Spawning Areas of Anguillid Eels in the World
Recent findings relevant to the spawning areas of the genus Anguilla are greatly changing our understanding of the migratory ecology and evolutionary history of these fascinating catadromous species. The anguillid larvae (leptocephali) of many sympatric tropical species are morphologically similar, so they have long been impossible to identify to species, and their spawning areas are virtually unknown. Progress in sequencing mitochondrial DNA (Minegishi et al. 2005) has enabled us to identify anguillid leptocephali, eggs, and newly hatched larvae to species. These techniques have been applied recently to determine the spawning areas and distribution of anguillid eel larvae in Indonesian waters and the Pacific and Indian oceans.
Small leptocephali of two tropical anguillid species, Celebes longfin eel A. celebesensis (12.3 mm total length [TL]) and A. borneensis (8.5 mm TL), were collected around Sulawesi Island, Indonesia (Aoyama et al. 2003). This indicated that A. borneensis, which is endemic to the east-central part of Borneo, spawns in the Celebes Sea and migrates back to their nearby freshwater growth habitats (Figure 1). This finding indicates that much shorter migrations of a few hundred kilometers are made by tropical eels to spawn in areas near their freshwater habitats. This clearly contrasts with the long distance migrations of their counterparts in temperate regions, such as European eel A. anguilla, American eel A. rostrata, and Japanese eel A. japonica, and suggests an evolutionary cline of migratory behavior in freshwater eels from tropical to temperate regions.
Interestingly, the small leptocephali of Celebes longfin eel were collected in two different, but neighboring areas, separated by the northern arm of Sulawesi Island: a 12.3-mm-TL specimen in the Celebes Sea and a 13-mm-TL specimen in Tomini Bay. This indicated that this species could have different spawning sites even in a narrow geographic range (Figure 1). Anguillid eels have been considered to be fully or mostly panmictic with single specific spawning sites, despite their wide geographic ranges (Dannewitz et al. 2005). However, it is also possible that Celebes longfin eel forms a single genetic population having multiple spawning sites or two clearly different genetic populations with respective spawning sites within a very small geographic range: these traits have never been found in anguillid eels. Gaining a greater understanding of Celebes longfin eel will give new insight into the ecology of the spawning migration of anguillid eels and the evolutionary processes associated with the development of these peculiar catadromous life histories.