Trophic Plasticity of Bombay Duck (Harpadon nehereus) in the South-Central East China Sea Based on Stable Isotope Evidence

High trophic plasticity is often invoked to explain the successful expansion of many aquatic species. Bombay duck (BD) outcompete other traditionally commercial fishes in the East China Sea (ECS) in recent years, displaying a continuous and sharp increase of catches, however, little is known whether the competition advantage of BD was related to its trophic plasticity. Using stable isotope analysis (SIA), we investigated the trophic ecology of BD in the south-central ECS in two periods. Significant differences in δ13C and δ15N values were found between sampling months and among body sizes. Modal (95% credibility limits) estimation showed that BD’s trophic position (TP) gradually increased from 3.56 (3.31–3.82) in G1 (120–150 mm for body size) to 4.09 (3.69–4.50) in G5 (241–270 mm) in November, and from 3.37 (2.92–3.88) in G2 (151–180 mm) to 3.66 (3.07–4.23) in G4 (211–240 mm) in April, respectively, suggesting obvious ontogenetic variation of δ15N and TP. More importantly, the TP of BD within the same body size showed wide amplitude, indicating BD possess high trophic plasticity, and can capture prey from low to high trophic levels. Further quantitative analysis based on the MixSIAR model showed that BD mainly fed on zooplankton and fish, but its feeding habit was characterized by the consumption of gradually reduced zooplankton and increased fish associated with increased body size. Interestingly, the fish’s contribution to BD showed obvious seasonal variation. It can be explained by varied food availability in ECS. Relatively low fish prey in April forced BD to adjust its diet, adding the ingestion of shrimps and cephalopods corresponding to temporal variations in food availability. The present study provides the evidence that high trophic plasticity exists in the feeding strategy of BD, which is reflected in the ability to adjust the feeding preference according to nutritional needs and food environmental changes; thus, it is competitively advantageous and may partly explain why BD is becoming the predominant species and displays an extensive distribution in the ECS.

Taxonomic, ecological and biogeographic re-evaluation of temperate stream fishes in Southern Africa : a case study of the enteromius anoplus complex

Worldwide, cryptic species, which comprise two or more distinct species that are classified as one due to their morphological similarity, are more common than previously thought. In South Africa, the chubbyhead barb, Enteromius anoplus (Weber, 1897), is the most widespread freshwater fish species. The widespread occurrence of this species across different river basins raised questions on its taxonomic status due to the likely occurrence of hidden diversity. This further raised the need to evaluate the ecological patterns associated with likely occurrence of such hidden diversity. Phylogenetic analyses of the cytochrome b gene of E. anoplus populations across South Africa revealed the occurrence of four genetically distinct species. These findings supported the restoration Enteromius cernuus (Barnard 1943) stat. rev. from the Olifants River system; the restoration of Enteromius anoplus sensu stricto (Weber 1897) stat. rev. from the Gouritz River system; the redescription of Enteromius oraniensis (Barnard 1943) stat. rev. from the Orange River system; and the description of a new species, Enteromius mandelai sp. nov. from the Eastern Cape rivers. Analyses of morphometric and meristic data revealed that these species were differentiated based on barbel length and number of lateral line scales. Ecological studies on Enteromius mandelai were conducted within headwater streams of major river systems in the Eastern Cape region. These headwater streams had different catchment characteristics ranging from near-natural to anthropogenically-perturbed mainly due to agriculture-related disturbances and invasion of riparian zones by non-native plants. Stable isotope analysis provided evidence of trophic plasticity for E. mandelai across the different headwater streams. Specifically, E. mandelai exhibited large isotopic niches in agriculture-disturbed headwater streams, whereas small isotopic niches were observed in headwater streams invaded by non-native plants. Because of the evidence of trophic plasticity for E. mandelai, this study further hypothesized that this species would exhibit variable environmental niche patterns related to its distribution, abundance and the presence of contra-specifics due to different abiotic factors across its range. Enteromius mandelai was generally most abundant at sites with high total dissolved solids and sand substratum, which was characteristic of large pool habitats, and least abundant at sites with high turbidity. In the Great Fish River, E. mandelai occurred as the only species in most headwater sites, whereas in the Keiskamma and Buffalo River systems, it co-occurred with Amatolacypris trevelyani and Sandelia bainsii. Enteromius mandelai was, nevertheless, found to be ubiquitous together with having wide environmental niches, and exhibited no discernible patterns in its habitat associations. The taxonomic re-evaluation of the E. anoplus complex, which provided evidence of the existence of new species, has implications for the biogeographic zonation of the ichthyofauna of the Cape Fold Ecoregion (CFE), Amathole-Winterberg Highlands Ecoregion (AWHE) and the Southern Temperate Highveld Ecoregion (STHE). For example, the currently recognised southern limit of the STHE, which encompasses the AWHE, is based on the occurrence of E. anoplus, Enteromius pallidus and Labeo umbratus. However, evidence from this and other recent studies indicate the occurrence of either new species or distinct lineages in the southern-most section of the STHE, which is consistent with the patterns observed in the CFE. Comparison of taxonomic composition showed that species in the eastern part of the CFE were similar to species in the AWHE and the southern-most section of the STHE. Because of the close affinities among most of the new taxa in these regions, this study proposes that the current delimitation of the CFE should be reviewed by extending it to include the AWHE and sections of the STHE.

