New observations of the gills of Placopecten magellanicus (Mollusca: Bivalvia), and implications for nutrition

The organization, general anatomy, and surface microanatomy of all regions of the gills of a representive bivalve mollusc, Placopecten magellanicus Gmelin, were studied using stereo-microscopic, histological, and scanning electron microscopic techniques. Individuals were collected in May and November 1985 from Chamcook Bay, New Brunswick, Canada. In addition to correcting earlier accounts of this structure, a number of new observations are reported. The orientation of the ciliated spurs appears to be responsible for the sinusoidal arrangement of the gill filaments. Micrographs showing the structure of the dorsal respiratory expansion are presented. The entire abfrontal surface of the principal filament, including the dorsal respiratory expansion, is densely ciliated and mucosecretory. These characteristics may aid in the establishment of a respiratory current and in the prevention of gill damage during escape responses. All nonciliated regions of the gill filaments are covered with microvilli, thus greatly increasing the surface area of the gill. The feeding mechanism is discussed in relation to the dorsal and ventral ciliated tracts. Symbiotic ciliate protozoans are constantly dislodged from the gill filaments and transported via the ventral mucus string to the buccal region. The nutritional implications of these observations are discussed.     

Distribution,structure and importance of the cephalic dorsal hump,a new sensory organ in calanoid copepods

The occurrence, external morphology and internal ultrastructure of a cephalic integumental organ in calanoid copepods were studied, using the specimens from the Pacific, Indian and Atlantic Oceans. This organ is located on the dorsoanterior surface of the cephalosome, and a name, cephalic dorsal hump (CDH) is proposed. Externally, it usually has two pores, anterior and apical, a dorsal plate, and a thin cuticle along the sides. CDH is found only in the male of Calanidae, Megacalanidae, Mecynoceridae and Paracalanidae, and showed some variation between species or species groups both in size and shape. Transmission electron microscopy (TEM) on the specimens from Sagami Bay, Central Japan, revealed that the CDH of Paracalanus parvus and Calanus sinicus consists of two dermal glands and a receptor, which is assumed to be chemosensory. A comparison of the distributions of CDH and prehensile fifth legs of male calanoid copepods suggests that it plays an important role in mate recognition.     

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.     

Labial palps of the blue mussel Mytilus edulis (Bivalvia: Mytilidae)

As a basis for understanding the functions of labial palps in the blue mussel Mytilus edulis, the structure and histology of palps were studied using light and scanning electron microscopy. Mussels used in the present study were collected in August 1993 and April 1994. The palp ridged surface is characterized by the presence of a smooth but densely ciliated dorsal fold, upon which rests the corresponding demibranch ventral region. The underside of the dorsal fold and the palp ridges fuse to form vestigial ciliated tracts. The dorsal fold is capable of contraction, allowing it to cover variable amounts of the ridged surface. Two different types of mucocyte are present on the palp ridged surface: subepithelial, glandular, acid-dominant secretion mucocytes and epithelial mucocytes characterized by neutral secretions. In histological section, these mucocytes appear to be concentrated on anatomical features known to intervene in particle handling. The anatomical and histological features of the smooth surface are typical of bivalve labial palps, except that the dense ciliation of the dorsal fold begins in the dorsal region of the smooth surface, indicating the possible origin of this feature. Previous studies on M. edulis point to the palps as the probable site for both ingestion volume control and particle selection; the anatomical basis of the present study should facilitate further research on these aspects.     

The effect of copper and zinc on the metabolism of the mussel Mytilus edulis

The effects of copper and zinc on the metabolism of the mussel Mytilus edulis (L.) and its component tissues were studied. 500 ppm copper sodium citrate inhibited oxygen consumption of the whole animal and gill tissue, but no similar effect was observed on digestive gland tissue. 500 ppm zinc sodium citrate exerted no effect upon gill or digestive gland respiration, and neither metal salt affected the respiration of homogenates of gill, digestive gland or gonad. Direct observation of gill tissues during exposure to the metals revealed that 500 ppm copper sodium citrate caused inhibition of ciliary activity; exposure of tissues to 2 ppm Cu for 24 h resulted in only partial inhibition of the cilía. It is suggested that metabolic suppression noted in whole animals and gill tissues is due to the inhibition of an energy-consuming process such as ciliary activity rather than interference with respiratory enzyme systems.     

