- Written by Nadia O'Carroll
This species of eel may reach 1600 mm in length, but is commonly seen at 1000 mm. Specimens are usually olive green to brownish-green on back and sides, with spots and blotches of a deeper shade. A carnivore, it has a large mouth and bands of fine teeth which are used to catch fish and crustaceans.
The eel is not a species that attracts public interest, probably because so many people find the eel’s snake-like appearance, large mouth and slimy skin quite repugnant. So most people would be surprised to know, that this rather ordinary looking creature, has one of the most extraordinary life cycles on Earth, and that the eels in our local creeks, actually start their lives, and will eventually perish, far away in the tropical seas near Vanuatu.
The eel’s life begins hundreds of metres below the surface of the ocean, near Vanuatu, where huge aggregations of eels spawn in the cold depths. The egg is transformed into a leaf-shaped larval stage known as leptocephalus, which develops and drifts in the ocean as plankton for several years. Eventually the leptocephalus approaches the coast and changes into a juvenile elver, which resembles a small transparent adult eel.
Once it reaches the coast, the elver has an urge to move up-current and seek fresh water. It wriggles and slips its way up watercourses, slithering along wet vegetation to avoid waterfalls and along the damp concrete of weirs and if necessary, travelling overland until it finds a suitable place to stop.
After a few months in fresh water it begins to eat again, then to grow in size and develop pigmented skin. For years the eel stays in these fresh waters, although it may wander, sometimes even travelling considerable distances overland over wet vegetation. At this stage of its life, it is solitary although it may tolerate other eels living in close proximity.
Eventually the time comes for the eel to spawn, it becomes bright silvery and descends downstream towards the sea, seeking salt water and even slithering across land to reach it. The exceptions are totally land locked eels or those with hormonal disturbances, they remain in the fresh water and may live for many years, eventually becoming very large specimens.
Once the sea is reached, the eel swims away from the coast, it no longer feeds, but travels towards its birthplace, where it joins huge aggregations, spawns, then dies. Long-finned eels have not been caught at sea, and it has always been thought that they swam at great depths. This has been confirmed in European eels, by tracking them with radio transmitters. Once the tracked eels left the continental shelf, they dived to depths of 400-500 metres and swam some 6,000 kilometres to the Sargasso Sea, where spawning took place at 400-830 metres below the surface, at water temperatures of around 20 deg C.
There are many question raised by this incredible migration. What guides the eels? They have no parental guidance or memory of fresh water on their first journey. How does a population that starts as an aggregation become distributed along such a long coastline? How do they navigate on land and sea?
This rather dull looking local inhabitant will never win any prizes for popularity, however their navigational feats and extraordinary life-cycle make them truly remarkable creatures.