, <25% of the total head length, which is deeply invaginated.     

Spermatozoan ultrastructure and spermatogenesis in aplousobranch ascidians,with some phylogenetic considerations

Spermatozoa and some stages of spermatogenesis were studied for four species of aplousobranch ascidians. Spermatozoa of Clavelina lepadiformis (Müller) (family Clavelinidae) are plesiomorphous in that they have apical acrosomal vesicles and a moderately elongated, cylindrical nucleus. The elongated mitochondrion is twisted ea. 11/2 times around the nucleus. In their ultrastructural morphology, C. lepadiformis sperm conform in some respects to the least-derived ascidian sperm, those of the phlebobranch ascidian Ciona intestinalis L.; however, the sperm of Clavelina lepadiformis have two apomorphies not shared with those of Ciona sp.: (1) the mitochondrion of Clavelina lepadiformis is long and spiralled along the entire nucleus rather than being comparatively compact and not at all helical; (2) the mitochondrial cristae are elongated parallel to the long axis of the nucleus, whereas in Ciona sp. sperm the cristae are unmodified. In Distaplia sp., Aplidium sp. and Synoicum pulmonaria (Ellis and Solander) the spermatozoa are more derived and consist of a proximal cylindrical and a distal corkscrew-like part. The mitochondrion in Distaplia sp. and Aplidium sp. contains electron-dense material and extends in a long thread around the nucleus. In S. pulmonaria the mitochondrion surrounds the anterior part of the nucleus in mature spermatozoa, and an elongated, dense structure displaying fine striation is enclosed in the mitochondrion in late spermatids. The sperm ultrastructural morphology observed in this study is consistent with the majority view that clavelinids are closest to the ancestral ascidian but is also consistent with other conclusions, particularly that the Cionidae are closest to the stem ascidian.     

Spermatozoa and spermatogenesis in a monoplacophoran mollusc,Laevipilina antarctica: ultrastructure and comparison with other Mollusca

Ultrastructural features of spermatozoa and spermatogenesis are described for the first time in a monoplacophoran and compared with data for other conchiferan molluses. Spermatozoa of Laevipilina antarctica Warén and Hain, 1992, are of the structurally simple, aquasperm type, featuring a conical acrosome, a compact nucleus with lacunae, a short midpiece and a single flagellum. The acrosomal vesicle shows an electron-dense inner zone, and a basal invagination (subacrosomal space) contains granular material but no axial rod. The nucleus exhibits a shallow indentation apically which contains subacrosomal material, and five (sometimes four) indentations posteriorly which partially accommodate the five (rarely four) midpiece mitochondria. Two centrioles are present, the distal connected to the annulus by satellite fibres and acting as a basal body for the flagellum (axoneme probably 9+2 structure). Spermatogonia, characterized by an oblong nucleus and one or two nucleoli, line the basal membrane of the testis wall; spermatids of varying stages of maturity occupy the remainder of the testis. Acrosome and flagellum production is already well advanced in spermatids and probably commences at the spermatocyte stage. Cytoplasmic bridges occur in all developmental stages, most visibly in spermatids. The spermatid chromatin condenses in large tracts, leaving electron-lucent lacunae. Mitochondria collect posteriorly and form, presumably by fusion, the five (or four) larger, spherical mitochondria which gather around the centrioles.     

Ultrastructure of sperm and spermathecae in Micromaldane spp. (Polychaeta: Capitellida: Maldanidae)

