Preliminary evidence that the feeding rates of generalist marine herbivores are limited by detoxification rates

Herbivores tend to increase feeding rate and fitness when consuming a mixed diet relative to a single diet. According to the detoxification limitation hypothesis (DLH), feeding choices and rates when confronted with chemically rich plants are determined by herbivore physiology, and specifically by the metabolic pathways that herbivores use to manipulate secondary metabolites. We tested two predictions of the DLH using two generalist herbivores, the urchin Arbacia punctulata and amphipod Ampithoe longimana. These herbivores have geographic ranges which overlap with brown seaweeds that produce diterpenes (Dictyota menstrualis, D. ciliolata) and a green seaweed that produces sesquiterpenes and diterpenes (Caulerpa sertularioides). As predicted by the DLH, herbivore consumption rates in no-choice feeding assays were limited by extract intake rates. This suggests an upper limit in the herbivores’ abilities to physiologically manipulate seaweed metabolites. Contrary to a second prediction of the DLH, urchins consumed equal amounts of foods coated with limiting concentrations of two seaweed extracts offered singly, as a mixture, or as a pairwise choice. This result suggests that secondary metabolites of these seaweeds are manipulated by a linked set of detoxification pathways. Improving our understanding of the mechanisms that underlie diet mixing depends on greater attention to the physiology of herbivore resistance to secondary metabolites.     

Feeding four species of pelagic copepods under experimental conditions

Different qualitative and quantitative aspects of feeding with varied compositions of diets have been studied in 4 species of pelagic copepods: Calanus helgolandicus, Centropages typicus, Temora stylifera and Acartia clausi. By feeding copepods different algal concentrations, it was shown that when food concentration increases grazing rate decreases; the ingestion rate remains fairly constant in the lower range of concentrations, but then increases, reaching a plateau at higher algal concentrations. There is a significant correlation between daily food intake and fecal pellet production. On a pluri-algal diet, selective grazing is observed: larger phytoplankton cells are more efficiently removed than smaller ones. Using Artemia nauplii, it is shown that the copepods studied are also able to eatch and ingest animal prey. Increased daily food intake affects respiration and oviposition. Metabolic requirements, gross growth-efficiency, and food assimilation have been ealculated in Calanus helgolandicus, Centropages typicus and T. stylifera for a large range of algal concentrations.     

A comparison of filtration rates among pelagic tunicates using kinematic measurements

Salps have higher filtration rates than most other holoplankton, and are capable of packaging and exporting primary production from surface waters. A method of kinematic analysis was employed to accurately measure salp feeding rates. The data were then used to explain how diverse body morphologies and swimming motions among species and lifecycle stages influence salp feeding performance. We selected five species, representing a range of morphologies and swimming styles, and used digitized outlines from video frames to measure body-shape change during a pulse cycle. Time-varying body volume was then calculated from the digitized salp outlines to estimate the amount of fluid passing through the filtering mesh. This non-invasive method produced higher feeding rates than other methods and revealed that body volume, pulse frequency and degree of contraction are important factors for determining volume filtered. Each species possessed a unique combination of these three characteristics that resulted in comparable filtration (range: 0.44–15.33 ml s⁻¹) and normalized filtration rates (range: 0.21–1.27 s⁻¹) across species. The convergence of different species with diverse morphologies on similar normalized filtration suggests a tendency towards a flow optimum.     

Comparison of gut-evacuation rates of feeding and non-feeding Calanus marshallae

Gut-evacuation rates were compared for feeding and non-feeding Calanus marshallae collected near Yaquina Bay, Oregon (44°37N; 124°04W) from June 1986 through May 1987. Evacuation rates were measured at four concentrations of Thalassiosira weissflogii from the decline of gut-pigment fluorescence following transfer of copepods to filtered seawater and from the loss of cells labelled with 68-germanium, a radioactive analog of silicon. There was no significant difference between gut-evacuation rates of feeding and non-feeding copepods over both short-term (20 min) and long-term (90 min) evacuation times. Furthermore there was no difference between rates obtained using either pigment or ⁶⁸Ge as tracers of gut food-passage. These results are discussed in the light of possible dilution of tracer in the guts of feeding copepods due to mixing with unlabelled food. Gut-evacuation rates measured at food concentrations ranging from 500 to 4000 cells ml^-1 were not significantly different, regardless of the technique employed.     

