A life history model for the San Francisco Estuary population of the Chinese mitten crab, <Emphasis Type="Italic">Eriocheir sinensis</Emphasis> (Decapoda: Grapsoidea)

First discovered in San Francisco Bay in 1992, the Chinese mitten crab,Eriocheir sinensis, has become established over hundreds of km² of the San Francisco Estuary. Ecological and economic impacts of this invasive species motivated our search for a greater understanding of the crabs life history as an important step in better management and control. Data for this life history model comes from the authors research and scientific literature. Juvenile crabs migrate from the Estuary into fresh water where they develop into adults. Environmental signals may stimulate gonad development that is followed by a downstream migration beginning at the end of summer. Mating occurs after the crabs reach saline water. Embryos are carried until hatching, and the larvae undergo five zoeal stages before settlement. Our model projects rates of development at various temperatures and growth increments, supports a minimum of 2 years in low salinity or freshwater habitat, and predicts that most California mitten crabs are at least 3 years old before becoming sexually mature. Environmental factors strongly influence the timing and duration of the crabs life stages, and are discussed in the context of a gradient of development times for worldwide populations of this important invasive species.     

Isolation and characterization of 12 dinucleotide microsatellites in the European eel, Anguilla anguilla L., and tests of amplification in other species of eels

Twelve polymorphic dinucelotide microsatellites in the freshwater eel Anguilla anguilla L. were isolated and characterized. Genetic diversity was assessed in eels from Lake Constance, Germany. Allele numbers ranged from five to 26 per locus with observed heterozygosities between 0.125 and 0.875. A portion of locus AangCT77 aligns with a transcribed region of the zebrafish gene crystallin beta B2. Cross-species amplification of most markers was possible for nine other Anguilla eel species. The newly developed primer pairs will facilitate population and conservation genetic studies in order to refine the understanding of the subtle population genetic structure typical of eels, and to identify interspecies admixture due to global trade.     

Genetic population structure of the catadromous Perciform: Macquaria novemaculeata (Percichthyidae)

Genetic population structure in the catadromous Australian bass Macquaria novemaculeata was investigated using samples from four locations spanning 600 km along the eastern Australian coastline. Both allozymes and mtDNA control region sequences were examined. Population subdivision estimates based on allozymes revealed low levels of population structuring ( G_st =0·043, P <0·05). However, mtDNA indicated moderate levels of geographic population structure ( G_st =0·146, P <0·0l). Phylogenetic analysis of mtDNA control region sequences (mean sequence divergence 1·9%) indicated little phylogeographic structuring. Results suggested that genotypic variation within each river population, while being affected primarily by genetic drift, was also prevented from more significant divergence by homogenizing levels of gene flow—synonymous with a one-dimensional stepping-stone model of population structure. The catadromous life history of Macquaria novemaculeata was considered to be influential on the pattern of population structure displayed. Results were compared to the few population genetic studies involving catadromous fishes, indicating that catadromy alone is unlikely to be a good predictor of population structure. A more comprehensive suite of biological characteristics than simple life-history traits must be considered fully to allow reliable predictive models of population structure to be formulated.     

Feeding habitat and silvering stage affect lipid content and fatty acid composition of European eel Anguilla anguilla tissues

Lipids, particularly fatty acids (FAs), are major sources of energy and nutrients in aquatic ecosystems and play key roles during vertebrate development. The European eel Anguilla anguilla goes through major biochemical and physiological changes throughout its lifecycle as it inhabits sea- (SW), and/or brackish- (BW) and/or freshwater (FW) habitats. With the ultimate goal being to understand the reasons for eels adopting a certain life history strategy (FW or SW residency vs. ‘habitat shifting’), we explored differences in lipid content and FA composition of muscle, liver and eyes from eels collected across Norwegian SW, BW and FW habitats, and at different lifecycle stages (yellow to silver). FW and SW eels had a higher lipid content overall compared to BW eels, reflecting differences in food availability and life history strategies. SW eels had higher proportions of certain monounsaturated FAs (MUFAs; 18:1n-9, 20:1n-9), and of the essential polyunsaturated FAs 20:5n-3 (eicosapentaenoic acid, EPA) and 22:6n-3 (docosahexaenoic acid) than FW eels, reflecting a marine-based diet. In contrast, the muscle of FW eels had higher proportions of 18:3n-3, 18:2n-6 and 20:4n-6 (arachidonic acid), as is typical of FW organisms. MUFA proportions increased in later stage eels, consistent with the hypothesis that the eels accumulate energy stores prior to migration. In addition, the decrease of EPA with advancing stage may be associated with the critical role that this FA plays in eel sexual development. Lipid and FA information provided further understanding of the habitat use and overall ecology of this critically endangered species.     

