Trophic Niche Dynamics and Diet Partitioning of King Crab Lithodes santolla in Chile’s Sub-Antarctic Water

The southern king crab Lithodes santolla is one of the most economically important fishery species in the southern waters of the Atlantic and Pacific Oceans. A combination of stomach content and stable isotope analyses was used to reveal the potential dietary characteristics, isotopic niche, overlap among maturity stages and sexes, and trophic relationships of an L. santolla population in the Nassau Bay, Cape Horn region. Stable isotope analyses indicated that L. santolla assimilated energy from a basal carbon source, the giant kelp Macrocystis pyrifera, forming the trophic baseline of the benthic food web. Moreover, the trophic position of L. santolla varied among late juveniles and adults, suggesting that the southern king crab does undergo an ontogenetic diet shift. L. santolla exhibited intraspecific isotopic niche variation, reflecting niche differentiation which allows the species to partition resources. The trophic relationships of L. santolla with the associated fauna suggested some potential interactions for food resources/habitat use when they are limited. This study is the first attempt to characterize the trophic dynamics of the southern king crab in the Cape Horn area and, by generating more data, contributes to the conservation of the king crab population and the long-term management of local fisheries that rely on this resource.

Interannual temperature variability is a principal driver of low-frequency fluctuations in marine fish populations

Marine fish populations commonly exhibit low-frequency fluctuations in biomass that can cause catch volatility and thus endanger the food and economic security of dependent coastal societies. Such variability has been linked to fishing intensity, demographic processes and environmental variability, but our understanding of the underlying drivers remains poor for most fish stocks. Our study departs from previous findings showing that sea surface temperature (SST) is a significant driver of fish somatic growth variability and that life-history characteristics mediate population-level responses to environmental variability. We use autoregressive models to simulate how fish populations integrate SST variability over multiple years depending on fish life span and trophic position. We find that simulated SST-driven population dynamics can explain a significant portion of observed low-frequency variability in independent observations of fisheries landings around the globe. Predictive skill, however, decreases with increasing fishing pressure, likely due to demographic truncation. Using our modelling approach, we also show that increases in the mean and variance of SST could amplify biomass volatility and lessen its predictability in the future. Overall, biological integration of high-frequency SST variability represents a null hypothesis with which to explore the drivers of low-frequency population change across upper-trophic marine species.

Growth and feeding ecology of coniform conodonts

Conodonts were the first vertebrates to develop mineralized dental tools, known as elements. Recent research suggests that conodonts were macrophagous predators and/or scavengers but we do not know how this feeding habit emerged in the earliest coniform conodonts, since most studies focus on the derived, ‘complex’ conodonts. Previous modelling of element position and mechanical properties indicate they were capable of food processing. A direct test would be provided through evidence of in vivo element crown tissue damage or through in vivo incorporated chemical proxies for a shift in their trophic position during ontogeny. Here we focus on coniform elements from two conodont taxa, the phylogenetically primitive Proconodontus muelleri Miller, 1969 from the late Cambrian and the more derived Panderodus equicostatus Rhodes, 1954 from the Silurian. Proposing that this extremely small sample is, however, representative for these taxa, we aim to describe in detail the growth of an element from each of these taxa in order to the test the following hypotheses: (1) Panderodus and Proconodontus processed hard food, which led to damage of their elements consistent with prey capture function; and (2) both genera shifted towards higher trophic levels during ontogeny. We employed backscatter electron (BSE) imaging, energy-dispersive X-ray spectroscopy (EDX) and synchrotron radiation X-ray tomographic microscopy (SRXTM) to identify growth increments, wear and damage surfaces, and the Sr/Ca ratio in bioapatite as a proxy for the trophic position. Using these data, we can identify whether they exhibit determinate or indeterminate growth and whether both species followed linear or allometric growth dynamics. Growth increments (27 in Pa. equicostatus and 58 in Pr. muelleri) were formed in bundles of 4–7 increments in Pa. equicostatus and 7–9 in Pr. muelleri. We interpret the bundles as analogous to Retzius periodicity in vertebrate teeth. Based on applied optimal resource allocation models, internal periodicity might explain indeterminate growth in both species. They also allow us to interpret the almost linear growth of both individuals as an indicator that there was no size-dependent increase in mortality in the ecosystems where they lived e.g., as would be the case in the presence of larger predators. Our findings show that periodic growth was present in early conodonts and preceded tissue repair in response to wear and damage. We found no microwear and the Sr/Ca ratio, and therefore the trophic position, did not change substantially during the lifetimes of either individual. Trophic ecology of coniform conodonts differed from the predatory and/or scavenger lifestyle documented for “complex” conodonts. We propose that conodonts adapted their life histories to top-down controlled ecosystems during the Nekton Revolution.

