Ultrastructure of carposporophyte development in the red algaCaulacanthus ustulatus (Gigartinales: Caulacanthaceae)

The ultrastructure of carposporophyte development is described for the red algaCaulacanthus ustulatus (Turner) Kützing collected at Epanomi (Gulf of Thessaloniki, Greece) and studied in 1988/1989. Following presumed fertilization the diploid nucleus is transferred to the auxiliary cell which expands and cuts off several multinucleate gonimoblast initials. The gonimoblast initials are distributed around the periphery of the cystocarpic cavity and divide further to produce generative gonimoblast cells. The latter cleave repeatedly to form clusters of carpospores. Thus, carposporophyte development proceeds inwardly. The first formed gonimoblast cell is transformed into a storage gonimoblast cell containing large quantities of starch granules and presumably functioning to supply nutrients. A fusion cell is never formed. Gonimoblast initials contain typical red algal proplastids and numerous sacs and/or vesicles originating from cytoplasmic concentric membranes.Please address all correspondence and requests for reprints to Dr S. G. Delivopoulos     

Ultrastructure of tetrasporogenesis in the red alga Osmundea spectabilis var. spectabilis (Rhodomelaceae: Ceramiales: Rhodophyta)

Tetrasporogenesis in Osmundea spectabilis (Postels and Ruprecht) K.W. Nam var. spectabilis proceeds through three developmental stages. A uninucleate tetraspore mother cell with synaptonemal complexes depicted during early prophase of meiosis I comprises the youngest stage. Subsequently, the nuclear envelope obtains a highly irregular profile and is surrounded by perinuclear endoplasmic reticulum (ER) and numerous mitochondria. Proplastids with peripheral thylakoid and numerous straight-profiled dictyosomes are present. Dictyosome vesicles discharge their contents, initiating tetraspore wall formation. A four-nucleate tetraspore mother cell is formed prior to tetrahedral cleavage. During the second stage of tetraspore formation the nuclei are very irregular in outline and move toward the central region where they fuse. The formation of fibrous vacuoles originates from the fibrous vacuole associated organelles (FVAOs). Thylakoid formation from concentric lamellar bodies (CLBs) seems to occur in plastids. Extremely hypertrophied dictyosomes with two tightly compressed and densely laminated mid-regions produce vesicles which contribute to enlargement of the fibrous vacuoles. During the second stage of tetraspore development striated vesicles are formed and starch granule formation increases. Mature tetraspores are characterized by numerous cored vesicles and abundant starch granules. Plastids have a fully developed system of internal thylakoids, and cleavage is usually complete at this stage. Mature tetraspores are surrounded by a bi-layered wall and the tetrasporangial wall. Unbranched, tubular structures connecting the plasmalemma with the peripheral sheets of ER are possibly involved in wall deformation. A distinct blebbing nuclear envelope appears to sequester vesicles into the cytoplasm.     

Ultrastructure of the corallinaceae. I. The vegetative cells of Corallina officinalis and C. cuvierii

A technique utilizing combined fixation and gentle decalcification has been employed to study the ultrastructure of the vegetative cells of the articulated calcareous coralline algae Corallina officinalis Linnaeus and C. cuvierii Lamouroux (Rhodophyta: Cryptonemiales). The epidermal cells are distinctive, with many cell wall inggrowths which pass between the chloroplasts. It is suggested that these cells function as transfer cells. The epidermal cells contain no starch, although the chloroplasts have well developed photosynthetic lamellae. Damage to these epidermal cells leads to formation of new cells by renewed division of sub-epidermal meristematic cells. The outer cortical cells have few small vacuoles and many plastids, with an extensive photosynthetic lamellar system. Deeper into the thallus, the vacuoles increase in size and free cytoplasmic starch grains occur. The medullary cells have a very large vacuole and in older tissue often appear dead. The long genicular cells have calcareous walls at either end while the wall in the middle of these cells is non-calcareous and has an inner fibrillar layer and a thin outer cuticle. In partially decalcified material, the orientation of the CaCO₃ (calcite) crystals next to the cells can clearly be seen. Immediately next to the cell the crystals are fairly small and arranged at right angles to the plasmalemma. Further away from the cell the crystal size is larger and their orientation is more random. The crystals are surrounded by organic material, and the possible rôle of this material in calcification in coralline algae is discussed.     

