American Eel - Life Cycle

Life Cycle

The American eel’s complex life history begins far offshore in the Sargasso Sea in a semelparous and panmictic reproduction . From there, young eels drift with ocean currents and then migrate inland into streams, rivers and lakes. This journey may take many years to complete with some eels travelling as far as 6,000 kilometers. After reaching these freshwater bodies they feed and mature for approximately 10 to 25 years before migrating back to the Sargasso Sea in order to complete their life cycle.

Life stages are detailed below.

1. Eggs: The eggs hatch within a week of deposition in the Sargasso Sea. McCleave et al. (1987) suggested that hatching peaks in February and may continue until April. Wang and Tzeng (2000) proposed, on the basis of otolith back-calculations, that hatching occurs from March to October and peaks in August. However, Cieri and McCleave (2000) argued that these back-calculated spawning dates do not match collection evidence and may be explained by resorption. Fecundity for many eels is between about 0.5 to 4.0 million eggs, with larger individuals releasing as many as 8.5 million eggs (Wenner and Musick 1974). The diameter of egg is about 1.1mm. Fertilization is external, and adult eels are presumed to die after spawning. None has been reported to migrate up rivers.

2. Leptocephali: The leptocephalus is the larval form, a stage strikingly different from the adult form the eels will grow into. Leptocephali are transparent with a small pointed head and large teeth and are frequently described “leaf-like”. The laterally compressed larvae are passively transported west and north to the coastal waters on the eastern coast of North America, by the surface currents of the Gulf Stream system, a journey that will last between 7 and 12 months (Schmidt 1922; Tesch 1977; Kleckner and McCleave 1982). Vertical distribution is usually restricted to the upper 350 m of the ocean (Kleckner and McCleave 1982; Castonguay and McCleave 1987). Growth has been evaluated at about 0.21 to 0.38 mm per day (Kleckner and McCleave 1985; Castonguay 1987; Tesch 1998; Wang and Tzeng 2000).

3. Glass eel: As they enter the continental shelf, leptocephali metamorphose into glass eels (juveniles), which are transparent and possess the typical elongate and serpentine eel shape (McCleave et al. 1987). The term glass eel refers to all developmental stages between the end of metamorphosis and full pigmentation (Tesch 2003). Metamorphosis occurs when leptocephali are about 55 to 65 mm long (Kleckner and McCleave 1985). Mean age at this metamorphosis has been evaluated at 200 days and estuarine arrival at 255 days; giving 55 days between glass eel metamorphosis and estuarine arrival (Wang and Tzeng 2000). Young eels use selective tidal stream transport to move up estuaries (Kleckner and McCleave 1982). As they enter coastal waters, the animals essentially transform from a pelagic oceanic organism to a benthic continental organism.

4. Elvers: Glass eels become progressively pigmented as they approach the shore; these eels are termed elvers. The melanic pigmentation process (Bertin 1951; Élie et al. 1982; Grellier et al. 1991) occurs when the young eels are in coastal waters. At this phase of the life cycle, the eel is still sexually undifferentiated. The elver stage lasts about three to twelve months. Elvers that enter fresh water may spend much of this period migrating upstream (Haro and Krueger 1991; Jessop 1998a). Elver influx is linked to increased temperature and reduced flow early in the migration season, and to tidal cycle influence later on (Tesch 1977; Kleckner and McCleave 1982; Martin 1995; Jessop 2003b).

5. Yellow eels: This is the sexually immature adult stage of American eel. They begin to develop a yellow color and a creamy or yellowish belly. In this phase, the eels are still mainly nocturnal. Those remained in estuarine environment continue to go through their life cycle more quickly than those traveled into freshwater. Those in freshwater, however, tend to live longer and attain much larger sizes. Sexual differentiation occurs during the yellow stage and appears to be strongly influenced by environmental conditions (Krueger and Oliveira 1997; Oliveira 1997; Krueger and Oliveira 1999). Krueger and Oliveira (1999) suggested that density was the primary environmental factor influencing the sex ratio of eels in a river, with high densities promoting the production of males. From life history traits of four rivers of Maine, Oliveira and McCleave (2000) evaluated that sexual differentiation was completed by 270 mm total length.

6. Silver eels: As the maturation process proceeds, the yellow eel metamorphoses into a silver eel. The silvering metamorphosis results in morphological and physiological modifications that prepare the animal to migrate back to the Sargasso Sea. The eel acquires a greyish colour with a whitish or cream coloration ventrally (Gray and Andrews 1971; Scott and Crossman 1973; Tesch 1977). The digestive tract degenerates (Pankhurst and Sorensen 1984; Durif 2003), the pectoral fins enlarge to improve swimming capacity (Pankhurst 1982a; McGrath et al. 2003; Durif 2003), eye diameter expands and visual pigments in the retina adapt to the oceanic environment (Vladykov 1966; Pankhurst 1982b; McGrath et al. 2003), the integument thickens (Tesch 1977; Pankhurst and Lythgoe 1982), percentage of somatic lipids increases to supply energy for migrating and spawning (Larsson et al. 1990; Tremblay 2004), gonadosomatic index (Verreault 2002; McGrath et al. 2003; Tremblay 2004) and oocyte diameter increase (Couillard et al. 1997), gonadotropin hormone (GTH-II) production increases (Durif et al. 2005), and osmoregulatory physiology changes (Dutil et al. 1987).