The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. 
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. 
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. 
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

1 Design and relationships.- 1.1 Introduction.- 1.2 The orders: morphology and life cycles.- 1.2.1 Stauromedusae.- 1.2.2 Coronatae.- 1.2.3 Semaeostomeae.- 1.2.4 Rhizostomeae.- 1.3 Relationships and origins of the class and orders.- 2 Locomotion.- 2.1 Introduction.- 2.2 Mesoglea.- 2.2.1 Fibre composition.- 2.2.2 Mechanics.- 2.3 Muscle.- 2.3.1 General muscular anatomy.- 2.3.2 Fine structure of muscles.- 2.3.3 Physiological properties of muscles.- 2.4 Sensory receptors.- 2.4.1 Structure of the marginal sense organs.- 2.4.2 Photoreception.- 2.4.3 Equilibrium reception.- 2.4.4 Other sensory responses.- 2.5 Nervous system.- 2.5.1 Nervous system of medusae.- 2.5.2 Marginal centres.- 2.5.3 Structure and function of the motor nerve net.- 2.5.4 Diffuse nerve net.- 2.5.5 Nervous system of polyps.- 2.5.6 Transmitters.- 2.6 Locomotion.- 2.6.1 Physical dynamics of swimming.- 2.6.2 Nervous control of swimming.- 2.6.3 Locomotion of polyps.- 2.6.4 Locomotion of planulae.- 3 Feeding.- 3.1 Introduction.- 3.2 Cnidae.- 3.2.1 Structure and classification.- 3.2.2 Formation and migration.- 3.2.3 Discharge.- 3.2.4 Toxins.- 3.2.5 Functions.- 3.3 Types of prey.- 3.3.1 Prey in diets of scyphomedusae.- 3.3.2 Prey of polyps.- 3.4 Contact with prey.- 3.4.1 Medusae encounter probabilities.- 3.4.2 Medusae attraction to prey.- 3.5 Feeding behaviour.- 3.5.1 Medusae prey capture.- 3.5.2 Polyp prey capture.- 3.5.3 Chemical induction of feeding.- 3.6 Feeding rates.- 3.6.1 Selection of prey types.- 3.6.2 Factors affecting feeding rates.- 4 Nutrition.- 4.1 Introduction.- 4.1.1 Units of intake.- 4.1.2 Dietary requirements.- 4.2 Digestion.- 4.2.1 Extracellular and intracellular digestion.- 4.2.2 Enzymes.- 4.2.3 Digestion rates.- 4.3 Circulation and translocation.- 4.3.1 Circulatory canals and ciliary currents.- 4.3.2 Endocytosis.- 4.3.3 Translocation.- 4.4 Uptake of dissolved organic material.- 4.5 Symbiosis.- 4.5.1 Identity and location of algal symbionts.- 4.5.2 Metabolic exchange between symbiont and host.- 4.5.3 Establishment and control of algal numbers.- 4.5.4 Ecological significance of symbiosis.- 5 Metabolism.- 5.1 Introduction.- 5.1.1 Definitions.- 5.1.2 Aerobic and anaerobic metabolism.- 5.2 Factors affecting oxygen consumption.- 5.2.1 Body size.- 5.2.2 Muscular activity.- 5.2.3 Food.- 5.2.4 Temperature.- 5.2.5 Oxygen availability.- 5.2.6 Effects of symbionts.- 5.3 Nitrogen excretion 131 5.3.1 Factors affecting rates of excretion.- 5.3.1 Factors affecting rates of excretion.- 5.4 Osmotic and ionic regulation.- 5.4.1 Water content.- 5.4.2 Buoyancy.- 6 Reproduction.- 6.1 Synopsis.- 6.1.1 Types of reproduction and trade-offs.- 6.1.2 Genetics.- 6.2 Gametogenesis.- 6.2.1 Gonad formation.- 6.2.2 Gamete production.- 6.2.3 Fertilization.- 6.3 Larval development.- 6.3.1 Embryogenesis and planulae.- 6.3.2 Brooding.- 6.3.3 Settlement including metamorphosis.- 6.3.4 Direct development.- 6.4 Polyp.- 6.4.1 Budding.- 6.4.2 Cysts including podocysts.- 6.4.3 Strobilation.- 6.5 Ephyra.- 7 Growth.- 7.1 Measurement of growth.- 7.1.1 Units.- 7.1.2 Methods.- 7.2 Organic composition of scyphozoa.- 7.3 Growth curves.- 7.3.1 Laboratory data.- 7.3.2 Field data.- 7.3.3 Life span.- 7.4 Starvation and regeneration.- 7.4.1 Degrowth and regrowth.- 7.4.2 Regeneration.- 7.5 Conversion efficiencies.- 7.6 Dietary requirements.- 7.6.1 Energy budget.- 7.6.2 Food supply.- 8 Physical ecology.- 8.1 Biomass.- 8.1.1 Measurement.- 8.1.2 Production.- 8.2 Mortality and adaptation to physical factors.- 8.2.1 Temperature.- 8.2.2 Salinity.- 8.2.3 Pollution.- 8.2.4 Oxygen.- 8.3 Depth.- 8.3.1 Vertical distribution.- 8.3.2 Diel migration.- 8.3.3 Changes with life cycle.- 8.4 Aggregation and horizontal migration.- 8.5 Zoogeography.- 9 Biological interactions.- 9.1 Predation.- 9.1.1 Natural predators: planktonic.- 9.1.2 Natural predators: benthic.- 9.1.3 Fisheries.- 9.1.4 Transparency and pigmentation.- 9.2 Parasites.- 9.2.1 Larval trematodes and cestodes.- 9.2.2 Hyperiid amphipods.- 9.3 Associations.- 9.3.1 Associations with fish.- 9.4 Bioluminescence.- 9.4.1 Anatomy of luminescent structures.- 9.4.2 Chemical basis of luminescence.- 9.4.3 Control of luminescence.- 9.4.4 Ecological significance.- 9.5 Trophic relationships.- 9.5.1 Impact on prey populations.- 9.5.2 Competition.- 9.5.3 Trophic levels.- Appendix: Classification of extant scyphozoa.- References.

