Biologie d'un copépode des mares temporaires du littoral méditerranéen français: Eurytemora velox

The feeding activity of Eurytemora velox, a brackish copepod from temporary lakes of the south of France, was studied in 1978–1979 using various foods (natural particles, monospecific algal cultures, and artificial food) under different conditions of temperature and salinity. Experiments with Amphidinium sp. or Tetraselmis maculata as food showed that the ingestion rate increased with food concentration according to an asymptotic or a linear relationship. Although of slightly smaller size, T. maculata was ingested at a higher rate than Amphidinium sp. Large maximum daily rations (up to 150% of body carbon with Amphidinium sp. and up to 250% with T. maculata) were attained. These values, which greatly exceed those generally obtained with marine copepods, could result from adaptation of the feeding processes of this copepod to its very rich trophic environment. A significant correlation was demonstrated between ingestion rate and fecal pellet production using T. maculata as food. Therefore, daily fecal production was used as an index of feeding activity in experiments carried out with natural food, T. maculata cultures and artifical food (Tetramin). Increased temperature generally resulted in an activation of grazing and filtration rates and of fecal production at low temperatures (10° to 15°C), but a strong decrease was observed over 22°C. Differences of 10 S over or under the natural salinity level led to a decrease in fecal production, suggesting unachieved acclimatization to salinity variation due to a too short acclimation period before the experiments. Fecal pellet production was higher during the day than during the night. It depended also on the quality of food used: high values were obtained with T. maculata, Phaeodactylum tricornutum, Rhodomonas sp. and Chlamydomonas sp., low values with Chlorella sp. and Amphidinium sp., and medium values with natural food material. The assimilation rate (A) was calculated by Conover's methods. A significant negative correlation was obtained between A and the ash content of the food. High assimilation rates were attained with chlorophycean algae, while natural particulate food produced variable assimilation rates, depending on the amount of inorganic material present.

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Biologie d'un copépode des mares temporaires du littoral méditerranéen français: Eurytemora velox

     

Effects of food-level acclimation on digestive enzyme activities and feeding behavior of Calanus pacificus

A recent hypothesis in the zooplankton literature states that zooplankton acclimate to ambient food concentrations such that higher digestive enzyme activities and, consequently, higher maximum ingestion rates are achieved at higher food levels. To test this hypothesis, adult female Calanus pacificus, collected from the main basin of Puget Sound, Washington, USA, in August 1979 and May 1982, were conditioned for 2 wk at different concentrations of the diatom Thalassiosira weissflogii (=fluviatilis). Ingestion rates and the activity of the digestive enzymes laminarinase, maltase, and cellobiase were measured periodically during acclimation and in a block-designed feeding experiment at the end of acclimation. Consistent with the hypothesis, maximum ingestion rate and digestive enzyme activity were positively correlated. However, in contrast to the hypothesized mechanism, this result arose because both maximum ingestion rate and digestive enzyme activity were negatively correlated with food concentration during acclimation. The enhanced ingestion of copepods following long-term (12 to 14 d) acclimation to low food is similar to that previously described for short-term (e.g. 1 d) starvation. It might be energetically optimal for copepods experiencing a patchy food environment to maintain higher levels of digestive enzymes at low food concentrations in order to exploit high concentrations of food when encountered.     

Food selection by copepods: discrimination on the basis of food quality

The copepod Acartia tonsa displayed nearly two-fold higher ingestion rates on faster-growing cells of the diatom Thalassiosira weissflogii compared to ingestion rates on slower-growing cells of that species at the same cell concentration. Ingestion rates on slow-growing cells were also enhanced by the addition of cell-free aliquots of algal exudate to the experimental feeding chambers. In addition, the faster-growing algal cells were selectively ingested by the copepod when the two cell types were mixed together in different proportions, indicating that physiological differences between growing cells are a critical factor in the food detection/selection process of zooplankton. Consideration of cell carbon, nitrogen, and protein composition suggests that the copepods are maximizing nitrogenous ingestion (total protein and/or nitrogen). Selectivity for cells with higher protein content results in a higher daily protein ration, even if the selection process results in a decreased rate of ingestion in mixtures of cell types.     

