Local surface differentiations at the vegetal pole of the eggs of Nassarius reticulatus,Buccinum undatum,and Crepidula fornicata (Gastropoda,Prosobranchia)

In the eggs of the gastropods Nassarius reticulatus, Buccinum undatum, and Crepidula fornicata, local surface differentiations are described, using transmission and scanning electron microscopy. The differentiations consist of special microvilli (Nassarius), ridges composed of microvilli (Buccinum), or extensive surface folds (Crepidula). They occur on the polar lobes of these eggs and it is argued that a relation may exist between the surface structures and the localization or expression of the morphogenetic factors present in the polar lobes. The presence of the surface differentiations before the appearance of the polar lobe and their persistence after resorption of the lobes are shown in Nassarius and Crepidula eggs. Evidence is presented that the differentiated area at the vegetal pole of the Nassarius eggs corresponds to the former contact area of the oocyte with the follicle cells in the ovary. This correspondence is considered as an argument in favor of the theory that the egg cortex may locally acquire special properties under the influence of the surrounding structures in the ovary.     

Sex and genetic structure across age groups in populations of the European marine invasive mollusc, Crepidula fornicata L. (Gastropoda)

In long-lived species, variance in allele frequencies over time may vary according to the number of generations contributing to progeny. Here, we investigate the temporal stability of genetic diversity and structure in relation to sex and age in introduced populations of Crepidula fornicata , an exotic gastropod that successfully invaded Europe . This protandrous species has the potential to change sex from male to female according not only to age, but also to local sex ratio (social environment). This mechanism may adjust the reproduction efficiency across different cohorts and thus decrease the likelihood of genetic drift in the following generations. Based on crude demographic structure analysis in two spatially closed introduced French populations, we demonstrate that recruitment is discontinuous. Although timing of sex change is different across populations, both populations have a similar age structure characterized by distributions of males and females changing across generations. Using five microsatellite loci, we show that both populations display a temporal genetic homogeneity and a stability in genetic diversity indices across age groups examined. Our results highlight that the social control of sex change in C. fornicata has strong implications to the maintenance of high genetic diversity by enhancing breeding across several generations at each reproductive season.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 365–374.     

Isolation and characterization of highly polymorphic microsatellite markers from the marine invasive species Crepidula fornicata (Gastropoda: Calyptraeidae)

Many marine organisms have pelagic larvae and these are often important agents of dispersal and establishment of exotic species. The larval phase and the hermaphroditic reproductive system of the invasive marine gastropod Crepidula fornicata are crucial in determining the success of this species along the European coasts. To analyse these factors, we developed seven microsatellite loci specific to C. fornicata. On the same set of individuals, the microsatellite loci exhibited a greater level of polymorphism than previously studied allozyme markers and, thus, will permit fine‐scale analysis and larval pool studies.     

Revision of the Arctic-Atlantic Rissoidae (Gastropoda, Prosobranchia)

The present paper contains a revision of the Arctic-Atlantic Rissoidae (Gastropoda, Prosobranchia). Twenty-two species are recognized. Two new species, Alvania obliqua and Alvania pseudoareolata are described, both previously wrongly identified. One new genus, Frigidoalcania , is established with Rissoa janmayeni as type-species.     

Reproduction in Crepidula philippiana (Gastropoda,Calyptraeidae) from Southern Chile

Species of the Clivinematini genus Admetus Distant, 1883 are revised. A new species, A. quercicola, from Mexico, is described and figured. Admetus mexicanus and A. fimbriatus are redescribed and a key to species is provided.     

Zur Fortpflanzungsbiologie der Naticiden (Gastropoda Prosobranchia)

Ohne Zusammenfassung(Mit 13 Abbildungen und 3 Tabellen)Herrn Prof. Dr. A. Bückmann zum 60. Geburtstag gewidmet.     

Zur Lokomotion bei Naticiden(Gastropoda Prosobranchia)

Ohne Zusammenfassung(Mit 2 Abbildungen)     

The formation of the pronuclei and their associated perinuclear cytoplasm; the determination of the phases of the first cleavage cycles in Crepidula fornicata (Mollusca,Gastropoda)

Summary

The behaviour of the male and the female pronuclei in Crepidula fornicata is studied, beginning at the formation of the second polar body. Shortly after the extrusion of the second polar body the female pronucleus is formed, and then the male pronucleus enters the yolk-free cytoplasm near the animal pole. Both pronuclei are enveloped by a typical nuclear membrane, and increase in size until the prophase; a zygote nucleus is not formed (“Ascaris type” of fertilization). In the meantime, the chromatin of both pronuclei is arranged in a meshwork in the centre of the pronuclei.