Trophic interactions of marine sponges

<p>Marine communities in the Anthropocene are changing rapidly with potentially severe consequences for ecosystem functioning. Recently, there has been increased interest in the ecological role of sponges, particularly on coral reefs, driven by evidence that sponges may be less affected by this period of environmental change than other benthic organisms. The Sampela reef system in the Wakatobi Marine National Park, Indonesia, is an example of a reef that has shifted to sponge dominance following a decline in hard corals and an increase in sponge density. Previous research suggests that the Sampela reef system may support a greater abundance of spongivorous fishes relative to surrounding reefs, however, uncertainties remain regarding spongivore identity and predated sponges. In addition, little is known about how shifts towards sponge dominance affect the trophic structure of reefs. The primary aim of my thesis was to investigate sponge trophic interactions to gain insight into the way sponge-dominated reefs of the future might function. This information is essential to predict the broader functional consequences of increasing sponge dominance on reefs in the Anthropocene. In my first data chapter, I measured the functional impact of spongivorous fishes by quantifying sponge biomass consumption on Wakatobi reefs. Video analysis identified 33 species from 10 families of reef fish grazing on Xestospongia spp., although 95% of bites were taken by only 11 species. Gut content analysis indicated that Pygoplites diacanthus and Pomacanthus imperator were obligate spongivores and Pomacanthus xanthometopon, Zanclus cornutus and Siganus punctatus regularly consumed sponges. In situ feeding observations revealed that sponges from the family Petrosiidae are preferred by P. diacanthus and Z. cornutus. Spongivores were estimated to consume 46.6 ± 18.3 g sponge 1000 m- 2 of reef day-1 and P. diacanthus had the greatest predatory impact on sponges. While estimates provided here are conservative and likely underestimate the true magnitude of spongivory on Indo-Pacific coral reefs, this chapter provides the first known estimate of reef wide sponge biomass consumption. Comparisons with published data estimating coral consumption by Chaetodontids in the Pacific suggests that biomass transferred through both pathways is similar in magnitude. Hence spongivory is an important, yet overlooked, trophic pathway on Indo-Pacific reefs. In my second data chapter, I developed genetic methods to identify sponges from the stomach contents of spongivorous angelfishes sampled in my first chapter. A range of primers and associated predator-blocking primers targeting the 18S rDNA gene were designed and tested on extracts of sponge and spongivore DNA. Sequences were successfully amplified from 14 sponges spanning 6 orders of Porifera, with the majority of samples identified belonging to the order Haplosclerida. This study is the first to successfully sequence sponges from the gut contents of spongivorous fishes. Sequence data indicated that Pygoplites diacanthus consumed sponges with considerable chemical defences and exhibited significant dietary plasticity within the Porifera phylum, similar to observations of angelfishes in the Caribbean and the eastern Pacific. In my third data chapter, I used stable isotope analysis to investigate differences in consumer niche widths and trophic diversity on the sponge-dominated Sampela reef system in comparison to an adjacent, higher quality reef. I measured the stable isotope ratios of coral reef fish representing different functional feeding groups, prey items and basal carbon sources at both sites. I used isotope data to calculate the trophic position and isotopic niches of each species and performed interspecific and inter-site comparisons. The fish assemblage had a significantly lower mean trophic position at the sponge-dominated site and the majority of species had wider isotopic niches, in accordance with optimal foraging theory which supports expansion in niche widths when per capita prey is low. The fish assemblage sampled at the sponge-dominated site used a significantly lower range of resources, had lower trophic diversity and obtained more carbon from benthic production than fish from the higher quality reef site. Results indicate a simpler trophic structure at the sponge-dominated site characterised by fish with more similar diets. Whilst trophic niche expansion may facilitate population survival in the short term, it can be expected to lead to intensified competition for increasingly scarce resources. In my final data chapter, I investigated niche partitioning and organic matter contributions to co-occurring temperate sponges. I sampled the stable isotope ratios of five abundant sponge species at 10 m and 30 m at two sites at opposing ends of Doubtful Sound, Fiordland. I also used an ROV to opportunistically sample sponges at depths >50 m and measured stable isotope ratios of picoplankton (</p>