Intracellular cyanobiont Richelia intracellularis: ultrastructure and immuno-localisation of phycoerythrin,nitrogenase, Rubisco and glutamine synthetase

In marine tropical or subtropical plankton the filamentous, heterocyst-forming cyanobacterium Richelia intracellularis forms a symbiosis with the diatom Rhizosolenia clevei. An ultrastructural analysis of the apex of Rhizosolenia clevei showed that the cytoplasm in that particular part of the cell was present only where the cyanobiont was located. The cyanobiont was, however, always outside the host cytoplasm. Vegetative cells as well as the heterocysts of the cyanobiont were devoid of gas vesicles and cyanophycin granules, while carboxysomes and large glycogen granules were common. The cyanobacterial cell wall apparently remained intact in both vegetative and heterocyst cells. In green excitation light the heterocysts and vegetative cells emitted a bright yellow fluorescence, indicating that both cell types possessed high concentrations of the pigment phycoerythrin (PE) commonly associated with photosystem (PS) II. The presence of this pigment in both cell types was verified by immunogold localisation. Using the same technique, the nitrogenase (dinitrogenase reductase) enzyme was shown to be exclusively present in the heterocysts, while Rubisco was localised primarily to the carboxy-somes, which were only detected in vegetative cells. Using an antiserum against the ammonia assimilating enzyme glutamine synthetase (GS), we could demonstrate very low levels of this enzyme, indicating repression of GS in the cyanobiont.     

Cell architecture of the dinoflagellate Symbiodinium sp. inhabiting the Hawaiian stony coral Montipora verrucosa

The Hawaiian stony coral Montipora verrucosa (Lamarck) Quoy et Gaimard (Anthozoa) harbours zooxanthellae of the genus Symbiodinium Freudenthal (Dinophyceae). The algae occur as coccoid cells when inside their host and produce periodically motile flagellate cells when in culture. Coccoid cells of cultured specimens were investigated in this study, using three-dimensional reconstructions in tandem with quantitative analyses after electron microscopy of serially sectioned cells, as well as freeze-fracture electron microscopy. The amphiesma normally consists of five membraneous layers with intermediate material of unknown composition. The intracellular morphology is characterized by a single, peripheral, multilobed chloroplast with a parallel thylakoid arrangement, polyhedral inclusions resembling carboxysomes, and one double-stalked pyrenoid, outlined by a triple-layered chloroplast envelope. Spherical, elongated or branched mitochondria are aggregated in the center of the cell, surrounded by the chloroplast. The nucleus is a large, spherical structure located rather ventrally and containing 26 chromosomes with ovoid to elongated shapes. Further structures found to be present include Golgi apparatus, fibrous bodies, centrioles, and vacuoles containing crystals. Cell models of Symbiodinium sp. are represented in order to uncover completely the cellular microarchitecture of a gymnodinioid zooxanthella.     

An electron-microscope study of periostracum formation in some marine bivalves. II. The cells lining the periostracal groove

The histology and ultrastructure of the epithelial cells of the outer and middle mantle folds of the bivalves Mytilus edulis (L), Cardium edule (L), Macoma balthica (L) and Nucula sulcata Bronn are described. The cells lining the inner face of the outer fold exhibit a prominent granular endoplasmic reticulum, and Golgi complexes which are concerned with the elaboration of granules and vesicles eventually incorporated into the periostracum. A gradual reduction in the protein synthetic apparatus occurs towards the tip of the fold. Within the cells, it is proposed that the ovoid inclusion bodies are lysosomes and that they control the rate of secretion. The cells of the middle fold are cuboidal in appearance. Those of M. edulis and N. sulcata exhibit prominent granules, whereas those of C. edule and M. balthica possess vesicles. The cells of M. edulis differ from the others in possessing stout bundles of filaments, which occupy large areas of the cell and constitute a cell web. The cells of the epithelium in all cases do not appear to be implicated in periostracum formation.     

