An electron-microscope study of periostracum repair in Mytilus edulis

The ultrastructural details of changes that occur in the inner face of the outer fold of Mytilus edulis during periostracum repair are described. Initially, after the periostracum is slit, increased secretory activity occurs in the cells, apparently controlled by lysosomes. Proto-ostracal material deposited outside the cells seems to be due to Golgi secretory granules, which dissociate and then re-assemble outside the cells to form an organised layer. During the later stages of repair, autophagic vacuole formation increases in the cells, possibly because of increased demands for synthetic material. When substantial amounts of proto-ostracal material are secreted and the outer fold is sealed off from the external environment, the cells return to their normal condition. During the early stages of repair, amoebocytes appear to be implicated in the laying down of proto-ostracal material.     

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.     

An electron-microscope study of periostracum formation in some marine bivalves. I. The origin of the periostracum

Electron microscopy has revealed the presence of specialised basal cells at the base of the periostracal groove in the bivalves Mytilus edulis (L), Nucula sulcata Bronn., Cardium edule (L) and Macoma balthica (L). A common basal cell type is observed which gives rise to a pellicle in all cases except N. sulcata. The pellicle is the first part of the periostracum to be elaborated, and it is suggestd that it may be an indication of specialisation in bivalves. In N. sulcata and C. edule, an additional cell type is present. In N. sulcata, this second basal cell type appears to be directly involved in periostracum formation, whereas in C. edule an indirect role is proposed.     

Incidence and distribution of phototrophic shell-degrading endoliths of the brown mussel Perna perna

The incidence, distribution and infestation sequences of four endolithic cyanobacteria in the shells of the brown mussel Perna perna (L.) were studied along the south coast of South Africa. The incidence of endolith-infested shells varied significantly among the 21 study sites (∼23 to 95%), with highest infestation rates occurring on promontories and headlands as compared to sites within bays. At a smaller scale, the incidence of infested shells also varied with tidal height, being high at the upper tidal levels of mussel distribution and low or absent on the lowshore. The observation that small-scale variability in infestation rates was positively related to water movement suggested that physical damage to the outer protective periostracum of mussels may have facilitated colonisation by endoliths. This hypothesis was supported by the fact that shells with artificially damaged periostraca became infested at a greater rate than did control shells. Once colonisation by the filamentous cyanobacteria Plectonema terebrans, Hyella caespitosa and Mastigocoleus testarum had taken place, endoliths spread throughout the shell, causing progressive shell deformation and damage. Only older shells that were infested by the late successional, cavity-forming Pleurocapsa sp., however, exhibited severe shell deformations, became brittle and eventually fractured near the structurally important site of adductor muscle attachment. Heterotrophic endoliths typically associated with shell degradation in previous studies were extremely rare and if present did not contribute to shell disintegration. Although it is generally perceived that endolithic algae and cyanobacteria are restricted to the surface layers of shells by light limitation within the substratum, it is clear from this study that the interaction of a combination of factors (i.e. erosion of the periostracum, successional sequence of colonists and mechanical properties of the shell) may result in phototrophic endoliths causing severe shell degradation and eventually mussel mortality. Received: 18 December 1998 / Accepted: 18 June 1999     

Shell repair in Nautilus macromphalus

The shell of Nautilus macromphalus, like that of N. pompilius, consists of an outer spherulitic-prismatic layer, a middle nacreous layer, and an inner semiprismatic layer. No periostracum was observed. When an area of shell 2 cmx2 cm over the living chamber was removed experimentally, it was replaced by shell of normal structure. Regeneration required 45 days. Observations of regenerated shell made with a scanning electron microscope showed that the spherulitic portion of the spherulitic-prismatic layer was formed from crystal grains that developed into spherulites. The nacreous layer was formed by the deposition of stacks of hexagonal crystals which, through lateral growth, made contact with crystals in adjoining stacks and formed continuous layers 1 crystal in thickness. The deposition of all the shell layers during regeneration was probably accomplished by a single mantle region rather than by different mantle regions as in normal shell growth.     