A functional biology of scyphozoa

There has been a dramatic increase in jellyfish biomass over the eastern Bering Sea shelf since the early 1990s, which was previously hypothesized to have been triggered by changing climate and ocean conditions. We examine the hypothesis that the presence of these large carnivores has affected fisheries resources, either through direct predation on larval stages, or through compe- tition for zooplankton prey. In this paper, we explore the impact of this jellyfish increase on zoo- plankton and fish communities based on field data on the composition of the jellyfish community, and the abundance, size, stable isotopic signatures, and feeding habits of the principal scyphomedusae in the region. These data, together with those on zooplankton biomass, are used to estimate the ecosys- tem impacts of this increase. The center of jellyfish biomass has shifted from the SE Middle Shelf Domain in the early 1980s to the NW in the late 1990s. In recent years, the species composition of large medusae caught in trawls was dominated (>80% by number and >95% by weight) by the scyphozoan Chrysaora melanaster. Dense aggregations of this species occupied the water column in daytime between 10 and 40 m. Their food habits consisted mainly of pelagic crustaceans (euphausi- ids, copepods, amphipods), although other jellyfish and juvenile pollock were also consumed. Based on stable isotope ratios, the trophic level of this scyphozoan is equivalent to, or higher than, that of Age 0 pollock. Preliminary estimates showed that medusae have a moderate grazing impact on zoo- plankton in the area around the Pribilof Islands; C. melanaster was estimated on average to consume seasonally about one-third of the standing stock and 4.7% of the annual production of zooplankton in this region. Daily consumption of Age 0 pollock was estimated to be 2.8% of the standing stock around the Pribilof Islands during 1999. A hypothesis for the increase in jellyfishes observed in the eastern Bering Sea, based on release from competition from planktivorous forage fishes, is proposed.

