Eelscalator – a Helping Hand
by Juliana Andrea Berner
Let’s get one thing straight! What a true eel is and what a fake eel is. Despite its misleading name, the electric eel is in fact not an eel but a knifefish and a member of the Gymnotiformes order, meaning they are closely related to carp and catfish rather than actual eel [1]. However, unlike true eels, electric eels are solely found in the freshwaters of South America. A further distinction between the two orders is that electric eels are air-breathers needing to surface to breathe compared to true eels that breathe underwater using gills [2]. The reason for the electric eels’ name is – you guessed it – they are electric while looking like an eel [1].
Who is this so-called true eel?
Eels are an underrated creature of the sea. Not much is known – but this slimy snake-like fish belongs to the Anguilliformes order with 15 families and over 800 species including moray, conger and freshwater eels. These critters are found at almost every depth and in a range of colours. Eels are primarily found in saltwater, but freshwater species, for example the European eel [3], live their adult life in freshwater and migrates approx. 5000 km from rivers in Europe to the Sargasso Sea to mate. However, the exact life cycle of the European eel – especially at sea – is still a big mystery. After successful mating, the adults die and the baby eels journey their way back to Europe [4].
The European eel lifecycle quick facts:
Fertilized eggs hatch and the larvae emerge; known as leptocephalus due to its leaf shape with a thin head and a wide body.
As the animal grows the body elongates and thins, becoming larger and more translucent; at this stage is known as glass eels (even at this stage the eels are harvested and eaten) [5].
The eels enter the freshwater streams, and transform again by gaining more pigmentation and growing slightly in size; at this stage known as elvers. Here is when they migrate back to freshwater rivers.
Over the next six to twenty years the eels gain weight and grow in length, and their underbellies become yellow, with the surprising name - yellow eels.
The eels keep swimming upstream towards cooler freshwater, staying here until they reach reproductive maturity where they change into what is known as silver eels, due to the more metallic sheen and large eyes.
When the eels reach this stage, they begin their long journey of thousands of kilometres back to the Sargasso Sea to spawn, lay eggs and finally die [6].
Extra fun fact: The lifespan of these European Eels spans between 7 and 80 years, depending on when the individual specimen reaches maturity to begin their migration [6].
Eel-ssential to our ecosystem
Although these animals are not the prettiest or the cuddliest of animals, they are highly important to our ecosystem. For without these slippery and somewhat ugly fellows, our ecosystem would be much uglier. Elvers and eels are of significant importance in maintaining a balanced riverine ecology as they function as both prey and predator. Furthermore, freshwater eels move between seawater and rivers, creating a crucial nutrient fluctuation. Despite being a keystone in the overall ecosystem, eels are overfished – both legally and illegally – highly susceptible to infections and disease transmitted between eels, thereby leaving them critically endangered [7].
A stepping stone
However, there are conservation efforts set in motion to help these animals to evade extinction. At the elver stage, dams and other anthropogenic creation threatens the livelihood of said animal. All the elvers crossed the dam - sounds like the beginning of a very standardized joke - using a ladder, a ramp, or a pass which in fact has been invented to assist these animals to cross the dam, helping these animals to continue their long journey safely past the obstruction. The ramp has a constant flow of fresh water and is fitted with either bristle, open weave (a form of mesh), studded substratum, or steps to interrupt the water flow, thereby helping these small elvers cross the dam. This helping hand, together with the new awareness of the critically dwindling number of wild eels, may help the population to stabilize [8, 9].
References
- Mermelstein, Claudia D. S., Costa, Manoel L. & Neto, Vivaldo M. “The cytoskeleton of the electric tissue of Electrophorus electricus, L.” (2000) Anais da Academia Brasileira de Ciências, Volume 72, Page 341-351.
- Johansen, Kjell, Lenfant, Claude, Schmidt-Nielsen, Knut & Petersen, Jorge A. “Gas exchange and control of breathing in the electric eel, Electrophorus electricus.” (1968) Zeitschrift für vergleichende Physiologie, Volume 61, Page 137–163.
- Castle, Peter H. J. “Eel” Britannica, N.D.
- Righton, David, Westerberg, Håkan, Feunteun, Eric, Økland, Finn, Gargan, Patrick, Amilhat, Elsa, Metcalfe, Julian, Lobon-Cervia, Javier, Sjöberg, Niklas, Simon, Janek, Acou, Anthony, Vedor, Marisa, Walker, Alan, Trancart, Thomas, Brämick, Uwe & Aarestrup, Kim. “Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea” (2016) Science Advances, Volume 2.
- van Ginneken, Vincent J. T. & Maes, Gregory E. “The European eel (Anguilla anguilla, Linnaeus), its Lifecycle, Evolution and Reproduction: A Literature Review.” (2005) Reviews in Fish Biology and Fisheries, Volume 15, Page 367–398.
- “European eel” Scotland’s Nature Agency Buidheann Nàdair na h-Alba, N.D.
- May, Duncan & Marshall, Shona. “West Sutherland Elver Survey” West Sutherland Fisheries Trust, 2008.
- Watz, Johan & Nilsson, Anders & Degerman, Erik & Tamario, Carl & Calles, Olle. “Climbing the ladder: an evaluation of three different anguillid eel climbing substrata and placement of upstream passage solutions at migration barriers.” (2019) Animal Conservation, Volume 22.
- The Spill. “The eel deal.” Hydro Tasmania, 24 Aug. 2018.
