Structural and functional peculiarities of the digestive system of Cucumaria frondosa (Echinodermata: Holothuroidea)

The fine structure of the intestinal epithelium of Cucumaria frondosa has been examined. The digestive cells possessed some specific ultrastructural peculiarities which testify to their participation in the transport of substances and in the processes of intracellular and cavitary digestion. The main units of the intestinal epithelium appear to be multifunctional digestive cells in which the phenomenon of compartmentalization among the organelles' groups, intended to carry out various functions, occurs. The heterogeneity of the structure of populations of the digestive cells is explained. Both primitive and progressive features of the structure of the digestive tract of C. frondosa are indicated.     

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 highly specialised gut of Fritillariidae (Appendicularia: Tunicata)

The morphology and some functions of the gut of Fritillaria pellucida and Fritillaria formica (Fritillaridae) were investigated by light and electron microscopy, and also by means of histochemical and immunohistochemical techniques. Fritillarids, very important for their abundance and ecological impact in marine ecosystems, have a very simplified gut: a straight oesophagus connects the pharynx to the digestive nucleus, composed of globular stomach and rectum, connected dorsally through a very short proximal intestine. The latter is characterised by a few (two to four) extremely specialised cells, completely filled with mitochondria associated tightly with membrane infoldings showing strong ATP-ase activity, and probably involved in the osmoregulation of internal body fluids. The gut is formed of an extremely low number of cells, which, although poorly diversified, are very large in the stomach and rectum. Food transfer along the gut depends on and is regulated by well-developed cardiac and pyloric valves, and signs of general digestive and absorptive activity are recognisable all along the brush border of the main tracts. The macroscopic organisation and cytological characters of the gut in fritillarids are completely different from those of the oikopleurids. In particular, fritillarids lack specialised cells for endocytosis and intracellular digestion, like those described in the genus Oikopleura. The general simplification and specialisation observed in Fritillaria gut may account for their elevated growth rate and abundant diffusion in all oceans.     

Digestion in a subtidal population of Mercenaria mercenaria (Bivalvia)

The digestive processes of Mercenaria mercenaria (Linn.) were studied in a natural subtidal population. Hourly sampling of 4 to 5 quahogs was conducted over one 25 h period in August, 1978, at Woods Hole, Ma., USA. Crystalline style length did not vary significantly with time, tide, particulate C, N or suspended material in the water. Changes in secretory activity of the style sac epithelium were not evident from histological sections. Based on cellular morphological differences, digestive tubules were individually classified into one of 4 categories, indicative of the state of intracellular digestion within the digestive cells. All 4 tubule types were present in individuals from each sample hour, signifying that intracellular digestion occurs continually within the digestive gland. A 3 h time lag is evident between times of peak C, N or suspended materials and apparent peak levels of absorptive (Type II) tubules.Contribution No. 76 Marine Science Institute, Northeastern University; Nahant, MA 01908, USA     

Digestive rhythms in the mussel Mytilus edulis

Mytilus edulis L., collected from a mid-tide level on the shore, showed rhythmic changes in mantle fluid pH, crystalline style pH, style length and total protein, and in the amylase activity in the digestive gland. These changes were correlated with the changes in tidal height. Style size may be related to extracellular digestion in the stomach. Style size and amylase content of the style were not significantly correlated with each other. The changes in amylase activity in the digestive gland confirmed the existence of a tidal rhythm for intracellular digestion in M. edulis.     

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.     

Co-ordinated rhythms of digestion,absorption and excretion in Mytilus edulis (Bivalvia: Mollusca)

