Their attacks shock you (which makes you unable to move), and they often stick in groups. If you get surrounded by these, you're basically dead. Don't use melee weapons, use a crossbow.

They drop bio toxins, which can be used for a new tranq dart that's more powerful AND shocks the target.

Once grabbed they won't let go till dead I had a Lvl 215 spino and was having a jolly swim then BANG WAM I get jumped by 8 jelly bastards and the spino died :(

Basilosaurus is the by far the best way to kill these things. Doesn't get shocked. High level ones harvest hundreds of biotoxin in minutes with no threat.