Metazoan Parasite Fauna Research Articles

Metazoan parasite communities of rock cod Eleginops maclovinus along southern Chilean coast and their use as biological tags at a local spatial scale

The composition of the metazoan parasite fauna of the rock cod Eleginops maclovinus from three locations in southern Chile was compared to assess the local spatial variation of the community component of their parasitic faunas. A total of 13 108 metazoan parasites (5267 endoparasites and 7841 ectoparasites) belonging to 34 taxa were collected from 268 specimens of E. maclovinus between October 2008 and March 2009. The populations and community quantitative descriptors were estimated. Altogether, 97.4% of the fish were infected with at least one parasite taxon. The most prevalent species were Hypoechinorhynchus magellanicus (Acanthocephala), Caligus rogercresseyi, Lepeophtheirus mugiloides, Clavella adunca (Copepoda) and Similascarophis sp. (Nematoda). Five species are new records for this host: Argulus araucanus, Hirudinea gen. sp1., Hirudinea gen. sp2., Benedenia sp. and Camallanidae gen. sp. A linear discriminant analysis (LDA) showed that the metazoan parasite fauna of E. maclovinus varied qualitatively and quantitatively among three locations, with 89.7% of fish being correctly assigned to their respective locations. This suggested that parasites could be a reliable tool to discriminate individual fish from geographically close locations. There was a weak relationship between the parasite fauna and fish size and there were no accumulations of parasites in the host over time, which could be associated with ontogenetic changes of diet associated with habitat use.

Parasite communities and feeding ecology of the European sprat (Sprattus sprattus L.) over its range of distribution

The metazoan parasite fauna and feeding ecology of 165 Sprattus sprattus (L., 1758) was studied from different geographic regions (Baltic Sea, North Sea, English Channel, Bay of Biscay, Mediterranean Sea). A total of 13 metazoan parasite species were identified including six Digenea, one Monogenea, two Cestoda, two Nematoda and two Crustacea. Didymozoidae indet., Lecithocladium excisum and Bomolochidae indet. represent new host records. The parasite species richness differed according to regions and ranged between 3 and 10. The most species-rich parasite fauna was recorded for sprats from the Bay of Biscay (North Atlantic), and the fishes from the Baltic Sea contained the lowest number of parasite species. More closely connected geographical regions, the North Sea, English Channel and Bay of Biscay, showed more similar parasite component communities compared with more distant regions. From the examined stomachs of S. sprattus, a total of 11 different prey items were identified, including Mollusca, Annelida, Crustacea and Tunicata. The highest number of prey organisms belonged to the crustaceans. The variety of prey items in the stomach was reflected by the parasite community differences and parasite species richness from the different regions. The feeding ecology of the fish at the sampled localities was responsible for the observed parasite composition and, secondarily, the zoogeographical distribution of the parasites, questioning the use of the recorded sprat parasites as biological indicators for environmental conditions and change.

Metazoan parasites of the Indian mackerel, Rastrelliger kanagurta (Scombridae) of Visakhapatnam coast, Bay of Bengal

The metazoan parasite fauna of the Indian mackerel Rastrelliger kanagurta of Visakhapatnam coast, Bay of Bengal comprised 15 species including three species of Monogenea, seven species of Digenea and five species of Crustacea. Digeneans were the dominant members in the parasite spectrum while infections with ectoparasitic monogeneans and crustaceans were rare. The digeneans Opechona bacillaris and Lecithocladium angustiovum which occurred with high prevalence and mean intensity are the typical parasites of the mackerel. The parasitic fauna in general is found to be a reflection of the planktonivorous diet of the host. Except for two species of digeneans, Lecithocladium angustiovum and Aponurus laguncula, all the remaining species of metazoans showed narrow specificity to R. kanagurta, indicating a high degree of host specialization. The parasitological data may prove useful for differentiating stocks of R. kanagurta.

