Seasonal patterns of food availability: influences on the reproductive output and body condition of the herbivorous crab Grapsus albolineatus

 Rocky shores in Hong Kong experience marked seasonal differences in climate resulting in seasonal changes in macroalgal assemblages. The tropical rocky shore crab, Grapsus albolineatus, feeds selectively on filamentous algae through the year but the abundance of these algae and foliose algae is greatly reduced during the summer when encrusting algae dominate the shores and the crab’s diet. This switch in diet may have implications for the reproductive output of this crab. Standing crop of algae varied greatly through the year, peaking in March and April, when the nutritional quality of the algae was also highest. On the shore, available algal protein and energy were both lowest in July. The crab selected an algal diet rich in nutrients than that available to it on the shore for all months of the year except September to December for protein, and July and August for energy. The input of animal matter considerably increased the protein content of the diet, but made little difference to the energy content. Growth and body condition were greatest during March and June, coinciding with the peak in algal biomass. Storage of nutrients in the hepatopancreas of G. albolineatus commenced in November, coinciding with the increase in biomass and quality of algae on the shores, and then, peaking in May and June, these nutrients were utilised by the reproductive organs during the reproductive season which ran from April to November. The middle of the reproductive season coincided with the period when the standing crop of algae on the shore was at its lowest levels and poorest quality. Cycles of growth, reproduction and storage in G. albolineatus appear to be directly influenced by seasonal patterns of algal food availability. Nutrient storage in the hepatopancreas, during periods when food is nutritionally rich and is most abundant, does not guarantee reproductive success of the crab, but appears to be a prerequisite. Selection of an optimal diet (energy- and protein-rich) in a seasonal environment by ingesting abundant nutrient-rich algal species and the opportunistic consumption of animal matter strongly influences growth and reproductive output of G. albolineatus. Received: 18 March 1996 / Accepted: 17 July 1996     

The Influence of Diet Composition on Fitness of the Blue Crab,Callinectes sapidus

The physiological condition and fecundity of an organism is frequently controlled by diet. As changes in environmental conditions often cause organisms to alter their foraging behavior, a comprehensive understanding of how diet influences the fitness of an individual is central to predicting the effect of environmental change on population dynamics. We experimentally manipulated the diet of the economically and ecologically important blue crab, Callinectes sapidus, to approximate the effects of a dietary shift from primarily animal to plant tissue, a phenomenon commonly documented in crabs. Crabs whose diet consisted exclusively of animal tissue had markedly lower mortality and consumed substantially more food than crabs whose diet consisted exclusively of seaweed. The quantity of food consumed had a significant positive influence on reproductive effort and long-term energy stores. Additionally, seaweed diets produced a three-fold decrease in hepatopancreas lipid content and a simultaneous two-fold increase in crab aggression when compared to an animal diet. Our results reveal that the consumption of animal tissue substantially enhanced C. sapidus fitness, and suggest that a dietary shift to plant tissue may reduce crab population growth by decreasing fecundity as well as increasing mortality. This study has implications for C. sapidus fisheries.     

The effects of diet mixing on consumer fitness: macroalgae, epiphytes, and animal matter as food for marine amphipods