Shallow-water benthic and pelagic metabolism:

Pelagic primary production and benthic and pelagic aerobic metabolism were measured monthly at one site in the estuarine plume region of the nearshore continental shelf in the Georgia Bight. Benthic and water-column oxygen uptake were routinely measured and supplemented with seasonal measures of total carbon dioxide flux. Average respiratory quotients were 1.18:1 and 1.02:1 for the benthos and water column, respectively. Benthic oxygen uptake ranged from 1.23 to 3.41 g O₂ m^-2 d^-1 and totalled 756 g O₂ m^-2 over an annual period. Water column respiration accounted for 60% of total system metabolism. Turnover rates of organic carbon in sediment and the water column were 0.09 to 0.18 yr^-1 and 6.2 yr^-1, respectively. Resuspension appeared to control the relative amounts of organic carbon, as well as the sites and rates of organic matter degradation in the benthos and water column. Most of the seasonal variation in benthic and pelagic respiration could be explained primarily by temperature and secondarily by primary productivity. On an annual basis, the shelf ecosystem appeared to be heterotrophic; primary production was 73% of community metabolism, which was 749 g C m^-2 yr^-1. The timing of heterotrophic periods through the year appeared to be closely related to both river discharge and the periodicity of growth and death of marsh macrophytes in the adjacent estuary. The results of this study support the estuarine outwelling hypothesis of Odum (1968).This is Contribution No. 530 from the University of Georgia Marine Institute. This work was supported by the Georgia Sea Grant College Program maintained by the National Oceanic and Atmospheric Administration, US Department of Commerce     

Prey, feeding rates, and asexual reproduction rates of the introduced oligohaline hydrozoan Moerisia lyonsi

Moerisia lyonsi Boulenger (Hydrozoa) medusae and benthic polyps were found at 0 to 5‰ salinity in the Choptank River subestuary of Chesapeake Bay, USA. This species was introduced to the bay at least 30 years before 1996. Medusae and polyps of M. lyonsi are very small and inconspicuous, and may occur widely, but unnoticed, in oligohaline waters of the Chesapeake Bay system and in other estuaries. Medusae consumed copepod nauplii and adults, but not barnacle nauplii, polychaete and ctenophore larvae or tintinnids, in laboratory experiments. Predation rates on copepods by medusae increased with increasing medusa diameter and prey densities. Feeding rates on copepod nauplii were higher than on adults and showed no saturation over the range of prey densities tested (1 to 64 prey l⁻¹). By contrast, predation on copepod adults was maximum (1 copepod medusa⁻¹ h⁻¹) at 32 and 64 copepods l⁻¹. Unexpectedly, M. lyonsi colonized mesocosms at the Horn Point Laboratory during the spring and summer in 4 years (1994 to 1997), and reached extremely high densities (up to 13.6 medusae l⁻¹). Densities of copepod adults and nauplii were low when medusa densities were high, and estimated predation effects suggested that M. lyonsi predation limited copepod populations in the mesocosms. Polyps of M. lyonsi asexually produced both polyp buds and medusae. Rates of asexual reproduction increased with increasing prey availability, from an average total during a 38 d experiment of 9.5 buds polyp⁻¹ when each polyp was fed 1 copepod d⁻¹, to an average total of 146.7 buds polyp⁻¹ when fed 8 copepods d⁻¹. The maximum daily production measured was 8 polyp buds and 22 medusae polyp⁻¹. The colonizing potential of this hydrozoan is great, given the high rates of asexual reproduction, fairly wide salinity tolerance, and existence of a cyst stage. Received: 29 October 1998 / Accepted: 3 March 1999     

