RNA/DNA ratio and expression of 18S ribosomal RNA, actin and myosin heavy chain messenger RNAs in starved and fed larval Atlantic cod (Gadus morhua)

Levels of total RNA, total DNA, 18S ribosomal RNA (rRNA), poly(A) messenger RNA (mRNA), and two mRNAs coding for abundant myofibrillar proteins were estimated in laboratory-reared Atlantic cod larvae (Gadusmorhua Linnaeus) under conditions of feeding and starvation. DNA probes specific for cod 18S rRNA, β-actin mRNA and myosin heavy chain mRNA were developed. In two experiments on newly hatched larvae in fed and starved treatments, changes in 18S rRNA and mRNA were similar to changes in total RNA during the first weeks after hatching. RNA levels in fed and starved larvae in both experiments were stable, or increased, over the first 3 d after hatching, and then decreased to minima at 9 d. RNA levels increased after 9 d, with the degree and timing of the increase varying among the individual classes of RNA. Complete mortality of starved larvae in both experiments was observed shortly after 11 d, corresponding to exhaustion of endogenous yolk reserves. Total RNA content, RNA/DNA ratio, 18S rRNA levels, total mRNA pool, and actin and myosin heavy chain mRNA levels showed significant differences in fed and starved first-feeding larvae after yolk exhaustion. In another experiment with 3- to 4-week-old cod larvae, 18S rRNA levels were significantly lower in starved versus fed larvae after 3 d. Total RNA responded to feeding and starvation within a similar time as 18S rRNA and the mRNAs examined. Analysis of bulk nucleic acids using fluorometric dyes was simpler and faster than analysis of individual RNAs using hybridization probes, and provides valuable information on recent growth and condition of individual larvae. However, analysis of specific RNAs can provide information on expression of the corresponding genes and reveal the changes underlying trends seen in bulk RNA. Received: 9 February 1996 / Accepted: 7 June 1999     

Use of nucleic acids as indicators of growth in juvenile white shrimp,Penaeus vannamei

The use of nucleic acids to estimate crustacean growth is not well documented, and may be complicated by biochemical changes associated with their molt cycle. The objectives of this study were to assess the effects of molt stage on nucleic acid concentrations in abdominal muscle tissue of juvenile white shrimp,Penaeus vannamei, and to examine the relationship between nucleic acid concentrations and growth rates of shrimp exposed to different feeding regimes throughout a 12 d feeding experiment. RNA and DNA concentrations and RNA:DNA ratios were not significantly different among the five major molt stages early postmolt, late postmolt, intermolt, early premolt, and late premolt. In the feeding experiment, RNA concentrations and RNA:DNA ratios accounted for >70% of the variation in shrimp growth on three different sampling days. In addition, RNA concentrations and RNA:DNA ratios were used successfully to discriminate between unfed, moderately-fed, and well-fed shrimp. These variables exhibited significant treatment differences in <24 h after the initiation of the different feeding regimes, whereas significant changes in whole-body weight took longer to detect. Rapid detection of significant treatment effects can be useful in ecological studes, especially those concerned with food-web interactions.     

Effects of feeding history on mobilisation and deposition of body constituents and on growth in juvenile Ambassis vachelli (Pisces: Chandidae)

Young Ambassis vachelli (Richardson) 40 to 50 d old, were used in a laboratory experiment to test the effects of starvation and subsequent re-feeding on body constituents and growth. Fish in three laboratory treatments (fed continuously; fed for 9 d and then starved for 15 d; starved for 9 d and fed for 15 d), were compared to fish from a local field population. Different body stores were mobilised at different times and rates during starvation. Carbohydrates were mobilised from the onset of starvation and were depleted after 3 d. Lipid and protein were mobilised at an increasing rate from the onset of starvation. The mortality in starved fish was relatively high (up to 70%) until re-feeding. Upon feeding, all body stores were restored rapidly in fish that were starved, with carbohydrate levels displaying an overshoot (carbohydrate level exceeding normal levels) in comparison to the levels in continuously fed fish. After 15 d of feeding, the starved fish had levels of constituents similar to those in continuously fed fish. Fish that were fed and subsequently starved were able to maintain themselves for at least 15 d prior to death, indicating a better degree of starvation resistance than fish without a history of feeding. This implies that feeding history in the early life of a fish is important in growth and survival but that young fish may have growth regimes flexible enough to survive relatively long periods of starvation.     