Assessing the possibility that external fertilization has re-evolved requires the study of monophyletic groups that exhibit various reproductive methods. Maldanid polychaetes show a range of reproductive mechanisms, though previous studies of reproduction have hitherto been restricted to larger species with external fertilization. Micromaldane pamelae Rouse and M. nutricula Rouse are small, gonochoristic maldanids that brood directly developing larvae. Both species have sperm with elongate nuclei and an acrosome extending down each side of the anterior end of the nucleus. A true midpiece is absent; two mitochondria extend along the posterior region of the nucleus. Spermatids develop synchronously in large clusters connected by a cytophore. In M. pamelae sperm are released into the water as spermatozeugmata. These are comprised of clusters of sperm with their tails oriented to the centre and the sperm heads facing outwards. Females of M. pamelae and M. nutricula bear pairs of spermathecae ventrally (M. pamelae three pairs, between setigers 10 and 11, 11 and 12 and 12 and 13 and M. nutricula two pairs, between setigers 10 and 11 and setigers 11 and 12). The blind sacs are epidermal invaginations bound closely together. The entrance to each spermatheca may only be 1 to 2 m across with each spermatheca holding several hundred sperm. This represents the first detailed study of spermathecae in the Capitellida. The occurrence and structure of spermathecae and spermatozeugmata in other groups are discussed and compared with Micromaldane spp. Comparisons are made with non-polychaetes with the purpose of discussing functional aspects of reproductive mechanisms in marine metazoans in general. Elongate sperm nuclei are associated with sperm storage and/or large egg size. The lack of an elongate sperm midpiece may be an indicator of having to swim in water but does not contraindicate sperm storage. Spermatozeugmata may serve as an indication of sperm storage and brooding of larvae. Speculations on the phylogenetic significance of these reproductive features are limited by the fact that supposedly modified (i.e., derived) states may reflect functional/structura, constraints of small body size.     

Sperm quantity and quality effects on fertilization success in a highly promiscuous passerine, the tree swallow Tachycineta bicolor

Sperm competition is widespread among animal taxa and considered a major force in sperm evolution. Recent comparative studies have indicated that sperm competition selects for high sperm production capacity and long and fast-swimming spermatozoa across species. Here, we examine the role of sperm quantity and quality for fertilization success of individual males in a Canadian population of tree swallows Tachycineta bicolor, a socially monogamous, but highly promiscuous passerine. Male fertilization success (the sum of withinpair and extrapair young) was significantly associated with the size of the cloacal protuberance (a proxy for sperm quantity), but not with sperm size or in vitro sperm swimming speed. In a multivariate analysis, both cloacal protuberance volume and relative sperm midpiece size (i.e. high mitochondrial loading) had significant effects on male fertilization success. However, relative sperm midpiece size was not associated with fertilization success in a simple regression. Further, both cloacal protuberance volume and relative midpiece size had significant effects on sperm velocity, both in simple regressions and in a multivariate analysis. The finding that males with large relative midpiece size had both higher fertilization success and faster swimming sperm, suggests an indirect link between sperm morphology and male fertility mediated through sperm velocity. In conclusion, both quantitative and qualitative sperm traits seem to affect male fertilization success in tree swallows.     

渔游蛇精子的超微结构

扫描电镜观察表明,渔游蛇（Ｎａｔｒｉｘ Ｐｉｓｃａｔｅｒ）精子总长度约１１０μｍ,分为头和尾两部分;头部呈前稍尖的细长圆柱形,略变曲,约长８．５μｍ,直径以经约０．６μｍ。尾部又分４段：颈段、中段、主段和末段;其与从不同之处是中段较长,约７３μｍ,占精子总长度的２／３多;线料体型鞘由近５００圈超过５０００个线粒体构成;透射电镜揭示了头部顶体体覆疬于核前约１／２;尾部颈段的结构较复杂,包括连接片的踝     

Structure of the sperm of some South African archaeogastropods (Mollusca) from the superfamilies Haliotoidea,Fissurelloidea and Trochoidea

The structure of the spermatozoa of 11 species from five families of archaeogastropod (Haliotidae, Fissurellidae, Trochidae, Turbinidae, Phasianellidae) has been examined using transmission electron microscopy. All sperm are of the primitive, or ect-aquasperm type and each species has a sperm head (nucleus and acrosome) with unique morphology. Furthermore, the results from the present study together with those published in the literature reveal that, although there are a few exceptions, the size (length to breadth ratio) and spape of the nucleus and acrosome of sperm of species within each family are similar. It is therefore possible to identify members of a family and differentiate between families using sperm morphology. The intrafamilial similarities and interfamilial differences in sperm structure indicate that a spermiocladistic study of the suborder Vetigastropoda could provide interesting insights into the phylogenetic relationships of this group.     