Ovarian maturation of the multi-spawning spider crab <Emphasis Type="Italic">Maja brachydactyla</Emphasis> (Decapoda: Majidae) with special reference to yolk formation

In the study of the reproductive biology of the spider crab Maja brachydactyla, the morphology of the female reproductive system and yolk formation have long been overlooked. Females spawn two or three times during their annual reproductive cycle in northern Spain (Galicia). The ovaries consist of two lobes. The right and left lobes are connected by a small cross-lobe at the level of the heart and merge at the posterior edge. Before merging, the ovaries descend to the ventral part of the body, joining the spermathecae in the vagina, which opens through a chitin tube to the gonopore, located in the sternite, at the level of the third walking leg. No morphological changes have been observed between either the different parts of the ovaries or the different annual spawning periods. At the start of vitellogenesis, the oocyte of M. brachydactyla is characterized by a large number of vesicles in the cytoplasm. These vesicles are surrounded by a unit membrane whose size increases as the oocyte matures and contain fine granular material including a variable number of ovoid, electron-dense granules. The vesicles are of diverse origin, although most of them develop directly from the mitochondria and the Golgi complex (endogenous phase of vitellogenesis). In a subsequent phase, a series of substances (principally lipoproteins) are incorporated into the ooplasma by means of micropinocytosis. These substances are also involved in yolk formation (exogenous phase of vitellogenesis). During vitellogenesis in M. brachydactyla, mitochondria play the most important role since they are not only the energetic centre of the cellule, but they also act as containers of high-energy reserve substances: the yolk granules.     

Morphological evidence for vertical transmission of symbiotic bacteria in the viviparous sponge<Emphasis Type="Italic"> Halisarca dujardini</Emphasis> Johnston (Porifera,Demospongiae, Halisarcida)

All stages of vertical transmission of symbiotic bacteria, from the penetration into oocytes to the formation of rhagon, were investigated in the White Sea (Arctic) representatives of Halisarca dujardini Johnston (Demospongiae). Small populations of free-living specific symbiotic bacteria inhabit the mesohyl of H. dujardini. They are represented by a single morphotype of small spiral gram-positive bacteria. Vertical transmission of symbiotic bacteria between generations in sponges may occur in different ways. In the case of H. dujardini the bacteria penetrate into growing oocytes by endocytosis. A part of the bacteria plays a trophic role for oocytes and the other part remains undigested in membrane-bound vacuoles within the cytoplasm. In cleaving embryos bacteria are situated between the blastomeres or in the vacuoles. In the blastula all bacteria are disposed in the blastocoel. The symbionts are situated in intercellular spaces in free-swimming larvae and during metamorphosis. Symbiotic bacteria do not play any trophic role in the period of embryonic and postembryonic development of H. dujardini. No signs of destruction and digestion of bacteria were revealed at any stage of development.Communicated by O. Kinne, Oldendorf/Luhe     

Male reproductive biology of the red frog crab,Ranina ranina,off Hachijojima,Izu Islands,Japan