Changes in nucleic acids over the molt cycle in relation to food availability and temperature in Homarus americanus postlarvae

Postlarval lobsters Homarus americanus Milne Edwards hatched from three females collected in 1989 from Block Island Sound, Rhode Island were reared individually in the laboratory under nine treatment combinations of temperature (15, 18 and 21°C) and feeding (starved, low ration and full ration). Total RNA, DNA (mgind.^-1), RNA:DNA ratios and molt stage were determined for individuals sampled at daily intervals. Postlarval lobsters had high resistance to starvation. A majority of the lobsters survive 12 d of food deprivation, with some surviving up to 24–29 d. During starvation, cell biomass (estimated from protein:DNA) decreased to a minumum size, whereas cell number (based on total DNA) was generally conserved. The molt cycle was arrested at molt stage C in the starved postalrvae. Instar duration was inversely related to temperature. However, the duration of the postlarval instar did not differ between the low and full ration treatments. Uncoupling of cell growth and the molt cycle was evident in the full and low ration treatments. In the full ration treatments, the postmolt through early premolt was characterized by a rapid increase in total RNA and DNA. Maximum cell biomass was attained by molt stage C or D₀. In the low ration treatments, total RNA and DNA were less than those in the full ration treatments and the maximum cell biomass was attained only at molt stage D_2-3. Notably, different feeding regimes resulted in different patterns in RNA:DNA ratios over the molt cycle during the postlarval instar. RNA:DNA ratio was a sensitive indicator of recent (2 to 3 d) food deprivation. However, this ratio was positively related to the level of feeding only at molt stages C to D₁ and was inversely related to temperature regime. These results indicate that the use of RNA:DNA ratios to estimate the relative nutritional state of postlarval tobsters must be qualified with respect to the period of the molt cycle and the temperature regime.     

Time-course of digestive-enzyme acclimation in the cockle Cerastoderma edule

We determined the temporal evolution of amylase, cellulase, laminarinase and protease in the digestive gland and crystalline style of cockles Cerastoderma edule held over 9 to 12 d in the presence and absence of food. Cockles were fed a constant diet of 1.5 mm³ l⁻¹ of Tetraselmis suecica for 9 to 12 d and were then starved for 6 to 8 d in late summer (September 1992) and in winter (January 1993). Feeding increased the dry weight and total cellulase, laminarinase and protease activities of the digestive gland irrespective of season, whereas amylase activity remained unchanged. In winter (i.e. when cockles are metabolically weak) the response was faster and stronger, especially for protease. An additional experiment in September starved cockles for 20 d before resuming feeding. In agreement with the seasonal differences, the presence of food after prolonged starvation induced a rapid and marked increase in protease in the digestive gland of the cockles. In winter, the possible effects of the biochemical composition of food on their enzymatic response were tested by feeding two groups of cockles with the same ration of T. suecica but harvested at different growth phases. A compensatory induction of cellulases occurred in cockles fed on T. suecica with a lower carbohydrate content. In the crystalline style, the protein level and carbohydrase fell during the first day of feeding and increased during the first day of subsequent starvation. These results indicate that the release of enzymes from the style prevails over the incorporation of enzymes during the early stages of feeding, whereas the opposite occurs during starvation. Received: 15 February 1998 / Accepted: 22 February 1999     

Effect of unfavorable trophic scenarios on amylase and protease activity of <Emphasis Type="Italic">Nephrops norvegicus</Emphasis> (L.) larvae during their first vertical migration: a laboratory approach