Seasonal differences in citrate synthase and digestive enzyme activity in larval and postlarval antarctic krill, Euphausia superba

Antarctic krill maintain large population sizes despite dramatic seasonal fluctuations in food availability, but the mechanisms for this are still debated. The aim of this study was to compare seasonal differences in enzyme activity and respiration rates of larval and postlarval krill to provide insights into their overwintering strategies. Respiration rates, activity of the metabolic enzyme citrate synthase (CS), and those of the digestive enzymes laminarinase and total proteinase were measured in austral summer west of the Antarctic Peninsula, and in autumn in the southwestern Lazarev Sea. The 100-fold difference in chlorophyll a concentrations between the two studies is representative of the classic transition from a summer bloom to sparse winter conditions. Correspondingly, adult krill showed reduced respiration rates and CS activity in autumn. However, their digestive enzyme activity was significantly higher, suggesting more efficient assimilation of food at low food levels. Similar-sized larvae showed no summer-autumn differences in respiration rates and enzyme activity, supporting suggestions that they need to feed and grow year-round. However, trends in enzymatic activity varied between the larval stages measured, implying ontogenetic changes in body structure and function. Published online: 1 August 2002     

Short-term changes in feeding and digestion by the copepodCalanus pacificus

The planktonic marine copepodCalanus pacificus exhibits an enhanced feeding rate, or hunger response, when exposed to food following short periods of starvation. In a scries of laboratory experiments with copepods collected from the main basin of Puget Sound, Washington, during 1982 and 1984, we measured maximum ingestion rate, assimilation efficiency, and digestive enzyme activity to determine the time scales over which the feeding behavior ofC. pacificus responds to increases in food. These laboratory results were then compared to field studies of diel fluctuations in digestive enzymes and gut fluorescence ofC. pacificus in Dabod Bay, a fjord of Puget Sound, during September, 1980, and the closely relatedC. marshallae off the Washington coast, in August, 1981. Laboratory experiments demonstrated that the hunger response ofC. pacificus lasts approximately 6 h before ingestion rate returns to a steady state level of about one-half maximum. On the order of 12h of starvation were required to induce the maximum ingestion rate of the hunger response. Digestive enzyme activities did not change over these time scales. Assimilation efficiency peaked within a few hours of the onset of feeding, with low initial rates possibly related to the period of starvation prior to feeding. These results were consistent with diel patterns observed in the field. The hunger response ofC. pacificus appears to be controlled by processes within the gut, and our results are discussed in relation to recent studies of the digestive processes of calanoid copepods.Contribution No. 1772 from the School of Oceanography, University of Washington, Seattle, Washington 98195, USA     

“Good” and “bad” diatoms: development,growth and juvenile mortality of the copepod <Emphasis Type="Italic">Temora longicornis</Emphasis> on diatom diets

We measured development, growth and juvenile mortality of the common copepod Temora longicornis on 11 different monospecific diatom diets in order to estimate (1) how common the negative effects of diatoms are on the development of this copepod and (2) whether the arrested development is connected to deleterious polyunsaturated aldehydes (PUA) or food nutritional quality. Four diatom species (Thalassiosira weissflogii, Thalassiosira rotula CCMP1647, Leptocylindricus danicus CCPM469 and Skeletonema costatum CCMP1281) supported complete development, whereas development failed in or before metamorphosis on seven diatom species/strains (Chaetoceros affinis CCMP158, C. decipiens CCMP173, C. socialis, T. rotula CCMP1018, Thalassiosira pseudonana CCMP1010 and CCMP1335). However, four out of these seven species were not ingested by nauplii, either due to morphology (Chaetoceros spp.) or large size (T. pseudonana CCMP1010). The growth rate did not correlate with the ingestion rate of PUA, neither with ingestion of food mineral (nitrogen) nor with biochemical (polyunsaturated fatty acids, sterols) components. We show that, although some diatoms are of inferior food quality, this is unlikely to be connected to toxicity or due to a direct limitation by a single food nutritional compound.     