Metazoan parasites of swordfish, Xiphias gladius (Pisces: Xiphiidae) from the Atlantic Ocean: implications for host stock identification

Abstract Garcia, A., Mattiucci, S., Damiano, S., Santos, M. N., and Nascetti, G. 2011. Metazoan parasites of swordfish, Xiphias gladius (Pisces: Xiphiidae) from the Atlantic Ocean: implications for host stock identification. – ICES Journal of Marine Science, 68: 175–182. The metazoan parasite fauna composition of the swordfish Xiphias gladius is reported from 157 fish caught in four areas of the Atlantic Ocean: northwest (NW), central north (CN), eastern tropical (ET), and central south (CS). Samples were collected in 2003, 2006, and 2007. In all, 18 parasite taxa were identified by morphological and genetic methods. These data, together with earlier analyses from the tropical equatorial (TEQ) Atlantic revealed the association of some parasite taxa with distinct areas based on their presence/absence and infection levels. Hysterothylacium corrugatum (s.l.) and Rhadinorhynchus pristis were mostly associated with swordfish caught in the NW, and Anisakis paggiae and H. incurvum with fish caught in the southern areas (ET, TEQ, and CS). Anisakis simplex (s.s.) could be used as a marker for all northern samples. Parasitological data on Atlantic swordfish seem to support the existence of at least two discrete stocks in the Atlantic, supporting swordfish genetic stock structure.

Parasite structure of the Ocean Whitefish Caulolatilus princeps from Baja California, México (East Pacific)

The metazoan parasite fauna of Caulolatilus princeps from northern Baja California, Mexico is quantitatively described for the first time. Further, the ecological aspects of prevalence, abundance, and intensity of infection are examined through an annual cycle. Six parasite species were recorded; 2 ectoparasites (1 monogenean and 1 copepod) and 4 endoparasites (2 digeneans and 2 nematodes). The digeneans Choanodera caulolatili and Bianium plicitum, the nematodes Anisakis sp. and Hysterothylacium sp., and the copepod Hatschekia sp. set new geographical and host records. The highest values of prevalence and abundance were in Anisakis sp. (prevalence = 93.3%, abundance = 12.4 ± 4.7 ind/host) and in Hysterothylacium sp. (prevalence = 86.6%, abundance = 16.5 ± 3.4 ind/host). The mean intensity of infection showed maximum values in summer (August = 14.2) and minimums in winter (February = 4.2). The mean intensity was higher in Hatschekia sp. (20.3 ± 7.8) followed by Hysterothylacium sp. (18.6 ± 1.4) and Anisakis sp. (12.9 ± 2.2). Larval stages of Anisakis and Hysterothylacium were particularly important due to their high abundance and prevalence, because they represent a human health risk (anisakiasis). In addition, the relationships between the metazoan parasites of C. princeps and host size and weight, fish condition and water temperature (bottom) are discussed.

Boreogadus saida (Lepechin) (Gadidae): a review of its metazoan parasite fauna from Greenland, eastern Canada, Alaska and the Russian Arctic

The metazoan parasite fauna of 50 specimens of Boreogadus saida (Lepechin) (Gadidae) from eastern Greenland is very similar to those of previous studies of the parasite fauna of B. saida from Greenland, eastern Canada, Alaska and the Russian Arctic. The digeneans Hemiurus levinseni, Derogenes varicus and Lecithaster gibbosus and cestode larvae were found at most stations. Single specimens of the nematode larvae Anisakis simplex were found at four stations. A comparison of the distribution of the larvae of Contracaecum sp. and Hysterothylacium sp. is difficult due to a possible confusion of the two genera. Most of the metazoan parasites of B. saida are generalist species found in several fish families. Boreogadus saida acquires most of its endoparasites by eating pelagic crustaceans, mainly copepods and amphipods. It plays an important role in the arctic ecosystem and its parasites are transferred to predatory fish, birds and mammals through the food web.

Metazoan fish parasites of Segara Anakan Lagoon, Indonesia, and their potential use as biological indicators

The present study reports metazoan fish parasites from Segara Anakan, a brackish water lagoon located at the southern Java coast, Indonesia. Seven economically important marine fish species (Mugil cephalus, Siganus javus, Scatophagus argus, Caranx sexfasciatus, Lutjanus johnii, Eleutheronematetradactylum and Johnius coitor) were examined at two different sampling sites within the lagoon for the occurrence of metazoan parasites. A diverse parasite fauna was found, consisting of 43 species/taxa. Ectoparasites (31) were more abundant than endoparasites (12). The fish species J. coitor, M. cephalus and S. argus harboured the most diverse metazoan parasite fauna with 11, 13, and 16 different parasite species, respectively. Prevalence and intensity of infection for each parasite species/taxon is given, including short descriptions for rapid diagnosis. For the first time, we discuss the utilisation of the sampled fish parasites as biological indicator organisms for fish and environmental health within this tropical mangrove ecosystem. Ecto- versus endoparasite ratio and endoparasite diversity are suitable tools to describe the environmental health status at a tropical brackish water locality, and might be applied also for other tropical and possibly non-tropical marine ecosystems.