Shortly after the formation of the second polar body a special cytoplasm, the “perinuclear cytoplasm”, is formed in the vicinity of each of the pronuclei. During the early stages of the first cleavage cycle this cytoplasm is composed of numerous Golgi complexes, small dense Golgi vesicles, smooth endoplasmic reticulum vesicles, mitochondria and rosettes of glycogen-like granules. At later stages, when the pronuclei have met and their plasms coalesced, the number of Golgi elements decreases; at the same time, the small dense Golgi vesicles increase in number and aggregate in clusters.

The phases of the first three cleavage cycles are determined by cytophotometry. The nuclear DNA of the male pronucleus and that in the nuclei of the blastomeres of the 2- and the 4-cell stage is reduplicated between 7 and 33% of the normalized cleavage cycles; the G₂-phase is between 33 and 57%, while the mitotic phase occupies the last part of each cleavage cycle and the first 7% of the next cleavage cycle. There is no G j-phase. Since the female pronucleus lies just beneath the polar bodies, its DNA content could not be measured separately.     

Spermiogenesis in the vermetid gastropod Dendropoma petraeum (Gastropoda,Prosobranchia)

The structure and maturation of the male gonad of the Mediterranean vermetid gastropod Dendropoma petraeum are described. Histological sections of the gonads were made throughout development and gonad activity was monitored at regular monthly intervals. During the autumn months the gonad is very small and is surrounded by a large quantity of connective tissue; it becomes more voluminous from December to August, with the highest growth peak in springtime. The stages of spermatogenesis were also observed and described.     

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.     

Locomotion of the filiform sperm of Littorina (Gastropoda,Prosobranchia)

Summary The filiform sperm of Littorina sitkana swims effectively in sea water and more viscous fluids, overcoming the problems of a non-uniform flagellar beat with an unusual mechanism, which involves three main events: (1) the sperm rotates anti-clockwise (looking from tail to head); then (2) stops rotating and stiffens itself to form a screw-shape, with the tail being held perpendicular to the middle piece, and finally; (3) reverses its rotation and propels itself forward in a clockwise spiral. The average velocity of sperm is approximately 185 ms with a rotational frequency of 24 revs. The mechanism of propulsion may involve two kinetic centers at opposite ends of the sperm, which coordinate their movements to produce anti-clockwise rotation, stationary twisting, or clockwise rotation, in a manner reminiscent of spirochaetes. Littorina sperm also exhibit slower methods of propulsion including swimming backwards (tail first) at 18 m, and gliding at about 30 m.The adaptive significance of the rapid propulsion is not obvious, because Littorina copulate and fertilize internally and at each stage in the transfer there are external aids to sperm transport, such as ciliary action (oviduct) and muscular expulsion (bursa and seminal receptacle). The filiform shape, however, is well-adapted for long-term storage in the female body. These points are discussed.     

Feeding in some Australasian Cymatiidae (Gastropoda:Prosobranchia)

Of the four common Australasian cymatiids studied, Cabestana spengleri and Mayena australasia feed almost entirely on simple ascidians. Monoplex australasiae feeds on bivalves as well as on ascidians and Charonia rubicunda feeds on a variety of echinoderms. All species locate their prey by chemoreception of current born prey substances. Charonia rubicunda is a nocturnal predator while the other species feed almost continually.     

The diet of the dog-whelk, Nucella lapillus (Gastropoda Prosobranchia)

The information at present available suggests that, when adult, Nucella lapillus may prey upon a considerable number of different species, but that the diet of recently hatched dog-whelks is rather more restricted. The food preferences of young Nucella have been investigated and it has been concluded that, while the diet of young and adult animals may differ to some extent, it is probably equally varied at all ages, and that cannibalism may be a frequent occurrence amongst all age groups.
It has been determined that the size of the prey influences the selective predation of Mytilus by Nucella. Dog-whelks show a marked preference for mussels in the 1 to 3 cm size range which may possibly be identified chemotactically.     

