Taxonomic studies on the 0-group eel larvae (Anguilla sp.) caught in the Sargasso Sea in 1979

More than 2000 0-group larvae (5.0–30.0 mm long) of both AtlanticAnguilla species were examined taxonomically. With regard to the total number of myomeres inA. rostrata andA. anguilla, an average difference between the two species of 6 to 8 myomeres was found in all size groups. 31 specimens (i.e. 1.76 %) exhibited 111 myomeres. The position of the last vertical blood vessel and the number of preanal myomeres turned out to be statistically different in both species; however, these differences cannot be used for species identification. The regression line for the position of the last vertical blood vessel according to the total number of myomeres indicates that individuals with a total of 111 myomeres may beA. anguilla. Measurements of total lengths revealed highly significant differences between the larvae of both eel species. It can be concluded that, on the average,A. rostrata ( ) hatched about two weeks beforeA anguilla ( ). On the other hand, results obtained from the biggestA. rostrata larvae (29.5 mm) andA. anguilla larvae (23.5 mm) make a spawning ofA. rostrata likely two months beforeA. anguilla, when findings from hatching experiments withA. japonica are taken as a basis. There is no difference in the relative length of the intestine in eitherAnguilla species.     

On the geographic distribution and migration of I- and II-group eel larvae as studied during the 1979 Sargasso Sea Expedition

During the 1979 Sargasso Sea Expedition, 423 larvae ofAnguilla anguilla and 5 larvae ofA. rostrata were caught on three Atlantic transects and two cruises in the Sargasso Sea. Results of the identification of the larvae by myomere counts, and limits of the occurrence of I- and II-group larvae are presented. Four standard fishing depths are compared. A range shallower than 25 m was found to be the optimal fishing depth by night for both larval length groups. The geographic distribution of length group I was observed in central and eastern North Atlantic. Available data indicate a migration of these larvae in a north easterly direction. Length measurements of the II-group larvae taken from catches on the European continental slope during the same expedition support this assumption.     

Identification of aquarium-raised muraenid leptocephali, <Emphasis Type="Italic">Gymnothorax minor</Emphasis>

Leptocephalus larvae of Gymnothorax minor collected from eastern Kyushu were identified by observation of metamorphosis procedure. The leptocephali were characterized by 135–142 total myomeres (31–42 predorsal, 87–95 preanal) and the last vertical blood vessel between myomeres 77–85. Diagnostic pigments were apparent on the head, the somatic (below intestine) and splanchnic (along pronephric ducts) regions, before the dorsal fin origin, ventrally on the spinal cord, and along the dorsal and anal fin bases. Such pigments were suitable for species identification of metamorphosing larvae because of their persistence during that process.     

Introduced American eels Anguilla rostrata in European waters: life‐history traits in a non‐native environment

This study investigated growth, condition and development of American eels Anguilla rostrata that were introduced into a European river to estimate their competitive potential in a non‐native habitat. Results demonstrate that A. rostrata develops normally in European waters and successfully competes with the native European eel Anguilla anguilla. In addition, A. rostrata appears to be more susceptible to the Asian swimbladder nematode Anguillicola crassus than A. anguilla and could support the further propagation of this parasite. Detected differences in fat content and gonad mass between Anguilla species are assumed to reflect species‐specific adaptations to spawning migration distances. This study indicates that A. rostrata is a potential competitor for the native fauna in European fresh waters and suggests strict import regulations to prevent additional pressure on A. anguilla and a potential further deterioration of its stock situation.     