Sequential alternation of extracellular digestion in the stomach and intracellular digestion in the diverticula appears widespread among bivalves. The present study documents some physiological consequences of such processes in Mytilus edulis L. collected during 1981 from Whitsand Bay, Cornwall, England. Pronounced temporal fluctuations in faecal deposition are described that relate, in terms of amplitude and period, to both sinusoidal rhythmicity established for ammonia excretion and changes in the morphology of digestive tubules. Although at least partially synchronised among replicate groups of mussels, these cycles bore no consistent relationship with exogenous influences. Hourly fluctuation in the net absorption efficiency for nitrogen, as evidenced by the mean percentage ±2 SE, measured over 24 h sampling periods, was considerable (16.0±53.7, 49.3±10.9 and 52.8±6.6 for mussels acclimated in March, June and October, respectively). This variation in absorption derived from an inverse relationship between the percentage nitrogen within faeces and the rate of faecal egestion. Accordingly, peaks of faecal deposition presumably represented the pulsed remnants of intracellular digestion. Co-ordinated rhythms of digestion, absorption and excretion were thus evident in M. edulis. These processes displayed seasonally dependent periodicities of approximately 8, 3 and 4 h in March, June and October, respectively. It was concluded that, at least for M. edulis, this previously unquantified rhythmicity of physiological processes warrants careful consideration during assays commonly undertaken in the complication of nutrient and energy budgets.     

Microbial activity, oxygen status and fermentation in the gut of the irregular sea urchin Echinocardium cordatum (Spatangoida: Echinodermata)

Microbial activity in the gut of the detrivore irregular sea urchin Echinocardium cordatum (Pennant) was studied by measurements of oxygen profiles, fermentation end products, C/N ratios, pH and concentrations of sulphide and sulphate in the gut segments, including the nodules of the intestinal caecum. The highest oxygen flux and consumption, and highest concentrations of short-chained fatty acids, and the lowest pH values occurred in the anterior stomach segments, including the anterior caecum. The C/N ratios indicate synthesis of microbial biomass in the caeca. The concentration of sulphate was high in the anterior stomach segments, whereas sulphide was only detectable in the nodules of the intestinal caecum. The anterior and intestinal caeca were the major sites for microbial activity, and oxidation of acetate and propionate produced in the two caeca corresponded to at least 9% of the total respiration of the sea urchin. Microbial fermentation, especially in the anterior caecum, seems to be important for the metabolism of E. cordatum, allowing the sea urchin to utilise refractile carbohydrates. The functions of the intestinal caecum are probably both residual fermentation and oxidation of accumulated sulphide. Received: 25 February 1998 / Accepted: 30 June 1998     

The histology,histochemistry and ultrastructure of the cement apparatus of three adult sessile barnacles,Elminius modestus,Balanus balanoides and Balanus hameri

The histology, histochemistry and ultrastructure of the cement gland cells of 3 species of sessile barnacles are described. Cells of both Elminius modestus Darwin and Balanus balanoides (L.) (membranous bases) are similar histologically and ultrastructurally, and within the cells there are 2 distinct regions. One is the site of secretion accumulation, the other the site of secretion synthesis. In Balanus hameri Ascanius (calcareous base) the cells are different in that the cytoplasm is more homogeneous. In all 3 species protein is the final secretion product (cement) of the cement cells. The mode of discharge of secretion from the cells differs between the membrane based and calcareous based barnacles. In the former, there is a series of collecting canals within the cytoplasm of the cement cells (intracellular canals) which join with the larger extracellular cement ducts. Secretion passes into the intracellular canals and is moved along to the larger cement ducts, which have an inner chitin lining. In Balanus hameri (calcareous base) there are no intracellular collecting canals. Secretions are thought to pass from the cement cells into the cement duct cells direct. The cement ducts in this species are not chitin lined.     

Cytoenzymatic investigation of intracellular digestion in the symbiont-bearing hydrothermal bivalve <Emphasis Type="Italic">Bathymodiolus azoricus</Emphasis>