The metazoan parasites of swordfish from Atlantic tropical–equatorial waters

The metazoan parasite fauna composition of the swordfish Xiphias gladius is reported from 145 (72 females and 73 males) specimens caught in Atlantic tropical–equatorial waters during June to July 2006 and August 2007. Seventeen metazoan taxa (two monogeneans, two digeneans, one acanthocephalan, four cestodes, seven nematodes and one copepod) were identified by morphological and genetic methodologies. Among those identified, Anisakis brevispiculata, Anisakis paggiae and the acanthocephalan Bolbosoma vasculosum are new host records. Dominant species were: the anisakid nematode larvae Anisakis physeteris, Anisakis brevispiculata, Hysterothylacium incurvum and Hysterothylacium corrugatum (s.l.), the adult cestode Fistulicola plicatus, the plerocercoid Hepatoxylon squali and the monogeneans Tristoma integrum and Tristoma coccineum, which showed high prevalences and intensities. The relationship between metazoan parasites and host size and sex was investigated for the first time. No statistically significant correlation was found between host sex and parasite burden. Infection levels with T. integrum and T. coccineum were observed to be significantly negatively correlated to host size. Infection levels by endoparasites observed in X.gladius from Atlantic tropical‐equatorial area may be related to host feeding behaviour and the availability of the parasites’ intermediate and final hosts in the area studied. Anisakids of the genus Anisakis, were identified genetically to species level, and were considered to be promising biological tags for the characterization of X.gladius stocks.

Parasite fauna of seabass (Lates calcarifer) under mariculture conditions in Lampung Bay, Indonesia

Summary Fish parasites have been repeatedly reported to be a major threat to the developing industry of finfish mariculture in Indonesia, due to severe parasite and disease outbreaks. The aim of this study was to identify the metazoan parasite fauna and trichodinid ciliates that infect Lates calcarifer in a representative mariculture farm in Indonesia. Examined were 105 L. calcarifer (seabass) for the metazoan parasite fauna and trichodinid ciliates. Thirty-five specimens each from the net cages of the National Sea Farming Development Centre (Balai Budidaya Laut, BBL) in Lampung Bay, South Sumatra, Indonesia were investigated in three consecutive seasons (two dry and one rainy season from 2002 to 2003). Nineteen parasite species were identified; all fish specimens were infected with two to 10 parasite species, demonstrating a species-rich parasite fauna. Protozoans (1 species), myxozoans (1), digeneans (3), monogeneans (5), cestodes (3), nematodes (5) and acanthocephalans (1) were found, including 11 new host records in cultured L. calcarifer from Indonesia. Larval and adult parasite stages were isolated, demonstrating that this fish species, although kept inside the net cages, still functions as an intermediate and final host for marine fish parasites. During all seasons, the six detected monoxenous (single host life cycle) parasite species showed a higher prevalence than the 13 heteroxenous (multiple hosts) species. Most abundant were the fish pathogenic monogeneans Pseudorhabdosynochus epinepheli, Pseudorhabdosynochus lantauensis, Benedenia epinepheli and Neobenedenia melleni with a high prevalence. Most heteroxenous parasites (Digenea, Cestoda, Nematoda and Acanthocephala) occurred with a low prevalence below 26%, caused by the specific culture conditions. Diversity of the heteroxenous parasites was higher in the dry seasons than in the rainy season. Though some seasonality could be observed for the fish pathogenic monogeneans, severe disease outbreaks of these ectoparasites cannot be excluded in either the dry or rainy season.

Biogeographic Patterns of Metazoan Parasites of the Bigeye Flounder, Hippoglossina macrops, in the Southeastern Pacific Coast

We examined the metazoan parasite fauna of the bigeye flounder, Hippoglossina macrops, in a latitudinal gradient of the southeastern Pacific (ca. 25 degrees S-33 degrees S) to assess whether their parasites show latitudinal and/or biogeographical patterns, and to determine if these patterns agree well with those observed for free-living organisms. In total, 331 fish from 6 localities were analyzed. Eighteen parasite species, including 5 ecto- and 13 endoparasites, were found in the bigeye flounder. Neoheterobothrium chilensis, Floridosentis sp., and Corynosoma australe were the most prevalent parasite species. Ecto- and endoparasites of H. macrops showed different latitudinal patterns. Only endoparasite species show a distributional pattern concordant with biogeographic areas recognized for free-living organisms; this finding was supported by cluster analyses showing 2 major divisions. The first joined the most northern localities (25 degrees S-26 degrees S), and the second consisted of 2 subgroups: (1) 28 degrees S-30 degrees S, and (2) 31 degrees S-32 degrees S. A multivariate analysis of infracommunities showed significant differences in the parasitic load of H. macrops from different latitudes. The latitudinal differences in the prevalence and abundance of the parasites might be explained by differing oceanographic conditions in water masses. These physical conditions could affect the dispersal ability of the infective stages of ecto-parasites, and produce zoogeographical breaks that could affect the distribution of intermediate hosts (invertebrate and vertebrate prey), subsequently affecting the transmission of endoparasites.

