Habitat preference, movements and growth of giant mottled eels, Anguilla marmorata, in a small subtropical Amami-Oshima Island river

Although anguillid eel populations have decreased remarkably in recent decades, few detailed ecological studies have been conducted on tropical eels such as the giant mottled eel whose range extends across the whole Indo-Pacific. This species was studied throughout the entire 0.5 km mainstem reaches of Oganeku River on the subtropical Amami-Oshima Island of Japan over a two-year period using four sampling periods to understand its habitat preference, early life-stage dispersal process, movements, and annual growth using a mark-recapture experiment conducted with quantitative electrofishing. A total of 396 juvenile growth-phase A. marmorata eels were caught and tagged, with 48 individuals being recaptured at least once. Their density irrespective of size of eels was most strongly determined by distance from the river mouth, followed by riverbank type according to random forest models. Eel density decreased with increasing distance from the freshwater tidal limit located about 100–150 m from the river mouth. Eels preferred vegetated riverbanks, while they avoided those of concrete and sand. The density of small eels (total length: TL < 240 mm) was also associated with depth and velocity, with small eels tending to prefer riffle or run habitats. In contrast, large eels (TL ≥ 240 mm) were found in habitats of any depth and velocity. The TL of eels had a minimum peak at around the tidal limit, and it increased with increasing distance from the tidal limit. The observed density and size gradients of eels in relation to the distance from the river mouth suggested that A. marmorata initially recruited to freshwater tidal limit areas and then dispersed in both downstream and upstream directions. The growth rate of eels varied greatly among individuals that were at large for various periods of time and ranged from 0 to 163.2 mm/year (mean ± SD of 31.8 ± 31.0 mm/year). Of the recaptured eels, 52.1% were recaptured in a section that was different from the original capture section, and their mean ± SD distance travelled was 46.5 ± 72.5 m (median = 20 m). 47.9% of the eels were recaptured from the original section of capture (i.e., <10 m distances travelled), suggesting that they had strong fidelity to specific habitats with limited movements. The distance travelled of eels that had moved was greater for small eels (range = 10–380 m; mean ± SD = 84.4 ± 121.9 m) than large individuals (range = 10–120 m; mean ± SD = 30.9 ± 31.0 m), which indicates that the mobility of the eels declines as they grow. This is the first clear detailed documentation of the spatial distribution, growth, and movements of tropical eels in a small river system in relation to environmental conditions that provides an example of how future studies can be conducted in other areas to understand how conservation efforts can be most efficiently targeted for maximum success.

Degeneration of the olfactory epithelium in the Anguilid eels by hormone treatment

While the olfactory cue hypothesis has been proposed for spawning migration of silver eels, it has been shown that olfactory cells and associated mucus cells degenerate in male and female eels after hormonally induced sexual maturation. However, the degeneration of the olfactory organ could be a real event in the sequence of maturation, or may be an unnatural side effect of the hormone treatment itself. We morphologically and histologically examined the olfactory rosettes of hormone-untreated and hormone-treated (mixture of hCG and PG) giant mottled eel (Anguilla marmorata) and Japanese eel (A. japonica). The olfactory rosette from all the hormone-treated specimens significantly degenerated at various degeneration levels even in sexually immature specimens, indicating the side effect of the hormone-treatment. However, a sexually immature non-hormone treated female A. marmorata (87.4 cm TL, 199.4 g BW, at less advanced maturity) had slightly degenerated olfactory rosette. Further studies should focus on conducting natural degeneration of the olfactory rosette during the sexual maturation in tropical eels.

Stable Species Boundaries Despite Ten Million Years of Hybridization in Tropical Eels

Genomic evidence is increasingly underpinning that hybridization between taxa is commonplace, challenging our views on the mechanisms that maintain their boundaries. Here, we focus on seven catadromous eel species (genusAnguilla), and use genome-wide sequence data from more than 450 individuals sampled across the tropical Indo-Pacific, morphological information, and three newly assembled draft genomes to compare contemporary patterns of hybridization with signatures of past gene flow across a time-calibrated phylogeny. We show that the seven species have remained distinct entities for up to 10 million years, despite a dynamic scenario of incomplete isolation whereby the current frequencies of hybridization across species pairs (over 5% of all individuals were either F1 hybrids or backcrosses) contrast remarkably with patterns of past introgression. Based on near-complete asymmetry in the directionality of hybridization and decreasing frequencies of later-generation hybrids, we identify cytonuclear incompatibilities and hybrid breakdown as two powerful mechanisms that can support species cohesion even when hybridization has been pervasive throughout the evolutionary history of entire clades.

Species composition, abundance and seasonal recruitment patterns of freshwater eels (Anguilla spp.) to Viti Levu, Fiji Islands, in the western South Pacific

To understand the inshore recruitment mechanisms of tropical eels (Anguilla spp.), we collected 1368 glass eels at the mouth of a small river at Namelimeli, ~2km east of Navua Town, Fiji Islands, between April 2015 and June 2016. Specimens were identified using both morphological characters and DNA barcoding to only three species: one short-finned Anguilla obscura and two long-finned eels A. marmorata and A. megastoma. Anguilla obscura was the most abundant species (55% of glass eels collected) with peak recruitment periods from February to April. Anguilla marmorata was the second most abundant species (41.4% of glass eels collected) with peak recruitment periods in April 2015 and September–October 2015. Anguilla megastoma only comprised 3.9% of glass eels collected, with peak recruitment periods in April 2015 and October 2015. Anguilla obscura and A. marmorata were present in samples almost throughout the year, suggesting that tropical eels recruit to some degree throughout the year. Results suggest that peak recruitment occurs during seasons of heavy rain, from September to October and from February to April commencing 1h after sunset on the day following a new moon.

Assessing the species composition of tropical eels (Anguillidae) in Aceh Waters, Indonesia, with DNA barcoding gene cox1.

The objective of the present study was to evaluate the species diversity of eels native to Aceh waters based on genetic data. Sampling was conducted in western coast waters of Aceh Province, Indonesia, from July to August 2016. Genomic DNA was extracted from the samples, a genomic region from the 5’ region of the cox1 gene was amplified and sequenced, and this was then used to analyse genetic variation. The genetic sequences were blasted into the NCBI database. Based on this analysis there were three valid species of eels that occurred in Aceh waters, namely Anguilla marmorata, A. bicolor bicolor, and A. bengalensis bengalensis.