Trophic Shift Research Articles

Patterns of fish diversity, community structure and ecological integrity of River Yamuna, India

The Yamuna River, one of the most important and sacred rivers of the northern plains of India, is highly polluted. The river originates at the Yamunotri glacier, traverses 120 km to emerge onto the Indo-Gangetic plains at Dakpathar in Uttarakhand, and finally joins the Ganges River at Allahabad after traversing 1376 km. During 2007–2010 this study assessed the fish diversity, community structure, and ecological status in a 1200 km stretch of the river from Dakpathar to Allahabad. The waters were alkaline (pH 7.6–8.0). Dissolved oxygen was high at most sites (7.3–9.8 mg l−1), except at Wazirabad and Agra (3.4–3.7 mg l−1). A total of 143 fish species were recorded in the river. Cyprinidae was the most abundant family followed by Schilbeidae, Bagridae, and Sisoridae, respectively. The abundance was dominated by 48 species with = > 250 individuals caught. The highest intersite species similarities were recorded between Yamunanagar and Panipat, followed by Wazirabad and Hamirpur, and Yamunanagar and Allahabad. The middle stretch of the river was dominated by small-bodied erytopic, indigenous, and exotic fish species with periodic and opportunistic life history strategies, and had reduced abundances of the large–bodied, prized Indian Major Carps. A trophic shift towards dominance of carnivore catfish species was evident. Multivariate analysis indicated 83.50% of the total variability in species composition in the river could be attributed to gradients of the environmental variables: dissolved oxygen, turbidity, conductivity, total dissolved solids, stream velocity, and water temperature. Insufficient flows in the middle stretch of the river have altered fish habitat availability, resulting in changes in fish composition and assemblage patterns. The study will aid efforts to conserve the aquatic communities and their habitats in the river.

Broad-scale trophic shift in the pelagic North Pacific revealed by an oceanic seabird.

Human-induced ecological change in the open oceans appears to be accelerating. Fisheries, climate change and elevated nutrient inputs are variously blamed, at least in part, for altering oceanic ecosystems. Yet it is challenging to assess the extent of anthropogenic change in the open oceans, where historical records of ecological conditions are sparse, and the geographical scale is immense. We developed millennial-scale amino acid nitrogen isotope records preserved in ancient animal remains to understand changes in food web structure and nutrient regimes in the oceanic realm of the North Pacific Ocean (NPO). Our millennial-scale isotope records of amino acids in bone collagen in a wide-ranging oceanic seabird, the Hawaiian petrel (Pterodroma sandwichensis), showed that trophic level declined over time. The amino acid records do not support a broad-scale increase in nitrogen fixation in the North Pacific subtropical gyre, rejecting an earlier interpretation based on bulk and amino acid specific δ15N chronologies for Hawaiian deep-sea corals and bulk δ15N chronologies for the Hawaiian petrel. Rather, our work suggests that the food web structure in the NPO has shifted at a broad geographical scale, a phenomenon potentially related to industrial fishing.

By more ways than one: Rapid convergence at hydrothermal vents shown by 3D anatomical reconstruction of Gigantopelta (Mollusca: Neomphalina)