Male red frog crabs, Ranina ranina, were collected year round in 1990 and 1991 off Hachijojima for histological study of the reproductive system and cycle. The testis containing the lobules and seminiferous ducts is surrounded by connective tissue. The seminiferous duct connects to the anterior end of the vas deferens, which can be histologically divided into three portions similar to one another in appearance. It was surrounded by fibrous connective tissue, muscle fibrils and columnar epithelium. Muscle fibrils were absent in the anterior portion. Multiple sperm masses were not formed in the vas deferens and ejaculatory duct, but the sperm mass was covered with a capsule composed of two layers. The outer layer of the capsule was periodic acid Schiff (PAS)-positive, but the inner layer was negative. Both layers were Alcian Blue negative, except the vacuoles in the outer layer that were stained blue. The small round androgenic gland was attached to the posterior end of the vas deferens of the coxa of the eighth thoracic appendage. The ejaculatory duct was distinguishable from the vas deferens by the absence of columnar epithelium and the presence of thick longitudinal muscle fibers. Spermatogenesis was histochemically examined. The acrosomal vesicle appeared to be derived from PAS-positive vesicles in the cytoplasm of the spermatid at the early stage of spermiogenesis. The arms were positive to the Feulgen reaction and the subacrosomal region was negative to PAS. Seasonal changes in reproductive cycle were inconspicuous histologically and microscopically. Sperm were always present in the testis and vas deferens throughout the year and occupied 5.1 to 19.6% of testis observed in cross sections. The minimum size of maturity is less than 39 mm carapace length, but the minimum size capable of successful mating was estimated to be ca. 55 mm.     

Morphological,gravimetric, and biochemical changes in<Emphasis Type="Italic"> Crepidula fecunda</Emphasis> (Gastropoda: Calyptraeidae) egg capsule walls during embryonic development

Walls of egg capsules of the gastropod Crepidula fecunda Gallardo, 1979 were examined at different developmental stages during the period of intracapsular development of embryos. The weight, biochemical composition, and structural features (using scanning and transmission electron microscopy) of the capsule walls were examined at several intervals during development of the embryos to the hatching stage. Biochemical analyses were also carried out on the intracapsular fluid to identify the possible transfer of organic material from the capsule wall to the intracapsular fluid. The capsule wall is composed principally of organic matter, primarily protein (91%), plus minor lipid and carbohydrate components. The capsule wall consists of a thin, fibrous external layer, which overlies a thicker, spongy inner layer. The spongy layer has almost disappeared by the end of the developmental period, losing about 90% of its thickness. The 40% loss in weight of the capsule walls over the developmental period is due to loss of organic matter as protein. This suggests that the inner layer of the capsules dissolves and/or disintegrates as larval development advances. The levels of dissolved and/or particulate proteins in the intracapsular fluid are much higher than those typical of the seawater surrounding the capsules. This suggests that, as embryonic development proceeds, the inner capsule walls could potentially provide extra nutrients to the embryos.Communicated by J.P. Grassle, New Brunswick     

Oogenesis in the marine mussel Mytilus edulis: an ultrastructural study

Ultrastructural changes occurring during the course of development in oocytes of Mytilus edulis are described for mussels collected at monthly intervals over a period of one year (September 1981 to October 1982) from a site in Cornwall, England. During early stages of oogenesis the oocyte is surrounded by a small number of follicle cells but, as development proceeds, the follicle cells are restricted to the stalk region which attaches the oocyte to the acinar wall. Contact between the follicle cells and the developing oocyte is maintained by means of desmosomelike gap junctions. Organelles and inclusion bodies present in the ooplasm during oogenesis include rough endoplasmic reticulum (RER), Golgi bodies, mitochondria, free ribosomes, Balbiani's vitelline body, annulate lamellae and yolk and cortical granules. The RER, in particular, varies considerably throughout the course of development. Evidence for uptake of exogenous macromolecules into oocytes by pinocytosis is presented; it occurs in the basal region of previtellogenic oocytes prior to the formation of the vittelline coat. Lipid-yolk granules invariably have mitochondria in close association and, during the winter months, develop in close proximity to small, apparently glycogen-rich vesicles possibly suggesting that conversion of glycogen to lipid takes place in developing oocytes. Oocyte degeneration was commonly observed and involves initial breakdown of the plasma membrane followed by rupture of the vitelline coat. The oocyte contents once released into the acinar lumen are resorbed by the epithelial cells of the gonoducts, which are prevalent throughout the mantle of ripe individuals.     

