Abstract
Freshwater eels have fascinated biologists for centuries due to the spectacular long-distance migrations between the eels’ freshwater habitats and their spawning areas far out in the ocean and the mysteries of their ecology. The spawning areas of Atlantic eels and Japanese eel were located far offshore in the Atlantic Ocean and the Pacific Ocean, respectively, and their reproduction took place thousands of kilometers away from their growth habitats. Phylogenetic studies have revealed that freshwater eels originated in the Indonesian region. However, remarkably little is known about the life histories of tropical freshwater eels despite the fact that tropical eels are key to understanding the nature of primitive forms of catadromous migration. This study found spawning-condition tropical freshwater eels in Lake Poso, central Sulawesi, Indonesia, with considerably high gonadosomatic index values and with histologically fully developed gonads. This study provides the first evidence that under certain conditions, freshwater eels have conditions that are immediately able to spawn even in river downstream. The results suggest that, in contrast to the migrations made by the Atlantic and Japanese eels, freshwater eels originally migrated only short distances of <100 kilometers to local spawning areas adjacent to their freshwater growth habitats. Ancestral eels most likely underwent a catadromous migration from local short-distance movements in tropical coastal waters to the long-distance migrations characteristic of present-day temperate eels, which has been well established as occurring in subtropical gyres in both hemispheres.
Highlights
Schmidt (1922) discovered that the spawning area for both the European eel Anguilla anguilla and the American eel A. rostrata was located far offshore in the Sargasso Sea of the Atlantic Ocean thousands of kilometers away from their growth habitats in Europe and North America, indicating that these two species of Atlantic freshwater eels make remarkably long-spawning migrations
Ecology and Evolution published by John Wiley & Sons Ltd
The gonadosomatic index (GSI) values >1.0 were classified as migrating silver stage in A. japonica; all of the A. japonica specimens from the coastal area were silver eels in this study
Summary
Schmidt (1922) discovered that the spawning area for both the European eel Anguilla anguilla and the American eel A. rostrata was located far offshore in the Sargasso Sea of the Atlantic Ocean thousands of kilometers away from their growth habitats in Europe and North America, indicating that these two species of Atlantic freshwater eels make remarkably long-spawning migrations. The spawning area of the Japanese eel, A. japonica, was discovered far offshore in the Philippine Sea of the western North Pacific (Tsukamoto 1992) where all of the oceanic stages of this species were first collected, including spawning adults, eggs, and recently hatched larvae (Tsukamoto et al 2011). These eels in both the Atlantic and Pacific spawn in similar westward flowing currents at the southern edges of the subtropical gyres in both oceans; their larvae (leptocephali) can be passively transported to coastal areas.
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