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.     

Attachment of mature oysters (Saccostrea cucullata ) to natural substrata

It has been suggested that mature oysters attach to their natural substrata by means of a combination of a modification of the prismatic outer-shell layer formed within the periostracum and a “pressing” action of the mantle (Yamaguchi 1994). However, marine surfaces are seldom smooth enough to allow adhesion without the addition of a fluid adhesive to allow electromagnetic interactions to hold the two bodies together. An electron microscope study of the attachment of the oyster Saccostrea cucullata to its natural substrata has confirmed the presence of a crystalline calcareous cement. The cement shows a range of spherulitic and irregular blocky textures that are reminiscent of diagenetic cement fabrics. Their form suggests that the cement crystallises from a calcium-carbonate-saturated liquor trapped between the underside of the shell and the substratum, with crystallites nucleating on all bounding surfaces of the void. Received: 30 May 1996 / Accepted: 9 August 1996     

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.     

Cell-wall morphology correlated with vertical migration in the non-motile marine dinoflagellate Pyrocystis noctiluca

 We report an ultrastructural study of the morphological changes in cells of the marine dinoflagellate Pyrocystis noctiluca Murray, which correlate with its vertical migration pattern. Cells alternate between a large, highly vacuolated, positively buoyant, vegetative cyst surrounded by a dinosporin-containing wall and a smaller, more compact, negatively buoyant, cellulose-bounded cell. The cyst wall is composed of two layers: a thin smooth outer layer, thought to be composed of dinosporin, and a thick inner layer that likely to be cellulosic. One or two thecate cells are formed from within the cysts. Thecate cells are smaller, more compact and contain many small translucent bodies. They are surrounded by a typical dinoflagellate amphiesmal layer composed of membranes and cellulose plates. The amphiesmal layer appears only in recently divided cells and exists for only one night. By the following day, the cellulose wall has been replaced by a new dinosporin wall synthesized from beneath the cellulose thecal layer. The cyst stage is suggested as being optimized for photosynthesis, whereas the compact, negatively buoyant, thecate form is thought to allow nutrient uptake in deeper waters. Vertical migration in this species is thus correlated with the presence of dinosporin wall during most of its stay in the upper waters, alternating with a brief thecate wall in deeper nutrient-rich waters. This is the first report correlating dinoflagellate vertical migration with changes in cell-wall composition. Received: 28 July 1999 / Accepted: 8 May 2000     

Surface sequestration of chemical feeding deterrents in the Antarctic sponge<Emphasis Type="Italic"> Latrunculia apicalis</Emphasis> as an optimal defense against sea star spongivory

Latrunculia apicalis is a spherically shaped demosponge that previous investigations have shown is rarely preyed upon by sea stars which are the dominant spongivores in antarctic benthic communities. Prior studies have also demonstrated that L. apicalis produces organic compounds that elicit a tube foot retraction response in the keystone spongivorous sea star Perknaster fuscus that can be used as a reliable assay for feeding deterrence. L. apicalis is known to contain discorhabdin alkaloids which serve, among other roles, as the source of its green coloration. To assess the defensive nature of the discorhabdin alkaloids toward P. fuscus, we have determined discorhabdin G concentrations in discrete sponge layers and evaluated those concentrations in the P. fuscus bioassay. In discorhabdin G-bearing sponges, we found a gradient of discorhabdin G that falls off rapidly toward the center of the sponge. On average, 52% of total discorhabdin G in a given sponge was found within 2 mm of the sponge surface. Tube foot retraction responses to extracts from the surface tissues (0–2 mm depth) of L. apicalis were compared to those of an inner layer (8–10 mm depth) and to a sample comprised of the same inner layer spiked with discorhabdin G at a concentration equivalent to that of the surface tissues. Tube foot retraction response times to extracts of the surface layers and the spiked inner layers were not statistically different, but were significantly greater than responses to the unaltered inner layer and controls. These results support the predictions of the optimal defense theory as L. apicalis sequesters its defensive chemistry (discorhabdin G) in its most vulnerable surface tissues, where the likelihood of predation from sea stars is highest. As antarctic sponges are generally preyed upon by extraoral feeding sea stars rather than deeper biting predators such as fish, surface sequestration may be uniquely adaptive in sessile macroinvertebrates occurring in antarctic marine benthic environments.Communicated by P.W. Sammarco, Chauvin     