Distribution and ecological aspects of leptocephali collected 1979–1994 in North- and Central Atlantic. I. Congridae

This is the first report on the leptocephalus catches made during the last 15 years in North-and Central Atlantic during the course of 19 cruises of five different ships from Germany and one from Poland. This report comprises identification, geographical occurrence and abundance and in some cases depth preference, migratory routes and spawning area of Congridae in the North Atlantic. The largest part of this collection consists of 876Conger conger larvae. The likelihood that this species spawns in the Mediterranean is again confirmed. Decreasing density as well as increasing size from Gibraltar west-and north westward showed migration to be occurring in that direction. Age was determined by counting “daily rings” on the otoliths of up to 120-mm long larvae. It is suggested to be more than 300 days and is calculated to be 1 1/4 years for the largest larvae (TL=140–160 mm). Some 126 specimens of three otherConger species, i.e.C. oceanicus, C. triporiceps, C. esculentus, were identified in the collection. The major part originated from the western North Atlantic. The most numerous larvae, belonging to another genus of Congridae, wereAriosoma balearicum (n=265). They showed, also in consideration of other studies, quite a wide range in number of myomeres. Their systematic status is therefore uncertain, as is also the status of those known from the NW Indian Ocean and the NW Pacific. One larva, probably ofAriosoma selenops, of un-usually large size (TL=467 mm) and captured in the Iberian Basin is described and compared with specimens known from the literature. Two larvae of unknown identity were captured off NW Africa; they resembledA. balearicum but had too high a number of myomeres. The identity of most of theGnathophis larvae caught in the East Atlantic is uncertain. Leptocephali ofParaconger notialis in the East Atlantic, up to the area north of New Guinea, exhibited a higher number of myomeres than those known from the West Atlantic. Larvae ofXenomystax congroides (n-29), belonging to a population with a relatively high number of myomeres, were captured in the Sargasso Sea.     

Vertical distributions, diel and ontogenetic vertical migrations and net avoidance of leptocephali of Anguilla and other common species in the Sargasso Sea

Day- and night-time vertical distributions and their ontogeneticchanges in Anguilla leptocephali and other common species ofleptocephali were determined and compared during five cruisesin the Sargasso Sea using an opening - closing 2-m ring netto sample discrete depth strata between 0 m and 350 m deep.No difference in vertical distribution was ever found betweenAnguilla rostrata (American eel) and A. anguilla (European eel).Anguilla leptocephali <5 mm long did not exhibit a diel verticalmigration, as they were distributed between 50 m and 300 m bothby day and by night. The vertical distribution of these smalllepto-cephali is probably roughly representative of the depthdistribution of adult spawning. Anguilla     

A pre-leptocephalus larva ofConger myriaster (family Congridae) collected from Suruga Bay,Central Japan

One specimen of leptocephalus larva, measuring 16.0 mm in total length, was collected from the northern part of Suruga Bay (34°56′N, 138°39′E) by the T/V Bosei Maru II of Tokai University on November 7, 1982. This larva is identified asConger myriaster because of having melanophores under the eye, simple gut with melanophores, 142 total number of myomeres and the position of the last vertical blood vessel at the 52nd myomere. Judging from the body length, appearances of teeth and fins, and disposition of the anus, the present larva belongs to the preleptocephalus stage. The presence of this early larva in this area suggests that one of the spawning grounds ofC. myriaster exists in or near Suruga Bay in autumn.     

A century of research on the larval distributions of the Atlantic eels: a re‐examination of the data