Spermiogenesis and spermatozoa in the relict bivalve genus Neotrigonia: relevance to trigonioid relationships,particularly Unionoidea

Spermiogenesis and spermatozoa in the relict trigonioid genus Neotrigonia are examined ultrastructurally. Mature Neotrigonia spp. spermatozoa exhibit the following features: (1) a blunt-conical nucleus; (2) an acrosomal complex composed of discoidal vesicles (9 to 15) arranged as a thin layer over the nuclear apex; (3) five (rarely four) spherical mitochondria positioned in depressions at the base of the nucleus; (4) proximal and distal centrioles (surrounded by the mitochondria); (5) a satellite complex anchoring the distal centriole and flagellum to the plasma membrane; and (6) a single flagellum. Spermatozoa of unionoids examined to date (Unionidae and Hyriidae only) exhibit similar features, including a nucleus with a blunt apex capped by a thin acrosomal complex (showing up to three vesicles). Among the Bivalvia, only Neotrigonia spp. and unionoid sperm possess an acrosomal complex composed of multiple vesicles. These data suggest that the Trigonioidea and Unionoidea are phylogenetically linked, but further work particularly on primitive unionoids is required to determine if this relationship is one of common ancestry or whether the Unionoidea have in fact been derived from the Trigonioidea.     

Variation in sperm morphometry and sperm competition among barn swallow (Hirundo rustica) populations

Spermatozoa vary greatly in size and shape among species across the animal kingdom. Postcopulatory sexual selection is thought to be the major evolutionary force driving this diversity. In contrast, less is known about how sperm size varies among populations of the same species. Here, we investigate geographic variation in sperm size in barn swallows Hirundo rustica, a socially monogamous passerine with a wide Holarctic breeding distribution. We included samples from seven populations and three subspecies: five populations of ssp. rustica in Europe (Czech, Italy, Norway, Spain, and Ukraine), one population of ssp. transitiva in Israel, and one population of ssp. erythrogaster in Canada. All sperm traits (head length, midpiece length, tail length, and total length) varied significantly among populations. The variation among the European rustica populations was much lower than the differences among subspecies, indicating that sperm traits reflect phylogenetic distance. We also performed a test of the relationship between the coefficient of between-male variation in total sperm length and extrapair paternity levels across different populations within a species. Recent studies have found a strong negative relationship between sperm size variation and extrapair paternity among species. Here, we show a similar negative relationship among six barn swallow populations, which suggests that the variance in male sperm length in a population is shaped by the strength of stabilizing postcopulatory sexual selection.     

Spermatogenesis inPyrosoma atlanticum (Tunicata: Thaliacea: Pyrosomatida): Implications for tunicate phylogeny

Colonies ofPyrosoma atlanticum were collected by submersible in October 1988 in the Caribbean Sea, and testes were studied by electron microscopy. Spermatogonia, spermatocytes and early spermatids have two centrioles. The proximal centriole subsequently disappears, its remains apparently persisting in the spermatozoon as dense material adjacent to the distal centriole, which gives rise to the axoneme. At the tip of early spermatids are several 50 nm proacrosomal vesicles, which disappear leaving no trace in early elongating spermatids. The spermatozoon lacks an acrosome and has a head 35µm long. The head is differentiated into a bulbous posterior portion 5µm long × 1µm wide, a thinner anterior portion 25µm long tapering from a width of 0.7µm to a width of 0.4µm, and a very thin anterior extension 5µm long × 0.5µm wide. At the start of elongation, the anterior extension begins to form just lateral to the proacrosomal vesicles as a spiral projection comprising part of the nucleus, covered by a thin sheath of cytoplasm. This sheath of cytoplasm undergoes a complex differentiation. Ultimately, the nucleus in the anterior extension is overlain by two membrane-bound sheaths of cytoplasm connected by a spiral flange of cytoplasm. Between these two sheaths is a spiral space, open to the exterior through a subterminal pore near the sperm tip. In early spermatids the mitochondria fuse into a single mitochondrion, which remains lateral to the nucleus. The cristae become modified late in spermatogenesis. Throughout elongation of the spermatid there are patches of dense material between the nucleus and mitochondrion. A manchette of microtubules transiently encircles the thin anterior portion of the nucleus during the last phase of elongation. A manchette is not present during most of elongation. In the spermatozoon the mitochondrion, which has reticulate cristae, spirals a few times about the nucleus and extends from the junction between the bulbous portion and the thinner anterior portion of the nucleus to the junction between the thinner anterior portion and the nuclear extension. Spermatogenesis inP. atlanticum, compared to that in other tunicates, most closely resembles that in colonial ascidians, and supports the majority view that pyrosomes arose from aplousobranch ascidians that lost their attachment to the substratum. Pyrosome sperm are more highly derived than doliolid sperm, which have an acrosome that is probably capable of exocytosis. When salp and pyrosome sperm are compared, both are highly derived, but neither shares any apomorphies with the other that it does not share with at least one other tunicate order. Thus, sperm morphology does not support the majority view that pyrosomes gave rise to doliolids and neither confirms nor denies the idea that pyrosomes are intermediate between aplousobranch ascidians and salps. Therefore, it is likely that the class Thaliacea is polyphyletic, with doliolids arising very early from the ascidian lineage and with salps and pyrosomes arising somewhat later.     