Herbivores are thought to achieve adequate nutrition by consuming numerous species of plants or by occasionally consuming animal tissue. Although active selection of diverse foods is common in nature, the relationship between diet mixing and consumer fitness is poorly understood, especially in marine environments. We studied the fitness-based consequences of dietary mixing in the sympatric amphipods Ampithoe marcuzzii, A. valida, Cymadusa compta, and Gammarus mucronatus by measuring survivorship, growth, and fecundity of these amphipods when they were offered single species of algae, a single animal food, a mixture of algal species, or a combination of algae and animal matter. For the more sedentary, tube-building amphipods A. marcuzzii, A. valida, and C. compta, fitness on mixed algal diets was matched by fitness on at least one of the monospecific algal diets, suggesting that they could benefit from preferential feeding on those algae in the field. The more mobile amphipod, G. mucronatus, survived and grew similarly on the mixed diets and on the filamentous brown alga Ectocarpus siliculosus. However, its fecundity was significantly higher when feeding on the algal and animal mixture than on Ectocarpus alone. Additionally, for G. mucronatus, fitness on mixed algae, mixed algae plus animal matter, and animal matter alone was equivalent, although female growth (but not gonad production) was slightly lower on animal matter alone than on the mixed algae combined with animal food. Thus the more mobile amphipod, G. mucronatus, was the only species able to perform well on animal food alone. In contrast, A. valida and C. compta experienced large negative effects when limited to consuming animal matter alone. For these two species, combining algae and animal matter did not enhance fitness over combining only algae. Fitness effects of specific algal diets showed some general similarities, but also considerable variance among the amphipods. For example, E. siliculosus was generally better food than other algae for all four amphipods, whereas Sargassum filipendula was generally poor. However, A. marcuzzii did not suffer negative effects of consuming only Sargassum. The red alga Polysiphonia sp. and the green alga Enteromorpha flexuosa decreased fitness in A. marcuzzii, C. compta, and G. mucronatus, but not A. valida, and the negative effects of Polysiphonia were considerably larger for A. marcuzzii than for the other amphipods. Our data show that nutritional requirements, even among related species (e.g., A. marcuzzii and A. valida), can be dramatically different. Diet mixing may benefit more mobile consumers like Gammarus that are better able to search for different foods, and may be less important for more sedentary herbivores like Ampithoe and Cymadusa that consume, and live in close association with, individual host plants. Received: 14 September 1998 / Accepted: 22 October 1999     

Diet,food preference,and algal availability for fishes and crabs on intertidal reef communities in southern California

Synopsis Herbivory by wide-ranging fishes is common over tropical reefs, but rare in temperate latitudes where the effects of herbivorous fishes are thought to be minimal. Along the west coast of North America, herbivory by fishes on nearshore reefs is largely restricted to a few members of the Kyphosidae, distributed south of Pt. Conception. This paper presents information on natural diets and results from feeding choice experiments for two abundant kyphosids from intertidal habitats in San Diego, California —Girella nigricans andHermosilla azurea, and similar data for the lined shore crab,Pachygrapsus crassipes, which also forages over intertidal reefs. These results are compared with the availability of algae in intertidal habitats measured during summer and winter, on both disturbed and undisturbed habitats. The diets of juveniles ofG. nigricans andH. azurea collected from nearshore habitats were dominated by animal prey (mainly amphipods), but adults of these fishes, andP. crassipes, consumed algae nearly exclusively, with 26, 10, and 14 taxa of algae identified fromG. nigricans, H. azurea, andP. crassipes, respectively. Algae with sheet-like morphologies (e.g.Ulva sp.,Enteromorpha sp., members of the Delesseriaceae) were the principal algae in the diets of the fishes, and calcareous algae (e.g.Corallina sp.,Lithothrix aspergillum) and sheet-like algae (Enteromorpha sp.) comprised the greatest identifiable portion of the shore crab's diet. Feeding choice experiments indicated that the fishes preferred filamentous algae (e.g.Centroceras clavulatum, Polysiphonia sp.,Chondria californica) and sheet-like algae (e.g.Enteromorpha sp.,Ulva sp.,Cryptopleura crispa) over other algal morphologies, whereas the shore crab chose jointed calcareous algae (e.g.Lithothrix aspergillum, Corallina vancouveriensis, Jania sp.) most frequently. The diets and preferences for algae by the fishes were generally most similar to the assemblage of algae available in early successional (disturbed) habitats during summer when sheet-like and filamentous algae are abundant. The shore crab exhibited the opposite trend with a diet more similar to late successional (undisturbed) habitats.     