Allometry of gill weights,gill surface areas,and foot biomass δ<Superscript>13</Superscript>C values of the chemoautotroph–bivalve symbiosis<Emphasis Type="Italic"> Solemya velum</Emphasis>

The protobranch bivalve Solemya velum Say, 1822 has large gills, which harbor chemolithoautotrophic bacteria that supply the majority of the clams organic carbon. A substantial portion of the CO₂, O₂, H₂S, and other nutrients necessary for symbiont autotrophy and host heterotrophy are acquired from the environment through the gills, whose large size may be necessary to facilitate the acquisition of sufficient O₂ from S. velums habitat to meet the combined demands of the host and symbionts. Large gills may also result in an oversupply of CO₂, which may in turn be responsible for the isotopically depleted ¹³C values observed in S. velum biomass (–31 to –34). Alternatively, gill hypertrophy may simply be an adaptation to house a large population of symbionts adjacent to their environmental source of dissolved gases and other nutrients. To better understand gill function in this symbiosis, gill weights, gill surface areas, and foot ¹³C values were measured as a function of total weights. S. velum gill weights were found to be a substantial portion of total clam weight, averaging 38% of wet weight, compared to nonsymbiotic protobranch bivalves Yoldia limatula Say, 1831 (5%) and Nucula proxima Say, 1822 (11%). Gill weights are a smaller percentage of total weight in larger individuals; the allometric equation for gill weight (G) as a function of total weight (M) is G=0.26M^0.85. Dry weights scale similarly. Gill surface areas are immense; the average gill surface area measured was 107 cm² g^–1 total soft tissue wet weight, the highest value for any marine invertebrate. Gill surface area (SA) also scales with size (SA=69.8M^0.85). When gill surface areas were calculated with respect to gill wet weights, they did not scale with size. The ¹³C values do not scale with size either, consistent with high rates of CO₂ supply at all sizes. Extraordinarily high rates of CO₂ supply relative to demand are supported by a model for CO₂ delivery based on Ficks law and the allometric relationship between surface areas and total weight, consistent with a role for large gill surface areas in the generation of isotopically depleted tissue ¹³C values.Communicated by J. P. Grassle, New Brunswick     

Early development of the gill and implications for feeding in Pecten maximus (Bivalvia: Pectinidae)

Despite the importance of understanding feeding in the early stages of bivalve development, little information is available concerning the organogenesis of the bivalve gill. The present study used histological and scanning electron microscopical techniques to present a detailed account of gill development in the early stages of the scallop Pecten maximus L. (Bivalvia: Pectinidae). Live specimens from larval cultures were observed daily using light microscopy, while five scallops were sampled for electron and light microscopy every 2 to 3 d from Day 18 to 35, then weekly to Day 56, with a final sampling on Day 58. Although development was continuous, four distinct stages were identified (1-primordia, 2-homorhabdic unreflected, 3-homorhabdic reflected, 4-heterorhabdic), partially recapitulating the presumed phylogenetic evolution of this character in the Pectinidae. The absence of a ventral grcove in all stages suggests that the particle transport mechanism of pectinids evolved independently of such a structure, which is found in other bivalve families. Similarly, the absence of latero-frontal cilia in all specimens up to the largest observed (4 mm) indicates that the single row found in adults is a later development, rather than a vestige of a more abundant ciliation in ancestral forms. The anatomical data, together with in vivo observations of feeding in postlarvae, suggest that the developmental stages of the P. maximus gill correspond to critical changes in gill function. The early life of P. maximus may thus be characterized by distinct functional changes in feeding.     

