Jellyfish-M are anti-personnel mines with skittering tentacles, able to rapidly move towards the enemy in a pack but are easily dispatched with assault rifle fire. When activated they have a purple light on their backs that turns red when about to explode. Let the Marines handle individuals, unless a particularly large group appears after the destruction of a Jellyfish-L. Then use the Shotgun or Assault Rifle. Save Grenades for the big ones.
Jellfish-L are very large area superiority guided mobile explosives easily capable of destroying a light bunker, basically the Jellyfish-M in larger form and with more armor. When heavily damaged the green lights on their back turn red and the robot starts moving faster, signaling it is close to exploding. If one detonates away from the intended target, a swarm of Jellyfish-M is released. Use the Heavy Machine Gun and grenades/EMP Emitters, anything to kill them as far from the IFV as possible.
The larger ones thankfully only appear in one section of one mission, but there are plenty of them to deal with, often in quick succession.