高比例棉粕饲料中补充蛋氨酸对中华绒螯蟹幼蟹摄食、生长及抗氧化酶活性的影响

以40%棉粕为主的混合蛋白源制成基础饲料, 分别在其中添加0.00%、0.14%、0.28%、0.42%、0.56%蛋氨酸(分别记为0.00%Met、0.14%Met、0.28%Met、0.42%Met和0.56%Met), 配制了5种等氮等能的试验饲料, 以全鱼粉组(64.4%鱼粉)作为对照, 探讨了在高比例棉粕饲料中补充蛋氨酸对中华绒螯蟹幼蟹(0.390.02) g的摄食率、生长率和机体抗氧化酶活性的影响。结果表明: 与全鱼粉对照组相比, 0.42%Met组的增重率、特定生长率和饲料系数均无显著差异(P0.05), 但其显著高于0.00%Met、0.14%Met和0.28%Met组(P0.05); 增重率和特定生长率则显著高于0.56%Met组(P0.05); 从幼蟹的摄食量和蛋白质沉积率来看, 0.42%Met和0.28%Met组的摄食量与对照组相似, 当蛋氨酸的补充量低于0.28%或高于0.42%时, 幼蟹的摄食量均有所下降; 统计表明, 对照组的蛋白质沉积率最高(23.20%), 在各试验组中, 0.28%Met和0.42%Met组之间的蛋白质沉积率无显著差异(P0.05), 但显著高于0.00%Met、0.14%Met和0.56%Met组(P0.05); 分析幼蟹的肠道胰蛋白酶活性, 发现0.42%Met组与对照组之间无显著性差异(P0.05), 但均显著高于其他各试验组(P0.05); 0.28%Met、0.42%Met和0.56%Met处理组的血清丙二醛含量、超氧化物歧化酶活性和谷胱甘肽过氧化物酶活性与对照组无显著差异(P0.05)。结果提示, 在高比例植物蛋白(40%棉粕)的基础饲料中, 补充0.42%Met能够显著提高中华绒螯蟹幼蟹的生长率、消化率和抗氧化酶的活性。       

Facultative mutualism between an herbivorous crab and a coralline alga: advantages of eating noxious seaweeds

Because encrusting coralline algae rely on herbivory or low light levels to prevent being overgrown by competitively superior fleshy algae, corallines are relatively rare in shallow areas with low rates of herbivory. In contrast to this general trend, the branching coralline alga Neogoniolithon strictum occurs primarily in shallow seagrass beds and along the margins of shallow reef flats where herbivory on macrophytes is low. This alga apparently persists in these habitats by providing refuge to the herbivorous crab Mithrax sculptus at mean densities of 1 crab per 75 g of algal wet mass. When crabs were removed from some host corallines, hosts without crabs supported 9 times the epiphytic growth of hosts with crabs after only 30 days. Crabs without access to a coralline alga were rapidly consumed by reef fishes, while most of those tethered near a host alga survived. These results suggest that the crabs clean their algal host of fouling seaweeds and associate with the host to minimize predation. However, to effectively clean the host, the crab must consume the wide array of macroalgae that commonly co-occur with coralline algae in these habitats, including chemically defended species in the genera Halimeda, Dictyota, and Laurencia. Crabs did readily consume these seaweeds, which were avoided by, and are chemically defended from, herbivorous fishes. Even though crabs readily consumed both Halimeda and Dictyota in whole-plant feeding assays, chemical extracts from these species significantly reduced crab feeding, suggesting that factors other than secondary chemistry (e.g., food value, protein, energy content), may determine whole-plant palatability. Having the ability to use a wide variety of foods, and choosing the most profitable rather than the least defended foods, would diminish foraging time, increase site fidelity, and allow the crab to function mutualistically with the host alga. Despite the obvious benefit of associating with N. strictum, M. sculptus did not prefer it over other habitats offering a structurally similar refuge, suggesting that these crabs are not N. strictum specialists, but rather occupy multiple habitats that provide protection from predators. Structurally complex organisms like N. strictum may commonly suppress competitors by harboring protective symbionts like M. sculptus. It is possible that diffuse coevolution has occurred between these two groups; however, this seems unlikely because both herbivore and host appear to respond most strongly to selective pressures from predators and competitors outside this association.     

