This is seedling of either an eggplant or a tomato:
There are some wavy lines and a general discoloration on the cotyledons. What does this incdicate? Is it a disease or malnutrition?
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This pattern is typical of leaf miners. Adults lay eggs inside the leaf where the soft tissues are, then the eggs hatch and the larvae chew out the inside of the leaf, leaving the tunnels that you can see from outside. It's possible that the larvae have now matured and left this plant to move on to other things - keep an eye on the plant as it produces more leaves and if the tunnelling persists only then consider treatments or discarding the plant.