Diatom Thalassiosira weissflogii in oligotrophic versus eutrophic culture: models and ultrastructure

Populations of marine diatom Thalassiosira weissflogii were grown in continuous cultures enriched with f/2 medium. One of the two contrasting cultures (‘eutrophic’) received 5.6 times more nutrients than the other (‘oligotrophic’). Two mathematical models are analyzed to estimate eutrophication differences. The second model based on the Michaelis–Menten uptake and Droop growth shows that cells in the eutrophic culture should have about 56% higher content of silica which is the limiting nutrient. Diatom samples were prepared for the transmission electron microscopy after cells have been kept in chemostats for 37 days. The structure of diatom cells was investigated and a comparison is made between cells grown in oligotrophic and eutrophic conditions. In eutrophic culture, dividing cells were encountered more frequently while cell concentration was approximately equal in both chemostats. The central vacuole of cells in eutrophic culture accumulated dispersed and compact material from amorphous to spherical shape. In some cells the large central vacuole had fibrilar and peppered dense materials in addition to translucent granules, vesicules and multivesicular bodies. In the cytoplasm we found increased number of multivesicular bodies, dense and lucent granules some of which enclose membrane particles and lucent vesicules. Dense material depositions observed in the vacuole are also seen in the cytoplasm associated with organelles, mitochondria and plasmalemma. Cells have well-developed, active and slightly increased number of dictyosomes (5–6). Some dictyosomes with dense secretory material in the cistern are apparently engaged in a granule formation process. Functional significance of dense material in the central vacuole, which has not been observed in cells grown in oligotrophic condition, is discussed.     

X-ray diffraction study of calcification processes in embryos and larvae of the brooding oyster Ostrea edulis

X-ray powder diffraction was used to study shell calcifications of the oyster Ostrea edulis, sampled in the Limski Kanal, Istria (Adriatic Sea), in May 1992. All the developmental stages were followed, from the embryonic stage through the transition between the trochophore and veliger larva (prodissoconch I and II) and later, after swarming, the pelagic free-swimming larval stages, up to their settlement and attachment (from the D-shaped to the fully formed pediveliger larva), and finally during metamorphosis and juvenile stages (dissoconch). In the first gastrula stage, only an amorphous tissue is present (a periostracum and organic matrix). The beginning of shell formation (at the end of gastrulation) in early trochophores is manifested by the appearance of calcite (up to 1–7% of total volume) and then aragonite (about 1%). In the later stage of the veliger larva the fraction of calcite decreases as well as the amorphous fraction, while the fraction of aragonite rapidly increases. In the prodissoconch II stage and during the whole pelagic period aragonite is dominant, accompanied by a very small amorphous fraction and traces of calcite. The shell mineral composition does not change until metamorphosis, whereupon the fraction of calcite rapidly increases and the fraction of aragonite decreases. The postmetamorphic valves of the juvenile and adult oyster consist mainly of calcite, except the resilium and myostracum which remain aragonitic, possibly as a continuation of the inner layer of the larval shell. Received: 28 May 1997 / Accepted: 1 July 1997     

The larva of Rhabdopleura compacta (Hemichordata)