Relationships between spacing behavior and growth rates: A field study of lizard feeding territories

Summary Relationships between spacing behavior and growth rates were investigated in a field experiment with juvenile lizards, Anolis aeneus. The behavioral variable most closely related to juvenile growth was distance moved per unit time. This variable had a curvilinear relationship with growth, such that juveniles moving approximately 400 cm/h grew more rapidly than those traveling either larger or shorter distances per unit time. Daily fluctuations in arthropod abundance were also related to growth rates, with restricted growth during periods of low food availability. Temporal fluctuations in prey and distance traveled per unit time had independent effects on growth; together these two variables accounted for 43% of the variance in growth rate for the juveniles in this study.Territory size, overlap and social status appeared to affect growth indirectly, by influencing distance traveled per unit time. Optimal travel distances of around 400 cm/h were most likely when a juvenile had a relatively exclusive territory of about 0.5 m². High ranking juveniles were more apt to achieve this spacing pattern than were low ranking juveniles, but some high ranking juveniles had very large territories, extensive overlap with subordinates, supraoptimal travel distances and relatively low growth rates. Low ranking juveniles tended to fall into two groups: subordinates, with a small home range overlapping that of a more dominant individual and low travel distances, and floaters, with a large home range overlapping several more dominant individuals and high travel distances. Although a few low ranking juveniles achieved travel distances permitting high growth rates, most had either supra or suboptimal travel distances and relatively low growth rates.     

Energy budgets and feeding rates of Coryphaenoides acrolepis and C. armatus

This study develops energy budgets and estimates feeding rates for two macrourid fishes, Coryphaenoides acrolepis, dominant in the bathyal eastern North Pacific, and the abyssal cosmopolitan species, Coryphaenoides armatus. Daily energy expenditure by C. acrolepis was nearly twice that of C. armatus. C. acrolepis allocated nearly equal amounts of energy to metabolism and growth. Once sexual maturity was reached reproduction became the dominant energetic cost. Either these costs are necessary to retain adequate numbers of eggs and larvae on the continental slopes, or this fish does not reproduce on an annual basis and the calculated costs are an overestimate. C. armatus allocated relatively more energy to metabolism than growth. It may be semelparous, and this strategy would be of great energetic savings in its food-poor but stable environment. Individual daily ration for C. acrolepis decreased from 0.31% to 0.07% of body weight (BW) and for C. armatus from 0.12% to 0.02% BW with increasing fish length. These rates are substantially lower than those for fishes living in cold waters on the continental shelves. The population feeding rates for C. acrolepis ranged from 0.8 to 15 kg km^-2 day^-1 and for C. armatus from 5 to 2,800 g km^-2 day^-1. The scavenging behaviour of C. acrolepis was used to investigate the role of carrion as a food supply to the deep-sea benthos. It was estimated that the carrion eaten by C. acrolepis is equivalent to 0.04 mg C m^-2 day^-1 or only 0.2-0.4% of the average small particulate flux. Carrion consumption is important for scavengers like C. acrolepis, but it is not an important component of the carbon flux into the deep-sea benthic environment.     

Nuptial feeding in tettigoniids male costs and the rates of fecundity increase

Summary A recent debate has centred on the importance of paternal investment for the origin and maintenance of nuptial feeding in insects. Some authors have argued that the rates of nutrient incorporation are likely to be too slow to allow a male to fertilize the eggs that he helps to produce and cannot be considered as paternal investment. Here I report the results of some experiments that show that the positive effects of nutrient donation on female reproduction for one species of spermatophylax producing tettigoniid can be realized within 24 to 48 h. Furthermore, mating was found to induce a refractory period in the female that was longer than the time taken for the incorporation of nutrients and oviposition. Thus, the nurturant male is likely to fertilize the eggs even if last-male sperm precedence is high. The cost of spermatophylax production was manifest as a 5-day recovery period between matings. Thus males and their donations are likely to be limited resources for females resulting in a reversal of the typical sex roles.     