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Increases in jellyfish biomass in the Bering Sea: implications for the ecosystem

The food webs of rocky infra-littoral ecosystems in the Mediterranean have been little studied. In this investigation stable isotopes and dietary data were compared in an attempt to describe features of the food webs concerned. δ13C and δ15N were determined for plants, invertebrates and fishes from the Bay of Calvi, Corsica. Dietary data were derived from the literature. δ13C of plants ranged from –8.59‰ to –33.74‰, of benthic invertebrates from –17.0‰ to –20.52‰, of planktonic invertebrates from –20.08‰ to –22.34‰ and of fishes from –16.27‰ to –19.59‰. δ15N was generally greater at higher trophic levels. δ15N of plants was 0.95–2.92‰, of benthic invertebrates 1.69–6.54‰, of planktonic invertebrates 3.51–6.82‰ and of fishes 4.63–9.77‰. 13C enrichment tended to be associated with benthic food chains and 13C depletion with planktonic chains. Stable-isotope data suggested more varied diets for many species than implied by gut-contents data. Omnivory and trophic plasticity were widespread, and many consumers fed lower down the food chain than previous studies had suggested. Both stable-isotope and gut-contents analysis resolved differences between fishes feeding on planktonic and benthic prey and indicated that the herbivorous fish Sarpa salpa fed on a diet substantially different from that of other fishes. Zooplankton were important in the diets of several consumers (both primary and secondary), as was plankton derived detritus. One species of fish previously identified as planktivorous was shown to feed largely on benthic organisms, whilst several species of benthic invertebrates may feed on plankton-derived detritus. Although herbivores seemed to obtain most of their C from macroalgae, δ15N data suggested that many of these animals supplemented their intake of N, although gut-contents analysis did not provide evidence for such uptake. The isotopic data have elucidated several features of the food web which we would not otherwise have detected.

Contributions of stable-isotope data to elucidating food webs of Mediterranean rocky littoral fishes.

Fishes and invertebrates were collected from the continental slope southwest of the Balearic Islands, Mediterranean, in October 1996 and May 1998 from 3 faunally-distinct depth zones: 200 to 800 m (upper slope), 800 to 1425 m (middle slope) and 1425 to 1800 m (lower slope). Differ- ences in δ 15 N and δ 13 C between October 1996 and May 1998 were observed only in the crustaceans Aristeus antennatus and Nematocarcinus exilis. There were few differences among depths in δ 15 N or δ 13 C of benthic fishes or crustaceans: the shrimps A. antennatus and Acanthephyra eximia showed higher δ 15 N values at greater depth, and δ 13 C values of the fishes Alepocephalus rostratus (Ale- pocephalidae) and Coelorhynchus coelorhynchus (Macrouridae) also tended to increase with in- creasing depth. Differences in plankton and suprabenthos were also observed among zones (at depth, greater δ 15 N values of gelatinous plankton, calanoid copepods and peracarids were recorded, along with smaller δ 13 C values of gelatinous plankton and peracarids). These are attributed to a microbially-mediated increase in δ 15 N and a reduction in δ 13 C of the particulate material remaining after preferential utilization of labile organic matter in the stable and relatively high (13°C) tem- perature of the Mediterranean water column. Strong positive correlations between δ 15 N and δ 13 C data for plankton, fishes and crustaceans indicated a single primary source material for these com- munities, which is attributed to marine snow.

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Feeding relationships in Mediterranean bathyal assemblages elucidated by stable nitrogen and carbon isotope data

In our field study we analyzed the C and H isotopic and biochemical (C, N, P, protein, lipid, carbohydrate) composition of the jellyfish Pelagia noctiluca (collected from the Gulf of Trieste in 1985 to 1986) and its presumed diet-net zooplankton. The mean δ 13C (-18.8‰) and δ D (-58.4‰) ratios of P. noctiluca showed enrichment in heavy isotopes relative to net zooplankton (∼2‰ for carbon and ∼30‰ for hydrogen). Both the jellyfish and net zooplankton were characterized by a linear correlation between δ 13C and δ D. C. N, P, protein, lipid, and carbohydrate contents of P. noctiluca were low on a dry weight basis as compared to net zooplankton. Significantly lower C:N and C:P ratios were found in jellyfish indicating a greater loss of carbon relative to nitrogen and phosphorus along the passage to a higher trophic level. Isotopic and biochemical evidence indicate that, though collected in nearshore waters, P. noctiluca depended on autochthonous marine organic matter.

J. Pezdič

Papers

Stable isotope and biochemical fractionation in the marine pelagic food chain: the jellyfish Pelagia noctiluca and net zooplankton