Fluid mechanics of the mussel gill: The lateral cilia

Quantitative data on the water currents produced by the ciliary tracts of the gill filaments are needed to understand the fluid mechanics of suspension feeding in bivalves, as well as in other ciliary suspension feeders. This paper investigates the water currents produced by the bands of lateral cilia, as studied on isolated gill filaments, gill fragments, and intact gills of the mussel Mytilus edulis L with severed adductor muscles. The metachronally beating cilia produce oscillatory currents near the oscillating enveloping surface of the ciliary bands and rectilinear currents, the interfilamentary through-currents, farther from the surface. It is suggested that the oscillatory currents play an important role in the fluid mechanical capture of suspended particles. In the intact gill the interfilamentary currents pass the bands of lateral cilia at velocities that are two or more times higher than those generated by the bands of isolated filaments. The mussel gill is compared with an optimized peristaltic pump.     

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.     

Persistence,morphology, and nutritional state of a gastropod hosted bacterial symbiosis in different levels of hydrothermal vent flux

The limpet, Lepetodrilus fucensis McLean, is found in prominent stacks around hydrothermal vents on the Juan de Fuca Ridge. L. fucensis hosts a filamentous episymbiont on its gill lamellae that may be ingested directly by the gill epithelium. To assess the persistence of this symbiosis I used microscopy to examine the gills of L. fucensis from sites representing its geographic range and different habitats. The symbiosis is present on all the specimens examined in this study, including both sexes and a range of juvenile and adult sizes. Next, I aimed to determine if patterns in bacterial abundance, host condition, and gill morphology support the hypotheses that the bacteria are chemoautotrophic and provide limpets with a food resource. To do so, I compared specimens from high and low flux locations at multiple vents. My results support the above hypotheses: (1) gill bacteria are significantly less abundant in low flux where the concentrations of reduced chemicals (for chemoautotrophy) are negligible, (2) low flux specimens have remarkably poor tissue condition, and (3) the lamellae of high flux limpets have greater surface area: the blood space and bacteria-hosting epithelium are deeper and have more folds than low flux lamellae, modifications that support higher symbiont abundances. I next asked if the morphology of the lamellae could change. To test this, I moved high flux limpets away from a vent and after 1 year the lamellar depth and shape of the transplanted specimens resembled low flux gills. Last, I was interested in whether bacterial digestion by the gill epithelium is a significant feeding mechanism. As bacteria-like cells are rarely apparent in lysosomes of the gill epithelium, I predicted that lysosome number would be unrelated to bacterial abundance. My data support this prediction, suggesting that digestion of bacteria by the gill epithelium probably contributes only minimally to the limpet’s nutrition. Overall, the persistence and morphology of the L. fucensis gill symbiosis relates to the intensity of vent flux and indicates that specimens from a variety of habitats may be necessary to characterize the morphological variability of gill-hosted symbioses in other molluscs.     

Changes in feeding mechanisms during early ontogeny in juveniles of <Emphasis Type="Italic">Crepidula fecunda</Emphasis> (Gastropoda,Calyptraeidae)

Feeding in early life stages of the sedentary snail Crepidula is effected both by the use of the radula and by filtering with the gill. The present study is a contribution to the knowledge of the mechanisms of feeding by early juvenile snails of Crepidula fecunda. Experimental observations were made on specimens of known ages as they were fed constant concentrations of microalgae in the presence of a primary biological film on a glass substrate. Feeding activity was filmed under the microscope, and images were digitized for the identification and quantification of feeding structures. A morphological analysis was made of the structures associated with this process. Results showed that the radula was functional beginning in 1-day-old juveniles, and the gill functioned in respiration. Gill function in filter-feeding began in juveniles of 9 days old when dorsal and ventral ciliation had developed, as well as the food pouch. The latter structure begins activity as soon as the gill starts food collection. Osphradia appeared simultaneous with development of the filter feeding capacity by the gill. In their earliest stages after metamorphosis, the young snails begin life by radular scraping of primary biofilms, gradually shifting to filter feeding as the gill developed a critical number of filaments and cilia.     

