Reproductive activity and physiological status of the calanoid copepods Calanus helgolandicus and Calanoides carinatus under food-limiting conditions

The reproductive activity and the physiological state of the calanoid copepods Calanus helgolandicus and Calanoides carinatus were investigated off the coast of NW Spain during autumn to evaluate the effect of short food resources on both populations. Phytoplankton biomass was low, and neither phytoplankton size distribution nor composition was suitable to support high reproductive rates. Accordingly, egg production rates (EPR) were much lower than maximum rates for both species, pointing to food limitation. The reproductive index (RI), which represents the proportion of females with mature gonads, was < 50% at each of the three zones into which the sampling area was divided (coast, shelf and ocean). Potential recruitment rates were very low except at some nearshore stations, where the highest concentrations of chlorophyll-a (Chl-a), diatoms, dinoflagellates and large cells were found. EPR of C. helgolandicus and C. carinatus were correlated with phytoplankton biomass and unaffected by temperature. Phytoplankton carbon ingestion explained ca. 50% of the variability in EPR for both species. At most of the stations, herbivory was insufficient to cover the carbon requirements for reproduction and respiration, so females probably fed on heterotrophic prey to meet their demands. However, given the low fecundity observed, this omnivorous diet did not seem to be optimum for reproduction, and a severe food limitation is thus suggested. Furthermore, the high C/N values measured point to a notable lipid storage, but given the low EPR found, lipid reserves were probably invested into female maintenance rather than into gonad maturation. C. helgolandicus and C. carinatus populations did not mirror phytoplankton biomass distribution, but they correlated well when considering only copepodites V (CV). The CV could be preparing for the overwintering, storing lipid reserves to ensure a successful diapause, and they could also be advected by the poleward current detected during the study. Females showed a diel feeding rhythm, with highest ingestion rates during night. From our results, it follows that C. helgolandicus and C. carinatus females did not perform diel vertical migrations. We suggest that this behaviour is likely due to the food-limiting conditions, which make it more advantageous to remain at the surface during daytime.     

Feeding trials ofAphelenchus avenae on soil bacteria and actinomycetes

Summary Aphelenchus avenae and various unidentified bacteria and actinomycetes were cultured from soil samples taken from a pine-forest and a wheat-field. The nematode failed to reproduce in significant numbers on cultures of these micro-organisms and was apparently unable to utilize them as food sources.     

The interactive effects of temperature,food level and maternal phenotype on offspring size in Daphnia magna

Invertebrate offspring are usually larger in colder environments. To test for possible effects of covariates (e.g. maternal phenotype and feeding conditions) on this pattern, we performed a laboratory experiment to look at the effect of temperature on newborn weight in the planktonic crustacean Daphnia magna. Three tempèratures (12°C, 16°C and 22°C) and two food levels (10,000 cells ml^–1 and 150,000 cells ml^–1) were used, and offspring were examined from the first five clutches of mothers that had been maintained under the constant experimental conditions for three generations. Preliminary analysis suggested that newborn weight was significantly affected by temperature although patterns in the data were not clear cut. In addition, the covariates mother weight and clutch size were positively and negatively correlated with newborn weight, respectively; and later clutches tended to contain heavier offspring. Therefore, in an effort to control for the effects of the covariates, repeated-measures analysis of covariance was performed using ratio values of newborn weight/mother weight (relative newborn weight) as the dependent variable, clutch size as the covariate and clutch number as the repeated measures term. Now, temperature as a main effect in an ANCOVA model did not significantly influence relative newborn weight. The repeatedmeasure term clutch number also became nonsignificant, indicating that when differences in mother weight due to age were accounted for there were no overall differences in relative newborn weight between clutches from a particular mother. Temperature effects on relative newborn weight were only significant as part of interaction terms with food concentration and with clutch number. Thus there were different weight responses to temperature within food levels, and between clutch numbers within food levels. Under the low-food conditions newborn were heaviest at 16°C, lightest at 12°C and intermediate at 22°C. Conversely, under the high-food condition newborn were lightest at 16°C, heaviest at 12°C and again intermediate at 22°C. However, newborn tended to be heavier under the low food condition, and food concentration was highly significant as a main effect. Mother growth rate showed no significant relationship with newborn weight. It is concluded that direct temperature effects on relative newborn weight are marginal and nonsignificant. Temperature effects through interactions with food concentration and clutch number are important determinants of newborn weight, but relatively speaking account for only a small proportion of observed variance in newborn weight (25%), compared with the direct effect of food concentration (67%).     

Variables Influencing the Effect of a Meal on Brain Tryptophan

Previous work from our laboratory points to plasma free tryptophan being a useful predictor of brain tryptophan concentration in many circumstances. Other work, in particular various studies on the acute effects of food intake, has emphasized the roles of plasma total tryptophan and of plasma large neutral amino acids that compete with tryptophan for transport to the brain. We have now studied associations between the above variables under different dietary conditions. Rats were allowed to feed for restricted periods during a 12-h light-12-h dark cycle. In the first study, rats were given access to a carbohydrate diet for 2 h midway through the light cycle and following an 18-h fast. The resultant rise of brain tryptophan was explicable largely by the associated fall in large neutral amino acids. In a second study, rats were adapted to a regimen whereby they were allowed access to the standard laboratory diet for 4 h during the dark cycle for 3 weeks. A postprandial decrease in brain tryptophan was associated with a fall in free tryptophan and of its ratio to competing amino acids. The brain change could be attributed neither to changes in plasma total tryptophan (which increased) nor to changes of its ratio to the competers (which remained unchanged). Results as a whole are thus consistent with changes of plasma free tryptophan and large neutral amino acid concentrations affecting brain tryptophan concentration under different dietary circumstances. It is suggested that these influences serve to maintain brain tryptophan when dietary supplies are defective.     