The spawning areas of the Atlantic freshwater eels were discovered about a century ago by the Danish scientist Johannes Schmidt who after years of searching found newly hatched larvae of the European eel, Anguilla anguilla, and the American eel, Anguilla rostrata, in the southern Sargasso Sea. The discovery showed that anguillid eels migrate thousands of kilometers to offshore spawning areas for reproduction, and that their larvae, called leptocephali, are transported equally long distances by ocean currents to their continental recruitment areas. The spawning sites were found to be related to oceanographic conditions several decades later by German and American surveys from 1979 to 1989 and by a Danish survey in 2007 and a German survey in 2011. All these later surveys showed that spawning occurred within a restricted latitudinal range, between temperature fronts within the Subtropical Convergence Zone of the Sargasso Sea. New data and re‐examinations of Schmidt's data confirmed his original conclusions about the two species having some overlap in spawning areas. Although there have been additional collections of leptocephali in various parts of the North Atlantic, and both otolith research and transport modelling studies have subsequently been carried out, there is still a range of unresolved questions about the routes of larval transport and durations of migration. This paper reviews the history and basic findings of surveys for anguillid leptocephali in the North Atlantic and analyses a new comprehensive database that includes 22612 A. anguilla and 9634 A. rostrata leptocephali, which provides a detailed view of the spatial and temporal distributions and size of the larvae across the Atlantic basin and in the Mediterranean Sea. The differences in distributions, maximum sizes, and growth rates of the two species of larvae are likely linked to the contrasting migration distances to their recruitment areas on each side of the basin. Anguilla rostrata leptocephali originate from a more western spawning area, grow faster, and metamorphose at smaller sizes of <70 mm than the larvae of A. anguilla, which mostly are spawned further east and can reach sizes of almost 90 mm. The larvae of A. rostrata spread west and northwest from the spawning area as they grow larger, with some being present in the western Caribbean and eastern Gulf of Mexico. Larvae of A. anguilla appear to be able to reach Europe by entering the Gulf Stream system or by being entrained into frontal countercurrents that transport them directly northeastward. The larval duration of A. anguilla is suggested to be quite variable, but gaps in sampling effort prevent firm conclusions. Although knowledge about larval behaviour is lacking, some influences of directional swimming are implicated by the temporal distributions of the largest larvae. Ocean–atmosphere changes have been hypothesized to affect the survival of the larvae and cause reduced recruitment, so even after about a century following the discovery of their spawning areas, mysteries still remain about the marine life histories of the Atlantic eels.     

A Quick Method for Species Identification of Japanese Eel (<Emphasis Type="Italic">Anguilla japonica</Emphasis>) Using Real-Time PCR: An Onboard Application for Use During Sampling Surveys

To compensate for the limited number of morphological characteristics of fish eggs and larvae, we established a convenient and robust method of species identification for eggs of the Japanese eel (Anguilla japonica) using a real-time polymerase chain reaction (PCR) that can be performed onboard research ships at sea. A total of about 1.2 kbp of the mitochondrial 16S ribosomal RNA gene sequences from all species of Anguilla and 3 other anguilliform species were compared to design specific primer pairs and a probe for A. japonica. This real-time PCR amplification was conducted for a total of 44 specimens including A. japonica, A. marmorata, A. bicolor pacifica, and 6 other anguilliform species. Immediate PCR amplification was only observed in A. japonica. We then tested this method under onboard conditions and obtained the same result as had been produced in the laboratory. These results suggest that real-time PCR can be a powerful tool for detecting Japanese eel eggs and newly hatched larvae immediately after onboard sampling during research cruises and will allow targeted sampling efforts to occur rapidly in response to any positive onboard identification of the eggs and larvae of this species.     

Midwater trawl catches of adolescent and adult anguilliform fishes during the Sargasso Sea Eel Expedition 1979

During the research program on the biology and migration ofAnguilla spp. carried out with F.R.V. Anton Dohrn in 1979, approximately 1300 adolescent and adult anguilliform individuals were caught covering 8 families, 10 genera and 12 species. Observations on each of these species, including horizontal and vertical distributional patterns, are dealt with herein. The appearance of various species in hauls and the absence of adultAnguilla spp. in the catches obtained are discussed.     