In Portuguese waters, Nephrops norvegicus larvae hatch at 400–800 m depth and need to perform a vertical migration to food-rich shallower waters to find suitable prey. The effect of suboptimal feeding on digestive enzymes activity of N. norvegicus larvae during this early period of their larval life remains unknown. Protease and amylase activities were investigated ex situ using flurometry in laboratory-hatched larvae exposed to different feeding and/or starving scenarios in the 24 h following hatching, the period during which they typically accomplish their upward vertical migration. Amylase activity was very low in comparison with protease activity, indicating that carbohydrates are not a primary energy reserve. Larvae starved for 12 h and subsequently fed displayed no increase in amylase activity, which suggests that feeding may be required before 12 h post-hatch to trigger amylase activity. Protease activity was high under all feeding conditions, and the increase in protease activity under sustained starvation indicated the catabolism of protein reserves. The ability of first-stage N. norvegicus larvae to metabolize protein reserves may play a decisive role for their survival during their first vertical migration, as it enables them to overcome the deleterious effects of short-term starvation and/or suboptimal feeding.     

The use of RNA:DNA ratios to predict growth limitation of juvenile summer flounder (Paralichthys dentatus) from Delaware and North Carolina estuaries

Nutritional indices were used to develop biochemical correlates of feeding and growth rates for juvenile summer flounder, Paralichthys dentatus (Linnaeus), from North Carolina (NC) and Delaware (DE). Six parameters (Fulton's condition K=10⁴xweight/(length³), wet weight/dry weight, [protein], [RNA], [DNA], and RNA:DNA) were related to feeding and growth rates of fish from previously reported 10 to 14-d experiments at temperatures ranging from 2 to 20 °C with varying feeding levels (0 to 100% and libitum). RNA:DNA ratios were the best predictors of growth rates, but inclusion of a temperature term improved the relationship between RNA:DNA ratios and growth rate for Delaware fish. Feeding rates were poorly correlated with all parameters. RNA:DNA ratios of fish in the laboratory changed significantly within 1 d of starvation and refeeding at 16 °C. RNA:DNA of juvenile summer flounder collected from one site in Indian River Bay, DE and two sites in the Newport River Estuary, NC, between January and June 1992 were used to estimate in situ growth rates following settlement. Predicted growth rates in both estuaries were close to maximum (suggesting ad libitum feeding) until early May. Growth rates of juveniles from Delaware were <0% d^-1 from December through early March, and were higher (0.6 to 3% d^-1) from April through early June. However, growth rates of DE juveniles during May were <50% of maxinum. North Carolina juveniles had growth rates of 2 to 5% d^-1 from February through early April. Juveniles from one of the Newport River sites (a marsh habitat) were also severely growth limited (<20% of maximum) after April. Prolonged periods of sub-optimal growth may be important to survival and recruitment of juvenile summer flounder in northern mid-Atlantic estuaries. A model is presented which illustrates the potential impact that small changes in temperature and growth limitation can have on recruitment success in both delaware and North Carolina estuaries.     

Episodes of starvation are recorded in the otoliths of juvenile Ambassis vachelli (Chandidae), a tropical estuarine fish

Juvenile Ambassis vachelli, 40 to 50 d old, were used to test the effects of starvation and recovery-feeding on the widths of otolith increments. Three laboratory treatments (fed continuously; fed for 9d then starved for 15 d; starved for 9 d then fed for 15 d), were compared. Fish were also collected throughout the experiment from a field site to assess the natural fluctuations of increment widths. Growth, starvation and recovery were reflected in the widths of otolith increments. The width of increments deposited during starvation were similar in all fish, regardless of prior feeding history, despite the lower mortality recorded in starved fish with a history of intense feeding. Increment-width data were confounded by an ontogenetic decrease in increment widths with age. The results indicate that starvation and recovery are recorded in the increment widths of otoliths and are distinguishable from the effects of ontogenetic development. However, ontogenetic changes in the width of otolith increments confounded the interpretation of otolith-increment series. Therefore, the ontogenetic pattern of otolith increments must be known for each species before valid interpretation of otolith microstructure is possible.     

