Eels are threatening and weird, they are in-between, hybrid creatures. Both snake and fish, they symbolize predators/danger/protection (they are like angry dogs and will bite you if you get too close to their temporary home, the rocks in which they hide most of their bodies. They will only show their head and most importantly their teeth). Eels are also magic & monsters, in their giant, mythical version, which, along with other characteristics, makes them tricksters.. They also symbolize flow (a strength which is not strictly physical, and which allows for resilience in that, like fish and water itself, it is adaptive to the elements): a great tool for survival and yet, they end up in sushis and are actually delicious.
In "Trickster Makes This World: Mischief, Myth, and Art", Lewis Hyde describes tricksters as boundary-crossers. Tricksters "violate principles of social and natural order, playfully disrupting normal life and then re-establishing it on a new basis."