Metazoan Parasite Fauna of Fish Species from Lake Kortowskie

Parasitological studies of fish from Lake Kortowskie were carried out from 2001 to 2004. In the current study, 381 fish representing ten species: perch, Perca fluviatilis L., bleak, Alburnus alburnus (L.), ruffe, Gymnocephalus cernuus (L.), silver bream, Abramis bjoercna (L.), pike, Esox lucius L., common gudgeon, Gobio gobio (L.), tench, Tinca tinca (L.), rudd, Scardinius erythrophthalmus (L.), bream, Abramis brama (L.) and roach, Rutilus rutilus (L.) were examined. The aim of the study was to review the parasites of selected fish species and to compare the infestation of bream, perch, and rudd in 1984-88, 1994 and 2001-2003. A total of 51 species of parasites belonging to Monogenea (27), Digenea (6), Cestoda (7), Nematoda (4), Acanthocephala (3), Crustacea (3), and Molusca (1) were recorded. The largest numbers of parasite species were found in roach (18), followed by bleak (14), perch and silver bream (13), bream (11), ruffe and pike (10), and the fewest in common gudgeon (4) and tench and rudd (2). Tetraonchus monenteron, Triaenophorus nodulosus, Ergasilus sieboldi, and molluscs glochidia occurred in 83.3% of the pike. Comparing the bream, perch, and rudd infestation parameters of over twenty years with the current results, it can be concluded that the changes are linked to environmental changes that have occurred as a consequence of the Lake Kortowskie experiment.

Local variability in metazoan parasites of the pelagic fish species, Engraulis ringens: implications for fish stock assessment using parasites as biological tags

Parasites have been used successfully as biological tags in population studies, mainly in marine fishes, but also in marine mammals, crustaceans and molluscs. Almost all published information dealing with parasites as biological tags evaluates differences between localities. However, local variability in the component community has not been assessed. In this work, we examined whether local variation of the metazoan parasite fauna of Engraulis ringens, extracted from five independent samples from two nearby localities in northern Chile, can be a factor causing bias in stock identification. Our results show that local variability, as estimated by a single sample, may suffice to represent component community variability with no need for replicated data.

Parasite fauna of bream Abramis brama and roach Rutilus rutilus from a man-made waterway and a freshwater habitat in northern Germany

Fifty specimens each of bream Abramis brama and roach Rutilus rutilus were examined for metazoan parasite fauna and trichodinid ciliates; 25 specimens of each species were collected from the Kiel Canal, a man-made waterway, and a nearby freshwater lake, the Dieksee. This is the first detailed parasitological examination of A. brama and R. rutilus at these locations: 30 parasite species were found, comprising 4 protozoans, 4 myxozoans, 5 digeneans, 3 monogeneans, 2 cestodes, 6 nematodes, 2 acanthocephalans, 3 crustaceans and 1 hirudinean. The crustacean Caligus lacustris occurred in both habitats while 2 other crustacean species, 2 acanthocephalans and 1 hirudinean were recorded exclusively for the lake habitat. Larval as well as adult stages of the different parasite species were found, indicating that both fish species act as intermediate and final hosts in both habitats. The Kiel Canal (total of 17 parasite species) showed a lower parasite species richness for A. brama and R. rutilus (14 and 10 parasite species, respectively) than the lake (25 parasite species). A. brama had a higher parasite richness (22 species) than R. rutilus (16 species) in the lake habitat. Most parasites collected were of freshwater origin. Consequently, the observed infection pattern of both fish species in the waterway is mainly influenced by the limited salinity tolerance of freshwater parasites, which are negatively affected even by a salinity of 2.3 to 4.5. In the central Kiel Canal, neither fish species was infected with marine parasites of low host specifity. These parasites are either limited by the low salinity at this sampling site (<4.5 to 6.0) or they cannot enter the canal due to the environmental conditions prevailing in this artificial brackish water habitat. Thus, the canal may comprise a natural barrier preventing the distribution of North Sea parasites into the Baltic Sea. However, the brackish water Baltic Sea nematodes Paracuaria adunca and Cosmocephalus obvelatus were found in R. rutilus from the canal, demonstrating the ability of some parasite species to invade and extend their range of distribution through this man-made shipping route from the Baltic to the North Sea.