BackgroundExtreme environments prompt the evolution of characteristic adaptations. Yet questions remain about whether radiations in extreme environments originate from a single lineage that masters a key adaptive pathway, or if the same features can arise in parallel through convergence. Species endemic to deep-sea hydrothermal vents must accommodate high temperature and low pH. The most successful vent species share a constrained pathway to successful energy exploitation: hosting symbionts. The vent-endemic gastropod genus Gigantopelta, from the Southern and Indian Oceans, shares unusual features with a co-occurring peltospirid, the ‘scaly-foot gastropod’ Chrysomallon squamiferum. Both are unusually large for the clade and share other adaptive features such as a prominent enlarged trophosome-like oesophageal gland, not found in any other vent molluscs.ResultsTransmission electron microscopy confirmed endosymbiont bacteria in the oesophageal gland of Gigantopelta, as also seen in Chrysomallon. They are the only known members of their phylum in vent ecosystems hosting internal endosymbionts; other vent molluscs host endosymbionts in or on their gills, or in the mantle cavity. A five-gene phylogenetic reconstruction demonstrated that Gigantopelta and Chrysomallon are not phylogenetically sister-taxa, despite their superficial similarity. Both genera have specialist adaptations to accommodate internalised endosymbionts, but with anatomical differences that indicate separate evolutionary origins. Hosting endosymbionts in an internal organ within the host means that all resources required by the bacteria must be supplied by the animal, rather than directly by the vent fluid. Unlike Chrysomallon, which has an enlarged oesophageal gland throughout post-settlement life, the oesophageal gland in Gigantopelta is proportionally much smaller in juveniles and the animals likely undergo a trophic shift during ontogeny. The circulatory system is hypertrophied in both but the overall size is smaller in Gigantopelta. In contrast with Chrysomallon, Gigantopelta possesses true ganglia and is gonochoristic.ConclusionsKey anatomical differences between Gigantopelta and Chrysomallon demonstrate these two genera acquired a similar way of life through independent and convergent adaptive pathways. What appear to be the holobiont’s adaptations to an extreme environment, are driven by optimising bacteria’s access to vent nutrients. By comparing Gigantopelta and Chrysomallon, we show that metazoans are capable of rapidly and repeatedly evolving equivalent anatomical adaptations and close-knit relationships with chemoautotrophic bacteria, achieving the same end-product through parallel evolutionary trajectories.

Differentiated stable isotopes signatures between pre- and post-flexion larvae of Atlantic bluefin tuna (Thunnus thynnus) and of its associated tuna species of the Balearic Sea (NW Mediterranean)

The trophic ecology of bluefin tuna larvae (Thunnus thynnus) from the Balearic Sea, together with its co-existing tuna species such as albacore (T. alalunga), bullet (Auxis rochei) and little tunny (Euthynnus alletteratus) were examined by nitrogen and carbon stable isotope analyses. A total of 286 larvae were analyzed for this study, of which 72 larvae corresponded to bluefin, 57 to albacore, 81 to bullet tuna and 76 to little tunny. Tuna larvae were separated into the pre-flexion and post-flexion developmental stages. Within the size 3–9mm standard length (SL), the stable isotope of nitrogen (δ15N) showed significant differences between species where bluefin tuna larvae ranked highest. Pre-flexion bluefin tuna and little tunny larvae showed significantly higher δ15N signatures than the post-flexion larvae. This effect is attributed to a biochemical trace of maternal δ15N signatures. However, neither albacore nor bullet tuna larvae showed this pattern in δ15N signatures, possibly owing to a compensation effect between lower maternal δ15N values transmitted to pre-flexion larvae and the early increase of δ15N values in post-flexion stages. One way ANOVA showed significant differences between species in the stable isotope ratio of carbon (δ13C) values, which suggests specific differences of carbon sources. Furthermore, a similar significant ontogenic effect between δ13C signatures of pre-flexion and post-flexion larvae is also evidenced in all four species. At pre-flexion stages, all species except bullet tuna larvae showed significant negative relationships between δ15N and larval standard length. At post-flexion stages, a significant linear relationship with larval size was only observed in albacore and bullet tuna larvae indicating a possible trophic shift towards early piscivory. With respect to δ13C values with larval size, all four species showed significant linear decreases. It may be explained by the metabolism of growth of somatic mass subject to modification of the relative carbon isotopic sources. In conclusion, the species' signatures of δ15N and δ13C indicate differentiated early life trophic niches. In addition, it is worth remarking the potential use of transgenerational isotopic transmission in future research applications.