Spatial distribution of 0-group eel larvae (Anguilla sp.) in the Sargasso Sea

Both Atlantic eel species (Anguilla anguilla andA. rostrata) were collected in the Sargasso Sea during the 1979 cruise of the F. R. V. Anton Dohrn and R. V. Friedrich Heincke. A total of 3,097 0-group larvae were caught during 80 hauls using the Isaacs Kidd Midwater Trawl (55 hauls) and the 9-fold opening and closing net MOCNESS (25 hauls). 11 hauls of the MOCNESS indicated that the larvae showed a preference for the 150–175 m water depth during daytime and for the 50–75 m depth during night. The northern distribution limit seems to coincide with the 18°C-water at the sea surface. The Antilla current characterised by higher temperatures and salinities could have been the southern distribution limit. The north-easternmost occurrence ofA. anguilla was noted at 50°W; the eastern distribution limit of this species could be farther east. The westernmost station at 69°W was positive for both species.A. anguilla, therefore, is very likely to occur beyond this area. The easternmost occurrence ofA. rostrata was noted at 52°W though sporadic and with increasing abundance towards the west. At the western end of the network of stations the highest concentration of larvae from both species was recorded. Oceanographic investigations reveal that the distribution of the smaller larvae (size-groups <7 mm and <10 mm) almost coincides with the assumed spawning area.     

TOTAL CELL NUMBER AND NUMBER OF THE PRIMARY MESENCHYME CELLS IN WHOLE, 1/2 AND 1/4 LARVAE OF CLYPEASTER JAPONICUS*

Total cell number and number of the primary mesenchyme cells of 1/2 and 1/4 larvae were counted at several developmental stages after hatching in comparison with those of a whole larva, using Clypeaster japonicus as material. To obtain partial larvae, blastomeres were isolated at the 2- or 4-cell stage in Ca-free sea water and cultured in natural sea water at around 23°C. Isolated blastomeres cleaved as in situ, namely, as a part of an embryo. Although each partial embryo tended to spread into a plate, it acquired spherical shape prior to hatching of control whole embryo and developed normally in terms of both developmental rate and morphogenesis. Total cell number of a whole larva was about 620 just after hatching and increased almost linearly until i t reached 1850 at the pluteus stage. A half and quarter larvae contained roughly 1/2 and 1/4, respectively, of the number of cells of whole larva through all stages counted. Numbers of the primary mesenchyme cells in the partial larvae, however, tended to be slightly larger than a half or a fourth of that in whole larva. In whole larva, 35, 50, 56 and 58 was counted at the mesenchyme blastula, early gastrula, late gastrula and pluteus stage, respectively.     

Early development of the Neotropical fish known as long sardine Triportheus auritus (Valenciennes 1850) (Characiformes,Triportheidae)

Larvae and juveniles of long sardine, Triportheus auritus, from the lower Amazon river was described, evaluating ontogenetic changes in their external mor­phology, pigmentation, fin development, morphometry, and meristics. A total of 93 individuals, 83 larvae and 10 juveniles were analyzed, they were captured monthly between 2014 and 2019 in the Amazon river channel and in macrophytes aquatic stands in the alluvial plains located in the lower Amazon River. From each specimen, morphometric and meristic data were measured and then the growth pattern between morphometric variables was analyzed. The larvae have an elongated body in a fusiform shape, superior mouth, simple nostril, pigmented spherical eyes and long intestine, surpassing the median region of the body. Initial pigmentation is scarce, but intensifies through development forming a pattern composed of three longitudinal bands concentrated in the ventral, cephalo-dorsal and lateral line regions. There are also pigments in the mandible, surrounding the mouth, under the swim bladder, intestine and fins. The sequence of complete fin formation is: caudal, anal, dorsal, pectoral and pelvic. The total number of myomeres ranged from 45 to 48 (25–29 preanal and 17–22 postanal). Morphometric relationships indicated differential growth for measurable morphometric parameters, with abrupt growth of snout length, head length and body height in the transition from flexion to postflexion stages. The pre-dorsal distance showed a decrease in the growth rate at the threshold from the larval to the juvenile period. The pre-pectoral and pre-anal distances showed negative allometric growth. In conclusion, the combination of body shape pigmentation pattern and, the formation sequence of fins allow the identification of the genus and coupled with the number of myomeres, morphometric relationships, and ray numbers of the anal fin ensure the differentiation of T. auritus from the other congeneric species. The metamorphosis occurred mainly at the end of the larval period and it is related to changes in the physiological and ecomorphological characteristics of the species.     