Egg production of the Arctic copepod Calanus glacialis: laboratory experiments

Egg production of single female Calanus glacialis Jaschnov fed with Thalassiosira antarctica at super-abundant concentrations (>300 g C l^-1) was determined over several weeks. Experiments were performed directly after collection from the East Greeland Current in June 1987 and 1988, and during resumed feeding after long-term starvation over 4 (October 1988), 4.5 (October 1987) and 6.5 (January 1988) mo. In addition, in June 1987, short-term starvation experiments of 5 and 15 d were conducted. Egg production was closely related to feeding in all experiments. While directly after collection eggs were produced within a few days, it took 2 wk (October 1987 and 1988) and 10 d (January 1988), respectively, to resume egg production after long-term starvation. During long-term starvation periods eggs were not laid. The decrease in total egg production with duration of starvation was due to decreasing clutch size and increasing spawning interval. In contrast, short-term starvation experiments only affected spawning interval. Interannual variability in egg production was high, with much higher clutch sizes in 1988. Average production rates in June 1988 correponded to 5% body C female^-1 d^-1, the maximum was 7.4% (1 274 eggs in 23 d). Carbon content of eggs was 0.40 g egg^-1. C. glacialis is well adapted to pulsed food events in the Arctic by its longevity; its ability to preserve its reproductive potential over several months; its rapid mobilization of ovaries; and by its high egg production rates. The implication of prolonged spawning capacity on life cycle studies is discussed.     

RNA:DNA ratios and growth of herring (Clupea harengus) larvae reared in mesocosms

Autumn-spawned North Sea herring larvae (Clupea harengus L.) were released in two outdoor mesocosms of 2500 m³ (A) and 4000 m³ (B). The mesocosms were monitored for temperature, salinity, oxygen, chlorophyll a, zooplankton and herring larvae abundance. The density of suitable prey for first feeding larvae (mainly copepod nauplii) was initially low in Mesocosm A (<0.11^-1) compared to in Mesocosm B (>11^-1). Half-way through the experiment the situation was reversed, with higher densities of prey in Mesocosm A (>31^-1) as compared to Mesocosm B (11^-1). The average temperature declined steadily in both mesocosms from 18°C at release to 11–12°C by the end of the experiment 60 d later. The RNA:DNA values of individual herring larvae were related to protein growth rates and temperature adjusted according to Buckley (1984). A corresponding DNA growth index (Gdi) was given as: Gdi=0.68 TEMP+3.05 RNA:DNA-9.92. The RNA:DNA based growth indices were significantly correlated with other somatic growth estimates. The average estimated protein growth rate in the two mesocosms followed the same temporal pattern as the somatic growth rate, but with a lag of 2 d or more. Residual analysis of the regression of ln RNA versus ln DNA also showed the same temporal pattern as the RNA:DNA ratios, but the shift in condition as estimated by this method occurred more in synchrony with the other somatic growth measures. Larvae in Mesocosm A had RNA:DNA values similar to the starvation control kept in the laboratory the first days after release, confirming that larvae in Mesocosm A initially were in poor nutritional condition. On the other hand, the majority of the herring from Mesocosm B were characterised as starving or in poor nutritional condition towards the end of the experiment. The assessment of growth and nutritional condition were in accordance with independent survival estimates which suggested that the majority of the total mortality occurred during the first 15 d in Mesocosm A and there-after in Mesocosm B.     

Effects of feeding history on the growth characteristics of a reef fish at settlement