Trophic Niche Differences in Arachnid Predators Between Field and Forest Ecosystems

ABSTRACT: Arachnids are important predators in arthropod assemblages that occur from early successional old fields to mature forests. As generalist predators, they may occupy varying trophic niches in different environments that have different prey available. Given that trophic niche can be quantified by the enrichment of 15N in an organism, we tested the null hypothesis that the trophic niche is invariant between old fields and forests in three groups of arachnids: cursorial and web weaving spiders (Araneae), and harvestmen (Opiliones), by comparing the stable isotope ratios of nitrogen (δ15N) in specimens of each group collected from field and forest. Cursorial spiders fed at the highest trophic level in both habitats, and likely consumed other predators as well as herbivores in the field. Web weavers showed slightly higher δ15N in the field than in the forest. Harvestmen and cursorial spiders had significantly lower δ15N in the forest, indicating a trophic shift downward during later succession when th...

By their own devices: invasive Argentine ants have shifted diet without clear aid from symbiotic microbes.

The functions and compositions of symbiotic bacterial communities often correlate with host ecology. Yet cause-effect relationships and the order of symbiont vs. host change remain unclear in the face of ancient symbioses and conserved host ecology. Several groups of ants exemplify this challenge, as their low-nitrogen diets and specialized symbioses appear conserved and ancient. To address whether nitrogen-provisioning symbionts might be important in the early stages of ant trophic shifts, we studied bacteria from the Argentine ant, Linepithema humile - an invasive species that has transitioned towards greater consumption of sugar-rich, nitrogen-poor foods in parts of its introduced range. Bacteria were present at low densities in most L.humile workers, and among those yielding quality 16S rRNA amplicon sequencing data, we found just three symbionts to be common and dominant. Two, a Lactobacillus and an Acetobacteraceae species, were shared between native and introduced populations. The other, a Rickettsia, was found only in two introduced supercolonies. Across an eight-year period of trophic reduction in one introduced population, we found no change in symbionts, arguing against a relationship between natural dietary change and microbiome composition. Overall, our findings thus argue against major changes in symbiotic bacteria in association with the invasion and trophic shift of L.humile. In addition, genome content from close relatives of the identified symbionts suggests that just one can synthesize most essential amino acids; this bacterium was only modestly abundant in introduced populations, providing little support for a major role of nitrogen-provisioning symbioses in Argentine ant's dietary shift.

Terrestrial discharges mediate trophic shifts and enhance methylmercury accumulation in estuarine biota.

The input of mercury (Hg) to ecosystems is estimated to have increased two- to fivefold during the industrial era, and Hg accumulates in aquatic biota as neurotoxic methylmercury (MeHg). Escalating anthropogenic land use and climate change are expected to alter the input rates of terrestrial natural organic matter (NOM) and nutrients to aquatic ecosystems. For example, climate change has been projected to induce 10 to 50% runoff increases for large coastal regions globally. A major knowledge gap is the potential effects on MeHg exposure to biota following these ecosystem changes. We monitored the fate of five enriched Hg isotope tracers added to mesocosm scale estuarine model ecosystems subjected to varying loading rates of nutrients and terrestrial NOM. We demonstrate that increased terrestrial NOM input to the pelagic zone can enhance the MeHg bioaccumulation factor in zooplankton by a factor of 2 to 7 by inducing a shift in the pelagic food web from autotrophic to heterotrophic. The terrestrial NOM input also enhanced the retention of MeHg in the water column by up to a factor of 2, resulting in further increased MeHg exposure to pelagic biota. Using mercury mass balance calculations, we predict that MeHg concentration in zooplankton can increase by a factor of 3 to 6 in coastal areas following scenarios with 15 to 30% increased terrestrial runoff. The results demonstrate the importance of incorporating the impact of climate-induced changes in food web structure on MeHg bioaccumulation in future biogeochemical cycling models and risk assessments of Hg.