Sperm morphology, swimming velocity, and longevity in the house sparrow Passer domesticus

Sperm competition exerts strong selection on males to produce spermatozoa with an optimal morphology that maximizes their fertilization success. Long sperm were first suggested to be favored because they should swim faster. However, studies that investigated the relationship between sperm length and sperm competitive ability or sperm swimming velocity yielded contradictory results. More recently, ratios of the different sections of a spermatozoon (the head, midpiece, and flagellum) were suggested to be more crucial in determining swimming velocity. Additionally, sperm ability to remain and survive in the female storage organs may also influence fertilization success, so that optimal sperm morphology may rather maximize sperm longevity than velocity. In this study, we investigated how sperm morphology is related to sperm velocity and sperm longevity in the house sparrow Passer domesticus. Sperm velocity was found to be correlated with head/flagellum ratio. Sperm with small heads relative to their flagellum showed higher swimming velocity. Additionally, shorter sperm were found to live longer. Finally, we found sperm morphological traits to vary substantially within males and the head/flagellum ratio to be unrelated to total sperm length. We discuss the hypothesis that the substantial within-male variation in sperm morphology reflects a male strategy to produce a diversity of sperm from long, fast-swimming to short, long-living sperm to maximize their fertilization success in a context of sperm competition.     

Spermatogenesis,sperm storage and comparative sperm morphology in nine species of Capitella,Capitomastus and Capitellides (Polychaeta: Capitellidae)

Light microscopy and transmission electron microscopy (TEM) were used to describe spermatogenesis and the morphology of mature sperm and sperm storage organs in five sibling species of Capitella, three species in the related genus Capitomastus, and one species in the genus Capitellides. These capitellids lack a well-developed testis, but young males have a few specialized regions of the peritoneum in the eighth setiger, where germ cells proliferate and spermatogonia are released into the coelom, and spermiogenesis is completed. Mature sperm are stored in the central regions of paired genital ducts (coelomoducts), which lie between the seventh and eighth setigers. The cells forming the walls of the coelomostome and central region of the duct are ciliated and have large glycogen deposits. The lumenal borders have extensive microvilli and there is evidence that they secrete glycogen-containing materials into the duct. All species have modified primitive sperm with a conical acrosome, elongated nucleus, and long middle piece extending along the proximal portion of the flagellum. A single ring-shaped mitochondrion encircles the centriolar region of the middle piece and the cytoplasm is filled with glycogen. The sperm of all nine species differ significantly in the lengths of their middle pieces, acrosomes and especially in their nuclear lengths. The nuclear lengths have a twofold range among the sibling species of Capitella and Capitomastus. Subtle differences in the shape and volume of the acrosomal vesicle and acrosomal space characteristic of the Capitella sibling species seem to correlate with a basic division of these species into those with diploid chromosome numbers of 20 or 26. Spermiogenesis, the number of sperm produced, and the method of sperm storage are appropriate for efficient sperm utilization in fertilization. No evidence indicates that spermatophores are formed and transferred between individuals and the method of sperm transfer is not understood. The differences in the dimensions and acrosome morphology of mature sperm, and the previously demonstrated specializations in the egg envelopes in the Capitella sibling species, are characteristic features of the reproductive isolation that exists among these capitellid species.     

