Migration Scales of Catadromous Eels: Diversity and Evolution of Larval Migration Based on Their Distribution, Morphology, and Early Life History
Mari Kuroki, Jun Aoyama, Heeyong Kim, Michael J. Miller, Shingo Kimura, and Katsumi Tsukamoto
The long distance migrations of catadromous eels of the genus Anguilla consist of two different components, the larval migration toward their freshwater growth habitats and the adult migration back to the spawning area. These components have been poorly understood due to the critical lack of information about the spawning areas and migration routes. We examined the morphology and otolith microstructure of anguillid leptocephali to estimate the diversity of migration and its evolutionary origins. Leptocephali were examined from the Indo-Pacific region (12 species, N = 759), and glass eels (9 species, N = 653) were obtained from all over the world, including temperate and tropical eels. Tropical eels compose two-thirds of the genus Anguilla, but compared to temperate eels, information has been scarce in tropical eels, especially their leptocephalus larvae (Jespersen 1942).
Distributional analysis of leptocephali (Figure 1) suggested that most tropical species such as the Celebes longfin eel A. celebesensis and A. borneensis spawn near their growth habitats like in the Celebes Sea or in Tomini Bay in Indonesian waters, and their larval migrations are only short distances (small-scale migration; Aoyama et al. 2003). Other tropical species like the giant mottled eel A. marmorata and the Indian shortfin eel A. bicolor pacifica spawn in the open ocean, but inhabit wide regions of the tropics (middle-scale migration; Kuroki et al. 2006). These migratory distances contrast with that of temperate eel species, which show migrations of thousands of kilometers (large-scale migration; Tsukamoto et al. 2002).
Numerical simulations were made of the larval migration routes of the temperate Japanese eel A. japonica and the tropical giant mottled eel in the western North Pacific region, (Figure 2) based on their slightly different spawning areas west of the Mariana Islands. The simulated transport trajectories showed that the Japanese eel had an expected long distance passive transportation by the North Equatorial Current and northward into the Kuroshio Current (Figure 2A), whereas the giant mottled eel showed transport by the North Equatorial Current and then southward transport into the Mindanao Current (Figure 2B). The spawning sites of the tropical Celebes longfin eels and A. borneensis were based on the collection locations of small leptocephali, and simulated transport trajectories resulted in their larvae remaining near the spawning areas for about 100 d (Figure 2C, 2D). These results corresponded well with the mean duration the larval period of tropical and temperate species estimated from the otolith microstructure of their glass eels (108.5–194.4 d in four tropical species and 203.0–351.2 d in five temperate species) as well as their geographic distributions.