Gill development, functional and evolutionary implications in the Pacific oyster Crassostrea gigas (Bivalvia: Ostreidae)

Development of the Crassostrea gigas gill was studied in order to better understand the feeding biology of early life stages, identify potentially critical developmental stages which may influence rearing success or recruitment to wild populations, and shed light on the evolution of the basic bivalve gill types. Larvae and juveniles were reared in an experimental hatchery, and larger specimens were obtained from a commercial hatchery. Specimens were relaxed, fixed, dried, and observed using scanning electron microscopy (SEM). The right and left gills developed symmetrically, via a “cavitation–extension” process from the gill buds. The inner demibranchs developed first (V-stage, 0.29–2.70 mm), in a sequential postero-anterior series of homorhabdic filaments. The outer demibranchs developed later (W-stage, from 2.70 mm), also as homorhabdic filaments, synchronously along the gill axis. The principal filaments (PF) developed from the progressive fusion of three ordinary filaments (OF), at a size of 7.50 mm, and the consequent plication was accentuated by the formation of extensive tissue junctions. Effective filament number (number of descending and ascending filaments) showed a marked discontinuity at the transition from the V- to the W- stage of the gill. Filament ciliation showed several important changes: establishment of OF ciliation in the homorhabdic condition (2.70 mm), ciliary de-differentiation of the PF in the heterorhabdic condition (7.50 mm), and establishment of a latero-frontal cirri length gradient from the plical crest to the PF base. Reversal of direction of ciliary beat is also necessary prior to adult functioning of the PF. Three major transitions were identified in C. gigas gill development, each potentially important in rearing success or wild population recruitment: (1) transition from velum to gill at settlement, (2) transition from a V- to a W-shaped gill (2.70 mm), and (3) transition from the homorhabdic to the heterorhabdic condition (7.50 mm). Complete gill development was much more prolonged than in species previously studied. The major ontogenetic differences between the C. gigas heterorhabdic pseudolamellibranch gill and the pectinid heterorhabdic filibranch gill suggest that the heterorhabdic condition evolved independently in these two bivalve families.     

Effects of UV-B radiation on the gills of Catla catla during early development

Scanning electron microscopic (SEM) study of gills of Catla catla catla (17-day-old) exposed to UV-B radiation (145?µW?cm^?2 at the water surface) for three different exposure times: 5, 10 and 15?min was conducted. Fish without UV-B exposure served as control. UV-B radiation damaged both gill filaments and lamellae. The intensity of damage was minimal in 5?min exposed fish, followed by 10?min exposed fish and maximal in 15?min. The gill epithelium was severely damaged in 15?min irradiated fish compared to control. Pavement cells (PVCs) were damaged and the numbers of microridges within PVCs decreased. The deep boundary of PVC was not clear. In some area of gill epithelium, PVCs were destroyed and mitochondrion-rich cells (MRCs) were exposed. The 5?min exposure reduced the number of microridges in the PVCs, but the boundary of PVCs was still visible. MRCs in the gill epithelium were not exposed in 5?min exposed fish. The damage to PVCs and subsequent exposure of MRCs in UV-B irradiated fish may hamper respiratory functions and disturb osmoregulation in catla.     

Ultrastructure of the gill of the hydrothermal vent bivalveCalyptogena magnifica,with a discussion of its nutrition

Calyptogena magnifica Boss and Turner, 1980, a new Vesicomyidae found during the Galápagos expedition in hydrothermal vents of the East Pacific Rise, was collected in the same Rise at 21°N during the Oasis expedition (March 1982), and samples of the gill were fixed for ultrastructural observations. The large size and structure of the gill indicate that this is the organ mainly involved in the nutritional processes ofC. magnifica. Despite the classic structural appearance of the external cilia of its gill, and an obvious production of mucus,C. magnifica is not a filter-feeder, as it does not use filtering processes to provide its major source of nutrition. Negligible particulate transfer is evidenced by reduction of the ciliary groove, of the labial palps and of the digestive tube, as well as by the absence of mucous strings. Histological and ultrastructural observations endorse the hypothesis that endocellular chemoautotrophic bacteria play an important role in the nutrition of the clam. Except for a superficial zone of ciliated cells, most of the gill tissue is comprised of cells which appear to be bacteriocytes, and which are perfectly integrated into the gill tissue and contain abundant and normally reproducing bacteria. The differences observed in the structure of the bacteriocytes suggest a cyclic process of their colonization by bacteria, their possible resorption, and their replacement by new bacteria-infected cells. Energetic substrates (sulfides and organic molecules) are probably directly absorbed by the bacteriocytes through the microvilli of the epithelial cells. Abundant fingerprint-like mitochondria in ciliate cells attest to a particularly high metabolic activity, perhaps related to active biosynthesis.     

