Molecular diet analysis of Anguilliformes leptocephalus larvae collected in the western North Pacific

Diet composition of wild columbiform birds: next-generation sequencing of plant and metazoan DNA in faecal samples

Accurate knowledge of a species' diets is fundamental to understand their ecological requirements. Next-generation sequencing technology has become a powerful and non-invasive tool for diet reconstruction through DNA metabarcoding. Here, we applied those methods on faecal samples of Common Woodpigeons Columba palumbus, European Turtle Doves Streptopelia turtur, and Stock Doves C. oenas to investigate their dietary composition. By applying primer pairs targeting both the ITS2 region of plant nuclear DNA and the mitochondrial COI region of metazoan DNA, we provide a complete picture of the food ingested and estimate the dietary overlap between the columbiform species during the breeding season. Animal DNA was present very rarely, and a diverse range of plants from the class Spermatopsida dominated the diet, with Asteraceae, Brassicaceae, Cucurbitaceae, Fabaceae, and Poaceae as the most frequently represented families. Generally, we detected a variability between species but also amongst individual samples. Plant species already known from previous studies, mainly visual analyses, could be confirmed for our individuals sampled in Germany and the Netherlands. Our molecular approach revealed new plant taxa, e.g. plants of the families Malvaceae for Woodpigeons, Lythraceae for Turtle Doves, and Pinaceae for Stock Doves, not found in previous studies using visual analyses. Although most of the plant species observed were of wild origin, the majority of cultivated plants found were present in higher frequencies of occurrence, suggesting that cultivated food items likely constitute an important part of the diet of the studied species. For Turtle Doves, a comparison with previous studies suggested regional differences, and that food items (historically) considered as important part of their diet, such as Fumitory Fumaria sp. and Chickweed Stellaria media, were missing in our samples. This indicates that regional variations as well as historic and current data on diet should be considered to plan tailored seed mixtures, which are currently proposed as an important management measure for conservation of the rapidly declining Turtle Dove.

Effectiveness of blocking primers and a peptide nucleic acid (PNA) clamp for 18S metabarcoding dietary analysis of herbivorous fish

The structure of food webs and carbon flow in aquatic ecosystems can be better understood by studying contributing factors such as the diets of herbivorous fish. Metabarcoding using a high-throughput sequencer has recently been used to clarify prey organisms of various fish except herbivorous fish. Since sequences of predator fish have dominated in sequences obtained by metabarcoding, we investigated a method for suppressing the amplification of fish DNA by using a blocking primer or peptide nucleic acid (PNA) clamp to determine the prey organisms of herbivorous fish. We designed three blocking primers and one PNA clamp that anneal to fish-specific sequences and examined how efficient they were in suppressing DNA amplification in various herbivorous fish. The results showed that the PNA clamp completely suppressed fish DNA amplification, and one of the blocking primers suppressed fish DNA amplification but less efficiently than the PNA clamp. Finally, we conducted metabarcoding using mock community samples as templates to determine whether the blocking primer or the PNA clamp was effective in suppressing fish DNA amplification. The results showed that the PNA clamp suppressed 99.3%-99.9% of fish DNA amplification, whereas the blocking primer suppressed 3.3%-32.9%. Therefore, we propose the application of the PNA clamp for clarifying the prey organisms and food preferences of various herbivorous fish.

Observation of the feeding behaviour of reared Japanese eel Anguilla japonica leptocephali fed picocyanobacteria Synechococcus spp