Environmental DNA metabarcoding provides enhanced detection of the European eel Anguilla anguilla and fish community structure in pumped river catchments

The European eel Anguilla anguilla (eel hereafter) is critically endangered and has a catadromous life cycle, which means adult eels that live in pumped catchments must pass through pumps during their downstream spawning migration. Policy makers are currently lacking detailed site-by-site eel distribution information to estimate the overall impact of individual pumping stations on eel escapement, and as such lack the data to enable informed prioritisation of pumping station management and targeted mitigation. This study investigated whether environmental DNA (eDNA) metabarcoding can provide increased detection sensitivity for eel and fish community structure in highly regulated pumped catchments, when compared directly to current standard practice fish survey protocols (seine netting/electric fishing). Eels were detected in 14 of 17 sites (82.4%) using eDNA metabarcoding in contrast to 3 of 17 sites (17.6%) using traditional catch methods. In addition, when using eDNA monitoring, species richness was higher in 16 of 17 sites (94.1%), and site occupancy was greater than or equal to traditional methods for 23 of 26 of the fish species detected (88.5%). Although eDNA methods presented significantly higher average species richness and species site occupancy overall, eDNA and catch methods were positively correlated in terms of species richness and site occupancy. It was therefore found that eDNA metabarcoding was a high-sensitivity method for detecting eels in pumped catchments while also increasing the detection of overall fish community structure compared to traditional catch methods. In addition, this study highlights how eDNA monitoring is especially suited to increase the detection of particular species, with traditional methods sufficient for others. This high sensitivity, coupled with the ability to sample multiple sites in a short time frame, suggests that eDNA metabarcoding workflows could be invaluable tools when prioritising pumping station management.     

Morphological variation in the catadromous Australian bass, from seven geographically distinct riverine drainages

The Australian bass Macquaria novemaculeata is a catadromous species restricted to coastal drainages of south-eastern Australia and recently has undergone reductions in population numbers. Morphometric variation among bass from seven distinct drainages was used to test for the presence of stock structuring. Males and females were treated separately. For both male and female bass, significant heterogeneity in morphology among sample locales was revealed by univariate statistics and discriminant function analysis, with morphological variation found to differ in a clinal pattern. The demonstration of population-specific morphological traits among bass from different river drainages may indicate that inter-populational movement is restricted and that each sample locale constitutes a separate stock of bass.     

The Invasion of the Chinese Mitten Crab (<Emphasis Type="Italic">Eriocheir sinensis</Emphasis>) in the United Kingdom and Its Comparison to Continental Europe

Owing to its catadromous lifestyle, the Chinese mitten crab, Eriocheir sinensis, allows comparison between a coastal and an inland biological invasion of the same species. Information about the distribution of this species in the United Kingdom has been collected from sightings made by governmental agencies, The Natural History Museum (London) collection, literature, and from the general public. This information indicated that the range of the species has expanded since the species’ arrival in 1973. The spread has been most marked along the east coast northwards to the river Tyne, on the south coast westwards to the river Teign. The expansion range was quantified and compared using geographic information software, and then compared to recorded spread in Europe. Mitten crabs dispersed along the coast at an average rate of 78 km per year (1976–1999), with a recent sharp increase to 448 km per year (1997–1999). These values are comparable with the historic outbreak in continental Europe where the average rate of dispersion along the Baltic Sea coast (1928–1935) was 416 km per year. Comparable figures for the North Sea coast (1923–1954) were 75 km per year with a peak of 168 km per year in 1927–1937. The upstream spread along rivers in the United Kingdom was 16 km per year in 1973–1998 with a marked increase since 1995 to 49 km per year (1995–1998). These data, in combination with population data published for the river Thames, indicate that the population has been increasing since the early 1990s, causing further range expansion into previously uninvaded river systems. The comparison of the spreading behaviour of the ongoing invasion in the United Kingdom with the historic invasion in northern Europe suggests that E. sinensis in future has the potential to establish itself in all major UK estuaries.     