The effect of trophic level on individual amino acid δ15N values in a terrestrial ruminant food web

ABSTRACTBulk collagen δ15N analysis is widely used to investigate past diet and trophic positions, but these values average the δ15N values of the constituent amino acids. Compound–specific isotope analysis of amino acids (AAs) can help elucidate the complex metabolic effects underpinning bulk δ15N values. Although trophic level effects on individual AA δ15N values have been investigated in aquatic and terrestrial invertebrate food webs, most archaeological applications involve terrestrial herbivores, hence a greater understanding of these effects between diet and consumer in this food chain is required. The North Wyke Farm Platform provided baseline nitrogen isotope information for cattle grazing on a Lolium perenne-dominated pasture. Bulk dentine δ15N values show a shift expected for a one trophic level increase, but obscure insight into the underlying metabolic processes that cause this change in value. However, determination of AA δ15N values of hydrolysable plant protein and cattle tooth dentine clarifies the trophic effect on consumer AA δ15N values. The observed trophic shift in the studied system is different from previously studied food webs, with a trophic enrichment factor, based on the δ15N values of glutamate and phenylalanine, of 4.0‰ compared to 7.6‰ commonly used in ecological and archaeological studies. This emphasises the need to understand the trophic shifts in the particular food web being investigated in order to apply isotopic investigations in archaeological contexts.

Mercury levels in herring gulls and fish: 42 years of spatio-temporal trends in the Great Lakes

Total mercury levels in aquatic birds and fish communities have been monitored across the Canadian Great Lakes by Environment and Climate Change Canada (ECCC) for the past 42 years (1974–2015). These data (22 sites) were used to examine spatio-temporal variability of mercury levels in herring gull (Larus argentatus) eggs, lake trout (Salvelinus namaycush), walleye (Sander vitreus), and rainbow smelt (Osmerus mordax). Trends were quantified with dynamic linear models, which provided time-variant rates of change of mercury concentrations. Lipid content (in both fish and eggs) and length in fish were used as covariates in all models. For the first three decades, mercury levels in gull eggs and fish declined at all stations. In the 2000s, trends for herring gull eggs reversed at two sites in Lake Erie and two sites in Lake Ontario. Similar trend reversals in the 2000s were observed for lake trout in Lake Superior and at a single station in Lake Ontario. Mercury levels in lake trout continued to slowly decline at all of the remaining stations, except for Lake Huron, where the levels remained stable. A post-hoc Bayesian regression analysis suggests strong trophic interactions between herring gulls and rainbow smelt in Lake Superior and Lake Ontario, but also pinpoints the likelihood of a trophic decoupling in Lake Huron and Lake Erie. Continued monitoring of mercury levels in herring gulls and fish is required to consolidate these trophic shifts and further evaluate their broader implications.

Dining dichotomy: aquatic and terrestrial prey capture behavior in the Himalayan newt Tylototriton verrucosus.

ABSTRACTTransitions between aquatic and terrestrial prey capture are challenging. Trophic shifts demand a high degree of behavioral flexibility to account for different physical circumstances between water and air to keep performance in both environments. The Himalayan newt, Tylototriton verrucosus, is mostly terrestrial but becomes aquatic during its short breeding period. Nonetheless, it was assumed that it lacks the capability of trophic behavioral flexibility, only captures prey on land by its tongue (lingual prehension) and does not feed in water. This theory was challenged from stomach content analyses in wild populations that found a variety of aquatic invertebrates in the newts' stomachs during their breeding season. Accordingly, we hypothesized that T. verrucosus actively changes its terrestrial prey capture mechanism to hunt for aquatic prey at least during its aquatic stage. In fact, the kinematic analyses showed that T. verrucosus uses lingual prehension to capture prey on land but changes to suction feeding for aquatic strikes. The statistical analyses revealed that terrestrial and aquatic strikes differ significantly in most kinematic parameters while behavioral variability does not differ between both behaviors. In turn, the movement patterns in suction feeding showed a higher degree of coordination between jaw and hyoid movements compared to the putative primary feeding mode, namely lingual prehension. We conclude that T. verrucosus, though relatively slow compared to trophic specialists, benefits from a high degree of behavioral flexibility that allows exploiting food sources efficiently from two very different habitats.