Differences in learning by foraging juvenile pumpkinseed and bluegill sunfish in a structured habitat

Synopsis We investigated the ability of two congeneric species of sunfish to learn to forage on a novel prey item in feeding arenas containing structured habitats. Eight bluegill sunfish and eight pumpkinseed sunfish were given the opportunity to forage on whiteworms daily for 10 days. Each day, several behavioural measures were recorded for each fish. Both species of sunfish learned to feed over the 10-day period but the bluegill sunfish learned to feed more quickly than the pumpkinseed sunfish. Pumpkinseeds, however, attained a higher level of foraging efficiency. The differences in learning and foraging efficiency were related to body morphology.     

Effects of selective cholecystokinin antagonists L364,718 and L365,260 on food intake in rats

The selective type A and B cholecystokinin (CCK) receptor antagonists L364,718 and L365,260 were used to identify the receptor subtype that mediates the satiety effect of endogenous CCK. Male rats (n = 12–13/group), fed ground rat chow ad lib, received L364,718 (0, 1, 10, 100, or 1000 μg/kg IP) or L365,260 (0, 0.1, 1, 10, 100, 1000, or 10,000 μg/kg IP) 2 h after lights off, and food intake was measured 1.5, 3.5, and 5.5 h later. L364,718 significantly stimulated 1.5-h food intake by more than 40% at 10 μg/kg and higher doses; cumulative intake at 3.5 and 5.5 h remained elevated by about 20% at 1000 and 100 μg/kg of L364,718, respectively. In contrast, L365,260 had no significant stimulatory effect on feeding at any dose. The potency of L365,260 for antagonizing gastrin-stimulated gastric acid secretion was examined in unanesthetized rats. Male rats (n = 14), prepared with gastric and jugular vein cannulas, received doubling doses of gastrin (G-17I) (0.16–5 nmol/kg/h IV), each dose for 30 min, and gastric juice was collected for each 30-min period. G-17I stimulated gastric acid output dose dependently; the minimal effective dose was 0.16 nmol/kg/h, while maximal output (5-fold above basal) occurred at 5 nmol/kg/h. L365,260 (0, 1, 10, 100, 1000, or 10,000 μg/kg IV), administered 30 min before continuous infusion of G-17I (1.25 or 5 nmol/kg/h), significantly inhibited acid output only at 10,000 μg/kg; cumulative 60-min output was decreased by 60%. These results suggest that CCK acts at CCK-A receptors to produce satiety during the dark period in ad lib-feeding rats.     

The effect of cropping and fertiliser nitrogen rates on nitrogen balance in soil

Summary Fertilizer/soil N balance of cropped and fallow soil has been studied in a pot experiment carried out with grey forest soil (southern part of Moscow region) at increasing rates of¹⁵N labelled ammonium sulfate (0; 8; 16; 32 mg N/100 g of soil). The fertilizer¹⁵N balance has been shown to depend upon its application rate and the presence of growing plants. Fertilizer N uptake efficiency was maximum (72.5%) and gaseous losses-minimum (12.5%) at the application rate of 16 mg N/100 g of soil. Fertilizer N losses from the fallow soil were 130–220% versus those from the cropped soil. At the application of fertilizer N the plant uptake of soil N was 170–240% and the amount of soil N as N–NH₄ exchangeable + N–NO₃ in fallow was 350–440% as compared to the control treatment without nitrogen (PK).After cropping without or with N fertilizer application at the rates of 8 and 32 mg N/100 g of soil, a positive nitrogen balance has been found which is likely due to nonsymbiotic (associative) N-fixation. It has been shown that biologically fixed nitrogen contributes to plant nutrition.     

An experimental removal of a territorial pomacentrid: effects on the occurrence and behavior of competitors

Synopsis Stegastes fasciolatus is the most common territorial damselfish in the shallow waters of Hawaii. Territorial defense was observed against other herbivorous fishes, especially acanthorids, scarids and one omnivorous chaetodontid. One acanthurid,Acanthurus nigrofuscus was found to differ in abundance and social behavior in areas whereS. fasciolatus was present, compared to areas where it was absent. The chaetodontid,Chaetodon quadrimaculatus was sheltered during the day in areas where the pomacentrid was abundant, apparently feeding at night. In other areas it fed during the day and at night, depending on the phase of the moon.S. fasciolatus were then experimentally removed from one study site, to test whether the differences in abundance and behavior of the other species were due to the presence of the damselfish. There was a significant increase in numbers of the surgeonfishAcanthurus nigrofuscus in the removal area, as well as changes in social behavior from schooling to defense of small territories. The butterflyfish,C. quadrimaculatus, was observed to forage during the day in the removal area. There were no significant changes in the control sites. The presence of the interspecifically territorial damselfish,S. fasciolatus, thus appears to be an important determinant of the behavior of these potential food competitors.     

Gravitropically-stimulated seedlings show autotropism in weightlessness

In a spaceflight experiment, autotropism by oat ( Avena sativa L.) coleoptiles following gravitropic responses was prominent in weightlessness: counter-reactions led to the straightening of the curved coleoptiles. This was not the case during clinorotation on earth. The autotropic reactions appeared to be related to the stimulus received during the stimulus period, i.e. the greater the response the greater the autotropic counter-reaction. Previous models of the gravitropic system which predicted that coleoptiles would not straighten in weightlessness are disproved. A modification to one of the models is proposed which includes the autotropic response observed in spaceflight. The nature of the counter-reactions in the absence of gravitropic stimulation is discussed.