There are several ways to tell the three New Zealand eel species apart.
The freshwater eel is elongated and slender-bodied. The dorsal and anal fins are united to form a continuous fin along the back around the tail and along the belly. The skin is thick and leathery with an embedded tiny mosaic of scales, which are more noticeable in the shortfin than the longfin.
Shortfin eels usually have a silvery belly compared to a yellowish one on longfins, but colours can vary considerably (even pure yellow and white shortfin eels have been caught). You can tell longfins and shortfins apart by looking at a couple of features:
- the length of the fins on the top (dorsal) in relation to the length of the fin on the bottom (anal)
- wrinkles - a longfin body "wrinkles" when bent
- the tooth patterns on the upper jaws.
The best way is to look at the fins. When viewed from the side, longfins have a dorsal (top) fin that extends well forward - towards the head - past the end of the anal fin. In shortfins, however, the ends of the dorsal and anal fins are almost the same length.
The tooth patterns from the upper jaws of eels have also been found to be unique to every Anguilla species worldwide. In longfins, the middle line of teeth are longer and more pointed compared to shortfins, whose middle line of teeth are shorter and more rounded. Next time you catch a tuna to eat, push some plasticine or playdough up against the roof of the eel's mouth to get a better look at the tooth patterns – make sure it's not still alive though!
Additionally, when you bend the body of a longfin the skin "wrinkles" on the inside curve. This is not observed in shortfins.
The Australian longfin looks like the New Zealand longfin but has black blotches along the top of its body.
References and further reading
Cairns, D. (1941). Life history of the two species of New Zealand freshwater eel. Part I. Taxonomy, age and growth, migration and distribution. New Zealand Journal of Science and Technology, Series B 23: 53-72.
Ege, W. (1939). A revision of the genus Anguilla Shaw, a systematic, phylogenetic and geographical study. Dana Rep 16: 1-256.
Schmidt, J. (1928). The Fresh-Water Eels of New Zealand. Transactions and Proceedings of the Royal Society of New Zealand 58: 379-388. http://rsnz.natlib.govt.nz/volume/rsnz_58/rsnz_58_00_003260.html
Tesch, F.W. (2003). The Eel. Fifth edition. Blackwell Science Ltd and The Fisheries Society of the British Isles, Oxford. 408 p.
