Prodissoconch morphology is environmentally modified in the brooding bivalve Lasaea subviridis

Embryonic shell sculpture of intertidal and laboratory brooded individuals of the direct developer Lasaea subviridis was studied using scanning electron microscopy. Intertidally brooded individuals develop a distinct prodissoconch I (PI) and prodissoconch II (PII) of unusual morphologies. The PI is relatively small (50–100 m in length), oval in outline, lacks a radial sculpture and is restricted to the initial pitted zone of the prejuvenile shell. A larger PII is formed (510–680 m in length) and it is dominated by pronouced commarginal striae but also includes a belt of radial ridges extending from the PI/PII boundary. Laboratory brooded individuals kept constantly submerged do not develop the pronounced commarginal striae characteristic of PII. This implies that PII formation in L. subviridis is not a direct result of the brooding habit, and can be modified by environmental factors, which possibly include low-tide exposure. There is no single feature of external prodissoconch morphology that unambiguously indicates a direct development mode combined with mantle cavity brood protection. General guidelines are presented to help recognize this developmental mode, based on prodissoconch morphology. In comparing prodissoconch morphologies of brooding bivalves, the habitats as well as the phylogenies of species should be considered, especially in comparisons of intertidal and subtidal species.     

Biomineralization markers during early shell formation in the European abalone <Emphasis Type="Italic">Haliotis tuberculata</Emphasis>, Linnaeus

Larval shell formation was investigated in the European abalone Haliotis tuberculata. Stages of mineralization as well as enzymatic and endocrine biomarkers were monitored throughout larval development, from hatching to post-larval stages. Polarized light microscopy and infrared spectroscopy analyses revealed the presence of crystallized calcium carbonate arranged in aragonite polymorphs from the late trochophore stage. A correlation between the main steps of shell formation and enzymatic activities of alkaline phosphatase and carbonic anhydrase was seen. The variations of these biologic activities were related to the onset of mineralization, the rapid shell growth, and the switch from larval to juvenile shell following metamorphosis. Furthermore, a strong increase in the level of calcitonin gene-related molecules was measured in post-larvae, suggesting that endocrine control takes place after metamorphosis. The changes measured for the three biomineralization markers together with mineralogical analysis allowed us to correlate physiologic mechanisms with early steps of abalone shell formation.     

Spatial variation of growth and gonadal developments of Maurolicus muelleri in the Norwegian Sea and in a Norwegian fjord

The epibenthic megafauna of the high-Arctic Northeast Greenland shelf was investigated by means of seafloor photography and Agassiz trawl catches. At 54 stations in water depths between 40 and 770 m, sequences of color slides, each depicting about 1 m² of the seafloor, were obtained along photographic transects of about 100 to 600 m length. The photographs were quantitatively analyzed for abundance of epibenthic organisms identified by comparison with specimens collected from trawl catches. Megabenthic biomass was estimated by multiplying density values with averge body mass figures. For five dominant brittle star species, the population oxygen uptake and, thus, organic carbon mineralization potential were approximated by applying individual respiration rates of average-sized specimens to density figures. Multivariate analyses of the megabenthic species distribution revealed a distinct depth zonation. Shallow shelf banks (<150 m), characterized by coarse sediments, many stones and boulders as well as negative bottom water temperatures, housed a rich epifauna (30 to 340 ind m^–2, 1.8 to 10.5 g AFDW m^–2), strongly dominated (80 to 98% by numbers) by the brittle stars Ophiocten sericeum and Ophiura robusta. The oxygen uptake by brittle stars ranged from 0.4 to 95 mol O₂ m^–2 h^–1 (i.e., assuming a respiratory quotient of 0.8, an organic carbon mineralization of 0.1 to 21.9 mg C m^–2 d^–1). At the bank flanks sloping to the shelf troughs (100 to 580 m), finer sediments prevailed, stones were rare, and bottom water temperatures were positive due to the inflow of Atlantic water. Compared to bank sites, total epibenthic abundances as well as carbon mineralization by brittle stars were roughly ten times and total biomass about four times smaller. In deep shelf depressions as well as at the continental slope (200 to 770 m), stones were completely lacking, and sediments very fine. Epibenthic standing stock and carbon mineralization were one to two orders of magnitude lower than on the banks. The estimation of brittle star oxygen uptake indicates that a considerable portion of the organic carbon produced in the polynya and partitioned to the benthos may be remineralized by epibenthic bank assemblages.     