Intra-clonal variation in the red seaweed Gracilaria chilensis

The phenotypic plasticity often found in seaweed populations has been explained only from the perspective of inter-population or inter-individual differences. However, many seaweeds grow and propagate by fragmentation of genetically identical units, each with the capacity to function on its own. If significant differences in performance exist among these supposedly identical units, such differences should be expressed upon the release and growth of these units. In this study we document two such types of variation in the red seaweed Gracilaria chilensis. Populations of sporelings, each grown under similar culture conditions and derived from carpospores shed by the same cystocarp exhibit significant differences in growth. In this species, each cystocarp develops from a simple gametic fusion, and cystocarp fusions occur too infrequently to account for the growth differences observed among recruits. In adult thalli, branches (ramets) derived from the same thallus (genet) and grown under similar conditions exhibit significant variation in growth rates and morphology. These findings have several implications. They suggest that carpospore production is not only an example of zygote amplification but that it also could increase variability among mitotically replicated units. Intra-clonal variability followed by fragmentation and re-attachment may increase intra-population variation which, in species of Gracilaria, is often larger than inter-population variation. In addition, the existence of intra-clonal variability suggests that strain selection in commercially important species may require a more continuous screening of highquality strains because of frequent genotypic or phenotypic changes in the various cultivars.     

Oocyte development in the kuruma prawn Penaeus japonicus

Female kuruma prawns (Penaeus japonicus Bate) with undeveloped, early developing, developing, nearly ripe and ripe ovaries, were collected from Ise Bay, Japan, in 1984. Oocyte development of the kuruma prawn was classified into ten stages according to morphological characters, namely: (1) synapsis stage, (2) chromatin nucleolus stage, (3) early perinucleolus stage, (4) late perinucleolus stage, (5) oil globule Stage I, (6) oil globule Stage II, (7) yolkless stage, (8) yolk granule stage, (9) prematuration stage, and (10) maturation stage. The synapsis stage is a multiplication stage. The chromatin nucleolus stage, early and late perinucleolus stages are previtellogenesis and primary growth stages. Oil globule Stage I is an initial stage of primary vitellogenesis and secondary growth. Follicle cells on the oil globule Stage I oocytes expand rapidly and reach maximum size during oogenesis. Yolk granule stage oocytes are in the initial stages of secondary vitellogenesis. Strongly acidophilic yolk granules accumulate within basophilic vesicles of the cytoplasm. The yolk granules are first concentrated in the inner part of the cytoplasm, then gradually spread to the periphery. Cortical crypts, which are separated from the oocyte cytoplasm by the cytoplasmic membrane, are situated outside of oocyte cytoplasm. Germinal vesicle breakdown (GVBD) is initiated in the late phase of prematuration and continues until the late phase of maturation immediately prior to spawning. At the beginning of the maturation stage, the oocytes are ovulated, after which the nuclei further shrink and migrate out-wards. After ovulation, meiotic division of the ovarian oocyte progressed up to the metaphase of primary maturation division. Finally, the meiotic metaphase is visible just beneath the cytoplasmic membrane in the mature oocyte. Though ovulation is synchronous within the same ovary, GVBD is not completely synchronous. Ovulated mature oocytes have many club-shaped cortical crypts in the peripheral part of the cytoplasm and contain extensive accumulations of yolk granules dispersed throughout the cytoplasm. The apical end of the club-shaped cortical crypts and cytoplasmic membrane are coated by the vitellin envelope in the mature oocyte.     

Route of egg yolk protein uptake in the oocytes of kuruma prawn,Penaeus japonicus

The route of egg yolk protein uptake into the oocytes of kuruma prawn, Penaeus japonicus, was studied using immunohistochemical and electron microscopical methods. Although a significant immunofluorescence with anti-vitellin-immunoglobulin was observed in the enlarged follicle cells surrounding oil globule stage oocytes of the early vitellogenic ovary, no fluorescence was detected in shrunken follicle cells surrounding oocytes in the yolk granule stage. Electron microscopically, yolk granule stage oocytes have an irregular surface with numerous well-developed microvilli. In contrast, the surface of follicle cells is relatively smooth. The irregular surface of yolk granule stage oocytes was covered with a layer of electron dense material. Similar dense material was found in the spaces between the neighboring follicle cells on the yolk granule stage oocytes. The outer surface of the follicle cells on yolk granule stage oocytes was covered by dense materials which were similar to those found on the irregular surface of oocytes. Micropinocytotic vesicles containing dense material were found in the ooplasm near the irregular surface with numerous well-developed microvilli. Dense material was concentrated in the peripheral part of the small forming yolk bodies of yolk granule stage oocytes. This suggests that the electron dense material, probably egg yolk protein, transferred to the surface of yolk granule stage oocytes from the spaces between the neighboring follicle cells may be incorporated into the ooplasm by pinocytosis through the microvilli and subsequently aggregate to form yolk bodies.     

