Temperature behavioral responses of the American eel,Anguilla rostrata (Lesueur), from Maryland

Acute temperature preference tests were conducted with American eels, Anguilla rostrata, collected from Maryland's eastern shore. Eels were acclimated to temperatures of 6, 12, 18, 24 and 30°C. Final temperature preferendum was 16.7°C. Data differ from the temperature responses of the majority of fishes tested to date in that acclimation temperature did not influence selected temperatures. Similar results were obtained for various other fishes (Oncorhynchus, Salmo, Salvelinus) by other investigators. Behavioral responses at various acclimation temperatures were observed.     

Furunculosis in cultured American eel Anguilla rostrata (Le Sueur)

Diseased American eels, Anguilla rostrata , were collected from an eel culture facility in South Carolina and examined. The disease was characterized by skin lesions located on the body. A bacterium identified as Aeromonas salmonicida was isolated from the lesions and shown to be the causative agent. The bacterium was also isolated from the kidney during later stages of the disease.     

Validation of daily otolith increments in glass-phase American eels Anguilla rostrata (Lesueur) during estuarine residency

Prior to making inferences from otoliths about the residence time and growth rate of glass-phase anguillid eels Anguilla in estuaries, it is necessary to validate the deposition rate of microincrements in the otoliths. Glass-phase American eels Anguilla rostrata (Lesueur), which had been captured near the mouth of an estuary in Maine, USA, prior to freshwater exposure, deposited increments at a daily rate at ambient temperature and salinity in a field and laboratory study. The regression for glass eels not possessing a transition ring was: I=0.976(D-1)+0.434, where I is the number of otolith increments distal to a fluorescent mark placed on the otolith at the beginning of the experiment, and D is the number of days in the experiment, which ranged from 7 to 49. The slope was not significantly different than 1. Unexpectedly, many glass eels deposited the transition ring during the experiment, although this ring had previously been thought to mark entry into fresh water. The regression for these glass eels was: I=0.961(D-1)-3.880, and the slope was not significantly different than 1. The negative intercept suggests that approximately 4 days were lost from the otolith record during deposition of the ring. This study demonstrated daily deposition of increments prior to freshwater exposure and demonstrated that deposition of the transition ring is not linked to freshwater entry.     

Retinal response in the post metamorphic American eel (Anguilla rostrata) (Pisces,Teleostei)

Retinas of light and dark adapted post metamorphic American eelAnguilla rostrata were examined. The retinal epithelial pigment migrates vitreally in light and sclerally in darkness. Two layers of rods and a layer of single cones are present. Some cones elongate slightly in the dark and contract in the light. The cone synaptic ribbons show no difference between the light and dark adapted stages. It appears that this eel stage is capable of functioning in bright and dim environments.     

The genetic consequences of spatially varying selection in the panmictic American eel (Anguilla rostrata)

Our understanding of the genetic basis of local adaptation has recently benefited from the increased power to identify functional variants associated with environmental variables at the genome scale. However, it often remains challenging to determine whether locally adaptive alleles are actively maintained at intermediate frequencies by spatially varying selection. Here, we evaluate the extent to which this particular type of balancing selection explains the retention of adaptive genetic variation in the extreme situation of perfect panmixia, using the American eel (Anguilla rostrata) as a model. We first conducted a genome scan between two samples from opposite ends of a latitudinal environmental gradient using 454 sequencing of individually tagged cDNA libraries. Candidate SNPs were then genotyped in 992 individuals from 16 sampling sites at different life stages of the same cohort (including larvae from the Sargasso Sea, glass eels, and 1-year-old individuals) as well as in glass eels of the following cohort. Evidence for spatially varying selection was found at 13 loci showing correlations between allele frequencies and environmental variables across the entire species range. Simulations under a multiple-niche Levene's model using estimated relative fitness values among genotypes rarely predicted a stable polymorphic equilibrium at these loci. Our results suggest that some genetic-by-environment interactions detected in our study arise during the progress toward fixation of a globally advantageous allele with spatially variable effects on fitness.     