Reproduction response of Chinese mitten-handed crab (Eriocheir sinensis) fed different sources of dietary lipid

The effect of feeding three semi-purified diets containing different lipid sources (anchovy oil, soybean oil and pork lard) on fecundity, hatchability and egg fatty acid composition of Chinese mitten-handed crab (Eriocheir sinensis) broodstock was compared with a fresh clam diet in a 6-month feeding trial. Broodstock crabs fed the diet containing pork lard showed poor fecundity and low hatchability. Crabs fed the diet containing soybean oil showed improved fecundity; however, no significant improvement in hatchability was observed. Broodstock fed the diet containing anchovy oil showed the highest fecundity and egg hatchability. Eggs from broodstock fed anchovy oil as sole dietary lipid had a higher n-3 polyunsaturated fatty acid (PUFA) content (33.3%) compared with those of crabs fed diets with soybean oil (20.1%) and pork lard (16.3%) as lipid sources. The results indicate a close correlation between: (1) the 20:5n-3 content of the egg lipid and fecundity; (2) the 22:6n-3 content and hatchability; and (3) fecundity, hatchability and n-3/n-6 fatty acid ratio. The results also suggest that each of these n-3 HUFAs may play different and specific roles in crab reproduction and that either must be adequate in the broodstock diet.     

Effects of the complete replacement of fish oil with linseed oil on growth,fatty acid composition,and protein expression in the Chinese mitten crab (<Emphasis Type="Italic">Eriocheir sinensis</Emphasis>)

Background

The finite marine resources make it difficult for us to obtain enough fish oil (FO) used in aquatic feeds. Another sustainable ingredients should be found to substitute FO. The effects of replacing FO with vegetable oil have been studied in a variety of crustaceans, but most studies have focused on the phenotypic effects. Little is known about the mechanisms of the effects.

Methods

To understand the molecular responses during the replacement of FO in Eriocheir sinensis, we investigated the effects of feeding FO or linseed oil (LO) on growth performance, digestive enzyme activity, fatty acid composition and protein expression in E. sinensis. Twenty-four juvenile crabs were fed diets containing FO or LO for 112 days. Weight, carapace length and width were recorded. Fatty acid composition of the diets and the hepatopancreas and protein expression in the hepatopancreas were analyzed.

Results

Growth performance and molting interval were unchanged by diet. Crabs fed FO and LO had same activity of lipase and amylase, but comparing with crabs fed LO, crabs fed FO had higher trypsin activity and lower pepsin activity. Hepatopancreas fatty acid composition changed to reflect the fatty acid composition of the diets. In total, 194 proteins were differentially expressed in the hepatopancreas between the diets. Expression of heat shock proteins was higher in crabs fed LO. Expression of fatty acid synthase, long-chain fatty acid transport protein 4, acyl-CoA delta-9 desaturase, and fatty acid-binding protein 1, was higher in crabs fed FO.

Conclusions

The substitution of FO with LO didn’t have any effects on the growth and molting of mitten crab, but could significantly decrease the ability of mitten crab to cope with stress. The high content of HUFAs in the hepatopancreas of mitten crab fed FO is due to the high abundance of the proteins relative to the transport of the HUFAs. These findings provide a reason of the high content of EPA and DHA in crabs fed with FO, and provide new information for the replacement of FO in diets of mitten crab.