Age,growth, and mortality of the prosobranch<Emphasis Type="Italic"> Zidona dufresnei</Emphasis> (Donovan, 1823) in the Mar del Plata area,south-western Atlantic Ocean

The volutid snail Zidona dufresnei is a benthic top predator in the Mar del Plata (Argentina) shelf area where it was subjected to unregulated commercial exploitation for more than 20 years. So far there is no stock management, and hitherto even the most basic information on population dynamics of this species is missing. Annual formation of internal shell growth bands visible by x-ray was confirmed by the stable oxygen isotope record in the shell carbonate that reflects seasonal oscillations in water temperature. A Gompertz growth function ( , K=0.211 year^–1, t₀=5.496) fitted 142 pairs of size-at-age data (30 shells) best. Maximum individual production amounted to 26.8 g shell-free wet mass (SFWM) at 115 mm shell length. Based on a size-frequency distribution derived from commercial catches, annual mortality rate of Z. dufresnei was estimated to be 0.61 (±0.21) year^–1.Communicated by O. Kinne, Oldendorf/Luhe     

Consequences of maternal isolation from salinity stress for brooded embryos and future juveniles in the estuarine direct-developing gastropod Crepipatella dilatata

At reduced salinities, brooding females of some gastropods and bivalves may isolate their mantle cavities from the environment for several days, maintaining internal osmotic concentration but causing severe declines in dissolved oxygen and pH, and increases in ammonia and other toxic substances in the mantle fluid. This study in November–December examined the immediate consequences of such stresses for brooded embryos of Quempillén estuary gastropod Crepipatella dilatata, in terms of time to juvenile emergence and rates of embryonic growth [measured as shell length (SL)]. Juveniles were also monitored for latent effects on feeding rates, oxygen consumption, and growth for the first 4 weeks after emergence into normal salinity seawater. An acute salinity stress lasting 3 days applied to females that were brooding pre-shelled or intermediate-shelled stages increased embryonic incubation periods, but without affecting SL at emergence. Growth rates were reduced for encapsulated embryos regardless of the stage at which the salinity stress was applied. Latent effects on juvenile development included slower shell growth and reduced rates of oxygen consumption and feeding. These effects were sustained for the first month after release from the female. The results suggest that marked reductions in salinity lasting for several days indirectly but negatively affect the development of brooded embryos of C. dilatata and also affect the juveniles for at least several weeks following their release, even after salinity has returned to normal.     

Biochemical and energetic composition of bathyal echinoids and an asteroid,holothuroid and crinoid from the Bahamas

The biochemical and energetic composition of body components of ten species of bathyal echinoids, and an asteroid, a holothuroid and a stalked crinoid were determined from individuals sampled from a variety of deep-water sites near the Bahamas (north Caribbean Sea) in October 1988. When compared with other studies of echinoderms, no geographic- or depth-related differences in biochemical or energetic composition were found. Body-wall tissues were composed primarily of skeletal material (mineral ash), but were comparatively high in organic material in the echinothuriid echinoids, and the asteroid and holothuroid. Gut tissues and pyloric cecae had high levels of lipid and protein, indicating their potential role in nutrient storage. Body-wall tissues were generally low in energy, but were highest in the echinoidsAraeosoma belli (7.7 kJ g^–1 dry wt) andSperosoma antillense (8.0 kJ g^–1 dry wt), the asteroidOphidiaster alexandri (8.9 kJ g^–1 dry wt), and the holothuroidEostichopus regalis (13.1 kJ g^–1 dry wt). Energy levels of gut and pyloric cecal tissues were two to three times higher than those of body-wall tissues. Total somatic tissue energy values varied greatly among species, ranging from 1.5 kJ in the echinoidAspidodiadema jacobyi to 142.1 kJ inE. regalis. As the bathyal echinoderms examined in this study occur in great abundance, they represent a significant reservoir of organic and inorganic materials and energy in deep-water benthic systems.     