Histology and regeneration of the radula of Pomacea bridgesi (Gastropoda,Prosobranchia)

Summary Histology, physiological regeneration, and degradation of the taenioglossan prosobranch radula and its concomitant epithelia were studied by light and electron microscopy (TEM, SEM), electron microprobe analysis, and autoradiography. Taenioglossa have seven multicellular odontoblastic cushions which produce the tooth matrix by apocrine secretion; many long microvilli are also incorporated. In contrast to pulmonates, the odontoblasts of prosobranchs are capable of division, and their mitoses contribute to the expansion of the cushions, but presumably also to the displacement of degenerating odontoblasts. The seven cushions are isolated from each other by separation cells. The radular membrane is produced from microvilli of membranoblasts and a substance secreted at the base of microvilli.Strands of the supraradular epithelium subsequently move in between the teeth and finally enclose them completely. They effect the hardening and mineralization of the teeth. The strands move together with the radula towards the anterior and are extruded at the opening of the radular sheath; their degeneration, however, has already started in the middle section of the sheath. Epithelial cells are produced by two completely separated mitotic centres which lie dorsolaterally at the end of the sheath.In the subradular epithelium, mitotic activity is scattered over the posterior half of the sheath but is not found in the region where the supramedian radula tensor muscle is inserted. The epithelial cells move forward, but at a much lower rate than the radula. At the opening of the sheath the subradular membrane is generated, while cells of the subradular epithelium lying between the lamellae of the subradular membrane are extruded.The subradular membrane is limited to the functional part of the radula. It is situated on the distal radular epithelium, which is obviously not a continuation of the subradular epithelium. In test animals treated with tritiated thymidine, there is a very strong stationary centre of labeled cells at the beginning of the epithelium, but so far no mitoses have been found in this centre and the labeled cells do not move on continually. In the middle of the distal epithelium mitoses do occur, and the labeled cells permit the assumption that these cells do not migrate at all to the anterior end. At least in Prosobranchia, the distal radular epithelium does not transport the radula to its degradation zone. The transport mechanism for the radula is still unknown.     

Catecholamines in larvae and juveniles of the prosobranch gastropod, Crepidula fornicata

We investigated roles of catecholamines in metamorphosis of the prosobranch gastropod, Crepidula fornicata. Levels of DOPA, norepinephrine (NE) and dopamine (DA) were measured by high-pressure liquid chromatography (HPLC) in competent larvae and juvenile siblings that metamorphosed in response to the natural adult-derived cue or to elevated K+. Competent larvae contained 1.58 +/- 0.26 (S.E.M.) x 10(-2) pmol DOPA, 0.91 +/- 0.45 x 10(-2) pmol NE, and 0.290 +/- 0.087 pmol DA (mean values per microg total protein, n = 4 batches of larvae). Levels of DA per individual were not different between larvae and juvenile siblings; levels of NE were higher in juveniles. The tyrosine hydroxylase (TH) inhibitor alpha-methyl-DL-m-tyrosine (alpha-MMT) depleted DOPA and DA to approximately half of control values without affecting levels of NE. Depletion of DOPA and DA was accompanied by inhibition of metamorphosis in response to the natural cue but not to elevated K+. The dopamine-beta-hydroxylase inhibitor diethyldithiocarbamate (DDTC) induced high frequencies of metamorphosis at concentrations of 0.1-10 microM. In juveniles induced by 10 microM DDTC, levels of both NE and DA averaged approximately 80% of those in control larvae. Catecholamines may function as endogenous regulators of metamorphosis in C. fornicata.     

Reproduction in some New Zealand Cymatiidae (Gastropoda:Prosobranchia)

The behaviour during reproduction of four species of New Zealand cymatiids is described revealing a year-long courtship period in Cabestana spengleri. Charonia species, Mayena australasia and Monoplex australasiae have only a brief period of courtship prior to egg laying. The female of all species remains with the egg mass until the embryos hatch. This period may be as long as three months in the case of Mayena australasia. Protection of the egg mass seems to be the reason for this brooding behaviour as development proceeds normally in the absence of the female. Anatomically, the genital system of these mesogastropods is approaching the condition seen in more advanced neogastropods particularly with regard to the female.     

Silver staining analysis on spermatozoa of Nucella lapillus (Gastropoda,Prosobranchia)

Summary

Sperm of Nucella lapillus was studied by electron microscopy, including the application of a cytochemical silver method. Using silver impregnation a dense precipitation of Ag granules in spermatocyte II nucleoli was seen over the fibrillar component and a slight one in the granular component. On longitudinal sections of the spermatozoon the results demonstrate that argyrophilic proteins are located in the external limiting zone of the acrosome in the anterior portion of the nucleus between the cytoplasmic and the nuclear membranes, in the posterior end of the nucleus and in the terminal portion of the middle region. These data indicate an affinity for silver in areas of the cytoplasm containing microtubules and in zones of transition.