A controlled-temperature plankton wheel

A controlled-temperature plankton wheel is described that is suitable for use on board a ship. The IMER plankton wheel system allows the use of various sizes of experimental bottles, up to 2.2 litres, the simulation of ambient light regimes and variable speed control for the rotation of the experimental bottles. The flexibility of the system was demonstrated by investigating the relationship between temperature and ingestion rate of an herbivorous copepod. Using four of the IMER plankton wheels simultaneously at four different temperatures (5°, 10°, 15° and 20° C), the ingestion rate of Calanus helgolandicus, feeding on Thalassiosira weissflogii, was shown to increase with increasing temperature; from a transformation of log_e (ingestion rate), this relationship was calculated as a Q₁₀ (10° to 20°C) for Copepodite Stage V (Q₁₀ 4.5) and adult female (Q₁₀ 2.7) C. helgolandicus. The possibility of damaging cells, by rotation at 2 rpm, was investigated using the spinose form of the diatom T. weissflogii. Such rotation did not cause any damage to the spines of T. weissflogii, but mixing this diatom with a magnetic stirrer bar did damage the spines to varying degrees, depending on the volume being mixed.     

Ontogenetic development of digestive enzyme activities in larval walleye pollock,Theragra chalcogramma

Activities of digestive enzymes trypsin, amylase and lipase in laboratory-reared walleye pollock, Theragra chalcogramma, were measured from hatching to Day 39 (just before notochord flexion) in 1993. All measurements were conducted individually or semi-individually (groups of two larvae of the same standard length). Close relationships between digestive enzyme activities and morphological development of digestive organs were observed. Activities of trypsin and lipase were low during the transition period from endogenous to exogenous energy. Amylase activity was constant with large variance during the same period. Specific enzyme activities of trypsin and amylase indicated high values with large variance during the early period. All three enzyme activities increased with age afterthe transition period, and the specific enzyme activities became constant. The existence of two types of lipase was suggested. One lipase showed a peak of specific activity at Day 4 and might be related to yolk-sac absorption. The activity of the other lipase increased with age after Day 14 and might be related to digestion of prey lipid. Our results suggest that digestive enzymes included in food organisms supplement larval pollock digestive enzymes.     

Responses of echinoid larvae to food patches of different algal densities

High densities of larvae have been found in areas of high primary production, but it remains unclear whether this is the result of hydrodynamics or of larval aggregative behaviour in the presence of food. In this study, we examined changes in the vertical distribution and swimming patterns of four-armed larvae of the sea-urchin Echinometra lucunter (Linnaeus) around food patches of a range of microalgal densities. We reared larvae in the laboratory in a high or low concentration of either single (Isochrysis galbana) or mixed (I. galbana, Dunaliella tertiolecta, Thalassiosira weissflogii) microalgal species. In Plexiglas cylinders, we experimentally constructed haloclines in which the salinity of the bottom water-layer was 33‰ and that of the top water-layer was 24‰. In a thin layer in the middle of the halocline, we inserted a food patch that consisted of 0, 2500, 5000 or 10 000 T. weissflogii cells ml⁻¹. The presence of a food patch had a pronounced effect on the vertical distribution of larvae. This effect depended upon the algal density of the food patch and varied with dietary conditioning. The number of larvae that were above or within the patch decreased with increasing algal density, and was greater if larvae were reared in low-ration or single-species diets than in high-ration or mixed-species diets. Tracking of individual vertical swimming paths showed that within a few minutes, larvae swam into the patches of low algal density, and to positions just below the patches of the two higher algal densities, and remained there until the end of the experimental period. The greater number of algal cells in the digestive tracts of larvae from treatments with a food patch than in those without a patch confirmed that larvae were feeding on the microalgal cells of the patch. To our knowledge, this is the first study to experimentally show an aggregative behavioural response of invertebrate larvae to a food patch. Such a response may reduce the probability of food limitation and therefore enhance larval survival. Received: 14 February 1997 / Accepted: 24 September 1997     

Biochemical composition and digestive enzyme activity in larvae and postlarvae ofPenaeus japonicus during herbivorous and carnivorous feeding

The growth, survival, digestive enzyme activity and biochemical composition ofPenaeus japonicus (Bate) larvae and postlarvae were measured under three feeding regimes. Larvae were reared through the protozoeal stages usingChaetoceros gracilis. From the first mysis stage, three feeding regimes were used; (A)C. gracilis plusArtemia sp. nauplii, (B)Artemia sp. nauplii alone or (C)C. gracilis alone. No significant difference was found in growth, survival, protein content or lipid content of postlarvae from the treatments receiving the single-feed type, despite the low protein (7%) and highly unsaturated fatty acid content of the alga. Growth of larvae receiving the mixed diet was significantly higher than in the other treatments. Trypsin activity was more strongly influenced than amylase activity by dietary treatment, and differences in the ratio of these enzymes between treatments suggest independent control of their secretion. Trypsin activity recorded in larvae feeding onC. gracilis was up to six time higher than in larvae feeding onArtemia sp. nauplii, apparently in response to the low protein content of the alga. Larvae receiving the mixed diet exhibited an intermediate level of trypsin activity; it is suggested that the ingestion of algae is necessary for optimal assimilation of the zooplankton component of the diet.     