A feeding experiment was conducted on the pelagic stages of the tropical goatfish Upeneus tragula (family Mullidae) to examine how food quantity affects growth characteristics and potential success at settlement. Pelagic goatfish were collected from aggregation rafts 3 nautical miles west of Lizard Island on the northern Great Barrier Reef during December 1990. Three tanks in each of four feeding regimes were stocked with 25 pelagic goatfish between 20 and 23 mm standard length (SL). The four feeding regimes were: fed ad lib. (fed): fed once per day (f1pd); starved every second day (stld); starved for 3 d and re-fed (st3d). Fishes were fed 36 to 48 h old Artemia sp. nauplii (Ocean Star strain). Fish were removed from tanks when they underwent metamorphosis, changed pigmentation and settled to the bottom of the tanks. Morphology, muscle development, time taken to settle and biochemical condition were examined. Growth attributes of the treated fish were compared to fish which settled within 24 h of capture (field). All attributes examined were significantly influenced by the feeding treatments. Fish within the fed and st3d treatments were significantly larger and heavier than fish in the less well-fed treatments (f1pd, st1d). Similarly, concentrations of total lipid, carbohydrate and protein in the settled fish were significantly higher in the fed and st3d treatments compared with the st1d and f1pd treatments. For all these morphological and biochemical attributes the st1d and f1pd fish did not significantly differ from the field fishes, but did differ from the fed and st3d fishes. Water content was significantly higher in the f1pd, st1d and field fishes compared with the fed and st3d fish. Furthermore, the average time taken to settle followed the pattern: fed (14d)U. tragula are physiologically well suited to exploiting a patchy food source, and that food availability within the pelagic stages can have a major influence on the growth characteristics of this reef fish at settlement. The ramifications of this finding are discussed in relation to survival and success once fish have recruited to the reef population.     

RNA:DNA ratios and growth of herring (Clupea harengus) larvae reared in mesocosms

Autumn-spawned North Sea herring larvae (Clupea harengus L.) were released in two outdoor mesocosms of 2500 m³ (A) and 4000 m³ (B). The mesocosms were monitored for temperature, salinity, oxygen, chlorophyll a, zooplankton and herring larvae abundance. The density of suitable prey for first feeding larvae (mainly copepod nauplii) was initially low in Mesocosm A (<0.11^-1) compared to in Mesocosm B (>11^-1). Half-way through the experiment the situation was reversed, with higher densities of prey in Mesocosm A (>31^-1) as compared to Mesocosm B (～11^-1). The average temperature declined steadily in both mesocosms from 18°C at release to 11–12°C by the end of the experiment 60 d later. The RNA:DNA values of individual herring larvae were related to protein growth rates and temperature adjusted according to Buckley (1984). A corresponding DNA growth index (Gdi) was given as: Gdi=0.68 TEMP+3.05 RNA:DNA-9.92. The RNA:DNA based growth indices were significantly correlated with other somatic growth estimates. The average estimated protein growth rate in the two mesocosms followed the same temporal pattern as the somatic growth rate, but with a lag of 2 d or more. Residual analysis of the regression of ln RNA versus ln DNA also showed the same temporal pattern as the RNA:DNA ratios, but the shift in condition as estimated by this method occurred more in synchrony with the other somatic growth measures. Larvae in Mesocosm A had RNA:DNA values similar to the starvation control kept in the laboratory the first days after release, confirming that larvae in Mesocosm A initially were in poor nutritional condition. On the other hand, the majority of the herring from Mesocosm B were characterised as starving or in poor nutritional condition towards the end of the experiment. The assessment of growth and nutritional condition were in accordance with independent survival estimates which suggested that the majority of the total mortality occurred during the first 15 d in Mesocosm A and there-after in Mesocosm B.     

Osmoacclimation in Enteromorpha intestinalis: long-term effects of osmotic stress on organic solute accumulation

The fate of the protease trypsin in intestines of individual herring larvae Clupea harengus L. was studied following digestion of the copepod Acartia tonsa. Trypsin was retained in the intestine during two consecutive pulses of feeding and defaecation of copepods. Quantification of herring trypsin in digested, defaecated copepods showed that ca. 1% of larval intestinal enzyme was defaecated along with 1 to 3 copepods. Following ingestion of a single meal, the level of intestinal trypsin post-ingestion declined to pre-ingestion levels within 1 to 2 d of starvation. All enzyme data thus indicated that trypsin, released in response to ingestion of a meal, was retained. In addition, analysis of fed subgroups of starved larvae clearly indicated that release of trypsin from the pancreas stopped after 6 to 8 d of starvation. As the fish still contained substantial amounts of trypsinogen, the underlying cause might be defective release mechanisms. Daily secretion of trypsin and processes responsible for enzyme retention in the gut are discussed. Assimilation efficiency in herring larvae was estimated for copepodite prey. Average carbon assimilation was 90%.     