Trophic interactions of marine sponges

<p>Marine communities in the Anthropocene are changing rapidly with potentially severe consequences for ecosystem functioning. Recently, there has been increased interest in the ecological role of sponges, particularly on coral reefs, driven by evidence that sponges may be less affected by this period of environmental change than other benthic organisms. The Sampela reef system in the Wakatobi Marine National Park, Indonesia, is an example of a reef that has shifted to sponge dominance following a decline in hard corals and an increase in sponge density. Previous research suggests that the Sampela reef system may support a greater abundance of spongivorous fishes relative to surrounding reefs, however, uncertainties remain regarding spongivore identity and predated sponges. In addition, little is known about how shifts towards sponge dominance affect the trophic structure of reefs. The primary aim of my thesis was to investigate sponge trophic interactions to gain insight into the way sponge-dominated reefs of the future might function. This information is essential to predict the broader functional consequences of increasing sponge dominance on reefs in the Anthropocene. In my first data chapter, I measured the functional impact of spongivorous fishes by quantifying sponge biomass consumption on Wakatobi reefs. Video analysis identified 33 species from 10 families of reef fish grazing on Xestospongia spp., although 95% of bites were taken by only 11 species. Gut content analysis indicated that Pygoplites diacanthus and Pomacanthus imperator were obligate spongivores and Pomacanthus xanthometopon, Zanclus cornutus and Siganus punctatus regularly consumed sponges. In situ feeding observations revealed that sponges from the family Petrosiidae are preferred by P. diacanthus and Z. cornutus. Spongivores were estimated to consume 46.6 ± 18.3 g sponge 1000 m- 2 of reef day-1 and P. diacanthus had the greatest predatory impact on sponges. While estimates provided here are conservative and likely underestimate the true magnitude of spongivory on Indo-Pacific coral reefs, this chapter provides the first known estimate of reef wide sponge biomass consumption. Comparisons with published data estimating coral consumption by Chaetodontids in the Pacific suggests that biomass transferred through both pathways is similar in magnitude. Hence spongivory is an important, yet overlooked, trophic pathway on Indo-Pacific reefs. In my second data chapter, I developed genetic methods to identify sponges from the stomach contents of spongivorous angelfishes sampled in my first chapter. A range of primers and associated predator-blocking primers targeting the 18S rDNA gene were designed and tested on extracts of sponge and spongivore DNA. Sequences were successfully amplified from 14 sponges spanning 6 orders of Porifera, with the majority of samples identified belonging to the order Haplosclerida. This study is the first to successfully sequence sponges from the gut contents of spongivorous fishes. Sequence data indicated that Pygoplites diacanthus consumed sponges with considerable chemical defences and exhibited significant dietary plasticity within the Porifera phylum, similar to observations of angelfishes in the Caribbean and the eastern Pacific. In my third data chapter, I used stable isotope analysis to investigate differences in consumer niche widths and trophic diversity on the sponge-dominated Sampela reef system in comparison to an adjacent, higher quality reef. I measured the stable isotope ratios of coral reef fish representing different functional feeding groups, prey items and basal carbon sources at both sites. I used isotope data to calculate the trophic position and isotopic niches of each species and performed interspecific and inter-site comparisons. The fish assemblage had a significantly lower mean trophic position at the sponge-dominated site and the majority of species had wider isotopic niches, in accordance with optimal foraging theory which supports expansion in niche widths when per capita prey is low. The fish assemblage sampled at the sponge-dominated site used a significantly lower range of resources, had lower trophic diversity and obtained more carbon from benthic production than fish from the higher quality reef site. Results indicate a simpler trophic structure at the sponge-dominated site characterised by fish with more similar diets. Whilst trophic niche expansion may facilitate population survival in the short term, it can be expected to lead to intensified competition for increasingly scarce resources. In my final data chapter, I investigated niche partitioning and organic matter contributions to co-occurring temperate sponges. I sampled the stable isotope ratios of five abundant sponge species at 10 m and 30 m at two sites at opposing ends of Doubtful Sound, Fiordland. I also used an ROV to opportunistically sample sponges at depths >50 m and measured stable isotope ratios of picoplankton (</p>