     

How diet influences energy partitioning in the regular echinoid Psammechinus miliaris; constructing an energy budget

The effect of diet (made from either animal or plant material) on the partitioning of energy in small (8-16 mm test diameter (td)) and large (29-37 mm td) Psammechinus miliaris was examined. Diet significantly affected ingestion, digestion and growth of both size groups. Assimilation rates of the different types of feed varied and they supported differential development of the body parts. Urchins fed on an algal diet showed poorer absorption efficiency, assimilation, gonadal and somatic growth than individuals fed on an artificial sea urchin diet (mixed plant and animal material) or on the animal based diets. Small urchins fed on a diet of salmon food utilised proteins as the primary energy source rather than carbohydrates or lipids as indicated by low O/N atomic ratios, and salmon diet promoted a higher energetic investment in reproductive development in both size classes. A negative energy balance resulted when large urchins were fed on the algal diet. Energy losses due to ammonia excretion were negligible and the metabolic losses of assimilated energy, measured as oxygen consumption, were between 0.2% and 1.5% with the different diets. A diet made of mussel flesh stimulated ingestion, gave the highest assimilation rates and best overall growth performance. Such detailed information should assist in the design of diets for the cultivation of sea urchins.     

The effect of dietary chitin supplementation on the survival and immune reactivity of the shore crab, Carcinus maenas

Adult male shore crabs (Carcinus maenas) were maintained on a fish-based diet supplemented with 0, 5 or 10% chitin for 11 weeks. Significantly greater mortality was found during this period in the control group (0% chitin) than those fed 10% chitin. Crabs fed 5 or 10% chitin had lower numbers of cultivatable bacteria in the hepatopancreas than those on the basal diet alone. The addition of chitin had no significant effect on the serum concentrations of protein and glucose, and the levels of glycogen in the hepatopancreas. The total number of circulating hemocytes in the blood was unaffected by the addition of chitin to the diet, however, at week 6 there were significantly more hyaline hemocytes in those crabs fed 10% chitin than the control group. The in vitro phagocytic activity of hemocytes was unaffected by chitin supplementation and crabs challenged with Vibrio alginolyticus showed a similar pattern of susceptibility in the three dietary groups (0, 5 or 10% chitin). Overall although crabs on a chitin-supplemented diet showed greater survival, this was not explained in terms of elevation in the cellular defences of these animals. The enhanced survival of crabs-fed chitin is probably as a result of the removal of potentially pathogenic bacteria from the hepatopancreas. Because chitin appears to 'purge' bacteria from the gut, this may prove to be a useful addition to diets on animals undergoing oral probiotic treatment.     

Nutritional value of detritus and algae in blenny territories on the Great Barrier Reef

The salariin blenny, Salarias patzneri, primarily ingests detrital aggregates and inorganic sediment, with small quantities of filamentous algae. Samples of particulate matter <125 μm and filamentous algae were collected from the territories of S. patzneri and the nutritional value of these resources assessed using organic content and protein/energy ratios calculated from protein, carbohydrate and lipid content. Samples were collected throughout the day, during both summer and winter, to enable temporal comparisons of nutritional value. The particulates <125 μm, which were predominantly amorphic detritus, accounted for 41±2% and 44±3% of the organic matter in the epilithic algal matrix (EAM) during the summer and winter, respectively, whilst filamentous algae accounted for 37±3% and 41±3% of the organic matter in the summer and winter, respectively. Carbohydrate and lipid concentration in the algal samples was significantly greater than in particulate matter <125 μm, although no significant difference was detected in protein levels of algal samples and particulates <125 μm. Consequently, the protein/energy ratio of particulates <125 μm in the summer (11±1 mg kJ⁻¹) and winter (10±1 mg kJ⁻¹) was similar to that of filamentous algae in the summer (9±1 mg kJ⁻¹) and winter (8±1 mg kJ⁻¹). These results suggest detrital aggregates are at least comparable to filamentous algae as a nutritional resource. The large contribution of high quality detritus to organic matter in the epilithic algal matrix collected from S. patzneri territories throughout the day and between seasons provides strong evidence that detritus is a valuable component of small territorial fish diets and is an integral part of coral reef food webs.     