Rhabdopleura compacta (Hineks) has a motile larva. It is evenly ciliated, and swims by rotating about its long axis. The larva is lecithotrophic, and contains a considerable amount of yolk within the blastocoel. The blastocoel is lined with a layer of flattened cells early in development, before gastrulation has begun. The endoderm is formed by invagination. Initially, the endoderm cells are tall, columnar, and contain much yolk. Nerve fibres can be seen amongst the ectoderm cells very early in development. The ectoderm cells are separated from the inner layers and yolk by a basement lamella. There is yolk within the cells as well as in the blastocoel. Some of the yolk within the blastocoelic cavity is contained within cells and some of it is extracellular. The larvae settle during gastrulation, attaching themselves to the substratum. They tend to settle in the highest parts of upturned, empty, lamellibranch shells. Soon afterwards the body regions of the adult become recognisable.     

X-ray diffraction study of the first larval shell of Ostrea edulis

X-ray powder diffraction was used to study the calcification of the first larval shell of Ostrea edulis (sampled in Limski kanal, Istria, Adriatic Sea in April 1986) from the trochophore stage to the veliger larvae (prodissoconch I), and development of the latter up to several days postfertilization (prodissoconch II). In the first stage, only the amorphous component is present (periostracum and organic matrix). The beginning of shell formation is manifested by the appearance of calcite (up to 1–4% of the total vol.) and then aragonite (2 to 7%). In a later stage of the veliger larvae the fraction of calcite decreases, as well as the fraction of the amorphous component, while the fraction of aragonite rapidly increases. In the prodissoconch II stage, aragonite is dominant, with a very small amount of amorphous component and traces of calcite. In contrast, the valves of the adult O. edulis are composed mainly of calcite, with traces of aragonite.     

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.     

A special adaptation to planktonic life in larvae of the Cassoidea (=Tonnoidea) (Gastropoda)

Living veligers of the Cassoidea have been observed to use a mantle appendage to form and resorb periostracum. Anatomical and histological examinations of a ranellid (Cymatium sp.) larva collected from the Red Sea in 1987 revealed the structure and location of the pallial appendage. The mantle edges of juvenile or adult species of the Cassoidea do not show a comparable specialization. It is demonstrated that cassoid larval conch characters are sufficient to prove the existence of a pallial appendage without anatomical confirmation. A mantle appendage is not known from teleplanic (long-living planktic) veligers of other gastropod superfamilies. In cases where the larval strategies of the latter are known they are totally different. Therefore it is suggested that the adaptation of cassoid larvae to pelagic life is unique among gastropods representing an autapomorphic character of the superfamily.     

Functional morphology of the gonads of the articulate brachiopodTerebratulina retusa

The reproductive anatomy, and ultrastructural features of the gonads of the articulate brachiopodTerebratulina retusa (Linnaeus), are documented based on collections made between October 1985 and October 1986 from the Firth of Lorn, west coast of Scotland. This species is dioecious, and maturity is achieved in both sexes at shell lengths greater than ~5.5 mm. There is no obvious external sexual dimorphism except for slight differences in the coloration of the gonads; testes are white/cream, ovaries are yellow/orange. The gonads occur as four palmate lobes, a pair in each valve. Gonads are formed within a mantle sinus (vascula genitalia), which is an anterior extension of the coelom, that opens posteriorly into the visceral cavity and to the exterior via a pair of metanephridia. The latter serve as gonoducts during spawning. Gametes are borne on genital lamellae formed from a reticulate lattice of connective tissue. The lamellae are an extension of the ileoparietal band and are fused along one margin to the inner mantle membrane. Developing oocytes are closely affixed to the genital lamellae and originate from a pool of proliferating germ cells at its base. Vitellogenic oocytes that are at an advanced stage are released from the genital lamellae, but are retained within thevascula genitalia. Liberated oocytes continue to accumulate yolk and eventually occlude thevascula genitalia, before being spawned. Coelomocytes were identified within the gonads. In spent females these cells appear to be phagocytic and involved in the resorption of necrotic material, while in the male they may serve as nutritive cells.     