Trophodynamic function of copepods,appendicularians and protozooplankton in the late summer zooplankton community in the Skagerrak

The study was carried out in the Skagerrak during late summer when population development in the pelagic cycle culminated in the yearly maximum in zooplankton biomass. The cyclonic circulation of surface water masses created the characteristic dome-shaped pycnocline across the Skagerrak. The large dinoflagellate Ceratium furca dominated the phytoplankton biomass. Ciliates and heterotrophic dinoflagellates were the major grazers and, potentially, consumed 43–166% of daily primary production. The grazing impact of copepods was estimated from specific egg production rates and grazing experiments. The degree of herbivory differed between species (14–85%), but coprophagy (e.g. feeding on fecal pellets) and ingestion of microzooplankton were also important. The appendicularian Oikopleura dioica was present in lower numbers than copepods, but cleared a large volume of water. The grazing impact of copepods and O. dioica was estimated to 57±24% and 12±12% of daily primary production, respectively. Sedimentation of organic material (30 m) varied between 169 and 708 mg C m^–2 day^–1, and the contribution from the mesozooplankton (copepod fecal pellets and mucus houses with attached phytodetritus of O. dioica) was 5–33% of this sedimentation. Recycling of fecal pellets and mucus houses in the euphotic zone was 59% and 36%, respectively. However, there was a high respiration of organic material by microorganisms in the mid-water column, and 34% of the sedimenting material actually reached the benthic community in the deep, central part of the Skagerrak.     

Sclerotized protein in the shell matrix of a bivalve mollusc

The epsilon-amino groups of lysine and phenolic groups of tyrosine are most heavily concentrated in the newlydeposited organic matrix of the shell of the bivalve Mercenaria mercenaria. A phenoloxidase enzyme which oxidizes L-dihydroxyphenylalanine is present only in this new area of the shell matrix. Scanning electron micrographs of calcified secretions of the shell show that accretion lines, thought to be layers of organic matrix separating diurnal acceretions of calcium carbonate, are not developed until up to 4 d after deposition of shell material. These results suggest that the shell matrix is hardened by some kind of polymerization, and that lysine and tyrosine residues in the matrix are involved in the process. Accretion lines in polished and etched sections become visible only after complete hardening of the polymer occurs.     

Participation of the C-type hemolymph lectin in mineralization of the acorn barnacle Megabalanus rosa

BRA-2, a major C-type lectin in the hemolymph of the acorn barnacle Megabalanus rosa was detected in shell-associated proteins. Immobilized BRA-2 promoted CaCO₃ crystallization as determined by an in vitro pH-drift mineralization assay. In nature, calcification occurs as wound repair in M. rosa after breakage of the shell plate. To observe this phenomenon, we inserted a rubber cube inside the mantle cavity of M. rosa through the basal plate, and kept the organisms in a tank for several weeks. The rubber cube was covered first with brownish-colored material, probably a melanin, and then with CaCO₃ 5 weeks after the insertion. In the CaCO₃-associated proteins, BRA-3, a minor component of M. rosa hemolymph lectins, was present in addition to BRA-2. The presence of both the lectins and their ligands on the hemocyte was also observed. These results suggest that the C-type lectins of the M. rosa hemolymph participate in mineralization as well as defense in these organisms.     

Growth increments in the shell of the living brachiopod Terebratalia transversa

Scanning electron micrographs revealed growth increments in the primary shell layer of the extant terebratulid brachiopod Terebratalia transversa collected from Anacortes, Washington, USA, during the summers of 1982–1984. The increments extend into the secondary shell layer, but only as poorly-defined continuations. Growth increments narrow and widen cyclically, producing patterns similar to tidally-induced, fortnightly growth patterns in bivalve molluscs. Asymmetric increments or doublets that consist of paired wide and narrow bands present additional evidence that the growth pattern is produced by the interference of tidal (24 h 50 min) and solar (24 h) daily environmental influences on growth, as is the case in bivalves. Growth increments are relatively well-defined in specimens from tidal habitats and are poorly-defined in subtidal specimens. Specimens grown in laboratory aquaria and subjected to simulated tidal emersions and submersions had well-defined increments, whereas those that were continuously submerged in aquaria had poorly-defined increments, paralleling the distinctness in growth increments produced at various depths in the natural habitat. Production of growth increments in T. transversa may be related to valve movement and changes in mantle-cavity pH. When the valves are closed, pH drops and, when they are open, pH rises. Highperformance liquid chromatography (HPLC) of mantle tissue and fluid failed to detect any single organic acid that could have been responsible for the pH changes, confirming that intermediate metabolism in T. transversa is very low compared to that of bivalve molluscs. Calculations indicate that cycles in intermediate metabolism may nevertheless be responsible for cycles in growth-increment production and shell dissolution.Please address all correspondence to Dr. G.D. Rosenberg in Indianapolis     