Changes in macroinvertebrate trophic structure along a land-use gradient within a lowland stream network

Running waters are among the most threatened ecosystems globally, having altered hydrological regimes, homogenized habitat, and impaired water quality. These multiple stressors impact aquatic biodiversity and ecosystem function across space and time, although a clear mechanistic understanding is still lacking. Here, we examined the trophic response of macroinvertebrates among streams in a Swiss lowland catchment encompassing a gradient of land uses. Clear compositional changes were observed as anthropogenic impacts increased from least-impacted to agricultural and urbanized sites. Taxonomic diversity was lowest at sites with morphological and water quality impairment (agricultural sites), whereas taxonomic identity (susceptible vs. generalist species) mainly changed due to water quality degradation (agricultural and urban sites) based on the SPEAR (pesticides) index. Using stable isotopes (δ13C, δ15N), a simplification in macroinvertebrate trophic structure was evident along the land use gradient. At a site receiving wastewater treatment effluent, stable isotopes also revealed trophic shifts in primary consumers that corresponded to changes in available food resources. Results further showed that some taxa losses, e.g., the mayfly Ecdyonurus, to land- use effects may be due to low trophic plasticity. The combination of analyses, including stable isotopes, provided an improved mechanistic understanding of community and population responses to land-use changes along river networks.

An ecological risk assessment for managing and predicting trophic shifts in estuarine ecosystems using a Bayesian network

Estuaries are dynamic systems at the transition between freshwater and marine ecosystems. In this study, a spatially and temporally explicit Bayesian network (BN) was developed for a tidally connected estuary in southeastern Australia. The BN provides an environmental risk assessment (ERA) for the probability of a shift to a eutrophied state based on markers of pelagic and benthic primary production. The model was created to provide an initial framework of system knowledge based on empirical data, with the intention that the model and its linkages be iteratively developed as more information becomes available. The BN was investigated for its potential to predict trophic shifts and provide a framework for evidence-based decision making. Model assessment was conducted through both sensitivity analysis and scenario tests. Through evaluation and updating, the BN can provide information on the key nutrients and bio-physical mechanisms regulating changes in trophic state in estuarine ecosystems.

Benthic algae compensate for phytoplankton losses in large aquatic ecosystems.

Anthropogenic activities can induce major trophic shifts in aquatic systems, yet we have an incomplete understanding of the implication of such shifts on ecosystem function and on primary production (PP) in particular. In recent decades, phytoplankton biomass and production in the Laurentian Great Lakes have declined in response to reduced nutrient concentrations and invasive mussels. However, the increases in water clarity associated with declines in phytoplankton may have positive effects on benthic PP at the ecosystem scale. Have these lakes experienced oligotrophication (a reduction of algal production), or simply a shift in autotrophic structure with no net decline in PP? Benthic contributions to ecosystem PP are rarely measured in large aquatic systems, but our calculations based on productivity rates from the Great Lakes indicate that a significant proportion (up to one half, in Lake Huron) of their whole-lake production may be benthic. The large declines (5-45%) in phytoplankton production in the Great Lakes from the 1970s to 2000s may be substantially compensated by benthic PP, which increased by up to 190%. Thus, the autotrophic productive capacity of large aquatic ecosystems may be relatively resilient to shifts in trophic status, due to a redirection of production to the near-shore benthic zone, and large lakes may exhibit shifts in autotrophic structure analogous to the regime shifts seen in shallow lakes.