The authors observed the feeding behaviour of artificially reared Japanese eel Anguilla japonica leptocephali, 7.5-19 mm total length (10-61 days post-hatch), fed Synechococcus sp., which is considered a potential food source of anguilliform larvae. Three strains of Synechococcus sp. (NIES-972, 976 and 979) were tested as the food material. Larvae across the entire length range could effectively ingest a suspension of pico-sized cyanobacteria (1-3 μm in diameter). Video observations of the mid-hindgut of larvae under an epifluorescence microscope confirmed that the movement of microvilli of the intestinal epithelium allowed the cell particles to circulate in the mid-hindgut, before becoming solidified in the anal region. Significant differences in food intake were observed between larvae fed two strains of Synechococcus (NIES-972 and 976), and among different cell densities, which suggests feeding selectivity and density dependence. Comparisons of feeding behaviour under the light group (9L:15D) and the dark group (24D) showed significantly higher food intake (measured as an index of intestinal fullness) in the light group, although substantial and continuous ingestion was observed in the dark group, indicating continuous feeding by swallowing sea water. The authors hypothesise that the feeding ecology of anguilliform leptocephali is based on a survival strategy whereby the larvae do not compete with various higher-trophic-level fishes for food in an oligotrophic environment.

A comprehensive biochemical characterization of settlement stage leptocephalus larvae of bonefish (Albula vulpes)

Little is known about early development of the near-threatened bonefish (Albula vulpes), a member of superorder Elopomorpha. Members of Elopomorpha are partially defined by their synapomorphic leptocephalus larval stage, for which the nutritional requirements are not well understood. Characterizing the nutritional profile, including major nutrients (such as lipids) used for energetic processes, can help to gain a better understanding of the nutritional requirements for leptocephalus larvae. A total of 24 settlement stage A. vulpes leptocephalus larvae were collected at Long Caye Island, Belize. Samples were used to determine various biochemical characteristics including lipid class, fatty acid and glycosaminoglycan compositions. Each of these biochemical components plays a role in early developmental processes such as cellular membrane formation and is crucial for healthy development. Biochemical characteristics of settlement stage A. vulpes leptocephalus are presented in this study for the first time. The dominant lipid classes and fatty acids detected in these samples were consistent with prior studies using closely related species like the Japanese eel, indicating possible similarities in diets at this stage. In the future, similar analyses can be applied to other species that share the leptocephalus life stage to determine if nutritional requirements at this stage of development are unique to this species. The findings in this study will also help to facilitate the establishment of adequate aquaculture systems for captive bonefish, ultimately leading to improved management strategies for wild bonefish habitats.

Histological Evidence of Multiple Spawning in Wild Female Japanese Eel Anguilla japonica

The post ovulatory follicle (POF) is an important and reliable tissue structure used to investigate the spawning history in teleost fish. Fresh POFs shortly after spawning are comprised of cellular (follicular cells) and acellular (basement membrane and fibrils such as elastic fibers) components. The cellular components are quickly disintegrated by means of apoptosis, while the acellular components persist for a longer period. Since cellular components are well visualized by conventional hematoxylin-eosin (HE) staining but acellular components are not stained well, old POFs that have lost cellular components are difficult to identify. In this study, periodic acid-Schiff and Victoria blue staining, which can distinctly visualize acellular POF components, were applied to the ovarian tissues of Japanese eel (Anguilla japonica) (n = 9) captured from June to August of 2008, 2009, and 2013 at the southern West Mariana Ridge, a spawning area for Japanese eels. Only new POFs were observed in seven females caught in June, and these females had ovaries with early-to mid-vitellogenic stage oocytes. Both fresh and old POFs were observed in a female caught in July, and only mid-vitellogenic stage oocytes were observed. Only old POFs and no vitellogenic stage oocyte were observed in a female caught in August. A progressive decrease in muscle lipid content, gonad somatic index, and condition factors was observed from June to August. Thus, the female Japanese eel can spawn at least twice or three times at most during spawning season, depending on energy reserve.