Comparative growth and survival of juvenile hard clams,Mercenaria mercenaria,fed commercially available diets

Hobbyist and researchers often use commercially available phytoplankton concentrates to maintain filter feeding organisms held in their ornamental or experimental tanks. This study investigated the nutritional value of 10 products available commercially for juvenile hard clams, Mercenaria mercenaria. Growth and mortality rates in clams fed these products were compared with those found in clams fed fresh cultures of the microalgae Isochrysis galbana, which is considered an industry standard for supporting growth of juvenile bivalves. Our results show a clear difference in feed nutritional value between non‐living and living commercial diets, and among commercial diets advertised as containing live algae. Overall, results showed that juvenile hard clams fed fresh cultures of I. galbana displayed the best growth and lowest mortality rates, followed by those fed the commercial diet DT's Live Marine Phytoplankton. Growth and mortality rates in unfed controls were similar to those found in clams fed commercial non‐living algae mixes or diets advertised as containing live algae (Phyto‐Feast Live product). Results also showed that the nutritional value of fresh algae (I. galbana) cultures is lost rapidly when cultures are maintained at 4°C, suggesting that algae present in some commercial diets may lose their nutritional value during processing or refrigerated storage. The commercial blend, DT's Live Marine Phytoplankton, seems to represent a good substitute to lab grown algae for clams held in ornamental or experimental aquariums. Zoo Biol 0:1–13, 2006. © 2006 Wiley‐Liss, Inc.     

Parasites of the shore crab Carcinus maenas (L.): implications for reproductive potential and invasion success

The European shore crab, Carcinus maenas, is one of the most successful marine invasive species. Its success has been in part attributed to the loss of parasites, rekindling an interest in host-parasite interactions and impacts on host fitness in this crab. In the present study, we investigated C. maenas populations from Europe, South Africa and Australia for parasites, and assessed their impact on the fitness of male crabs. For the shore crab, testes weight along with success in mating competition is traded off against other life-history traits. We therefore used this parameter as an indicator both for reproductive fitness and a possible resource trade-off in response to parasite infestation. In the native range, crabs infested with Sacculina carcini showed significantly lower testes weight than uninfected crabs. However, helminth parasites did not generally cause reduced testes weights. Crab populations from South Africa and Australia were either parasitized at very low prevalences, or were completely parasite free. However, no population level effect of this parasite release was reflected in testes weight. These findings do not support a severe fitness impact of helminth parasites on C. maenas, which questions the role of parasites on its population dynamics, both in the native area and for invasive success.     

Coupling between microphytobenthic biomass and fiddler crab feeding

Among the organic matter ingested by fiddler crabs, microphytobenthos is of fundamental importance because it is their main N source. Microphytobenthos abundance generally develop semilunar changes as the dynamics of tidal exposures and day-night cycle are not held constant across days, modifying the balance between growth and mortality. In this study we explored the coupling between temporal dynamics in microphytobenthos abundance and crab feeding activity. We measured the Chlorophyll a content in the 2 mm surficial sediment surrounding the burrows and the crab feeding activity over two semilunar cycles. Chlorophyll a and crab feeding activity showed biweekly cyclic dynamics. Crabs did not concentrate feeding activity around days with maximum abundance of microhytobenthos. This phase difference between both dynamics could be the result of the crab feeding impact, but a crab experimental exclusion showed that the temporal dynamics of Chlorophyll a content stayed unchanged when feeding activity was removed. Comparisons between fed and unfed sediment suggest that the feeding efficiency changes with tidal dynamic. Crabs achieved more than 50% of Chlorophyll a extraction during days of highest feeding activity, and less than 30% during days of low feeding activity or low microhytobenthos abundance. Furthermore, comparisons of fed sediment between consecutive days indicated that Chlorophyll a was completely replenished during days with high flooding tides, but partially replenished during days near neap tides. Environmental conditions affecting feeding efficiency may select crabs to concentrate feeding activity before days with the highest microhytobenthos abundance. The low feeding impact on microphytobenthos dynamics suggests that fiddler crabs would not control microhytobenthos abundance and thus unable to absorb the increasing eutrophication of studied estuarine areas.     