Polymorphisms of GA/GT Microsatellite Loci from Anguilla japonica

Six novel microsatellite loci, containing (GA)_15\N17 or (GT)_10\N19 perfect tandem repeats, were isolated and characterized for the catadromous eel Anguilla japonica. The allelic size of the 6 loci ranged from 79 to 226 bp in length. All loci were polymorphic with a mean number of 14.7 alleles per locus and a mean heterozygosity of 0.67, suggesting higher polymorphism than that of freshwater and anadromous fishes, but lower than that of marine fishes. Genotype diversity of the 6 loci ranged from 0.22 to 0.61 with a mean value of approximately 0.5. Cross-species amplification showed that 5 of the 6 microsatellite primers proved to be useful in addressing questions of population genetics for all Anguilla species. Received October 11, 2000; accepted December 29, 2000.     

White mullet Mugil curema population structure from Mexico and Brazil revealed by otolith chemistry

The white mullet Mugil curema supports several fisheries in the neotropical region; nevertheless, the population structure is still elusive. The aim of this study was to assess the presence of adult management units and nursery areas from five sampling sites throughout the Gulf of Mexico and northern Brazil using otolith microchemistry. The Li/Ca, Na/Ca, Mn/Ca, Sr/Ca, Ba/Ca and Pb/Ca ratios were measured in otolith core (juvenile stage) and edge (adult stage) (N = 131) using laser ablation–inductively coupled plasma–mass spectrometry. Several ratios were significantly different between sampling sites for core and edge (P < 0.05). For otolith edge, permutational multivariate analysis of variance showed significant differences (P < 0.05) between all sampling sites from Mexico (except between Mecoacán and Tamiahua, P > 0.05) and between Mexico (pooled samples) and Brazil. Quadratic discriminant analyses showed jackknifed classification higher in the edge (66.6% and 99.5% for Mexico and Brazil plus Mexico, respectively) than in the core (46.3% and 76.5% Mexico and Brazil plus Mexico, respectively). The two cluster analyses based on the core microchemistry (Mexico and Brazil plus Mexico) produced three main clusters, which did not coincide with catchment areas. These results support the segregation of the M. curema adult life stages among several sampling sites from Mexico and Brazil; moreover, core analysis suggested that the nursery areas did not correspond to the capture sites or adults stocks.     

A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla)

The European eel (Anguilla anguilla) is a catadromous fish that spawns in the Sargasso Sea. As larvae, eels cross the Atlantic Ocean and reach the continental slope of Europe, where they metamorphose into post‐larval glass eels. These reach the continent, where some enter fresh water, some remain in marine waters, and others move between fresh and marine waters. After 5–25 years, as adult silver eels, they migrate back from fresh water to the Sargasso Sea to spawn and die. The glass eel stage is a critical step during which the eels cross the continental shelf and recruit to estuaries, where they facultatively transition to fresh water. Extensive research has been conducted to understand the behavioural mechanisms and environmental cues that aid and guide glass eels' migration. Glass eels follow odours and salinity gradients, they avoid light, and they change orientation and depth according to the tides. Recent work revealed that European glass eels also use Earth's magnetic field and lunar cues to orient. However, while we understand many aspects of their orientation behaviour, a unifying theory describing how glass eels migrate from the continental slope to fresh water is lacking. The goal of this review is to develop a comprehensive hypothesis on the migration of European glass eels, integrating previous knowledge on their orientation behaviour with recent findings on magnetic and celestial orientation. This review follows the journey of a hypothetical glass eel, describing the nature and the role of orientation cues involved at each step. I propose that, although glass eels have the sensory capacity to use multiple cues at any given time, their migration is based on a hierarchical succession of orientation mechanisms dictated by the physical properties of the environments that they occupy: (i) lunar and magnetic cues in pelagic water; (ii) chemical and magnetic cues in coastal areas; and (iii) odours, salinity, water current and magnetic cues in estuaries.