Replacement and aging of chloroplasts inStrombidium capitatum (Ciliophora: Oligotrichida)

The planktonic ciliateStrombidium capitatum (Leegaard, 1915) Kahl, 1932 retains functional chloroplasts derived from ingested algal cells. Chloroplast replacement and aging were experimentally investigated in cultured ciliates provided with a cryptophyte (Pyrenomonas salina), a prymnesiophyte (Isochrysis galbana) and a prasinophyte (Pyramimonas sp.) as sources of plastids. All three algae were ingested and chloroplasts from all were retained by the ciliate. Within 15 min of exposure to the cryptophyte, this alga was taken up by the ciliates. Initially, most of the cryptophyte chloroplasts were in intact algal cells in ciliate vacuoles. By 2 h, cryptophyte plastids were commonly found free in the ciliate cytoplasm. WhenS. capitatum was switched from a diet containing cryptophytes to a non-cryptophyte diet, most cryptophyte chloroplasts were diluted out of the ciliates by cell division and/or replaced by non-cryptophyte chloroplasts within 9 h. When the ciliates are not provided with algae, they decrease in size and number. However, the starving ciliate cells contain some chloroplasts for as long as they live (40 h or more). Under these conditions, cryptophyte chloroplasts persist longer than the other chloroplast types. Our observations suggest that chloroplast retention times inS. capitatum depend on the type of chloroplast as well as the availability of phytoplankton containing suitable new chloroplasts, and probably also on the physiological states of the ingested algae and the ciliates. It is interesting that we were not able to grow this ciliate when we provided it only with prey that lacked chloroplasts.Contribution No. 7241 from Woods Hole Oceanographic Institution. This research was supported by NSF grants OCE-8600765 and OCE-8709961 to D.K.S     

The biology of colonial hydroids. I. The morphology of the polyp of Eirene viridula (Thecata: Campanulinidae)

The colonial marine hydroid Eirene viridula Peron and Lesueur (Thecata, Campanulinidae) is readily maintained under laboratory conditions and reproduces in both vegetative and sexual modes. Polyps from these colonies displayed 4 distinct body regions, which were studied by comparison of light and electron micrographs of semi-thin and ultra-thin sections. Attention was directed chiefly to the epitheliomuscular and digestive muscle cells, found in all 4 regions and characterized by the presence of a central vacuole. Particular emphasis has been placed upon the coated vesicles and discoidal coated vesicles, which appear side by side in the digestive muscle cells of the stomach, and upon the central vacuoles typical of the muscle cells of the contractile parts of the body. The possible functional significance of these structures is discussed.This work was supported by the Deutsche Forschungsgemeinschaft.     

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     

Degrees of vacuolation of the absorptive intestinal cells of five Sagitta (Chaetognatha) species: possible ecophysiological implications

