Plants are the foundation of an Ecosystem. In the full game, they will spread in accordance to their own preferences - some prefer more light, for instance. Every so often they'll disperse seeds, depending on their health.
We can now see a visualisation of this fish's inner most desires: delicious seaweed. It knows where it wants to go and what it wants to eat. Steering seems fine, if only it could propel itself a little faster...
Many of the creatures in this particular Ecosystem seem to have developed a flat body shape, streamlining themselves against the drag of the water. Many real-life sea creatures have a similar evolutionary strategy.
The scissor-like motion that these creatures have evolved is a balanced & efficient method of forward movement. It pushes effectively against the water to propel the creature in its intended direction.
The flow of water currents around the ocean floor and the objects in it is an important part of Ecosystem's simulation. It will transport nutrients and creature eggs around the map, giving a realistic representation of the fluid dynamics of the sea.
These creatures have evolved a sleek form to appear rather eel-like, with one notable exception - they have a protrusion from below their midsection that they can control. What purpose do you think it has?