Information on the spawning migration, spawning ecology and life history of tropical eels in the Indo-Pacific region is very limited. The physiological and morphological characteristics of tropical freshwater eels, Anguilla bicolor bicolor and A. bengalensis bengalensis collected in Malaysia were examined in relation to their downstream migration patterns. A total of 455 eels were collected over monthly intervals between February 2014 and January 2016 and we examined both gonadosomatic index and gonad histology features. In both species, close positive relationships between advanced maturation stages and eye, fin, gonadosomatic indexes were found in males and females. A. bengalensis bengalensis was found to be larger and heavier than A. bicolor bicolor at the time of seaward migration. The final stage of maturation for seaward spawning migration occurred throughout the year in A. bicolor bicolor, although that of A. bengalensis bengalensis was estimated to six months due to the limited number of samples. These results suggest that year-round spawning in the open ocean occurs in the tropical eel. This non-seasonal spawning ecology is notably different from that of temperate eels, which are known to follow a well-defined spawning season, with spawning migrations generally taking place during autumn months.
Along with the mysteries of their ecology, freshwater eels have fascinated biologists for centuries. However, information concerning species diversity, geographic distribution, and life histories of the tropical anguillid eels in the Indo-Pacific region are highly limited. Comprehensive research on the species composition, distribution and habitat use among tropical anguillid eels in the Peninsular Malaysia were conducted for four years. A total of 463 specimens were collected in the northwestern peninsular area. The dominant species was A. bicolor bicolor constituting of 88.1% of the total eels, the second one was A. bengalensis bengalensis at 11.7%, while A. marmorata was the least abundant at 0.2%. A. bicolor bicolor was widely distributed from upstream to downstream areas of the rivers. In comparison, A. bengalensis bengalensis preferred to reside from the upstream to midstream areas with no tidal zones, cooler water temperatures and higher elevation areas. The habitat preference might be different between sites due to inter-species interactions and intra-specific plasticity to local environmental conditions. These results suggest that habitat use in the tropical anguillid eels might be more influenced by ambient environmental factors, such as salinity, temperature, elevation, river size and carrying capacity, than ecological competition, such as interspecific competition.