Leafminer. The critter is between the leaf surfaces.

As soon as you see a mine developing, look for the end of the trails that's the largest, then squish the critter.

In case you're wondering -- Sprays won;t help.

Thanks, jean.

To what extent will they effect the health or fruiting of the plant?

They seldom affect it at all because the damage is so limited. Even so, squish any new mines you see.