Nucleic acid and protein levels in strobilating polyps of Chrysaora quinquecirrha and Aurelia aurita

Levels of DNA, RNA, and protein were measured in scyphistomae of the scyphozoans Aurelia aurita and Chrysaora quinquecirrha during strobilation. In synchronously developing populations of A. aurita, the amount of DNA per polyp increases 41 to 260% during strobilation, whereas RNA and protein remain relatively constant. The RNA/DNA ratio drops by about 50% during strobilation in both C. quinquecirrha and A. aurita, and protein/DNA decreases by 50% in A. aurita. Specific RNA reserves are probably not accumulated in advance of strobilation, since strobilation is blocked immediately in both species by 1 to 3 g/ml actinomycin D. The size and DNA content of polyps are usually increased by prolonged incubation at temperatures of 12° to 15°C, which facilitates strobilation; however, the RNA/DNA and protein/DNA ratios of such polyps are not significantly increased by the cold conditioning.Contribution No. 472 from the Virginia Institute of Marine Science.     

Feeding ecology and interannual variations in diet of larval jack mackerel,Trachurus declivis (Pisces: Carangidae), from coastal waters of eastern Tasmania

Changes in the biochemical composition of the digestive gland and in the proteins of the mantle muscle of Sepia officinalis L, collected in September 1989 from the Ria de Vigo (northwest Spain), were measured during periods of 2, 4, 10 and >53 d starvation. The digestive gland lost weight faster than the rest of the body throughout the whole period of starvation. In the digestive gland, carbohydrate and protein contents did not change during starvation; however, lipid levels decreased significantly after 53 d. Phospholipid content increased during longterm starvation. The content of free fatty acids rose after 16 d. Sterols, diacylglycerylethers, triacylglycerols and carotenoids contents did not change significantly. Of the total fatty acids, 18:0, 20:2n6, 20:4n6 and the monounsaturated moieties were preferentially consumed; others, such as 22:5n3, 22:6n3 and 16:4n1, were selectively retained. In the mantle muscle, water content increased and total protein content decreased. The myofibrillar proteins decreased after 53 d starvation, whereas the sarcoplasmic fraction did not change and the stromatic proteins increased. No changes were observed in the electrophoretic patterns of sarcoplasmic and myofibrillar proteins. The digestive gland of S. officinalis does not seem to be an important reserve organ during long-term starvation, but does seem to be important during shortterm starvation.     

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.     

RNA:DNA ratio during the critical period and early larval growth of the red drum Sciaenops ocellatus

Eggs from two separate spawning stocks of the red drum Sciaenops ocellatus (Linnaeus) were hatched, and the larvae were reared in the laboratory for 2 wk under closely controlled conditions. Total RNA, DNA, and soluble protein were measured in each population daily in triplicate pooled samples of larvae from each of three tanks. Growth rate in mm d^-1 was determined for each population at 2 d intervals. Growth rate explained 72 and 95% of the variation in the RNA:DNA ratios of the two populations individually, and 86% of the variation in the RNA:DNA ratio when data from the two populations were combined. The RNA:DNA ratio appeared to be most effective as an indicator of growth in rapidly growing larvae, and to lose some resolution when growth was intermittent. The rates of deposition of RNA, DNA, and protein into tissue were all highly correlated with growth rate and with each other. Mean population RNA:DNA ratios of red drum yolk-sac larvae decreased from Day 1 post-hatch until larvae initiated successful feeding behavior, and then increased steadily throghout the remainder of the experimental period. This pattern of change in the RNA:DNA ratios during the yolk-sac stage appears to be an intrinsic developmental pattern of red drum ontogeny. The lowest values for the RNA:DNA ratio were observed just prior to the initiation of feeding or during the critical period, indicating that red drum larvae experience a decrease in capacity for protein synthesis as they initiate feeding. Intrinsic variation in the RNA:DNA ratio during development suggests that caution be used when comparing the RNA:DNA ratios of yolk-sac larvae to a critical ratio calculated from Buckley's general model.     