Effects of dietary fatty acids on the reproductive success of the calanoid copepod<Emphasis Type="Italic"> Temora longicornis</Emphasis>

Egg production and hatching success of the copepod Temora longicornis were measured in laboratory experiments and in the field (North Sea). In the laboratory, ingestion of four algal species (Thalassiosira weissflogii, Phaeocystis globosa, Isochrysis sp. and Dunaliella tertiolecta) was followed and the content of fatty acids in the algae was determined. The two food types (T. weissflogii and Isochrysis sp.) that provided the highest ingestion of carbon and long chain fatty acids also resulted in the highest egg production rate (E_r) and hatching success (H%). In contrast, D. tertiolecta led to both low ingestion of carbon and long chain fatty acids, resulting in low reproductive success. There was a positive relationship between the amount of eicosapentaenoic fatty acid [20:5(n-3), EPA] ingested and E_r and H%, and of the ratio between docosahexaenoic and eicosapentaenoic fatty acid [22:6(n-3)/20:5(n-3), DHA/EPA] in the ingested food and H%. In the field, chlorophyll a and specific fatty acids were measured and protists were enumerated, in order to investigate the link between these factors and the reproductive success of T. longicornis. Hatching time was found to be related to temperature and exceeded 120 h at 6°C. No relationship was found between chlorophyll a and reproductive success (E_r or H%). E_r correlated with the concentration of diatoms and ciliates, which were the dominating protists in early spring, indicating that food quantity was the limiting factor for E_r. As in the laboratory experiments, H% was dependent on the fatty acid DHA and the ratio of DHA/EPA, which indicates that the quality of eggs (H%) is linked to the quality of food.Communicated by M. Kühl, Helsingør     

Activity of the principal digestive enzymes in the detritivorous apodous holothuroid Leptosynapta galliennei and two other shallow-water holothuroids

Digestive ability by means of hydrolytic enzyme activities was studied in Leptosynapta galliennei (Herapath, 1865) and two other shallow-water deposit-feeding holothuroids: L. inhaerens (O. F. Müller, 1776) and Holothuria (Panningothuria) forskali (Delle Chiaje, 1828). The holothuroids were collected throughout the year at Roscoff (Britanny, France) from 1980 to 1985, except for L. inhaerens, which was collected in Galway Bay (Ireland) in June/July 1980. Using 25 substrates, the activity of 21 enzymes were recorded to detect hydrolysis of esters (5 substrates), glycosidic bonds (12 substrates) and peptide bonds (8 substrates). The activities of the homogenates of various parts of the gut as well as the digestive juice were measured. Esters and relatively long-chained fatty acids (up to C₁₄) were hydrolysed. Significant hydrolysis of naturally occurring disaccharides and starch (reserve carbohydrate) occurred. The holothuroids were also able to digest hydrolysis products of cellulose and chitin. This may also indicate a capacity to digest the products of complex compounds such as glycolipids or glyco-proteins. Exopeptidases capable of hydrolyzing peptides were present, but endopeptidases able to digest proteolytic chains were not. Most enzymatic activities occurred in all parts of the gut, but some were strictly localized, usually to the anterior intestine. Extracellular digestion of fatty acids, starch and saccharose occurred only in this latter digestive segment. Disaccharidases and peptidases are linked to the plasma membrane of enterocytes. Enzymatic activities associated with lysosomes occur throughout the gut. The diet of shallow-water holothuroids is omnivorous, with a marked preference for food of vegetable origin. Annelids and holothuroids possess a similar capacity to hydrolyse most types of bonds, including peptide bonds. The chemical nature of detritus and its relationship to the nutritional requirements of detritivores must be defined, and gut-retention time ascertained, to discover if different digestive strategies are developed by different detritivores to exploit the same food source in a given environment.     