Functional variation of sperm morphology in sticklebacks

Some theoretical models of sperm competition make the assumption that in fish species with external fertilisation, sperm length relates positively to swimming speed at the expense of sperm longevity. Few studies have tested this assumption. We used the three-spined stickleback, Gasterosteus aculeatus L., to study functional sperm morphology. In this study, the relationship between males’ mean sperm length and fertilisation rate was investigated in vitro in a non-competitive situation. Fertilisation at different time points after sperm release was taken into account, and sperm morphology was quantified from scanning electron microscopy images. The time series of artificial fertilisations demonstrated that males which produced sperm with a longer tail fertilised faster, but their sperm had a shorter lifespan (or activity period). It was further suggested that males that produced sperm with a larger midpiece had greater fertilisation chances later on in the fertilisation process. Thus, in sticklebacks, there exists functional variation in sperm morphology, and sperm tail length is traded off against sperm longevity (or activity).     

Sexual reproduction in the compound ascidian Diplosoma listerianum (Tunicata). I. Metamorphosis,storage and phagocytosis of sperm in female duct

In the colonial ascidian Diplosoma listerianum (collected in the Lagoon of Venice in 1986 and 1991), ovulating eggs detach themselves from the ovary wall and segregate in the tunic without exposure to seawater. With the aim of finding evidence of the pathway followed by spermatozoa to approach the oocyte, histological and ultrastructural observations were made. The results showed that the hollow ovary elongates in a fertilization canal which flanks the sperm duct and opens externally, next to the anus. Intercellular extended tight junctions isolate the lumen of this canal from the blood. Sperm were found in the fertilization canal, both free and in intracellular vacuoles of wandering cells (phagocytes). These sperm showed differences in comparison to those from the sperm duct, which are possible signs of a sperm reaction. The main modifications were shown by the dense groove, a narrow invagination of the plasmalemma bound to the nuclear envelope by dense material. In non-modified sperm, the groove runs spirally all along the head, while in those in the fertilization canal, it coils in the anterior half of the head, pressing back the long mitochondrion and endoplasmic tubules. These modifications were interpreted to be result of the release of the dense groove, considered to be a sort of stretched, contractile spring. The presence of sperm in the fertilization canal and particularly in phagocytes is discussed in relation to the capacity for exogenous sperm storage by zooids and to the necessity of waste clearance in order for successive waves of fresh sperm to be able to move towards fully grown oocytes. The origin of the female fertilization canal, the role of its tight junctions, and the complex sperm morphology are discussed as adaptations to internal fertilization in D. listerianum.     

Dorippids are Heterotremata: Evidence from ultrastructure of the spermatozoa ofNeodorippe astuta (Dorippidae) andPortunus pelagicus (Portunidae) Brachyura: Decapoda

Ultrastructural comparison between the sperm of the dorippid crabNeodorippe astuta (Fabricius, 1793) and the portunidPortunus pelagicus (Linnaeus, 1766) from Queensland, Australia, supports placement of dorippids with portunids and their relatives in the heterotreme section of the Eubrachyura (the Heterotremata - Thoracotremata or the Oxyrhyncha - Cancridea - Brachygnatha assemblage) and not withRanina ranina (in the Archaeobrachyura or the Oxystomata). Similarities between spermatozoa ofN. astuta and ofP. pelagicus (and other Eubrachyura) andR. ranina include: the large spherical, multi-layered, capsule-bound acrosome vesicle; the electron-dense operculum capping the vesicle; an invaginated core, or perforatorium; concentric zonation of the contents of the vesicle; a layer of cytoplasm, between the acrosome vesicle and the nucleus, which contains mitochondria (mostly degenerating) and lattice-like lamellar complexes or membrane remnants; a diffuse nucleus which is bounded externally by a combined nuclear and plasma membrane and cups the scanty cytoplasm and the large acrosome vesicle; and lateral arms into which the chromatin extends. Characteristic eubrachyuran features of theN. astuta sperm absent fromR. ranina are the long perforatorium (short and conical with a unique subacrosomal chamber inR. ranina) extending almost to the operculum; presence in the perforatorium of longitudinally arranged convoluted tubules; a zone of acrosomal rays forming the outer part of an inner dense zone; the presence of a thickened ring surrounding the basal part of the perforatorium; and, basally, two centrioles (absent fromR. ranina but also from some eubrachyurans). The sperm ofN. astuta is more similar to that ofP. pelagicus than to that of other investigated Brachyura. A heterotreme status ofN. astuta is thus unequivocally supported. Both species lack the posterior median process seen in the nucleus of majids andR. ranina.     