Availability of salts is not a limiting factor for the land crab Gecarcinus lateralis (Freminville)

The land crab Gecarcinus lateralis (Freminville) is restricted to within ≈ 250 m of the shore on Bermuda. Previous work demonstrated that availability of water did not account for the exclusion of crabs from inland habitats. The crabs' inability to produce dilute urine led to the hypothesis that availability of salts becomes limiting away from the shore. In the laboratory, osmoregulatory abilities of land crabs were tested under several ecologically realistic conditions. Salts were provided in interstitial water of various salinities in damp-sand “burrows”, in “drinking water” of various salinities, or in natural and artificial foods. Crabs maintained hemolymph concentrations very well even when exposed to salinities < 1% seawater (SW), particularly when given a choice of how much contact to have with test salinities (“drinking-water” regime). Rates of ion loss were apparently extremely low, even when low-ion artificial food (flavored filter paper) was passing through the gut. In the field, potential sources of salts were compared with the crabs' osmoregulatory abilities. All available water (rain, dew) was very dilute, and there was no evident gradient in availability of salts at increasing distances from shore that could account for the limited distribution of crabs. Concentrations of ions in forage plants were also low, and also showed no gradient correlated with the crabs' range limit. Crabs that were ion-depleted and then fed vegetation from inland of the normal range showed significant recovery in hemolymph concentrations. We concluded that availability of salts is adequate beyond the existing range of the land crabs, and is not the range-limiting factor. This is paradoxical, given the limited osmoregulatory abilities of crab antennal glands, and raises the question of how land crabs accomplish such extraordinary conservation of ions; the physiological mechanism (extrarenal modification of urine) will be described elsewhere. Factors that limit how far inland land crabs live remain enigmatic; hypotheses for further work are presented.     

Effects of poor food quality on copepod growth are dose dependent and non‐reversible

Herbivores regularly face imbalanced diets on a variety of timescales. They respond to such diets with reductions in growth and reproduction. The effect of exposure time to poor quality diets and recovery potential in herbivores has yet not been intensively studied. In order to investigate the response of herbivores growth and nutritional condition to phosphorus limited algal food, we fed the copepod Acartia tonsa with the autotrophic flagellate Rhodomonas salina, which was cultured towards high and low phosphorus content. To test the effect of the duration of the exposure to low and high P food on copepod growth, we also switched a subset of the copepods from high P to low P food for two to 12 days before putting them back on their original diet. Phosphorus limited prey clearly reduced copepod nutritional condition, expressed as their RNA:DNA ratio and consequently their growth rates. Several days after re‐feeding on high P algae, the copepods were able to reach growth rates comparable to the control groups, but they were in no case able to over shoot the growth rates of the control groups. A dose of two days feeding on low P algae resulted in slightly less than one stage growth reduction compared to copepods exclusively reared on high P algae. Our results show that phosphorus limited food drastically reduces secondary production, and no compensatory growth occurs in the investigated copepod species. Hence, even short term exposure of herbivores to poor food has a lasting effect on secondary production.     

SOME ASPECTS OF THE ECOLOGY OF THE FRESHWATER CRAB (POTAMONAUTES PERLATUS MILNE EDWARDS) IN THE UPPER REACHES OF THE BUFFALO RIVER,EASTERN CAPE PROVINCE,SOUTH AFRICA.