Ingestion-independent rates of ammonium excretion by the copepod Calanus pacificus

The relationship between food ingested and NH ₊ ⁴ excretion rate was investigated for female Calanus pacificus collected in August, 1982, from the San Juan Archipelago, Washington State, USA. The copepods were preconditioned to 6 densities of the diatom Thalassiosira weissflogii (0 to 10⁴ cells ml^–1) for 30 h before the experiment. The experiment was conducted with nutrients added in excess to maintain equal rates of NH ₊ ⁴ uptake by the diatoms at all densities. Although ingestion rates of C. pacificus varied from 0 to over 20% of body N d^–1 at the different food levels, excretion was a constant 6.6 nM NH ₊ ⁴ copepod^–1 h^–1 or about 10% of body N d^–1. This ingestion-excretion relationship, which is consistent with previous respiration and fecundity studies, suggests that the ecological dominance of C. pacificus only under conditions of high food abundance may be due to a dramatic increase in its growth efficiency as ingestion increases above the level supporting a constant metabolic rate. The maintenance of a constant level of metabolism during relatively short periods of low food abundance may be advantageous if it allows the copepod to exploit more effectively short-term variability in its food resulting from environmental heterogeneity or vertical migration.Contribution No. 1360 from the School of Oceanography, University of Washington, Seattle, Washington 98195, USA     

Diel variability of feeding activity in haddock (Melanogrammus aeglifinus) larvae in the East Shetland area, North Sea

Investigations of factors affecting feeding success in fish larvae require knowledge of the scales of variability of the feeding process itself and the indices used to assess this variability. In this study, we measured short-term (diel) variability in feeding rates of wild haddock (Melanogrammus aeglifinus) larvae four times per day during a 10-d cruise in the northern North Sea. Feeding activity was evaluated using indices of gut fullness, prey digestive state and biochemical measurements (tryptic enzyme activity). The gut fullness and the enzyme activity indices indicated moderate to high rates of food consumption throughout the cruise. Time series analysis of the three indices showed significant diel variability in all indices and enabled identification of significant lags between food uptake and peak digestive enzyme activity. The typical pattern of food consumption and digestion was characterized by maximal ingestion of prey early in the evening (19:00 hrs) and peak digestive enzyme activity at 01:00  hrs. The time scale over which enzyme activities reacted to prey ingestion was ca. 6 h, and is consistent with expectations from controlled laboratory experiments with other larval fish species. Significant diel variability in tryptic enzyme activity suggests that attempts to relate this measure of feeding success to other variables (e.g. food concentrations) should take care to accommodate natural cycles in feeding activity before making statistical comparisons. Received: 29 October 1998 / Accepted: 18 June 1999     

A microinjection technique using a pH-sensitive dye to determine the gut pH of Calanus helgolandicus

By applying a microinjection technique previously used in cell biology, a pH-sensitive dye, 2,7-bis(2-carboxyethyl)-5,6-carboxyfluorescein) (BCECF), was injected into the gut of the copepod Calanus helgolandicus to determine pH under a range of feeding conditions. The median gut pH of the fore- and hindguts of starved individuals was 6.86 and 7.19, respectively. This was a consistently lower pH than that observed in copepods feeding on either the diatom Thalassiosira weissflogii, the dinoflagellate Prorocentrum micans, or the coccolithophorid Emiliania huxleyi (Strain 92D), all of which had a median gut pH>7.0. In all treatments, the median pH of the foregut was more acidic than the median of the hindgut, implying that the foregut is the site of acid secretion. The results, which demonstrate pH values down to 6.11, are discussed in terms of coccolith dissolution, pigment degradation and digestive enzyme activity.     