Environmental controls on denitrifying communities and denitrification rates: insights from molecular methods.

The advent of molecular techniques has improved our understanding of the microbial communities responsible for denitrification and is beginning to address their role in controlling denitrification processes. There is a large diversity of bacteria, archaea, and fungi capable of denitrification, and their community composition is structured by long-term environmental drivers. The range of temperature and moisture conditions, substrate availability, competition, and disturbances have long-lasting legacies on denitrifier community structure. These communities may differ in physiology, environmental tolerances to pH and O2, growth rate, and enzyme kinetics. Although factors such as O2, pH, C availability, and NO3- pools affect instantaneous rates, these drivers act through the biotic community. This review summarizes the results of molecular investigations of denitrifier communities in natural environments and provides a framework for developing future research for addressing connections between denitrifier community structure and function.     

Specialized feeding mechanism in the pelagic copepod genus Heterorhabdus (Calanoida: Heterorhabdidae), with special reference to the mandibular tooth and labral glands

Scanning and transmission electron microscopy and gut-content analysis revealed the fine structure of the mandibular ventralmost tooth and labral glands of the carnivorous copepods Heterorhabdus spp. on the basis of the specimens collected in the waters near Nansei Islands, Southwestern Japan, in November 1993. The ventralmost tooth is hollow with a subterminal pore and a basal opening. The distal region of the tooth is like a hypodermic needle and is strengthened by opal layers. The tip of the tooth including the subterminal pore usually protrudes from between the labrum and the paragnath, while the basal region of the ventralmost tooth and the whole parts of the other teeth are semi-enclosed in the preoral food chamber formed by the labrum and the paragnath. There is a large glandular cell in the basal region of the mandibular gnathobase, but no direct connection exists between the gnathobasal cells and the tubular lumen of the ventralmost tooth. The posterior surface of the labrum carries a lateral pair of large glandular pores, which are located close to, and appear to fit exactly the basal openings of the ventralmost teeth. Each of these glands has two massive secretory cells, and the secretory materials from these cells appear to be mixed before discharge from the pore. A specialized feeding mechanism is proposed, wherein venom or anaesthetic is injected from the labral-gland pores into the tubular lumen of the mandibular ventralmost tooth, and then relcased into the prey from its subterminal pore immediately after capture using the stout maxillae of prey macrozooplankters such as copepodids and polychaetes.     

Predator-induced female behavior in the absence of male incubation feeding: an experimental study

Parental care plasticity is critical to understanding the ecological and evolutionary influence of nest predation on life history strategies. In birds, incubation imposes a trade-off between the requirements of females (i.e., food) and egg requirements (i.e., heat and protection from predators). However, studies on this topic are rare and usually restricted to species where the male feeds the incubating female, relaxing her incubation costs. Males and females can reduce their activity at the nest to avoid detection by predators. However, females could follow two alternative antipredator strategies: to delay their return to the nest to avoid attracting attention from the potential predator or to return to the nest as soon as possible to enhance nest concealment. In this study, we manipulated the perceived risk of nest predation of incubating common blackbirds (Turdus merula), a species without incubation feeding, to study female behavioral changes induced by nest predation risk. We show experimentally that female blackbirds can reduce their nest visits in the situation with higher nest predation risk. In addition, we confirm that females significantly delay their return to the nest in the presence of a nest predator, contradicting the nest concealment hypothesis. However, our results could be interpreted as a passive antipredator response (to minimize clues given to predators) or as an active antipredator response (to search for predators to expel them from their territories).     