Diet-driven mercury contamination is associated with polar bear gut microbiota

The gut microbiota may modulate the disposition and toxicity of environmental contaminants within a host but, conversely, contaminants may also impact gut bacteria. Such contaminant-gut microbial connections, which could lead to alteration of host health, remain poorly known and are rarely studied in free-ranging wildlife. The polar bear (Ursus maritimus) is a long-lived, wide-ranging apex predator that feeds on a variety of high trophic position seal and cetacean species and, as such, is exposed to among the highest levels of biomagnifying contaminants of all Arctic species. Here, we investigate associations between mercury (THg; a key Arctic contaminant), diet, and the diversity and composition of the gut microbiota of polar bears inhabiting the southern Beaufort Sea, while accounting for host sex, age class and body condition. Bacterial diversity was negatively associated with seal consumption and mercury, a pattern seen for both Shannon and Inverse Simpson alpha diversity indices (adjusted R2 = 0.35, F1,18 = 8.00, P = 0.013 and adjusted R2 = 0.26, F1,18 = 6.04, P = 0.027, respectively). No association was found with sex, age class or body condition of polar bears. Bacteria known to either be involved in THg methylation or considered to be highly contaminant resistant, including Lactobacillales, Bacillales and Aeromonadales, were significantly more abundant in individuals that had higher THg concentrations. Conversely, individuals with higher THg concentrations showed a significantly lower abundance of Bacteroidales, a bacterial order that typically plays an important role in supporting host immune function by stimulating intraepithelial lymphocytes within the epithelial barrier. These associations between diet-acquired mercury and microbiota illustrate a potentially overlooked outcome of mercury accumulation in polar bears.

Reconstructing the diet, trophic level and migration pattern of mysticete whales based on baleen isotopic composition

Baleen from mysticete whales is a well-preserved proteinaceous material that can be used to identify migrations and feeding habits for species whose migration pathways are unknown. Analysis of δ 13 C and δ 15 N values from bulk baleen have been used to infer migration patterns for individuals. However, this approach has fallen short of identifying migrations between regions as it is difficult to determine variations in isotopic shifts without temporal sampling of prey items. Here, we apply analysis of δ 15 N values of amino acids to five baleen plates belonging to three species, revealing novel insights on trophic position, metabolic state and migration between regions. Humpback and minke whales had higher reconstructed trophic levels than fin whales (3.7–3.8 versus 3–3.2, respectively) as expected due to different feeding specialization. Isotopic niche areas between baleen minima and maxima were well separated, indicating regional resource use for individuals during migration that aligned with isotopic gradients in Atlantic Ocean particulate organic matter. Phenylanine δ 15 N values confirmed regional separation between the niche areas for two fin whales as migrations occurred and elevated glycine and threonine δ 15 N values suggested physiological changes due to fasting. Simultaneous resolution of trophic level and physiological changes allow for identification of regional migrations in mysticetes.

Extreme bill dimorphism leads to different but overlapping isotopic niches and similar trophic positions in sexes of the charismatic extinct huia

The New Zealand huia (Heteralocha acutirostris) had the most extreme bill sexual dimorphism among modern birds. Given the quick extinction of the species, the cause of the dimorphism could only be hypothesised to reflect different trophic niches and reduce male/female competition. We tested that hypothesis by combining museum specimens, geometric morphometrics, and isotopic analyses. We used geometric morphometrics to describe bill shape; measured bulk (δ15Nbulk) and (δ13Cbulk) values from feather as proxies of the birds’ foraging habitat and diet; and compared compound-specific stable isotopes analyses (CSIA) of nitrogen in amino acids (δ15NAA) in male–female pairs to estimate their trophic position. Sexes had significantly different, but overlapping feather δ15Nbulk and δ13Cbulk values, but δ15NAA indicated identical trophic positions and δ15Nbulk was not related to bill shape. Trophic position was less variable among females, consistent with a specialised foraging behaviour and, thus, supporting a partial male/female foraging segregation.

Introduced European minnow Phoxinus phoxinus in alpine lakes may increase total mercury concentration in brown trout Salmo trutta

In Norway, the cyprinid European minnow Phoxinus phoxinus has been spread far outside its previous natural distribution area, with lots of establishments in mountain lakes where brown trout Salmo trutta originally was the only fish species. We have analysed δ15N and total mercury (THg) concentration in brown trout from eight lakes, situated between 1031 and 1244 m a.s.l. on the Hardangervidda mountain plateau, southern Norway. One of the lakes is inhabited by brown trout and European minnow, while in the other seven lakes, brown trout is the only fish species. δ15N of brown trout were significantly higher in the population with co-existing European minnow, indicating a higher trophic position of brown trout in this population than in the allopatric populations, probably caused by piscivory, as indicated by frequent occurrence of European minnow in brown trout diet. The mercury concentrations in brown trout from this lake had values up to around 0.4 mg THg per kg wet weight. The concentrations were significantly higher than in the lakes without European minnow, and together with the δ15N values, indicating that translocation and establishment of European minnow may increase the trophic position of brown trout in previously allopatric populations, and thereby also increase the mercury level.