Estimation of Arctic copepod grazing rates in vivo and comparison with in-vitro methods

Gut evacuation rates were measured in Calanus hyperboreus and C. glacialis from two stations in Jones Sound, Northwest Territory (NWT) and one station in an Ellesmere Island Fjord during late summer of 1984. Gut content decreased exponentially with a rate constant, that, for Stage V C. glacialis at least, was independent of food type and time of day. Gut filling rates were measured in Stage V C. glacialis in the light and in the dark, at noon and midnight. Nighttime gut filling rates were very similar for both light intensities, and also similar to the daytime rate in the dark, whereas the daytime rate in the light was much lower. Ingestion rates were calculated for these latter experiments, including a rate term for defecation, and these results were compared to the values obtained from the observations of gut filling rates in vivo as reported in Head et al. (1985) and from long-term (2–3 d) bottle incubations as reported in Head et al. (in press). The following points were made: (1) in-vivo and in-vitro ingestion rates were very close if appropriate in-vitro experimental conditions were used with respect to light intensity and time of day; (2) copepods could fill their guts at a rate apparently higher than their normal nocturnal ingestion rate; and (3) the calculated rations were dependent on the shape of the observed diurnal feeding patterns.     

Ontogenetic changes in digestive enzyme activity of the spiny lobster,<Emphasis Type="Italic"> Jasus edwardsii</Emphasis> (Decapoda; Palinuridae)

Digestive enzyme profiles of puerulus, post-puerulus, juvenile and adult stages of the spiny lobster Jasus edwardsii Hutton were determined in order to identify ontogenetic changes in digestive capabilities and assess capacity to use dietary components and to exploit diets to meet nutritional requirements. Juvenile and adult lobsters exhibited a diverse range of enzymes, suggesting they can exploit varied diets. Proteolytic activity of trypsin, chymotrypsin and carboxypeptidase A (1.86–3.70 U mg^–1 digestive gland protein) was significantly higher than carbohydrase activity of amylase, -glucosidase, cellulase and chitinase (0.0014–0.38 U mg^–1 digestive gland protein), thus showing that dietary protein was more important than carbohydrate and that the lobster is carnivorous. These conclusions are consistent with other studies that found spiny lobsters to feed predominantly on crabs, bivalves, ophiuroids and sponges. Lipase activity (0.371 U mg^–1 digestive gland protein) was also relatively high, thus showing the importance of dietary lipid. Total activities (units per digestive gland) of every enzyme assayed increased significantly with lobster carapace length. There were few significant ontogenetic trends in specific enzyme activity (U mg^–1 digestive gland protein). Amylase and laminarinase specific activity was significantly higher in small lobsters than large lobsters (regression analyses, respectively, F_(1,23)=9.84, P=0.005; F_(1,11)=19.65, P=0.001), suggesting that carbohydrates including laminarin are more important in the diet of small juveniles. Trypsin, amylase and lipase activities were detected in all non-feeding puerulus stages, suggesting that feeding is not a cue for digestive enzyme production in J. edwardsii. Significant variations in total and specific activities of amylase (F_(1,3)=14.2, P=0.00; F_(1,3)=14.2, P=0.00) and trypsin (F_(1,3)=8.8, P=0.00; F_(1,3)=21.41, P=0.00) and a declining trend in lipase total activity between non-feeding puerulus stages suggests that they may be hydrolysing endogenous energy reserves to sustain their onshore swimming activity prior to settlement. Profiles suggest carbohydrate and lipid are utilised first, followed by protein. Consistently high levels of lipase in all puerulus stages (0.24–0.7 U digestive gland^–1; P>0.05) confirm the importance of lipid as a major energy substrate.Communicated by G.F. Humphrey, Sydney     

Acid and alkaline phosphatase activities in the clam Ruditapes philippinarum

Acid and alkaline phosphatase activities have been partially characterized in Ruditapes philippinarum (Adams and Reeve, 1850). Two activity peaks at pH=4.5 and pH 10.5 were detected in the gill, digestive gland, mantle, siphon and foot. Acid phosphatase activity was higher than that of alkaline phosphatase. The highest activity for both enzymes was observed in the digestive gland and, in decreasing order, the gill, foot, siphon and mantle. Alkaline phosphatase activity was similar in the mantle, siphon and foot. K _m values were determined for both enzymes in the gill and digestive gland. Hill coefficients were near 1, indicating no allosteric behaviour for either enzyme in the two organs. The optimum temperature was the same for acid phosphatase in both gill and digestive gland (50 °C), while for alkaline phosphatase it differed for these two organs (gill, 40 °C; digestive gland, 35 °C). The apparent activation energy was obtained from Arrhenius plots, and ranged from 8.61 kcal/mol for alkaline phosphatase in the gill, to 10.84 kcal/mol for acid phosphatase in the digestive gland. The effects of metals (1 mM conc) on both enzyme activities were assayed in vitro. Hg strongly inhibited the enzyme activities in the gill and digestive gland, probably because of its affinity for the sulphydryl group. Histochemically, acid phosphatase in the gill was located in a granular form throughout the gill cells, but was undetectable in the ciliate epithelium of the gill filaments. Alkaline phosphatase was located in the gill skeleton. Clam size and phosphatase activities were inversely related, probably reflecting a decrease in shell deposition with inereasing size. As a function of season, both enzymes were present in lowest amounts in winter, when undifferentiated sex cells were predominant in the germinative epithelium, and highest in summer, when ripe individuals of both sexes were more frequent.     