Ultrastructural localization of metal binding sites in the kidney of the Antarctic scallop Adamussium colbecki

The functional morphology of kidney cells and the intracellular localization of metal storage sites were studied by electron microscopy and X-ray microanalysis in the Antarctic scallop Adamussium colbecki (Smith) collected at Gerlache Inlet, Terra Nova Bay, Antarctic in December 1990. Kidney cells of A. colbecki show the same basic cytoarchitecture of other Pectinidae, but the nephrolithes localized in the main vacuoles show a low degree of mineralization. The sites of heavy metal storage are: (1) the main vacuole of the renal cell, where Fe, Zn and Cu are associated with concretions, (2) the basal membranes and (3) the lysosomes of amoebocytes, in which Ag, Se, Fe and Cu occur in electron-dense particles. Finally a hypothesis for the formation and role of the Ag–Se rich particles is discussed.     

Biochemical and energy changes during embryogenesis in the rock scallopCrassadoma gigantea

The biochemical composition of eggs spawned from rock scallopsCrassadoma gigantea (Gray) collected from Clayoquot Sound, Vancouver Island (British Columbia, Canada) in the winter of 1987 and the summer of 1988, differed significantly only in lipid and soluble-ash contents. Major macronutrients were catabolized simultaneously and linearly with time of embryogenesis. Total egg energy expended during development was 11.8 kJ g^–1 derived 46.7, 9.8 and 43.5%, from lipid, carbohydrate, and protein, respectively. Glucose in the egg was the dominant source of carbohydrate energy. Deposition of shell was linear with time of embryogenesis and the formation of 44.8% dry wt of shell in the early larval stage, referred to as the D-larva, consumed 64.4% of the total energy expended. Energy required for shell formation was 17.55 kJ g^–1. The RNA:DNA ratio declined exponentially from 18.6 in the egg to 2.21 in the D-larva, which was consistent with that of adult bivalves. Many changes in fatty acid composition were linear with time of embryogenesis, and the percentage ofn3- andn6-higher unsaturated fatty acids to total polyethylenic acids exhibited significant declining and increasing trends, respectively. The linear rate of catabolism of eicosapentaenoic acid, 20:5n3, and the constant level of docosahexaenoic acid, 22:6n3, illustrated an energy and a structural role, respectively, for these dominant acids in the embryonic development of the rock scallop.     

Ultrastructural examination of chamber morphogenesis and biomineralization in the planktonic foraminiferOrbulina universa

Pre-sphere, trochospiralOrbulina universa (d'Orbigny) were hand-collected between June and September, 1982 to 1984, from surface waters of the Pacific Ocean ca. 2 km off Santa Catalina Island, California, USA. The formation of a spherical chamber by this planktonic foraminifer was studied with light and electron microscopy. Chamber morphogenesis is preceded by the secretion of a cytoplasmic layer rich in mitochondria. The layer expands away from the pre-existing foraminiferal shell surface in a spherical pattern. Full cytoplasm expansion is followed by the secretion of an organic matrix (OM) approximately 200 m above the original shell surface. Cytoplasm, mitochondria and/or vesicles may play a role in OM secretion. Calcification and chamber thickening, new spine growth and pore development are documented and continue for a period of 1–7 d. Approximately 24 h prior to gamete release, the foraminiferal spines are resorbed and a veneer of calcite is deposited on the shell surface. An extensive intracellular organelle unique to foraminifera, the fibrillar system, is examined and is hypothesized to play a role in the biomineralization process. The taxonomic and functional significance of theO. universa spherical chamber is discussed with respect to other foraminiferal species.     

