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.
This study reports the first occurrence of partial albinism in a tropical anguillid eelAnguilla bengalensis bengalensisfrom Malaysia. This paper also describes the first record of albinism in the genusAnguilla. The occurrence of albinism in our specimen ofAnguillamight have been caused by three factors: (1) contamination effects; (2) random genetic alterations; or (3) genetic alteration due to small population size. The present results suggest that the albinism inA. bengalensis bengalensisis probably caused by random genetic alteration. Partial albinism may not be a handicap in the life of the present specimen because the eel could still potentially grow to more than 1 m in total length, just like a normal adult eel.
Abstract. Although tropical anguillid eels account for two-thirds of all species in the genus Anguilla, the information on the species diversity, geographic distribution, and life histories of the tropical eels is very limited. Recent studies suggested that accurate species identification in the tropical anguillid eels needs a validation by molecular genetic analysis after morphological observation. Two anguillid eels found in Sabah, Borneo Island, were firstly identified as Anguilla marmorata Quoy et Gaimard, 1824 using morphological analysis and further analysis of mitochondrial 16S ribosomal RNA (16S rRNA) sequences confirmed the morphological species identification. The presently reported study represents the first description of A. marmorata in Sabah, Borneo Island. One-year survey suggests that representatives of A. marmorata found in the region might belong to the North Pacific population in the westernmost distribution.
Recent studies suggested that accurate species identification in the tropical anguillid eels based on morphological examination requires confirmation by molecular genetic analysis. Previous studies found that two tropical anguillid eels, Anguilla
bicolor
bicolor and A.
bengalensis
bengalensis, were found in peninsular Malaysia (West Malaysia) based on morphological and molecular genetic analyses. This study is the first record of A.
marmorata in peninsular Malaysia confirmed by both morphological and molecular genetic analyses. The present study also suggests that accurate tropical eel species identification is difficult by morphological identification alone; therefore, molecular genetic analysis is needed for precise species confirmation.
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