Tumor-like formations on the shells of Japanese scallops Patinopecten yessoensis (Jay)

Three types of tumor-like formations on the shell of commercial Japanese scallop Patinopecten (= Mizuhopecten) yessoensis were identified. The first type of tumor-like formation (Type I) is a brown prominent knob on the inner shell surface. This is the result of penetration of aggregations of the polychaete worms of genus Polydora into the scallop cavity usually through the upper valve of scallop and subsequent covering of their boreholes by shell calcite carbonate. The second type (Type II) is tumor-like lamination of the shell with the inclusion of organic matter between calcite layers on the inner surface of the upper and lower valves. These laminations have no outlet to the outer shell surface. The third type of scallop shell tumor-like lesion (Type III) mainly built up on the inner surface of the lower valve. It is a cone-shaped lump, which seemed to consist of aggregated coarse-grained sand. It has no outlet to the outer shell surface. Tumors of the different types are not equally distributed. Type I tumors are the most common, followed by Type II and Type III. Other kinds of tumor-like lesions on the inner shell surface are not found in the species studied. On the outer surface of scallop shell tumor-like formations were not found. The shell tumor-like lesions occurred significantly more frequently on scallops that are inhabiting muddy bottom sediments than sand. A positive relationship was found between scallop age and occurrence of shell lesions. The tumor-like formations of all types adversely influence scallops. The shell height and total, soft tissue and adductor muscle weights were lower for scallops with tumor-like formations on the shell than for healthy individuals.     

Ultrastructure of the spermatozoa of Hymenosomatidae (Crustacea: Brachyura) and the relationships of the family

The spermatozoa of the genus Odiomaris Ng and Richer de Forges, 1996 (=Amarinus Lucas, 1980) have the components typical of eubrachyuran (Heterotremata + Thoracotremata) sperm, but differ significantly from all other investigated eubrachyurans in at least ten characteristics: (1) presence of an epiopercular dome; (2) separation of all but the central region of the operculum from the remainder of the acrosome by an infra-opercular rim; (3) the fact that the acrosome is smaller in volume than the nucleus; (4) the acrosome is strongly emergent from the nucleus, being surrounded only basally by nuclear material; (5) the cytoplasmic sheath, ending anteriorly with the nucleus, is also basal; (6) division of the acrosome contents into an inner and outer acrosome zone is scarcely apparent in longitudinal section as the inner zone is narrow and of doubtful homology; (7) the thin, putative inner acrosome zone is anteriorly almost septate owing to several longitudinal corrugations; (8) basally there is a unique “fringe zone”; (9) the acrosome, including the epinuclear dome, is longer than wide; (10) the unique helical and posterolateral disposition of the nuclear arms. From a purely spermatological viewpoint, Odiomaris (as exemplified by O. pilosus and O. estuarius), and provisionally the Hymenosomatidae, are thus excluded from the Thora- cotremata, in which they were formerly placed, nor are they readily placeable in the Heterotremata. Received: 30 December 1996 / Accepted: 4 February 1997     

“Glandular pockets” of the integument and feeding mechanism inPagurus bernhardus (Crustacea: Anomura)

The benthic anomuran crabPagurus bernhardus (L) were collected from the North Sea near Helgoland, West Germany, during the years 1986–1987. The histological study of the integument ofPagurus bernhardus reveals a double structure with dorsal and ventral cuticular layers and with two epithelial cell layers. Additional structures have been developed within the integument to trap micro food organisms. The structure and function of the cupshaped glandular pockets of the integument is described. The cells which secrete mucus are also described. Histological and histochemical methods have been employed to study the chemical nature of various cells of the integument and the secretions of the glandular pockets. The ecological significance of the sensory hairs, glandular pockets, cuticular layers and the supra-branchial groove is discussed with reference to the crab's feeding mechanism.