CO2 Saturation and Trophic Shift Induced by Microbial Metabolic Processes in a River-Dominated Ocean Margin (Tropical Shallow Lagoon, Chilika, India)

ABSTRACTAn effort has been made for the first time in Asia's largest brackish water lagoon, Chilika, to investigate the spatio-temporal variability in primary productivity (PP), bacterial productivity (BP), bacterial abundance (BA), bacterial respiration (BR) and bacterial growth efficiency (BGE) in relation to partial pressure of CO2 (pCO2) and CO2 air–water flux and the resultant trophic switchover. Annually, PP ranged between 24 and 376 µg C L−1 d−1 with significantly low values throughout the monsoon (MN), caused by light limitation due to inputs of riverine suspended matter. On the contrary, BP and BR ranged from 11.5 to 186.3 µg C L−1 d−1 and from 14.1 to 389.4 µg C L−1 d−1, respectively, with exceptionally higher values during MN. A wide spatial and temporal variation in the lagoon trophic status was apparent from BP/PP (0.05–6.4) and PP/BR (0.10–18.2) ratios. The seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter inputs via rivers, enhanced the bacterial metabolism during the MN, as evident from the high pCO2 (10,134 µatm) and CO2 air–water flux (714 mm m−2 d−1). Large variability in BGE and BP/PP ratios especially during MN led to high bacteria-mediated carbon fluxes which was evident from significantly high bacterial carbon demand (BCD >100% of PP) during this season. This suggested that the net amount of organic carbon (either dissolved or particulate form) synthesized by primary producers in the lagoon was not sufficient to satisfy the bacterial carbon requirements. Lagoon sustained low to moderate autotrophic–heterotrophic coupling with annual mean BCD of 231% relative to the primary production, which depicted that bacterioplankton are the mainstay of the lagoon biogeochemical cycles and principal players that bring changes in trophic status. Study disclosed that the high CO2 supersaturation and oxygen undersaturation during MN was attributed to the increased heterotrophic respiration (in excess of PP) fuelled by allochthonous organic matter. On a spatial scale, lagoon sectors such as south sector, central sector and outer channel recorded “net autotrophic,” while the northern sector showed “net heterotrophic” throughout the study period.

Herbivory Promotes Dental Disparification and Macroevolutionary Dynamics in Grunters (Teleostei: Terapontidae), a Freshwater Adaptive Radiation.

Trophic shifts into new adaptive zones have played major (although often conflicting) roles in reshaping the evolutionary trajectories of many lineages. We analyze data on diet, tooth, and oral morphology and relate these traits to phenotypic disparification and lineage diversification rates across the ecologically diverse Terapontidae, a family of Australasian fishes. In contrast to carnivores and most omnivores, which have retained relatively simple, ancestral caniniform tooth shapes, herbivorous terapontids appear to have evolved a variety of novel tooth shapes at significantly faster rates to meet the demands of plant-based diets. The evolution of herbivory prompted major disparification, significantly expanding the terapontid adaptive phenotypic continuum into an entirely novel functional morphospace. There was minimal support for our hypothesis of faster overall rates of integrated tooth shape, spacing, and jaw biomechanical evolution in herbivorous terapontids in their entirety, compared with other trophic strategies. There was, however, considerable support for accelerated disparification within a diverse freshwater clade containing a range of specialized freshwater herbivores. While the evolutionary transition to herbivorous diets has played a central role in terapontid phenotypic diversification by pushing herbivores toward novel fitness peaks, there was little support for herbivory driving significantly higher lineage diversification compared with background rates across the family.

Potential drivers of seasonal shifts in fish omnivory in a subtropical stream

The trophic structure of fish assemblages often varies seasonally, following the changes in food availability and supposedly water temperature. To unveil potential drivers of trophic shifts, we studied changes in fish trophic structure at both whole-assemblage and species levels at contrasting food availability and water temperatures in a subtropical stream. We analysed the diet of the most abundant omnivorous species (Bryconamericus iheringii) monthly along the year, searching for relationships with environmental variables changing seasonally (i.e. temperature and water level) and with fish reproductive stage. We ran a single-species food choice field experiment with fixed animal and vegetal food availability in contrasting seasons to test food availability as driver of diet shifts. At the assemblage level, we found a higher consumption of vegetal during summer, reflecting the increased proportion of vegetal in the diet of omnivores, which dominated the assemblage. At the species level, the enhanced vegetal consumption was related to increases in temperature and reduction in water level. Moreover, fish selected for vegetal during summer and for animal food in winter under experimental conditions. Our findings support the role of temperature driving food web dynamics by increasing fish herbivory towards warmer scenarios, with potential strong implications for whole-assemblage trophic structure.