UV-absorbing substances in the tunic of a colonial ascidian protect its symbiont, Prochloron sp., from damage by UV-B radiation

The infaunal bivalve Ruditapes decussatus L. was collected from Ria Formosa, Faro, southern Portugal, and subjected to a range of hypoxic conditions and anoxia. Physiological measurements, clearance rates, respiration rates and absorption efficiency were undertaken at slightly different oxygen partial pressures (11, 6, 3 and 1.2 kPa for clearance rates and absorption efficiency and 12, 7, 5, 1.9 and 0.9 kPa for respiration rates). Metabolic rates under hypoxia were measured as oxygen consumption and anoxic metabolism was measured using direct calorimetry. Increasing hypoxia resulted in lower clearance rates, leading to lower ingestion rates and reduced faeces production. Clearance and ingestion rates declined below ˜6 kPa, reflecting decreasing ventilation and feeding activity, although complete cessation was not observed even at 1.2 kPa. Under extreme hypoxia (< 2 kPa) clams showed an irregular behaviour, with valves either closed or only slightly open, and siphons compressed or retracted. Clearance rate was 12% and respiration rate was 35% of normoxic rates. R. decussatus responded to increasing hypoxia by lowering its metabolic rate. Regulation of respiration was absent through moderate hypoxia (˜␣7␣kPa), but was observed in the lower hypoxia range (7 to 0.9 kPa). Under anoxia, rates of heat dissipation were 3.6% of normoxic rates. The low anoxic metabolic rate is indicative of a reduced energy expenditure, and this energy-saving mechanism is common in bivalves. Scope for growth was always pos itive, and even at low oxygen levels clams did not have to utilize their energy reserves. The ability to reduce metabolic costs but still meet the maintenance costs by aerobic catabolism enables R. decussatus to tolerate hypoxia. Such conditions can occur, particularly in the summer, in southern Portugal. Received: 19 July 1996 / Accepted: 17 September 1996     

Chitinolytic enzymes in the digestive system of marine fishes

Chitinase, exo-N-acetyl--D-glycosaminidase (NAGase) and lysozyme activities were assayed in the digestive tract of 6 species of marine fishes: Myxine glutinosa (cyclostome), Chimaera monstrosa (holocephalan), Squalus acanthias, Etmopterus spinax, Raja radiata (elasmobranchs) and Coryphaenoides rupestris (teleost). Strong chitinase activity was found in the gastric mucosa of the elasmobranchs (S. acanthias, E. spinax and R. radiata) and the teleost (Coryphaenoides rupestris). A remarkably high chitinase activity occurred in the pancreas of the stomachless holocephalan fish Chimaera monstrosa. NAGase activity was strong in the digestive tract of all species. It could be concluded that marine fishes with diets consisting largely of chitinous invertebrates may display high chitinase and NAGase activities in their digestive system; however, only low chitinase activity was found in the intestine of the cyclostome Myxine glutinosa. Coryphaenoides rupestris gastric mucosa chitinase had one optimum activity at pH 1.25, whereas S. acanthias chitinase had two optima, at pH 1.6 and 3.6. The NAGase pH-activity curves from S. acanthias and R. radiata gastric mucosa displayed similar optima, at pH 4.5 and 4.25 respectively. Chimaera monstrosa pancreatic chitinase had a very strong optimum around pH 8 to 10, and one less strong at pH 3. These enzyme activities could not be separated by gel filtration or isoelectric focusing. The pI (isoelectric point) was approximately 4.9 for both enzymes. The molecular weight of the C. monstrosa pancreatic chitinase was estimated to be approximately 43 000. Lysozyme activity was absent or extremely weak in the material studied.