Feeding habits of the spotted rose snapper, Lutjanus guttatus, (Actinopterygii, Perciformes, Lutjanidae), in the central Gulf of California, BCS, Mexico

The spotted rose snapper, Lutjanus guttatus (Steindachner, 1869), is an important resource for the coastal fisheries of the Gulf of California, mainly due to its high commercial value. Despite this, there are no management measures for this species, owing in part to a lack of information on its basic biology and its trophic ecology in the area. In this context, the presently reported study had as objective to describe the feeding habits of L. guttatus through stomach content analyses, as well as to describe possible changes linked to sex, size, and season. Specimens were caught monthly from June 2016 to September 2017 with gillnets in Santa Rosalía, BCS, Mexico. The abundance, weight, and frequency of occurrence of each prey were assessed, and these parameters were integrated into the index of relative importance (%IRI) to determine the importance of each prey item in the L. guttatus diet. The Levin’s index was used to assess the trophic niche width of the species, the feeding strategy was evaluated using Costello’s graphic method and the trophic level was calculated. Finally, to establish whether there were significant differences in the diet by sex, size, or season a PERMANOVA test was used with a 95% confidence level. A total of 202 L. guttatus stomachs were analyzed, 191 of which contained food. A total of 26 prey items were identified. According to the %IRI, the most important prey were the teleost fishes Harengula thrissina (Jordan et Gilbert, 1882) (45.7%) and Sardinops sagax (Jenyns, 1842) (34.8%), the euphausiid Nyctiphanes simplex (13.4%), and the crustacean Penaeus spp. (5.6%). The PERMANOVA analysis resulted in significant differences between the analyzed categories; however, there were no significant differences in the interactions among the categories. According to Levin’s index, L. guttatus had a narrow trophic width, with changes in the main prey consumed by the different categories. According to our results, L. guttatus can be considered a benthopelagic opportunistic carnivorous predator with a narrow trophic niche, presenting mostly quantitative variations in its diet according to sex, size, and season. Its trophic plasticity allows it to take advantage of the most available and abundant food resources.

Tracing the Trophic Plasticity of the Coral–Dinoflagellate Symbiosis Using Amino Acid Compound-Specific Stable Isotope Analysis

The association between corals and photosynthetic dinoflagellates is one of the most well-known nutritional symbioses, but nowadays it is threatened by global changes. Nutritional exchanges are critical to understanding the performance of this symbiosis under stress conditions. Here, compound-specific δ15N and δ13C values of amino acids (δ15NAA and δ13CAA) were assessed in autotrophic, mixotrophic and heterotrophic holobionts as diagnostic tools to follow nutritional interactions between the partners. Contrary to what was expected, heterotrophy was mainly traced through the δ15N of the symbiont’s amino acids (AAs), suggesting that symbionts directly profit from host heterotrophy. The trophic index (TP) ranged from 1.1 to 2.3 from autotrophic to heterotrophic symbionts. In addition, changes in TP across conditions were more significant in the symbionts than in the host. The similar δ13C-AAs signatures of host and symbionts further suggests that symbiont-derived photosynthates are the main source of carbon for AAs synthesis. Symbionts, therefore, appear to be a key component in the AAs biosynthetic pathways, and might, via this obligatory function, play an essential role in the capacity of corals to withstand environmental stress. These novel findings highlight important aspects of the nutritional exchanges in the coral–dinoflagellates symbiosis. In addition, they feature δ15NAA as a useful tool for studies regarding the nutritional exchanges within the coral–symbiodiniaceae symbiosis.