A functional interpretation of cilia and mucocyte distributions on the abfrontal surface of bivalve gills

The lack of fundamental data on the abfrontal surface of bivalve gills has prompted a comparative study of cilia and mucocytes on this surface, using scanning electron microscopy and histology on eight species of bivalves, representing seven families and the four major gill types: Mytilus edulis, Modiolus modiolus, Arca zebra, Placopecten magellanicus, Crassostrea virginica, Spisula solidissima, Mercenaria mercenaria, and Mya arenaria. Abfrontal cilia and mucocytes were found in all species studied, with types and densities differing within and between gill types. The three species of homorhabdic filibranchs presented different densities of abfrontal cilia and mucocytes, from very dense in M. edulis to sparse in A. zebra. The heterorhabdic gills had intermediate cilia and mucocyte densities, with highest concentrations of both abfrontal cilia and mucocytes on the principal filaments. The eulamellibranchs showed low ciliary densities together with high mucocyte densities, especially in S. solidissima, where the abfrontal mucocytes were glandular. These results indicate that: (1) the abfrontal surface is a vestigial mucociliary surface; (2) the abfrontal surface cannot participate in water pumping in most species, due to low ciliary densities; and (3) species with high densities of abfrontal mucocytes could utilize abfrontal mucus to reduce drag, especially in the highly fused gills, such as those of the eulamellibranchs. The differing distributions of abfrontal cilia and mucocytes may reflect different selective pressures acting on the gills within the various taxa. Received: 12 February 2000 / Accepted: 10 September 2000     

全氟辛烷磺酸（PFOS）急性暴露对斑马鱼鳃显微结构的影响

采用光镜和电镜的方法,探讨了高浓度全氟辛烷磺酸(PFOS)急性暴露对斑马鱼(Danio rerio)鳃显微结构的影响.暴露组PFOS浓度分别为0.5、1、2mg·L-1,同时设对照组,连续暴露7d,取鳃制备石蜡切片,进行光镜和扫描电镜观察.结果表明,7d后PFOS暴露组斑马鱼鳃部均有不同程度的损伤,石蜡切片上可观察到暴露组鱼鳃上皮细胞坏死脱落,鳃小片断裂或融合,并伴有鳃丝充血,且病变程度随PFOS浓度的升高而加重;扫描电镜下可观察到暴露组鱼鳃鳃丝表面分泌物增多,鳃丝细胞脱落或水肿,鳃小片前端细胞破损.高浓度PFOS急性暴露可在短期内对斑马鱼鳃组织造成严重损伤,且损伤程度存在剂量依赖效应.     

Phenotypic variations in the gills of the symbiont-containing bivalve Lucinoma aequizonata

The marine bivalve Lucinoma aequizonata (Lucinidae) maintains a population of sulfide-oxidizing chemoautotrophic bacteria in its gill tissue. These are housed in large numbers intracellularly in specialized host cells, termed bacteriocytes. In a natural population of L. aequizonata, striking variations of the gill colors occur, ranging from yellow to grey, brown and black. The aim of the present study was to investigate how this phenomenon relates to the physiology and numbers of the symbiont population. Our results show that in aquarium-maintained animals, black gills contained fewer numbers of bacteria as well as lower concentrations of sulfur and total protein. Nitrate respiration was stimulated by sulfide (but not by thiosulfate) 33-fold in homogenates of black gills and threefold in yellow gill homogenates. The total rates of sulfide-stimulated nitrate respiration were the same. Oxygen respiration could be measured in animals with yellow gills but not in animals with black gills. The cumulative data suggest that black-gilled clams maintained in the aquarium represent a starvation state. When collected from their natural habitat black gills contain the same number of bacteria as yellow gills. Also, no significant difference in glycogen concentrations of the host tissues was observed. Therefore, starvation is unlikely the cause of black gill color in a natural population. Alternative sources of nutrition to sulfur-based metabolism are discussed. Denaturing gradient gel electrophoresis (DGGE) performed on the different gill tissues, as well as on isolated symbionts, resulted in a single gill symbiont amplification product, the sequence of which is identical to published data. These findings provide molecular evidence that one dominant phylotype is present in the morphologically different gill tissues. Nevertheless, the presence of other phylotypes cannot formally be excluded. The implications of this study are that the gill of L. aequizonata is a highly dynamic organ which lends itself to more detailed studies regarding the molecular and cellular processes underlying nutrient transfer, regulation of bacterial numbers and host–symbiont communication. Received: 1 September 1999 / Accepted: 1 February 2000     