Invertebrates harbouring endosymbiotic chemoautotrophic bacteria are widely distributed in a variety of reducing marine habitats, including deep-sea hydrothermal vents. Bathymodiolids are dominants of the biomass at geochemically distinct vent sites of the Mid Atlantic Ridge (MAR) and thus are good candidates to study biological processes in response to site-specific conditions. To satisfy their nutritional requirements, these organisms depend to varying extent on two types of chemoautotrophic symbionts and on filterfeeding. The quantitative relationships of the nutritional modes are poorly understood. Using enzyme cytochemistry, electron microscopy and X-ray microanalysis, the structural and functional aspects of the cellular equipment necessary for lysosomal digestion was studied. We provide evidence for the following: (1) the basis of intracellular digestion of symbionts in Bathymodiolus azoricus from two geochemically distinct vent sites was not mainly in the large lysosomal bodies as previously thought (based on the membranous content resembling bacteria); (2) senescent bacteria are autolysed, possibly by bacterial acid phosphatase, that is more likely a cell cycling of the symbionts rather than an active lysosomal digestion by the host; (3) the consistent absence of hydrolases may indicate the improper use of the name “lysosome” for large vesicles at the base of the gill bacteriocytes (4) nutrient transfer in B. azoricus, therefore, may more likely be accomplished through leaking of metabolites from the symbiont to the host, not excluding lysosomal resorption of dead bacteria as an auxiliary strategy for organic molecule transfer; (5) evidence is provided for microvillar transfer of substances from the seawater that may indicate filter-feeding, in non-symbiotic ciliated gill cells of mussels from Lucky Strike; (6) two types of lysosomal vesicles can be distinguished in digestive cells based on their enzymatic content and their elemental composition.     

Morphogenesis of the digestive system during metamorphosis of the nudibranch Doridella steinbergae (Gastropoda): Conversion from phytoplanktivore to carnivore

The structural changes undergone by the digestive system of a phytoplanktotrophic nudibranch larva during metamorphosis into a benthic carnivore are described using histological and electron microscopic techniques. The relative positions of the stomach, digestive gland, and distal end of the intestine are rearranged at metamorphosis by the actions of the larval retractor muscle and the accessory pedal retractor muscle. Although the anus and distal end of the intestine are secondarily displaced to the posterior end of the gastropod, the stomach undergoes further torsional displacement at metamorphosis. The tissues of the larval stomach and distal end of the larval esophagus undergo drastic alteration at metamorphosis. The larval stomach consists of a ciliated vestibule, which receives the openings of the esophagus and left digestive gland, a gastric shield, a style sac, and an intestinal groove. All of these areas, except the vestibule, are destroyed by cell dissociation at metamorphosis. The vestibule becomes the ventral stomach of the benthic stage and the proximal end of the intestine becomes enlarged and muscularized to form the dorsal stomach of the benthic stage. The metamorphic changes involving the distal end of the esophagus include the continued development of the radula and oral lip glands, which both appear in rudimentary form during the larval stage, and differentiation of the buccal pump, salivary glands, and oral lips.     

Cytological study and immunohistochemical location of PSP toxins in foot skin of the ormer, Haliotis tuberculata, from the Galician coast (NW Spain)

The ormer, Haliotis tuberculata Linnaeus, 1758, is able to accumulate paralytic shellfish toxins (PST) in the foot side skin. Epidermal and secretory cells are described separately for the pedal side and sole epithelia, as notable differences have been observed. Melanin and photosynthetic pigment granules in the epidermal cells of the foot side epithelium give this skin its appearance and characteristic color. This epidermis also has abundant secretory cells containing at least three kinds of secretory granules. Conversely, the epidermal cells of foot sole epithelium do not have pigment granules, and the secretory cells are mostly subepithelial. Immunohistochemical reaction against PST (saxitoxin and its derivatives) showed that the toxins are located in specific cells of the foot side epithelium, which are distinct from the epidermal cells. Immunofluorescence also reveals toxins throughout many areas of the mucilage covering the epithelium, suggesting the secretory nature of the cells containing the toxins. These results do not show any apparent relationship between melanin or photosynthetic pigment granules and PST content. PST were not detected in the foot sole epithelium either immunohistochemically or by HPLC. Considering the toxic characteristics of ormers described in previous studies and the results presented here, the authors discuss the role of toxins retained or produced by the foot side epithelium Received: 25 July 2000 / Accepted: 7 November 2000     

Physiology of the digestive tract of the sedentary polychaete Terebellides stroemi