Mucosal galectin genes in all freshwater eels of the genus Anguilla

In this study, we determined the genomic DNA sequences of the mucosal galectin-encoding genes from all 19 species and subspecies of the genus Anguilla. The nucleotide sequences of the galectin genes were c. 2.3–2.5 kb long and the organisation of their four exons and three introns was conserved in all species. An unusual sequence was found in the fourth exon of Anguilla reinhardtii, resulting in a unique deduced amino-acid sequence at the C-terminus. All six amino-acid residues important for β-galactoside binding were conserved in three species, while one residue (R⁷³) was substituted to K⁷³ in the other 16 species–subspecies, including Anguilla marmorata. However, this substitution did not appear to affect the sugar-binding ability of galectins because the galectin of A. marmorata was previously shown to bind to lactose. We also discuss the molecular evolution of galectins among Anguilla spp. and the homologues previously identified in Conger myriaster.     

Feeding ecology of a vertebrate assemblage inhabiting a stream of NW Spain (Riobo; Ulla basin)

The food resource use of a stream in NW Spain by fish (Salmo trutta L. and Anguilla anguilla L.), birds (Cinclus cinclus L. and Motacilla cinerea L.) and mammals (Galemys pyrenaicus G. and Neomys anomalus C.) was studied. Data on seasonal diets and stream benthos prey were used to determine prey selection patterns.Caddisfly larvae are the main resource for Cinclus and Galemys, but these predators also consumed other benthic prey. Salmo fed on a wide range of benthic invertebrates, emergent pupae and terrestrial prey, whereas Anguilla consumed primarily benthic invertebrates, especially Lumbricids. Neomys fed mainly on terrestrial prey (Gasteropods and Lumbricids), but also consumed aquatic prey. Motacilla captured aquatic insects both in larval and aerial stages, as well as terrestrial prey.Both prey availability and selection led to seasonal differences in the use of food resources. All species showed a marked prey selection of aquatic taxa. Prey size plays an important role in this selection, most species consuming the largest of available prey sizes. In spite of the fact that all species feed upon freshwater invertebrates, substantial resource partitioning was observed in all seasons. This partitioning may be attributable to morpholological and physiological differences. Nevertheless, Anguilla and Galemys, two quite different animals, did feed on the same prey much of the time.     

Isolation and characterization of two different 5S rDNA in Anguilla anguilla and in Anguilla rostrata: possible markers of evolutionary divergence

We cloned and sequenced the HaeIII 350‐bp 5S ribosomal DNA (rDNA) band of Anguilla rostrata and designed specific primers from this sequence. Polymerase chain reaction performed with these primers is able to distinguish DNA samples obtained from European (Anguilla anguilla) and American (Anguilla rostrata) eels. Two amplicons of 1200 bp and 600 bp were obtained, respectively, from A. rostrata and A. anguilla, and the whole 5S rDNA repeated unit from these eels was cloned and sequenced. Southern blot experiments, using four different restriction enzymes and the 5S nontranscribed spacers regions as probe, are able to point out specific diversity in these eels.     

Leptocephalus larvae of two moray eels (Anguilliformes; Muraenidae), Gymnothorax sagmacephalus and Gymnothorax albimarginatus,identified from morphometric and genetic evidence