Surface morphology of dogfish (Scyliorhinus canicula) gill epithelium,and surface morphological changes following treatment with zinc sulphate: A scanning electron microscope study

The surface morphology of the gill epithelium of the dogfish Scyliorhinus canicula L. (collected near Barcelona, Spain, in February–March, 1981) was studied by scanning electron microscopy. Pavement cells exhibited either surface microvilli or microridges, which were randomly distributed on both the primary (afferent and efferent sides and interlamellar spaces) and secondary epithelium. Chloride cell apical regions on the afferent side displayed characteristics closer to freshwater than to marine teleosts: no apical pits were detected; chloride cells displayed longer microvilli than those of adjacent cells. Two morphologically different cell types were identified: a large chloride cell and a smaller cell (probably a chloride cell too), measuring 4 to 7 m and 1 m, respectively, the latter being dominant in the interlamellar spaces. Apart from pavement cells, the mucous cell was the prevalent cell type on the efferent region. The respiratory epithelium consisted of a mozaic of typical epithelial cells; some chloride and mucous cells were present, mainly located at the base of the secondary lamellae. Surface morphological changes were monitored after exposing the dogfish to subacute zinc treatment: 10 ppm Zn (ZnSO₄) for 3 wk. The chloride cell was the only cell type that underwent any modifications: microvilli became longer and tips were swollen following Zn treatment. The results are discussed in relation to a previous study on the effects of zinc sulphate on chloride cell response and heavy metal distribution in excretory organs of the dogfish.     

Respiration and nitrogen excretion of zooplankton. II. Studies of the metabolic characteristics of starved animals

Respiration and nitrogen-excretion studies were carried out on several species of zooplankton (Meganyctiphanes norvegica, Phronima sedentaria, Acartia clausi and Sagitta setosa) under starvation. Although all the species were mainly ammonotelic, apparently a significant amount of organic nitrogen was excreted; the validity of the measurements and their significance are discussed. The effect of duration of starvation showed for M. norvegica and A. clausi two different patterns of behaviour, which were chiefly a function of the rate of biomass turnover of the species studied. The rates of metabolism, chemical composition, and reaction to starvation varied with season in M. norvegica. The physiological balance of the experimental animals was examined by calculating the protein carbon equivalent to respiratory and excretory catabolism, and by use of atomic O:N ratios. Starved individuals catabolized more protein carbon than can be accounted for by the amount of respiratory oxygen utilized. A hypothesis, which suggests that there are three levels of resistance to starvation, is proposed to explain this paradox, and its metabolic basis is discussed.     

Biorheological properties of intertidal organic fluff on mud flats and its modification of gill ventilation in buried sole <Emphasis Type="Italic">Solea solea</Emphasis>

Intertidal mudflats are important nursery grounds for juveniles of many fish species. However, they are being used increasingly to farm bivalve molluscs, which produce large amounts of organic “fluff”, overlying the mud. Fish such as sole, Solea solea, hide in this fluff from potential predators, but the energy consumed by respiring the fluff may be high due to its biorheological properties. We developed an ichthyoviscometer. It incorporates a freshly killed fish as a viscometer, and we developed it to measure the rheological properties of fluids and suspensions, including fluff, at scales encompassing those in gill ventilation. We have shown that the rheological behaviour of fluff is close to that of a gel with a yield stress strongly dependent on particulate organic matter concentration ([POM]). This has allowed us to model fluff flow through the gill channels in living sole as a function of fish size and [POM], showing that in a 26-g sole, fluff would halve flow at a [POM] value of 3.2 g l⁻¹, and stop it at 3.4.