Metazoan parasites in wild fish and farmed salmon from aquaculture sites in southern Chile

Parasitology of cultivated salmonids from southern Chile is currently focussed on the agents causing diseases, whereas little attention has been placed on the vector potential of native fish stocks surrounding aquaculture areas. This study describes the variation of both population and community descriptors for metazoan parasite fauna in seven species of wild native fish and in the farmed Atlantic salmon ( Salmo salar) obtained from aquaculture sites in southern Chile. Fish were collected by standard methods, identified, measured and gutted for stomach content determination. Each parasite species was rated for prevalence and abundance, and parasite community richness was determined at the infracommunity and component level. Nine species of ectoparasites and 21 species of endoparasites were found in native fish. Although the diet of S. salar overlapped with that, of some native fish, they only harboured two parasite species. Richness of the component parasite community was higher than in other parts of Chile, and was generally related to host body size and diversity of habitat use. Results suggested that salmon farming affects distribution, feeding habits and ingested food items of native species, and may explain the higher species richness of the component community of native fish inhabiting areas surrounding salmon farms when compared to those of north and central Chile where salmon is absent. Native fish species may infect farmed salmonids with two ectoparasites and one endoparasite. However, the natural patterns of migration of some native fish species may enhance the increase of this number, or may cause outbreaks of parasitic species currently under control.

The Metazoan Parasite Fauna of Grebes (Aves: Podicipediformes) and its Relationship to the Birds' Biology

The Metazoan Parasite Fauna of Grebes (Aves: Podicipediformes) and its Relationship to the Birds' Biology

The Metazoan Parasite Fauna of Grebes (Aves: Podicipediformes) and its Relationship to the Birds' Biology

The Metazoan Parasite Fauna of Grebes (Aves: Podicipediformes) and its Relationship to the Birds' Biology

Metazoan parasite fauna of the bigeye flounder, Hippoglossina macrops, from Northern Chile. Influence of host age and sex.

The metazoan parasite fauna of Hippoglossina macrops (n = 123) from northern Chile (30 degrees S) is quantitatively described for the first time, and the role of host age and sex was evaluated. Twelve parasite species were recovered, including 5 ectoparasites (2 Monogenea, 2 Copepoda and 1 Piscicolidae) and 7 endoparasites (1 Digenea, 3 Cestoda, 2 Acanthocephala, and 1 Nematoda). The copepod Holobomolochus chilensis, the monogenean Neoheterobothrium sp., the adult acanthocephalan Floridosentis sp. and the hirudinean, Gliptonobdella sp. are new geographical and host records. The most prevalent ectoparasitic species were the monogenean, Neoheterobothrium sp. and the copepod, H. chilensis. Among endoparasites, the acanthocephalans Floridosentis sp. and Corynosoma australe were most prevalent and abundant. Prevalence and mean intensity of infection for most parasitic species were not affected by host sex, however the prevalence of Floridosentis sp. was significantly greater in males. Intensity of infection was positively correlated with host age for Neoheterobothrium sp., and negatively correlated for Floridosentis sp. and H. chilensis. The helminth species richness of the host H. macrops was lower compared to related flatfishes from the Northern Hemisphere. The relationship of the helminth fauna of H. macrops, its feeding habits and ecological habitats are discussed.

Gnathostoma sp. as an etiological agent in Mexican Gnathostomiasis: an morphological study by SEM

Parasitology of cultivated salmonids from southern Chile is currently focussed on the agents causing diseases, whereas little attention has been placed on the vector potential of native fish stocks surrounding aquaculture areas. This study describes the variation of both population and community descriptors for metazoan parasite fauna in seven species of wild native fish and in the farmed Atlantic salmon (Salmo salar) obtained from aquaculture sites in southern Chile. Fish were collected by standard methods, identified, measured and gutted for stomach content determination. Each parasite species was rated for prevalence and abundance, and parasite community richness was determined at the infracommunity and component level. Nine species of ectoparasites and 21 species of endoparasites were found in native fish. Although the diet of S. salar overlapped with that, of some native fish, they only harboured two parasite species. Richness of the component parasite community was higher than in other parts of Chile, and was generally related to host body size and diversity of habitat use. Results suggested that salmon farming affects distribution, feeding habits and ingested food items of native species, and may explain the higher species richness of the component community of native fish inhabiting areas surrounding salmon farms when compared to those of north and central Chile where salmon is absent. Native fish species may infect farmed salmonids with two ectoparasites and one endoparasite. However, the natural patterns of migration of some native fish species may enhance the increase of this number, or may cause outbreaks of parasitic species currently under control.