The cell organization and ultrastructure of the intestine have been studied in five species of Sagitta, one epiplanktonic (S. bipunctata Quoy and Gaimard, 1827), four mesoplanktonic (S. megalophthalma Dallot and Ducret, 1969, S. decipiens Fowler, 1905, S. minima Grassi, 1881, S. zetesios Fowler, 1905). The intestinal epithelium is composed of two ciliated cell types. The first (S-cell) principally occurs in the anterior intestine and represents typical secretory cells. The second (A-cell) occurs in the median and posterior intestine and displays ultrastructural features involved in absorption and intracellular digestion, that is, coated pits, endocytotic vesicles, cytoplasmic tubules, and two distinct types of digestive vacuoles. Whereas S-cells exhibit few ultrastructural differences among the five species, the vacuolar volume of the A-cells located in the mid portion of the intestine is higher in the mesoplanktonic species. In S. zetesios and S. megalophthalma, each side of the median intestine comprises several hypervacuolated A-cells visible in transverse section; their degree of vacuolation is inversely proportional to their number. The increased volume of the intestinal vacuoles is most marked in S. decipiens and S. minima; in both these species, each lateral side of the median intestine displays a single ultravacuolated A-cell. The possible ecophysiological implications of intestinal vacuoles are discussed in relation to patterns of the vertical distribution of several Sagitta species. The vacuoles are presumed to regulate buoyancy, enabling the mesoplanktonic species to make vertical diel migrations. Received: 22 January 2000 / Accepted: 28 July 2000     

The osphradium in Placopecten magellanicus and Pecten maximus (Bivalvia,Pectinidae): histology,ultrastructure, and implications for spawning synchronisation

The bivalve osphradium is a band of putatively sensory tissue located in the gill axis, whose function is uncertain. In the present study, extending from 1987 to 1994, anatomical, histological, and electron microscopical techniques were used to elucidate the structure and ultrastructure of the osphradium in hatchery Pecten maximus L. and Placopecten magellanicus (Gmelin) (collected from Passamaquoddy Bay, New Brunswick, Canada). The osphradium consists of two distinct regions which run longitudinally on both sides of each gill axis: the osphradial ridge, and the dorsal tuft cilia region. The osphradial ridge was largely devoid of cilia other than those of the few free nerve fibres. The dorsal tuft cilia region contained free nerve fibres and ciliary tufts, separated by undifferentiated epithelial cells. No paddle cilia were observed under isosmotic fixation conditions, although under hypotonic conditions such cilia were quite common, suggesting an artefactual nature. Most of the cells of the osphradial ridge were highly secretory, the principal products being large pigment granules (in Pecten maximus) directly secreted by the Golgi bodies, and numerous small, electron-dense vesicles. These vesicles were arranged along extensive microtubule arrays in the basal region, indicative of axonal transport. These data support and extend Haszprunar's hypothesis of the role of the osphradium in the reception of chemical spawning cues and in the synchronization of gamete emission. Together with independent data on nerve pathways, osphradial sensory modalities, and monoamine localisation, an anatomical pathway and neurophysiological mediator are postulated.     

Naked Dictyocha speculum — a new type of phytoplankton bloom in the Western Baltic

Chlamydomonas pulsatilla Wollenweber, a euryhaline, marine flagellate (isolated from rockpool at St. Andrew's, New Brunswick, Canada in 1980 by J. A. Hellebust), shows decreasing rates of activity of its four contractile vacuoles in the salinity range of 0 to 15% artificial seawater (ASW). Electron microscopy shows that the contractile vacuole complex persists as a spongiome (collection of small vesicles or tubules) in cells grown at salinities above the range for operation of contractile vacuoles. From calculations of rates of water expulsion, based on size and frequency of contraction of individual vacuoles determined by light microscopy, the time necessary to empty one cell volume increased from ca 20 min at 1% ASW to ca 600 min at 15% ASW. Analysis of inorganic and organic solute contents of cells grown at 1 and 5% ASW allowed the calculation of internal osmotic pressures. Estimates of hydraulic conductivities based on rates of water expulsion via contractile vacuoles and differences in internal and external osmotic pressures resulted in values ranging from 1.1 to 1.4×10^-14 m s^-1 Pa^-1 for individual cells. Growth experiments at low photon flux densities over a salinity range of 1 to 15% ASW, over which contractile vacuole activity varied by a factor of 30, showed little difference in growth rates. This indicates that the cost for operation of contractile vacuoles must be very low. The secretion of large molecular weight organic substances does not appear to be associated with the functioning of contractile vacuoles.