Sex in the beach: spermatophores, dermal insemination and 3D sperm ultrastructure of the aphallic mesopsammic Pontohedyle milaschewitchii (Acochlidia, Opisthobranchia, Gastropoda)

Sperm transfer via spermatophores is common among organisms living in mesopsammic environments, and is generally considered to be an evolutionary adaptation to reproductive constraints in this habitat. However, conclusions about adaptations and trends in insemination across all interstitial taxa cannot be certain as differences in mode of insemination via spermatophores do exist, details of insemination are lacking for many species, and evolutionary relationships in many cases are poorly known. Opisthobranch gastropods typically transfer sperm via reciprocal copulation, but many mesopsammic Acochlidia are aphallic and transfer sperm via spermatophores, supposedly combined with dermal fertilisation. The present study investigates structural and functional aspects of sperm transfer in the Mediterranean microhedylacean acochlid Pontohedyle milaschewitchii. We show that spermatophore attachment is imprecise. We describe the histology and ultrastructure of the two-layered spermatophore and discuss possible functions. Using DAPI staining of the (sperm-) nuclei, we document true dermal insemination in situ under the fluorescence microscope. Ultrastructural investigation and computer-based 3D reconstruction from TEM sections visualise the entire spermatozoon including the exceptionally elongate, screw-like keeled sperm nucleus. An acrosomal complex was not detected. From their special structure and behaviour we conclude that sperm penetrate epithelia, tissues and cells mechanically by drilling rather than lysis. Among opisthobranchs, dermal insemination is limited to mesopsammic acochlidian species. In this spatially limited environment, a rapid though imprecise and potentially harmful dermal insemination is discussed as a key evolutionary innovation that could have enabled the species diversification of microhedylacean acochlidians.     

Sperm ultrastructure of the giant clam Tridacna maxima (Tridacnidae: Bivalvia: Mollusca) from the Great Barrier Reef

Using transmission electron microscopy, spermatozoa from a member of the Tridacnidae, or giant clams, are described for the first time and compared with spermatozoa of other bivalves, especially other heterodonts. The acrosomal vesicle of Tridacna maxima (Röding, 1798) is short (0.37?μm), blunt-conical, and exhibits a prominent basal ring. A narrow apical elaboration of the nucleus, the nuclear peg, projects deep into the basal invagination of the acrosomal vesicle. Aside from this specialization, the nucleus is a solid elongate-cylindrical structure (7.66?μm) that exhibits several small irregular lacunae. Four or occasionally three round-ovate mitochondria surround a pair of orthogonally-arranged, triplet-substructure centrioles. The proximal centriole is connected to a small indentation of the nuclear base by a thin layer of granular pericentriolar material, whereas the distal centriole is anchored to the plasma membrane by nine terminally-forked satellite fibres. The 9?+?2 pattern axoneme of the tail is continuous with the distal centriole. Comparison with other bivalves indicates a very close relationship between tridacnids and cardiids based on sperm ultrastructure. Specifically, the presence of a nuclear peg links Tridacna spp. with the cardiid genus Cerastoderma, but further information on the many unstudied genera is required to test the exact nature of this relationship. The sperm ultrastructure provides additional support for the recently proposed hypothesis that the Tridacnidae may be no more than a specialized subfamily of the Cardiidae.