The ultrastructure of the integument of the American eel,Anguilla rostrata

Summary The morphology and ultrastructure of the lateral body integument of the leptocephalus, glass eel, pigmented elver, and adult stages of the American eel, Anguilla rostrata, were examined with light and electron microscopy. The integument consists of an epidermis separated by a basal lamina from the underlying dermis. Three cell types are present in the epidermis in all stages. Filament-containing cells, which are the principal structural cell type, are increasingly numerous at each stage. Mucous cells, which secrete the mucous that compose the mucous surface coat, are also more numerous in each subsequent stage and are more numerous in the anterior lateral body epidermis than in the posterior lateral body epidermis of the adult. Club cells, whose function is unknown, are most numerous in the glass eel and pigmented elver. Chloride cells are common in the leptocephalus which is marine and infrequent in the glass eel. They are not present in the pigmented elver and adult which inhabit estuaries and fresh-water. Lymphocytes and melanocytes are also present in some stages. The dermis comprises two layers: a layer of collagenous lamellae, the stratum compactum, and an underlying layer of loose connective tissue, the stratum spongiosum.There is a progressive increase in epidermal thickness at each stage which is paralleled by an increase in the thickness of the stratum compactum. Rudimentary scales are present in the dermis of the adult. The increase in the number of epidermal filament-containing cells, epidermal thickness and stratum compactum thickness is correlated with an increased need for protection from abrasion and mechanical damage as the eel moves from a pelagic, oceanic habitat to a benthic, freshwater habitat. The increase in mucous cell numbers is likewise correlated with an increased need for the protective and anti-bacterial action of the mucous surface coat in the freshwater environment.This investigation was supported by NIH research grant NS-11276 from National Institute of Neurological Diseases and Stroke to Dr. J.D. McCleave and by N.S.F. Grant GD 38933 to the Bermuda Biological Station, St. Georges West, Bermuda. Bermuda Biological Station Contribution No. 668     

Evidence for Environmental Sex Determination in the American eel, Anguilla rostrata

Males have predominated among migrating silver eels in the Annaquatucket River, Rhode Island, for at least two decades, with no significant variation in mean total length in either sex. Because the species is panmictic (random breeding), this consistency suggests environmental sex determination (ESD). Most yellow (feeding phase) eels <300mm total length in the Annaquatucket are sexually undifferentiated, and in contrast to all other published sex ratios, males greatly outnumber females (3:1) among differentiated yellow eels. Estimates of yellow eel population densities are 4–10 times greater than published values for other habitats. We propose that this crowding results in a long period of undifferentiation and the suppression of femaleness. Published field and experimental evidence indicates that high population density results in high proportions of males in Atlantic Anguilla, and that low population density results in the predominance of females. This ESD may be adaptive, resulting in vast numbers of small males in coastal habitats, relatively close to the spawing area, and much larger and more fecund females that occupy most of the available eel habitat.     

Draft genome of the American Eel (Anguilla rostrata)

Freshwater eels (Anguilla sp.) have large economic, cultural, ecological and aesthetic importance worldwide, but they suffered more than 90% decline in global stocks over the past few decades. Proper genetic resources, such as sequenced, assembled and annotated genomes, are essential to help plan sustainable recoveries by identifying physiological, biochemical and genetic mechanisms that caused the declines or that may lead to recoveries. Here, we present the first sequenced genome of the American eel. This genome contained 305 043 contigs (N50 = 7397) and 79 209 scaffolds (N50 = 86 641) for a total size of 1.41 Gb, which is in the middle of the range of previous estimations for this species. In addition, protein‐coding regions, including introns and flanking regions, are very well represented in the genome, as 95.2% of the 458 core eukaryotic genes and 98.8% of the 248 ultra‐conserved subset were represented in the assembly and a total of 26 564 genes were annotated for future functional genomics studies. We performed a candidate gene analysis to compare three genes among all three freshwater eel species and, congruent with the phylogenetic relationships, Japanese eel (A. japanica) exhibited the most divergence. Overall, the sequenced genome presented in this study is a crucial addition to the presently available genetic tools to help guide future conservation efforts of freshwater eels.     

