Ecology and evolution of migration in the freshwater eels of the genus Anguilla Schrank, 1798

Abstract

Scientists have long sought to uncover the secrets of the migration of anguillid eels, genus Anguilla. As catadromous fishes, anguillid eels spend most of their lives in freshwater until they return to their spawning grounds in the tropics, although part of the population never enters freshwater and instead resides in brackish and marine areas close to coastlines. Molecular phylogenetic research suggests that anguillid eels originated from deep-ocean midwater marine anguilliform species and that tropical eels originating from the Indo-Pacific region are the most basal species of anguillid eels. Anguillid eels left the tropical ocean to colonize temperate areas. The yearly spawning of tropical species and constant larval growth throughout the year extend to periods of recruitment in continental habitats to last all year for tropical eels. Tropical eels such as A. celebesensis and A. borneensis have relatively short migrations periods of less than 100 km to their spawning grounds. Conversely, the temperate European eel A. anguilla travels the longest distances and migrates more than 5000 km across the Atlantic Ocean to spawn in the Sargasso Sea. The ancestral state of migration in the genus Anguilla may have been local, short-scale and nonseasonal spawning migration throughout the year as defined in tropical eels. With the expansion of dispersion of global oceanic migration across the world, migration scales can gradually change. Temperate anguillid eels migrate thousands of kilometres from spawning areas to coastal and inland water habitats while retaining spawning areas in tropical areas, accompanied by seasonal downstream and spawning migrations with consequences for seasonal recruitment. Recent advances and the availability of electronic tags such as pop-up satellite archival tag could reconstruct the entire spawning migration from continental growth habitats to spawning sites with detailed migration behaviours and routes. Migration ecology and mechanisms throughout the life of anguillid eels have gradually been revealed in recent decades.

Keywords: Agricultural science; Anguilla; Animal science; Biological sciences; Catadromy; Continental migration; Diversity; Ecology; Environmental science; Oceanic migration; Spawning; Zoology.

Figure 1

Geographical distribution of the 19 species and subspecies of genus Anguilla.

Figure 2

Potential dispersal routes of ancestral anguillid eels into the Atlantic Ocean and distributions of the tropical anguillid eels Anguilla borneensis and A. mossambica which are thought to be the most basal species of the anguillid eels.

Figure 3

Scenario illustrating the evolution of anguillid eel migration from local short-distance movements with year-round spawning, recruitment and downstream migration for tropical eels to long-distance migration with seasonal spawning, recruitment and downstream migration for temperate eels.