Evolutionary Consequences of Eel Migration

Fish migration is a regularly occurring habitat transition between the spawning area and growth habitat of a species. A conceptual model that superimposes the life cycle of migratory fishes onto a closed route of migration connecting the spawning area and growth habitat has been defined as a “ migration loop” (McDowall 1992; Tsukamoto et al. 2002). In principle, each fish species has a migration loop specific to its life history and geographic distribution. Therefore, the differentiation of a new migration loop has the potential to cause reproductive isolation and hence speciation. This concept helps for an understanding of the evolutionary processes of fish migration as well as the migratory behavior and life cycle of fishes.

Diadromy is a migratory strategy in fishes that involves a regular migration pattern between fresh and salt water (McDowall 1992). It is a relatively rare behavioral trait, occurring in perhaps 250 out of the some 25,000 known fish species (McDowall 1993). Diadromous migrations have been observed in several taxa of fishes, suggesting that diadromy originated independently throughout fish evolution (McDowall 1992, 1993). Therefore, each migratory fish species might have experienced different selection pressures that resulted in variations in its life history. Diadromous fishes undertake two major habitat shifts in every generation: a migration from fresh water to the ocean, and another migration in the opposite direction. Some diadromous fish are semelparous (one single reproductive migration per generation), others are iteroparous (two to several reproductive events per generation). Anadromy, catadromy and amphidromy are all variants of diadromy. Anadromy refers to the migration patterns of fish, such as salmonids, that live in the ocean but return to fresh water to spawn. In amphidromy, such as observed in gobiids, the migrations are not directly tied to spawning, but to some other activity, such as feeding. Catadromy refers to the migration patterns of fishes, such as the European eel, that live in fresh water but return to the ocean to spawn. The evolution of these life-strategies is triggered by several abiotic and biotic factors, such as glaciation, continental drift, habitat suitability, food availability and selection. For example, the ancestor of the Anguillidae might have been a tropical marine species with a migration loop extending to coastal waters. From there they incidentally visited estuaries and eventually obtained a reproductive advantage because of the greater amount of food available in estuaries and by extension in fresh water (Tsukamoto et al. 2002). Thus the ancestor probably developed an adaptive behavior of regularly migrating upstream as a result of a gradient in food abundance between the ocean and fresh water in the tropics. In order for this process to occur, first there must have been a euryhaline marine species and then an oceanic amphid-romous adaptation before the appearance of catadromous eels (Tsukamoto et al. 2002) (Fig. 17.1). A similar scenario applies to anadromous fishes.