Biology

Species and Characteristics

Freshwater eels are widely distributed throughout the world, mostly in the areas around the Atlantic and Indo-Pacific oceans (Fig. 1) (2). A total of 16 species and six subspecies are recorded, namely, Anguilla anguilla (European eel), A. rostrata (American eel), A.japonica (Japanese eel), A. marmorata, A. bicolor (subspecies: A. bicolor pacifica and A. bicolor bicolor), A borneensis, A. celebesensis, A, an-cestralis, A. obscura, A. australis (subspecies: A. australis australis and A australis schmidti), A. megastoma,A. mos-sambica, A. nebulosa (subspecies A. nebulosa labiata and A. nebulosa nebulosa), A. reinhardti, A. interioris, and A. dieffenbachi. However, A. celebesensis and A. ancestralis are classified as synonyms (3). There are several characteristics that distinguish the world's eel species, including dorsal fin length, vertebral number, color, size, head shape, habitat, and distribution (Table 1).

Dorsal Fin Length. Eels are generally divided into two types according to the proportion of the length from the

30° 60° 90° 120° 150° 180° 150° 120° 90° 60° 30° 0° 30°

A. ang. Anguilla anguilla A. an.

A.ro. Anguilla rostrata A.me.

A. d. Anguilla dieffenbachi A. bo.

A. re. Anguilla reinhardti A. n.n.

A. c. Anguilla celebesensis A. mo.

Anguilla ancestralis A. b.b.

Anguilla megastomaa A.b.p.

Anguilla interioris A. o.

Anguilla borneensis A. a.a.

Anguilla nebulosa A. a.s. Anguilla labiata Anguilla mossambica

Anguilla bicolor Anguilla b. pacifica Anguilla obscura Anguilla a. australis Anguilla a. schmidti

30° 60° 90° 120° 150° 180° 150° 120° 90° 60° 30° 0° 30°

A. ang. Anguilla anguilla A. an.

A.ro. Anguilla rostrata A.me.

A. d. Anguilla dieffenbachi A. bo.

A. re. Anguilla reinhardti A. n.n.

A. c. Anguilla celebesensis A. mo.

Anguilla ancestralis A. b.b.

Anguilla megastomaa A.b.p.

Anguilla interioris A. o.

Anguilla borneensis A. a.a.

Anguilla nebulosa A. a.s. Anguilla labiata Anguilla mossambica

Anguilla bicolor Anguilla b. pacifica Anguilla obscura Anguilla a. australis Anguilla a. schmidti

Figure 1. Natural distribution of freshwater eels. Source: Ref. 2.

anterior base of the dorsal fin to the anus and the total body length. The long-fin type has a proportion of 7-17%, while the short-fin type has a proportion of 0-5%. Thirteen of the eel species belong to the long-fin category, three species belong to the short-fin type.

Vertebral Number. Another taxonomic criterion for eel types is the number of vertebrae. Eels have 100-119 vertebrae; the number varies among the different species. The number even varies within each species, which increases with higher latitudes.

Color. Eels can be mottled or plain. There are seven mottled and nine plain species.

Size. There is a considerable difference between female and male eels. The growth of female eels in inland water is much more superior than that of male eels. The females also stay longer in inland water before returning to the sea. Eels can grow at maximum sizes of 27 kg and 200 cm.

Head Shape. Eels are also classified according to the shape of their head. Those with a narrow head and thin, narrow lips are classified as narrow-head type while those with broad head and thick, broad lips are classified as broad-head type (4,5).

Distribution

Of the 16 species of eels, 14 are distributed in the Indo-Pacific areas. Some of these eels inhabit the tropical zone, while others abound in the temperate zone. Eels in the

Indo-Pacific Ocean are distributed in both the southern and northern hemispheres. The northernmost boundary is at about 45° N in Hokkaido, Japan, while the southernmost limit is at about 50° S in the Auckland Islands, New Zealand. The two other species, the European eel and the American eel, inhabit the Atlantic Ocean and abound in the temperate zones of Europe and North America, respectively (Fig. 1).

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