Origins of the freshwater attractant(s) of migrating elvers of the American eel,Anguilla rostrata

Synopsis Origins of the freshwater attractant(s) of migrating elvers of the American eel were investigated by assaying elvers' responses to rinses of plants, animals, and inanimate objects collected from a Rhode Island (U.S.A.) brook with a sizable elver run. Odor rinses were tested in a Y-maze at naturally occurring concentrations against both blank and brook water. Many items were attractive, several were repulsive, and some caused a reduction in elvers' rheotactic behavior, suggesting that elvers respond to a bouquet of odors. The odor of abundant decaying leaf detritus was highly attractive as were odors of the surfaces of aquatic plants, submerged stones, and migrating alewives. Conspecific odor was only weakly attractive. Because unattractive leaves became attractive when cultured with stream water, microorganisms responsible for detrital decomposition and present in/on most stream objects are thought to be the major source of the attractant(s). Decaying detritus and its associated microorganisms are abundant in most freshwater streams, where they often constitute the ecosystem's primary energy source; their odor could serve as an index of environmental suitability for migrating eels.     

Densities and swimbladder development of juvenile American eels, Anguilla rostrata (Lesueur) as related to energetics of migration

Glass eels and elvers of the American eel were negatively buoyant. Those adapted to sea water were more dense (1.072 ± 0.001 g cm⁻³) than those adapted to fresh water (1.061 ± 0.001 g cm⁻³). Adaptation to fresh water increased relative body water content, but did not account for the observed decrease in total body density. Histological examination revealed the presence of a potentially functional swimbladder in the glass eels, although this hydrostatic organ did not becomegas-filled until after freshwater residency had occurred. Calculation of lift as used in the selective tidal transport mechanism suggests that hydrodynamic compensation for horizontal swimming during the estuarine phase of migration is energetically adaptive.     

Habitat use of American eel (Anguilla rostrata) in a tributary of the Hudson River,New York

American eel Anguilla rostrata populations are declining over much of their native range. Since American eels spend extended periods in freshwater, understanding their habitat requirements while freshwater residents is important for the management and conservation of this species. As there is little information on American eel habitat use in streams, the ontogenetic, diel, and seasonal habitat use as well as habitat selectivity of three size groups (i.e. ≤199 mm total length, 200–399 mm, ≥400 mm) of eel were examined in a tributary of the Hudson River. American eels in Hannacroix Creek exhibited ontogenetic, diel, and seasonal variation in habitat use as well as habitat selection. During both summer and autumn all sizes of American eels used larger substrate and more cover during the day. American eels ≤199 mm exhibited the strongest habitat selection, whereas eels 200–399 mm exhibited the least. During the autumn all sizes of American eels occupied slower depositional areas where deciduous leaf litter accumulated and provided cover. This may have important implications for in‐stream and riparian habitat management of lotic systems used by American eel.     

Electrophysiological responses from the peripheral olfactory system of the American eel,Anguilla rostrata

Summary A method is described for recording with microelectrodes from central neurones in locusts,Schistocerca gregaria americana, that are free to perform a large fraction of their behavioural repertoire. This tethered preparation has been used to examine the individual responses of large neurones in the neck connectives to a range of sensory stimuli.From differences in the responses of the units examined and from their positions in the connective, as determined by dye iontophoresis, 31 separate neurones have been identified. The axons of these cells had relatively constant diameters and cord positions in different animals and appeared in both right and left connectives but with their positions mirror reversed. The majority of these 31 cells carried descending information from the head ganglia and under our experimental conditions, 7 were found to have wind stimulation as their strongest sensory input, 17 had visual stimulation, 4 had sound stimulation and 3 had proprioceptive input.Abbreviations DCMD descending contralateral movement detector (neurone) - DIMD descending ipsilateral movement detector (neurone)     