Two forms of muraenid leptocephali, collected from the western Pacific Ocean, were identified as Gymnothorax sagmacephalus Böhlke 1997 and Gymnothorax albimarginatus (Temminck and Schlegel 1846) on the basis of morphometric and genetic analyses. The leptocephali of each species were characterized, respectively, by counts of 172–175 and 186–191 myomeres, 43–44 and 47 predorsal myomeres, 109–113 and 127–134 preanal myomeres, and 100–104 and 118–119 last vertical blood vessel myomeres. Gymnothorax sagmacephalus leptocephali had minute melanophores over much of the head and body, closely resembling the condition in Gymnothorax minor (Temminck and Schlegel 1846), whereas those of G. albimarginatus not only had minute melanophores over much of the head and body, but also a pair of melanophore groups on the posteroventral and posterodorsal aspects of the head. Such groups are here considered to represent highly specific characters. Although a previous opinion postulated that G. sagmacephalus is a juvenile of G. albimarginatus, and the adult morphologies of the two species have a lot in common, they clearly differ in both leptocephalus morphology and genetic sequence. Therefore, G. sagmacephalus was concluded as being a valid species.     

Population demographics of American eels Anguilla rostrata in two Arkansas,U.S.A., catchments that drain into the Gulf of Mexico

The goal of this study was to compare American eel Anguilla rostrata life history in two inland river systems in Arkansas, U.S.A., that ultimately discharge into the Gulf of Mexico via the Mississippi River and the Red‐Atchafalaya catchments. From 21 June 2011 to 24 April 2014, 238 yellow‐phase A. rostrata were captured in the middle Ouachita River and tributaries using boat electrofishing and 39 in the lower White River using multiple sampling gears. Most of them were caught downstream of dams in both basins (61%). Medium‐sized A. rostrata ranging from 225 to 350 mm total length (L_T) were the most abundant size group in the Ouachita River basin, but they were absent from the White River. Mean L_T at age 4 years (i.e. youngest shared age) was 150 mm greater for the White River than the Ouachita River basin. Anguilla rostrata appeared to have a greater initial L_T (i.e. minimum size upon arrival) in the White River that allowed them to reach a gonado‐somatic index (I_G) of 1·5 up to 4 years earlier, and downstream migration appeared to occur 5 years earlier at 100 mm greater L_T; these differences may be related to increased river fragmentation by dams in the Ouachita River basin. Growth and maturation of A. rostrata in this study were more similar to southern populations along the Atlantic coast than other inland populations. Adult swimbladder nematodes Anguillicoloides crassus were not present in any of the 214 swimbladders inspected. Gulf of Mexico catchments may be valuable production areas for A. rostrata and data from these systems should be considered as range‐wide protection and management plans are being developed.     

Description and ecology of larvae and juveniles of three native cypriniforms of coastal southern California

Larval series of the Santa Ana sucker, Catostomus santaanae (Federally Threatened), arroyo chub, Gila orcutti (California Species of Special Concern), and Santa Ana speckled dace, Rhinichthys osculus (California Species of Special Concern) are described from wild-caught specimens from the Los Angeles and Santa Ana river drainages. Santa Ana sucker larvae are elongate, having 41–46 myomeres and a distinctive paired-triangle patch of melanophores over the midbrain. Melanophores present on the snout, dorsal body, lateral midline, dorsal gut, postanal ventral body, and caudal fin. Preanal length 74–79% in body length (BL), typical of catostomids. Arroyo chub larvae relatively deep-bodied, 36–39 myomeres, and a heart-shaped patch of melanophores over the midbrain with a line of melanophores trailing posteriorly. Heavy pigment present on the snout, lower jaw, dorsal body, lateral midline, gill arches, dorsal gut, postanal ventral body, and caudal fin; short preanal length of 65–72% BL, typical of native North American cyprinids. Santa Ana speckled dace are similar to arroyo chub except for having less pigment on the ventral gut, large distinct melanophores on the ventrolateral caudal peduncle, a wedge-shaped patch of midbrain melanophores with no distinct line trailing posteriorly, and lateral midline melanophores that do not extend anteriorly. These three species often occur together and with nonnative cyprinids. Characters distinguishing them from other local larvae, including southern fathead minnow, Pimephales promelas confertus and red shiner, Cyprinella lutrensis, are discussed with their habitat preferences.