Micro-size plankton abundance and assemblages in the western North Pacific Subtropical Gyre under microscopic observation

While primary productivity in the oligotrophic North Pacific Subtropical Gyre (NPSG) is changing, the micro-size plankton community has not been evaluated in the last 4 decades, prompting a re-evaluation. We collected samples over three years (2016–2018) from depths of 10 to 200 m (n = 127), and the micro-size plankton were identified and counted to understand the heterogeneity of micro-size plankton community structure. The assemblages were consistent to the those of 4 decades ago. Dinophyceae (dinoflagellates) were the most numerically abundant, followed by Cryptophyceae and Bacillariophyceae (diatoms). The other micro-size plankton classes (Cyanophyceae, Haptophyceae, Dictyochophyceae, Euglenophyceae, and Prasinophyceae) were not always detected, whereas only Trichodesmium spp. was counted in the Cyanophyceae. Other unidentified autotrophic and heterotrophic flagellates were also significantly present, and their numeric abundance was higher than or at the same level as was that of the Dinophyceae. In the Dinophyceae, Gymnodiniaceae and Peridiniales were abundant. The chlorophyll a concentration and these class-level assemblages suggested micro-size plankton is not a major primary producer in this area. We applied generalized additive models (GAMs) and principal coordination analyses (PCoAs) to evaluate the habitats of every plankton group and the heterogeneity of the assemblages. The GAMs suggested that every classified plankton abundance showed a similar response to salinity, and we observed differences in habitats in terms of temperature and nitrate concentrations. Based on the PCoAs, we observed unique communities at the 200 m depth layer compared with those at the other sampling layers. The site scores of PCoAs indicated that the micro-size plankton assemblages are most heterogeneous at the 10 m depth layer. At such depth, diazotrophic Cyanophyceae (Trichodesmium spp.) are abundant, particularly in less-saline water. Therefore, nitrogen fixation may contribute to the heterogeneity in the abundance and assemblages in the western NPSG.

18S rRNA gene sequences of leptocephalus gut contents, particulate organic matter, and biological oceanographic conditions in the western North Pacific

Eel larvae apparently feed on marine snow, but many aspects of their feeding ecology remain unknown. The eukaryotic 18S rRNA gene sequence compositions in the gut contents of four taxa of anguilliform eel larvae were compared with the sequence compositions of vertically sampled seawater particulate organic matter (POM) in the oligotrophic western North Pacific Ocean. Both gut contents and POM were mainly composed of dinoflagellates as well as other phytoplankton (cryptophytes and diatoms) and zooplankton (ciliophoran and copepod) sequences. Gut contents also contained cryptophyte and ciliophoran genera and a few other taxa. Dinoflagellates (family Gymnodiniaceae) may be an important food source and these phytoplankton were predominant in gut contents and POM as evidenced by DNA analysis and phytoplankton cell counting. The compositions of the gut contents were not specific to the species of eel larvae or the different sampling areas, and they were most similar to POM at the chlorophyll maximum in the upper part of the thermocline (mean depth: 112 m). Our results are consistent with eel larvae feeding on marine snow at a low trophic level, and feeding may frequently occur in the chlorophyll maximum in the western North Pacific.

Lipid Composition and Utilization in Early Stage Leptocephalus Larvae of Bonefish (Albula vulpes)

In order to encourage the survival of both captive and wild populations of bonefish (Albula vulpes), a deeper understanding of the species' early developmental characteristics is necessary. During ontogenesis, bonefish utilize lipids as a source of energy before the start of exogenous feeding. The goal of this study is to gain insight into the energetic needs of bonefish leptocephalus larvae in the post-hatch larval stage. Broodstock were collected in the wild and hormone induced. Spawning yielded eggs that were fertilized and were then incubated until hatching. Larval development was monitored throughout the duration of the trial until all larvae perished. Samples of larval tissue were taken to the lab for lipid analysis and composition was compared at different developmental stages. Larval lipid composition was significantly different across sample groups showing a change in lipid content related to development. After hatching, larvae gradually depleted wax esters-sterol ester (WE-SE) reserves over a period of 4 days, while simultaneously increasing hydrocarbon (HC). The role of WE-SE is seemingly tied to both buoyancy and energy reserves due to its high abundance immediately post-hatch and selective catabolism. As larvae weaned off of the nutrition provided by the yolk, exogenous feeding began to diversify lipid composition and overall lipid reserves were depleted. Future directions included the development of optimal larval feeds based on this analysis in order to gain more insight into the nutritional needs and requirements during the critical leptocephalus stages.