Cholinoceptor-mediated control of catecholamine release from chromaffin cells in the American eel,Anguilla rostrata

The cholinergic agonist-induced secretion of catecholamines from chromaffin cells in the American eel, Anguilla rostrata, was assessed using a salineperfused posterior cardinal vein preparation. Direct membrane depolarization with 60 mmol·l^-1 K⁺ caused a significant release of catecholamines (adrenaline + noradrenaline) into the perfusate which was unaffected by pre-treatment with the ganglion blocker, hexamethonium (final concentration = 10^-3 mol · l^-1). The nicotinic receptor agonist, 1,1-dimethyl-4-phenylpiperazinium iodide, evoked catecholamine release in response to several doses exceeding 10^-7 mol; at 10^-5 mol the response was abolished by pre-treatment with the ganglion blocker, hexamethonium (final concentration = 10^-3 mol · l^-1). The muscarinic receptor agonist, pilocarpine, did not elicit catecholamine release in response to any of the doses administered (10^-8–10^-4 mol). A single injection of the mixed nicotinic/muscarinic cholinoceptor agonist, carbachol (10^-5 mol), caused the release of catecholamines which was abolished by pre-treatment with hexamethonium but which was unaffected by pre-treatment with the muscarinic receptor antagonist atropine (final concentration = 10^-5 mol · l^-1). The results of this study indicate that the process of cholinergic agonist-induced catecholamine secretion from the chromaffin cells in the American eel is mediated exclusively by activation of nicotinic receptors with no involvement of the muscarinic receptor.Abbreviations DMPP 1,1-dimethyl-4-phenylpiperazinium iodide - MS222 ethylaminobenzoate - HPLC high-performance liquid chromatography - PCV posterior cardinal vein - SEM standard error of the mean     

Validation of the occurrence of the American eel Anguilla rostrata (Le Sueur, 1817) in free-draining European inland waters

The screening of 2,735 eels from European waters and aquaculture farms was conducted using mitochondrial Cytochrome b and 16S rRNA gene fragments amplified by polymerase chain reaction. Reaction products were either sequenced directly or subjected to analysis using restriction fragment length polymorphism which resulted in species-specific restriction patterns. Beside the expected European eel, Anguilla anguilla (Linnaeus, 1758), the American eel, Anguilla rostrata (Le Sueur, 1817), was also identified in samples from both aquaculture (N = 40 out of 1,025) and from natural waters (N = 44 out of 1,710). The life stages of American eels identified from several German waters draining to either the Baltic Sea and the North Sea ranged from elver to silver eels. This indicates that stocking with glass eels or elvers must have occurred several times most likely in the period from 1998 to 2002. The application of a fast and precise method for species identification and genetic monitoring of eels delivered for stocking is therefore essential for maintaining the autochthonous species composition in future. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Spawning migration of American eel Anguilla rostrata from pristine (1843-1872) to contemporary (1963-1990) periods in the St Lawrence Estuary, Canada

A key for three putative species apparently found in three geographic areas, i.e. Coregonus clupeoides (in Scotland), Coregonus stigmaticus (in England), and Coregonus pennantii (in Wales) given in a recent review was tested quantitatively using 544 individuals from nine populations. The classification success of the key was very low (27%). It was concluded that there is currently no robust evidence for the recognition of the three putative species. Furthermore, the use of phenotypic characters alone to distinguish putative species in postglacial fish species such as those of the genus Coregonus that show homoplasy in many of these traits is questioned. In the absence of further evidence, it was concluded that a single highly variable species best describes the pattern of phenotypic variation in these U.K. populations. On this basis it is argued that taxonomic subdivision of U.K. European coregonids is inappropriate and that Coregonus lavaretus should prevail as the species name applicable to all populations.