Parasites of flatfish in relation to sewage sludge dumping

Variations in the occurrence of gastrointestinal metazoan parasites in flatfish were examined in relation to sewage sludge dumping at an ‘accumulating’ and two ‘dispersing’ dump sites in Scottish coastal waters. The gross disturbance of the benthic communities at the Garroch Head disposal site in the Firth of Clyde and absence of the intermediate hosts of particular parasites was not reflected in the parasite fauna of the long rough dab, Hippoglossoides platessoides. Results indicate that this flatfish host is highly mobile and intermixing within the population masked any localized effects of sludge dumping on parasite transmission at the dump site. The common dab, Limanda limanda, is considered to be relatively static at the St Abb's Head and Bell Rock disposal sites off the Firth of Forth where no evidence of effects of sewage sludge dumping on its metazoan parasite fauna was detected. This is consistent with the results of chemical and biological monitoring of the sites. The value of a parasite‐based index of the biological effects of sewage sludge dumping is discussed.

WHAT METAZOAN PARASITES TELL US ABOUT THE EVOLUTION OF AMERICAN AND EUROPEAN EELS.

Two hypotheses have been forwarded to explain the divergence of American (Anguilla rostrata) and European (Anguilla anguilla) eels from a common ancestor. The hypothesis proposes that eels formerly inhabited either North America or Europe. Divergence occurred on the spawning grounds in the Sargasso Sea, with new oceanic currents arising during the Pleistocene carrying leptocephalid larvae to the new continent. The vicariant hypothesis proposes that an ancestral eel population inhabited the North Atlantic regions, including Greenland, Iceland, and Scandinavia. This population was forced southwards by the Pleistocene glaciation and separated into European and American components. Examination of the freshwater metazoan parasite fauna specific to eels on both continents forces rejection of the hypothesis. Six parasites specific to eels (Gyrodactylus anguillae, Pseudodactylogyrus anguillae, Bothriocephalus claviceps, Proteocephalus macrocephalus, Daniconema anguillae, Paraquimperia tenerrima) infect eels in freshwaters of both Europe and North America. This transcontinental distribution of freshwater parasites specific to eels is incompatible with an oceanic separation of eels on the spawning grounds, because adult eels die after spawning and leptocephalid larvae cannot acquire these parasites at sea. The disjunct distribution of these parasites implies that at one time their ranges were continuous, thus supporting the vicariant speciation hypothesis. The American eel, Anguilla rostrata (LeSueur), and the European eel, Anguilla anguilla (L.), are sister species inhabiting separate continents. They differ only in the number of vertebrae, the frequency of certain enzymatic alleles, and the frequency of several restriction endonucleases in the mtDNA genotype (Avise et al. 1986, 1990). Although it has been established that these eels are distinct populations, some doubt exists as to whether they constitute distinct biological species (Williams and Koehn 1984). Although Anguillidae is an ancient family, the fossil record is virtually nonexistent (Cavender 1986). The earliest known Anguilla in North America dates from the late Pleistocene, while in Europe, to the Upper Miocene (Cavender 1986). These findings, together with the close relatedness of the sisterspecies Anguilla rostrata and Anguilla anguilla, suggest that the two species were recently separated. Both species grow in coastal, estuarine, and freshwater habitats, and then undergo a spawning migration to the southwestern Sargasso Sea, where they spawn in a largely overlapping area (McCleave et al. 1987). Leptocephalid larvae migrate to the continent of parental origin, partially by passive dispersal in oceanic currents. The nature of their sympatric spawning grounds has created difficulty in determining the origin of the two species. Two contrasting hypotheses have been forwarded recently to explain the divergence of these two anguilliforms from a common ancestral stock and to account for their present distributions (Avise et al. 1990). The hypothesis states that eels were native only to one side of the Atlantic, that is, Europe or North America (Avise et al. 1990). During the Pleistocene epoch, changes in ocean currents occurred that carried some eel larvae from the spawning grounds in the Sargasso Sea to the new continent. These eels matured in the new habitat and returned to the spawning grounds. However, as they migrated from a new direction, that of the newly colonized continent,