Chemical composition,metabolic rates and feeding behavior of the midwater ctenophore Bathocyroe fosteri

Individuals of the midwater ctenophore Bathocyroe fosteri (0.01 to 1.6 g dry weight, DW) were collected from Bahamian waters by the submersible Johnson-Sea-Link during May and September/October 1983 and October/November 1984 from 530 to 700 m depth. Metabolic rates were measured and showed oxygen consumption to be in the range of 0.01 to 0.18 mg O₂ g^-1 DW h^-1 at temperatures ranging from 9° to 12°C. Ammonium excretion (0.01 to 0.14 g-at N g^-1 DW h^-1) was typically low. Energy expenditures estimated from respiration data (ca. 7% body C d^-1) indicated that one to three midwater crustacean prey (ca. 150 g C d^-1) could provide the daily maintenance ration required by a 40 mm ctenophore. These metabolic characteristics complemented in situ observations of poor locomotor ability and passive feeding behavior.     

Processing of allochthonous macrophyte subsidies by sandy beach consumers: estimates of feeding rates and impacts on food resources

Allochthonous subsidies of organic material can profoundly influence population and community structure; however, the role of consumers in the processing of these inputs is less understood but may be closely linked to community and ecosystem function. Inputs of drift macrophytes subsidize sandy beach communities and food webs in many regions. We estimated feeding rates of dominant sandy beach consumers, the talitrid amphipods (Megalorchestia corniculata, in southern California, USA, and Talitrus saltator, in southern Galicia, Spain), and their impacts on drift macrophyte subsidies in field and laboratory experiments. Feeding rate varied with macrophyte type and, for T. saltator, air temperature. Size-specific feeding rates of talitrid amphipods were greatest on brown macroalgae (Macrocystis, Egregia, Saccorhiza and Fucus). Rates for large individuals of both species ranged from ∼40 mg wet wt individual⁻¹12 h⁻¹ on brown macroalgae to negligible feeding by M. corniculata on a vascular plant (surfgrass). Amphipod growth rates were also greatest on Macrocystis and lowest on surfgrass, Phyllospadix. For a Californian beach with substantial inputs of macrophyte wrack (>70 kg wet wt m⁻¹ month⁻¹ in summer), we estimated that the population of talitrid amphipods could process an average of 55% of the palatable Macrocystis input. Our results indicate that talitrid amphipod populations can have a significant impact on drift macrophyte processing and fate and that the quantity and composition of drift macrophytes could, in turn, limit populations of beach consumers.     

A compensation mechanism for siting noxious facilities: Theory and experimental design

In locating noxious facilities, such as a trash disposal plant or a hazardous waste disposal incinerator, the host community frequently incurs all the costs while other communities in the region receive the benefits. We propose a mechanism for sharing the benefits with the potential loser. Each community submits a sealed bid indicating the minimum amount it would require to host the facility. The site providing the lowest bid obtains the facility and receives its bid as compensation. This compensation is financed by the other communities. If there are N candidate communities, then each of the other communities would pay 1/(N − 1) of their acceptance bid. A series of controlled laboratory experiments show that the outcomes of this low-bid auction come close to predictions from a theoretical model based on maxi-min rules. Equity and efficiency considerations also are discussed in the context of the noxious facility problem.     

A method for the determination of copepod feeding rates during short time intervals

An experimental technique for measuring the feeding behavior of copepods on small time scales is important because grazer/micro-patch interactions in the ocean are probably short (minutes to hours). By placing copepods with empty guts into a known food concentration and monitoring the rate of increase in gut contents until defecation begins, usually 20 to 100 min, the volume of water swept clear of particles per min can be calculated. This process is easily quantified by using phytoplankton as food and measuring the gut contents by fluorometry. The utility of this technique is demonstrated in the laboratory and at sea.     