The digestive tract of the sedentary polychaete Terebellides stroemi, collected from 88 m depth on the French Catalan Coast, was studied using histological, biochemical, cytological and spectrographic techniques. The ciliated oesophagus possesses a ventral pharyngeal pouch with a muscle bulb. The stomach is divided into two parts, the fore stomach is ciliated, but the hind stomach takes the place of a gizzard with a thick peritrophic membrane and a thick muscle mantle. The digestive gland enters the digestive tract at the boundary of the two stomachal areas; it is comprised of epithelial lamellae with a glandular epithelium. The gland cells contain a spherocrystal secretion with a high calcium and phosphorus content. The cells are also absorbant due to their microvillar brush border and to the storage of glycogen and lipids in the basal epithelium. A biochemical study using the Apizym System confirmed the secretion of enzymes such as phosphatases, esterases and various carbohydrases. Tryptic and chymotryptic enzymes were visualized by biochemical hydrolysis of synthetic substrates. The epithelium of the fore intestine is similar in appearance to the epithelium of the digestive gland, but consists of a single epithelial layer. Hind intestine and rectum are only excretory systems. The presence of this digestive gland does not seem to have been described elsewhere in polychaetes but is similar to the Morren's gland of some oligochaetes. The study was carried out during 1982 and 1983.     

Ultrastructure of olfactory mucosa in the homing baltic sea trout Salmo trutta trutta

Scanning and transmission electron microscopy techniques have been used to study the olfactory organs of adult fresh-water adapted Baltic sea trout Salmo trutta trutta L. homing back to the River Umeälven in Västerbotten, Sweden. Parr and smolt show only minor differences in comparison to the adult mucosa. All fish have polymorphism of receptors, supporting cells and basal cells, and a dynamic cell population with primary cells basally. These mature and die, and are either extruded or phagocytized. The mucosa has a remarkably large population of loose connective tissue cells (plasma cells, macrophages and lymphoid wandering cells) as well as all types of leucocytes. The olfactory organs of sea trout have labyrinth cells, a cell type unique for vertebrates. These cells probably help to maintain an optimal ion balance, which is of great ecological importance for this migrating species. The mucosa cilia have small tubercles arranged in a spiral pattern and correlated with transitional filaments from the 9 peripheral fibrils. A hypothesis on the relationship of primary supporting cells, primary nonsensory cells, immature fibroblasts and Schwann cells is presented.     

The causes and consequences of partial prey consumption by wolves preying on moose

For a wide range of taxa, partial prey consumption (PPC) is a frequent occurrence. PPC may arise from physiological constraints to gut capacity or digestive rate. Alternatively, PPC may represent an optimal foraging strategy. Assessments that clearly distinguish between these causes are rare and have been conducted only for invertebrate species that are ambush predators with extra-intestinal digestion (e.g., wolf spiders). We present the first strong test for the cause of PPC in a cursorial vertebrate predator with intestinal digestion: wolves (Canis lupus) feeding on moose (Alces alces). Previous theoretical assessments indicate that if PPC represents an optimal foraging strategy and is not caused by physiological limitations, then mean carcass utilization is negatively correlated with mean kill rate and the utilization of individual carcasses is uncorrelated with time between kills. Wolves exhibit exactly this pattern. We explore how the typical portrayal of PPC by wolves has been not only misleading but also detrimental to conservation by promoting negative attitudes toward wolves.     

Gastric evacuation of different foods byNephrops norvegicus (Crustacea: Decapoda) and estimation of soft tissue ingested,maximum food intake and cannibalism in captivity

Gastric evacuation, maximum food intake and mortality during starvation were studied inNephrops norvegicus (L.) by means of simple laboratory experiments. Lobsters were collected by trawling off Barcelona (northwest Spain) at 400 m depth in winter 1986, and were held individually or in groups and fed different types of food:Nereis spp. (soft tissue, muscle), some species of Crangonidae (mostly hard tissue, carapace) andEngraulis encrassicolus (soft and hard tissues, combined muscles and bones). The digested matter was subsequently examined and analyzed from the stomach contents. Laboratory results were compared with the stomach contents of individuals collected from the field. Gastric evaluation was calculated as weight of stomach contents at Timet = stomach-content weight × 100:weight of food ration — residual weight of food, and the characteristic appearance of identifiable food items at different digestion times (0.5, 1, 2, 3, and 5 h) was recorded and illustrated, providing a basis for establishing the time of ingestion of the last meal and the underestimation of soft, compared to hard tissues. Underestimation of soft tissues may yield errors which constitute up to 10% of the soft food actually ingested. Maximum food intake was estimated at 0.25 g g^–1 body wt d^–1, and feeding did not recommence until 24 h after a previous meal. Finally, we present data on mortality resulting from cannibalism which occurred in conjunction with starvation: this ranged from 0% in isolated individuals to 36.6% in individuals held in groups.     