Effect of starvation on the energy budget of two Asian horseshoe crab species: <Emphasis Type="Italic">Tachypleus tridentatus</Emphasis> and <Emphasis Type="Italic">Carcinoscorpius rotundicauda</Emphasis> (Chelicerata: Xiphosura)

Energy budget is one of the most studied parameters in aquatic animals under environmental challenge. To examine how prolonged starvation would affect their energy budget, respiration rate (RR), ammonia excretion rate (ER), oxygen consumption to ammonia–nitrogen excretion (O:N) ratio and scope for growth (SfG) representing the balance between energy intake and metabolic output, two Asian horseshoe crab species, Tachypleus tridentatus and Carcinoscorpius rotundicauda, were investigated in two feeding regimes (fed and starved) over a period of 7 weeks. No significant effects of species and time course, as well as their interaction, on absorption efficiency were observed in the fed treatments. For both species, RR and ER of the starved treatments significantly decreased, while their O:N ratio significantly increased during the experiment. However, such values for the fed treatments remained relatively stable over the study period. A rapid reduction in SfG was only apparent in the first week of the starved treatments for both species; thereafter, their SfG remained relatively constant. In the fed treatments, SfG of T. tridentatus was significantly lower than that of C. rotundicauda throughout the experiment. In general, C. rotundicauda showed a greater decrease in SfG under starvation than T. tridentatus, suggesting that they may have a more competitive life-history strategy for adjusting to poor nutritional conditions.     

A comparison of meal size and feeding rate of the lysianassid amphipods Anonyx nugax,Onisimus (=Pseudalibrotus) litoralis and Orchomenella pinguis

Lysianassid amphipods were collected in 1987 from Frobisher Bay, Baffin Island, and from the Mingan Archipelago, Gulf of St Lawrence. Meal size and feeding rate of Anonyx nugax (Phipps), Onisimus (=Pseudalibrotus) litoralis (Krøyer) and Orchomenella pinguis (Boeck) were estimated directly, gravimetrically and/or from predictive equations. Size-specific ingestion was greatest in A. nugax, which fed swiftly and efficiently in comparison to O. litoralis and O. pinguis. These two latter species dispersed some bait while feeding and crawling on its surface. Groups of lysianassids fed more wastefully than single individuals. Meal size of females of O. litoralis decreased with increasing maturity, while berried females of O. pinguis consumed less food than mature males. Up to 30 d of starvation had no effect on survival and feeding ability of A. nugax, but 10 to 15 d of starvation dramatically reduced feeding ability or killed O. litoralis and O. pinguis. Differences between meal size, feeding rate and survival point to divergent feeding patterns, which also have been evidenced elsewhere by analysis of gut contents. O. litoralis and O. pinguis are best characterized as facultative scavengers, while large A. nugax are possibly obligate carnivores. Results emphasize the importance of lysianassid amphipods, particularly A. nugax, as bait stealers and as predators of commercial species trapped by various fishing gear.     

Measuring RNA:DNA ratios in individual <Emphasis Type="Italic">Acartia tonsa</Emphasis> (Copepoda)

Acartia tonsa Dana is a dominant copepod in coastal waters and is therefore an important link in the food web between microplankton and higher trophic levels. RNA:DNA ratios have been used to describe growth and nutritional condition of field-collected copepods and to show strong correlation between RNA:DNA ratios and group egg production (EP). A method was developed using a sensitive, nucleic acid-specific fluorescent dye, and automated microplate fluorometer to measure DNA, RNA, and the RNA:DNA ratio of individual A. tonsa. DNA, RNA, and RNA:DNA ratios and EP were all significantly higher in copepods fed Thalassiosira sp. compared to starved copepods. There was a general trend toward an increase in RNA:DNA ratios with increase in EP, but due to the high degree of variation in both RNA:DNA ratios and EP among individual copepods there was no significant correlation between RNA:DNA ratios and EP. Significant differences in RNA:DNA ratios between fed (7.2) and starved (3.3) copepods were found after 2 days. This assay may be applied to other species of copepods sampled in the field to provide an index of the health of planktonic food webs.