Burrow morphology,tube formation,and microarchitecture of shell dissolution by the spionid polychaete Polydora websteri

Burrow morphology, detrital tube formation within the burrow, and ultrastructure of the surface of the burrow of Polydora websteri Hartman, 1943, are described in the shells of Crassostrea virginica (Gmelin) and of Mytilus edulis Linné. Observations were made on live worms living normally in artificial preparations of polished shell covered with transparent plastic film. P. websteri is capable of settling wherever there are crevices in shell surfaces, and slowly penetrates the shell forming a U- or flask-shaped cavity. A detrital tube is formed within the burrow from detritus collected and pressed in place by the worm. During this process, the modified setae of the 5th setiger are thrust against the wall of the inner tube and help to keep the diameter of the inner tube constant throught its length. Individuals of P. websteri burrow into the shell when placed on preparations of polished shell of Mytilus edulis under plastic film. As they do so, they secrete a viscous fluid which dissolves interprismatic and interlamellar organic matrices and then dissolves the exposed crystals. Etched prisms and lamellae reveal a complex pattern of internal dissolution. Enlargement of the burrow by the worm thus takes place by chemical dissolution of shell and also probably by flushing of loosened partially dissolved prisms and lamellae from the tube. Some shell fragments become incorporated in the walls of the detrital tube. There is no evidence of setal abrasion on the shell walls of the burrow even at the ultrastructural levelUniversity of Delaware College of Marine Studies Contribution No. 99.     

Observations on the life history of the blue-ringed octopus Hapalochlaena maculosa

The life-history of the blue-ringed octopus Hapalochlaena maculosa (Hoyle) was observed in laboratory aquaria. Eggs from a brooding female were bred through to the next generation. The life cycle lasts approximately 7 months–4 months from hatching to maturity, 1 month from copulation to egg-laying, and an estimated 2 months for embryonic development. This venomous octopus has several unique and interesting habits. Eggs are not attached to a substratum but are carried by the mother throughout their embryonic life. She assumes a brooding posture for the greater part of the time and cradles the eggs between her raised skirt and body. However, she can move froely with her clutch of eggs, conveying them individually or in clusters along her arms from one sucker to the next as occasion demands. Embryos reverse position within their capsules 1 week prior to hatching, and hatch at the distal end of the capsule, mantle-first. Development is direct; there is no planktonic stage. The young immediately assume a benthic habit and, within a few hours, consume the remnant of the yolk-sac which they carry with them from the egg. Juveniles begin to feed on pieces of crab within a week of hatching, and to kill and eat live crabs within a month. They pierce the carapace of their prey at the abdominal articulation and with the aid, apparently, of venom from their posterior salivary glands suck out the partially pre-digested tissues. The ink gland is relatively small in size and is mainly used between the second and the fourth week of juvenile life. Chromatophores begin to function during embryonic life. After hatching there is a progressive change in coloration, and the iridescent blue rings which characterize the species appear at the age of about 6 weeks. Mating is a more active process than has been recorded for other octopuses. The male mounts the female and clasps her securely. After a short struggle the female becomes quite passive, almost inert. Copulation lasts about one hour. The unusual life-history of H. maculosa suggests that it is a highly evolved octopus species. Its direct development, simple diet, and rapid rate of growth make the animal relatively easy to cultivate for experimental or pharmacological purposes.     

The gastropod statolith: a tool for determining the age of<Emphasis Type="Italic"> Nassarius reticulatus</Emphasis>

The microstructure, shape and appearance of the growth rings in statoliths of Nassarius reticulatus (L.) were investigated. This species possesses two statocysts, each containing a single spherical statolith of calcium carbonate of up to 0.22 mm in diameter in the largest animals. The relationship between statolith diameter (SD) and total shell height (TSH) is exponential [ln(TSH)=26.3SD–0.842], although the function is site specific. Statoliths of the largest whelks (>29 mm) contained three or four clearly defined rings, corresponding to TSH values of ~1.1, 4.6–5.3, 12.0–13.5 and 18.5 mm, respectively. The first ring likely represents the metamorphic ring that was deposited at the time of larval metamorphosis when the post-larval whelk adopted a benthic lifestyle. The estimated size of the whelks at formation of the second, third and fourth statolith rings closely matched the TSH inferred from the shell rings. It is concluded that the patterns of growth rings present in statoliths can provide information about the age and growth of N. reticulatus.Communicated by O. Kinne, Oldendorf/Luhe     

