The anguillid eels found in Peninsular Malaysia were identified using a morphological analysis and that identification was further validated as Anguilla bengalensis bengalensis and Anguilla bicolor bicolor by an analysis of the eels' mitochondrial cytochrome oxidase subunit I (COI) sequences. Because of the difficulty of accurately identifying tropical eels solely on morphological analyses, previous studies had reported the occurrence of the tropical eel species Anguilla marmorata in Peninsular Malaysia. This study suggests the occurrence of Anguilla bengalensis bengalensis in Malaysian waters, confirmed by both morphological and molecular genetic analyses. Anguilla bicolor is further confirmed as the subspecies of Anguilla bicolor bicolor by molecular genetic analyses. The present study also suggests that accurate tropical eel species identification requires validation by molecular genetic analysis after a morphological observation.

Previous studies indicated that a tropical mottled eel Anguilla marmorata Quoy & Gaimard, 1824 had been found in Peninsular Malaysia. After reexamination of a number of key characters of a preserved specimen, the species was identified as Anguilla bengalensis bengalensis Gray, 1831. This is the first record of the occurrence of this species in Malaysian waters. There was no eel aquaculture around the eel habitats. Thus, A. bengalensis bengalensis is a native eel species in Malaysia. The new record is suggestive of an offshore spawning area and migration route to the Malaysian coast.

During their complex life history, anguilliform eels go through a major metamorphosis when developing from a fresh water yellow eel into a deep-sea silver eel. In addition to major changes in body morphology, the visual system also adapts from a fresh water teleost duplex retina with rods and cones, to a specialized deep-sea retina containing only rods. The history of the rods is well documented with an initial switch from a porphyropsin to a rhodopsin (P5232 to P5011) and then a total change in gene expression with the down regulation of a “freshwater” opsin and its concomitant replacement by the expression of a typical “deep-sea” opsin (P5011 to P4821). Yellow eels possess only two spectral classes of single cones, one sensitive in the green presumably expressing an RH2 opsin gene and the second sensitive in the blue expressing an SWS2 opsin gene. In immature glass eels, entering into rivers from the sea, the cones contain mixtures of rhodopsins and porphyropsins, whereas the fully freshwater yellow eels have cone pigments that are almost pure porphyropsins with peak sensitivities at about 540–545 nm and 435–440 nm, respectively. However, during the early stages of metamorphosis, the pigments switch to rhodopsins with the maximum sensitivity of the “green”-sensitive cone shifting to about 525 nm, somewhat paralleling, but preceding the change in rods. During metamorphosis, the cones are almost completely lost.

About 60 papers and reports dealing with heavy metal contents in different eel species were reviewed in order to document various patterns of uptake, elimination and accumulation of these metals and to underline their detrimental effects (lethality, histo-pathology). Two main toxicological topics were considered: observations on contaminant levels in eels taken from the wild, mainly from contamined waters, and experimental studies under laboratory conditions.