Jellyfish in Defender of the Future live in small schools. They usually have a reddish color and some can even glow in the dark. Some are even transparent and can only be seen from a short distance away. They can be defeated with Ecco's charge attack and the same attack can go through multiple jellyfish. If Ecco touches them he will either get poisoned or take damage.
See also: Jellyfish (original series)