Summary

Freshwater crabs (Potamonautes perlatus) are the largest naturally occurring invertebrates in southern African rivers. The ecology of these animals in South African riverine ecosystems is little understood. This study investigates some aspects of the population and feeding ecology of P. perlatus in the upper reaches of the Buffalo River. The densities of crabs (carapace width larger than 25 mm) at two sites in the river ranged between 1.72 and 5.25 crabs m², higher than previously recorded for freshwater crabs in rivers in southern Africa and southern Italy. Twenty-four hour observations revealed that P. perlatus has a nocturnal habit. The diet of P. perlatus was established by examination of the contents of the fore-gut. Crabs with a carapace width smaller than 40 mm fed predominantly on aquatic invertebrates while larger crabs fed on allochthanous material (leaf litter), detritus and algae. Through feeding, crabs reduced the surface area of leaves in the stream by 99.95%.     

The incidence of autotomy in Carcinus maenas (L.)

A total of 1023 individuals of the common shore crab, Carcinus maenas (L.), were obtained from the environs of Whitby Harbour and Robin Hood's Bay. The bulk of the sample was taken from the sublittoral zone, but crabs from a sheltered shore were included. Each crab was measured, sexed and examined for the incidence of autotomized and regenerated limbs.There was a positive correlation between the incidence of autotomy and carapace width for crabs in both the sublittoral zone and the exposed intertidal zone. In sublittoral crabs the males suffered a higher incidence of autotomy than did the females and the incidence of cheliped autotomy was higher than for the walking limbs. The discrepancy between the loss of the chelipeds and walking limbs was reflected in a similar disparity between the occurrence of the regenerated chelipeds and walking limbs.Crabs on an exposed shore suffered a higher incidence of autotomy than did those from a sheltered shore. There were more crabs with cumulative limb losses than would be expected which suggests that once a crab enters into a state of autotomy it becomes increasingly vulnerable to the loss of another limb. Only 1 % of the population of sublittoral crabs would be expected to carry the loss of five limbs at once. No crabs were found with six or more limbs missing.     

The effect of reservoir trophic status on the feeding and growth of <Emphasis Type="Italic">Labeo umbratus</Emphasis>

Synopsis We compared the diet and growth of moggel, Labeo umbratus, in an oligotrophic system to that in a eutrophic system to determine if the trophic status of the environment in which it occurs effects its growth rate. The diet of moggel in both reservoirs was similar and consisted of (in order of importance) detritus, algae and sand. The protein content of the diet was significantly higher in the eutrophic reservoir as a consequence of the elevated concentration of algae in the detrital aggregate. Diatoms were more easily ruptured by the pharyngeal plates of moggel than blue-green algae. Cell rupture may have been assisted by the large amount of sand in the diets. Sand may also have contributed to the breakdown of algal cells in the alimentary canal by compaction. Although there was no difference in the temperature regime of the two reservoirs, moggel grew significantly faster in the eutrophic system. The slower growth rate of moggel in the oligotrophic system was attributed to the poorer nutritional value of the diet as a result of the lower concentrations of algae in the detrital aggregate. Trophic status therefore had an effect on moggel growth by directly influencing the nutritional value of their diet.     

Changes in the selection differential exerted on a marine snail during the ontogeny of a predatory shore crab

Empirical estimates of selection gradients caused by predators are common, yet no one has quantified how these estimates vary with predator ontogeny. We used logistic regression to investigate how selection on gastropod shell thickness changed with predator size. Only small and medium purple shore crabs (Hemigrapsus nudus) exerted a linear selection gradient for increased shell‐thickness within a single population of the intertidal snail (Littorina subrotundata). The shape of the fitness function for shell thickness was confirmed to be linear for small and medium crabs but was humped for large male crabs, suggesting no directional selection. A second experiment using two prey species to amplify shell thickness differences established that the selection differential on adult snails decreased linearly as crab size increased. We observed differences in size distribution and sex ratios among three natural shore crab populations that may cause spatial and temporal variation in predator‐mediated selection on local snail populations.