Measuring sedimentation in tidal marshes: a review on methods and their applicability in biogeomorphological studies

It is increasingly recognised that interactions between geomorphological and biotic processes control the functioning of many ecosystem types as described e.g. by the ecological theory of ecosystem engineering. Consequently, the need for specific bio-geomorphological research methods is growing recently. Much research on bio-geomorphological processes is done in coastal marshes. These areas provide clear examples of ecosystem engineering as well as other bio-geomorphological processes: Marsh vegetation slows down tidal currents and hence stimulates the process of sedimentation, while vice versa, the sedimentation controls ecological processes like vegetation succession. This review is meant to give insights in the various available methods to measure sedimentation, with special attention to their suitability to quantify bio-geomorphological interactions. The choice of method used to measure sedimentation is important to obtain the correct parameters to understand the biogeomorphology of tidal salt marshes. This review, therefore, aims to be a tool for decision making regarding the processes to be measured and the methods to be used. We, subdivide the methods into those measuring suspended sediment concentration (A), sediment deposition (B), accretion (C) and surface-elevation change (D). With this review, we would like to further encourage interdisciplinary studies in the fields of ecology and geomorphology.     

In situ feeding rates and grazing impact of zooplankton in a South African temporarily open estuary

Recent studies in temporarily open estuaries of South Africa have shown that phytoplankton biomass is at times low, when compared to the high standing stock of the grazers. In situ grazing rates of the dominant zooplankton species were estimated at the Mpenjati Estuary once during the winter closed phase, in August 1999, and once during the summer open phase, in February 2000. The study aimed at determining what proportion of the energetic demands of the dominant grazers of the estuary is met by the available phytoplankton. Results show that the gut of all species exhibited higher pigment concentrations during the night than during the day, both in winter and summer. Gut pigment contents ranged from 0.27 to 5.38 ng pigm individual^-1 in the mysid Gastrosaccus brevifissura, from 0.16 to 1.63 ng pigm individual^-1 in the copepod Pseudodiaptomus hessei, from 0.12 to 0.45 ng pigm individual^-1 in the copepod Acartia natalensis, and from 0.8 to 5.44 ng pigm individual^-1 in the caridean Palaemon sp. [where pigm is the sum of chlorophyll-a (chl-a) and phaeopigments]. During the winter closed phase, gut evacuation rates for G. brevifissura, P. hessei, and A. natalensis were 0.62, 0.42, and 0.46 h^-1, respectively. In summer, gut evacuation rates were 0.68, 0.48, and 0.46 h^-1 for G. brevifissura, P. hessei, and Palaemon sp., respectively. The rate of gut pigment destruction for G. brevifissura was 99.6% of the total ingested, one of the highest values ever recorded for any crustacean. A gut pigment destruction of 79.0% was measured for Palaemon sp., 95.7% for P. hessei, and 93.8% for A. natalensis. During winter the total grazing impact of the dominant zooplankton species ranged from 5.05 to 22.7 mg chl-a m^-2 day^-1 and accounted for 34-69% of the available chl-a in the water column. During summer, the grazing impact ranged between 0.45 and 0.65 mg chl-a m^-2 day^-1, accounting for 17-41% of the available chl-a in the water column. This shows that the dominant zooplankton species of the Mpenjati have a very high grazing impact on algal cells. At times this may exceed 100% of the available phytoplankton production, suggesting that the zooplankton community may often resort to other food sources to meet all its energetic demands.     

Effects of rising temperature on pelagic biogeochemistry in mesocosm systems: a comparative analysis of the AQUASHIFT Kiel experiments

A comparative analysis of data, obtained during four indoor-mesocosm experiments with natural spring plankton communities from the Baltic Sea, was conducted to investigate whether biogeochemical cycling is affected by an increase in water temperature of up to 6?°C above present-day conditions. In all experiments, warming stimulated in particular heterotrophic bacterial processes and had an accelerating effect on the temporal development of phytoplankton blooms. This was also mirrored in the build-up and partitioning of organic matter between particulate and dissolved phases. Thus, warming increased both the magnitude and rate of dissolved organic carbon (DOC) build-up, whereas the accumulation of particulate organic carbon (POC) and phosphorus (POP) decreased with rising temperature. In concert, the observed temperature-mediated changes in biogeochemical components suggest strong shifts in the functioning of marine pelagic food webs and the ocean’s biological carbon pump, hence providing potential feedback mechanisms to Earth’s climate system.