Larval and post-larval morphogenesis in the gutless protobranch bivalve Solemya reidi (Cryptodonta: Solemyidae)

Structural changes occurring in the alimentary tract, perivisceral cavity, foot, hypobranchial gland, and gills of larval and post-larval Solemya reidi, a gutless protobranch bivalve, were examined using both light and electron microscopy at 1, 3, 5, 18, 34 and 42 d after fertilization. A fully developed mouth, esophagus, and anus, together with the rudiments of a stomach and rectum are present before metamorphosis. At metamorphosis, the cells making up the dorsal and lateral walls of the stomach dissociate, and by 18 d the mouth and anus are the only remaining portions of the alimentary tract. The larval test is ingested into the perivisceral cavity at metamorphosis and its autolysis continues through at least the 42nd day after fertilization. The foot, gills, and hypobranchial gland, poorly developed at metamorphosis, develop slowly and are still undergoing extensive morphogenesis at 42 d.Harbor Branch Oceanographic Institution Contribution No. 601.     

Herbivory by the myctophid fish Ceratoscopelus warmingii

During a study of midwater fish feeding in the oligotrophic North Pacific gyre in August 1978, nine specimens of a tropical-subtropical myctophid, Ceratoscopelus warmingii, were found to have been feeding on mats of diatoms, Rhizosolenia spp. Chemical analyses of the intestinal contents of several additional specimens showed significantly elevated levels of diatom degradation products. Therefore, C. warmingii, although known to feed on zooplankton, appears to be adapted for occasional herbivory. Such an adaptation is exceptional within a major ecological group of oceanic fishes that was previously thought to be exclusively carnivorous. Like omnivorous and herbivorous fishes in general, C. warmingii has a longer intestine than other myctophid species. As an adaptive response to competition from a diverse fish fauna in a low-productivity environment, occasional herbivory by C. warmingii runs counter to the theoretical expectation by expanding rather than narrowing the range of potential food types.     

Responses of an estuarine population of the blue mussel Mytilus edulis to heated water from a steam generating plant

An entire bed of the blue mussel Mytilus edulis, consisting of 5,000 individuals/m², died during June, 1971 in the effluent canal of a steam generating plant when the temperature increased above 27°C. Similarly, the population in the intake canal disappeared when temperatures rose above 27°C in August. Laboratory studies showed that M. edulis could not tolerate continuous temperatures above 27°C, and feeding stopped shortly after the mussels were exposed to 25°C. Histopathological studies indicated that the cause of death of this bivalve was associated with degeneration of the frontal and laterofrontal cilia of the columnar epithelium of the gill filaments. In conjuction with this, there was necrosis and sloughing of the epithelium of the intestinal diverticula. Extensive amoebocytic infiltration was noted in the byssogenous cavity, gill filaments and stomach wall.This paper represents part of a dissertation by the first author, submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, University of Rhode Island, January 1973.     

Protective skin structures in shell-less marine gastropods

Skin and stomach epithelia of the four suborders of nudibranch gastropods (Mollusca, Opisthobranchia) are characterized by large intracellular ovoid disks, here called spindles. These spindles are an evolutionary novelty in the nudibranchs; in shell-less sacoglossan species they are missing. We here examined whether the distribution and occurrence of the spindles is consistent with the assumption of a protective role against discharging nematocysts of co-habiting and prey Cnidaria. Spindles were abundant in epidermal cells of regions exposed to nematocysts, such as the cerata, the lips, the edges of lamellate rhinophores, the surfaces of non-retractile gill-like organs, as well as in the stomach epithelium of eolid and dendronotacean species which feed on Cnidaria. While cells packed with spindles almost exclusively formed the epidermis of eolid and arminacean species, they were less numerous in the skin of dendronotacean and dorid species, where glandular cells predominated. The preponderance of either glandular or spindle cells suggests a dual complementary defense strategy, on the one side the production of mucus coats and aversive glandular secretions, on the other structural defensive devices that are cushion-like entities filled with inert grains.