Shell structure and behaviour related to cementation in oysters

Shell microstructure and mantle behaviour relating to shell cementation was studied on adult oysters,Crassostrea gigas, Saccostrea mordax, andS. kegaki (collected from Shirahama, Wakayama Prefecture, Japan, in 1989 and 1990). At the place of cementation, the prismatic structure of the outermost shell layer is modified to a significant structure. This structure, named the ridge-and-furrow structure, consists of calcified ridges a few microns wide separated by furrows, both arranged parallel to the shell growth direction. The furrows are ultimately filled by shell material. The prismatic and ridge-and-furrow structures gradually merge in a transitional area where an intermediate type of the two structures occurs. The small size of the crystal units of the ridges and furrows is due to the close distribution of crystal seeds, especially close to pre-existing ridges. This is the basis of the difference between the ridge-and-furrow and the prismatic structures and also makes the former structure functional for cementation, in contrast to the latter. At the site of ongoing shell cementation, the mantle margin adpresses the shell margin onto the substrate. Experiments show that this pressing action is essential for cementation and probably also for the formation of the ridge-and-furrow structure. Even the right valve, which oysters nerve use to cement in natural conditions, forms the ridge-and-furrow structure and cements to the substrate if the pressing action of its mantle margin is induced under artificial conditions. Behavioural changes probably led oysters to switch from byssal attachment to cementation within a short time span when they acquired their cementing habit.     

Mangrove forest structure and productivity in the Fly River estuary,Papua New Guinea

In April, July and August 1989 and February 1990, the delta region of the Fly River was surveyed to establish the aerial extent of mangrove forests, their species composition, tree densities and basal areas, and potential net primary production. Mangrove forests cover 87 400 ha, mainly on islands within the delta. Twentynine mangrove plant species were recorded, but there were only three major forest types in the delta.Rhizophora apiculata-Bruguiera parviflora forests (hereafterRhizophora-Bruguiera forests) predominated in regions where river water salinities were >10. These forests covered 31 500 ha and had mean total tree densities and basal areas of 2027 stems ha^–1 and 21 m² ha^–1, respectively. Forests of the palmNypa fruticans (hereafterNypa forests) covered 38 400 ha, mainly in regions where river salinities were ~1 to 10, and had mean total densities and basal areas of 4431 stems ha^–1 and 38 m² ha^–1, respectively. Forests dominated byAvicennia marina and/orSonneratia lanceolata (hereafterAvicennia-Sonneratia forests) predominated on accreting banks of sediment and covered 17 500 ha. In very low-salinity (< 1) regions there are large monospecific stands ofS. lanceolata. Mean total densities and basal areas forAvicennia-Sonneratia forests were 7036 stems ha^–1 and 22 m² ha^–1, respectively. Mean net primary productivity (kg C ha^–1 d^–1) was estimated to be 26.7, 27.1 and 19.0 forRhizophora-Bruguiera, Nypa andAvicennia-Sonneratia forests, respectively. Total daily net primary production by all mangrove forests was estimated at 2214 t carbon. Using assumptions based on work in tropical Australia, it was estimated that ~678 t carbon (or 31% of primary production) were exported daily from mangrove forests to the waters of the delta.Contribution No. 550 from the Australian Institute of Marine Science     

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.     

Degradation of diuron by the electro-Fenton process

The degradation of the herbicide diuron has been undertaken by electrochemical advanced oxidation in aqueous solution. This process generates catalytically hydroxyl radicals that are strong oxidizing reagents for the oxidation of organic substances. Hydroxyl radicals degrade diuron in less than 10 min. Kinetic results evidence a pseudo-first-order degradation, with a rate constant of reaction between diuron and hydroxyl radicals of 4.8x10⁹ M^–1 s^–1. Several degradation products were identified by chromatography-mass spectrometry (LC-MS). The mineralization degree of a 1.7x10^–4 M diuron solution reached 93% at 1,000 coulombs.