Approach to determine individual trophic level and the difference in food sources of Japanese anchovy Engraulis japonicus in Sagami Bay, based on compound-specific nitrogen stable isotope analysis of amino acids

Compound-specific stable isotope analysis of amino acids (AA method) has recently been used to estimate the trophic level (TL) of animals. The individual TL (TLGlu/Phe) of Japanese anchovy Engraulis japonicus was precisely determined using the AA method without sampling or analysis of primary producers. Previously, TLs of organisms were determined using bulk nitrogen stable isotopic composition (δ15Nbulk) of tissues (bulk method). The δ15Nbulk for Japanese anchovies varied widely because of their migratory behavior, and the TL of anchovies in Sagami Bay remained ambiguous. We showed that the TLGlu/Phe of anchovy was almost 3 and that food-web structures differed between inshore and offshore waters. Furthermore, there was little difference in TLGlu/Phe for representative Japanese anchovies in Sagami Bay, despite the large intra-sample variation in δ15Nbulk for anchovies ranging over a trophic shift of 3–4 ‰ in Sagami Bay. We speculated that this variation in δ15Nbulk resulted from differences in food sources among the different food webs. Estimation of individual TLs in some simple food chains using the AA method may enable the determination of the food-web structure of migratory fishes, which is difficult to determine using the traditional bulk method.

Insight on how fishing bats discern prey and adjust their mechanic and sensorial features during the attack sequence.

Several insectivorous bats have included fish in their diet, yet little is known about the processes underlying this trophic shift. We performed three field experiments with wild fishing bats to address how they manage to discern fish from insects and adapt their hunting technique to capture fish. We show that bats react only to targets protruding above the water and discern fish from insects based on prey disappearance patterns. Stationary fish trigger short and shallow dips and a terminal echolocation pattern with an important component of the narrowband and low frequency calls. When the fish disappears during the attack process, bats regulate their attack increasing the number of broadband and high frequency calls in the last phase of the echolocation as well as by lengthening and deepening their dips. These adjustments may allow bats to obtain more valuable sensorial information and to perform dips adjusted to the level of uncertainty on the location of the submerged prey. The observed ultrafast regulation may be essential for enabling fishing to become cost-effective in bats, and demonstrates the ability of bats to rapidly modify and synchronise their sensorial and motor features as a response to last minute stimulus variations.

More rapid shift to a benthic niche in larger Gadus morhua juveniles.

Trophic use by Atlantic cod Gadus morhua juveniles was examined early and late in the shift from pelagic to benthic habitats. Changes in the proportion of pelagic copepods, estimates of benthic prey indicated by isotope mixing models and stable-isotope values between sample periods suggested a gradual shift towards a benthic niche. Values of the trophic proxies, however, changed most markedly in the largest juvenile group, suggesting a more rapid trophic niche shift, and in turn competitive advantage, of larger juveniles.

Trophic Variation in Coastal Plain Stream Predatory Fishes

Abstract Unique morphologies along with associated differences in habitat use and feeding behavior can result in fish at the top of piscine food chains differing in trophic level. Broad size ranges inherent within large species provide opportunity for size-related trophic shifts. Such relationships between size and trophic level can be species specific. Furthermore, individual-based diet variation can bring about differences among similar-sized organisms. A challenge to aquatic ecologists is deciphering these patterns of trophic change both between and within species. Stable isotope analysis has emerged as a powerful tool for evaluating such patterns. Employing stable isotope analyses, we assessed trophic differentiation in 4 large predatory fish species from a coastal-plain stream. We established the trophic base by including 2 herbivorous invertebrates in the analysis and identified a trophic hierarchy among species, with 2 specialized, generally open-water piscivores, Lepisosteus osseus (Longnose Gar) ...