This might be a strange question, but why do sea turtles lay eggs on land and not for example dig up holes inside the ocean? They live their whole lives in the ocean, so why do they lay eggs on land? Why travel so far just to lay eggs?
Same goes for some salmon, why do they leave the oceans and lakes, and go upstream on rivers and not lay their eggs where they live?
It is probably something to do with protecting their offspring, but it seems to me that they still have predators that hunt their offspring fairly easily where they hatch/lay their eggs, so maybe there is another reason as well?
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Sometime animals evolve to go to a different environment than their ancestors , but their babies still live in the same environment as their ancestors.
It’s the case of amphibian whose ancestors and babies live in water, but adults live on land.
It’s the case of sea mammals, penguins and sea reptiles including turtles whose ancestors and children need land, but adults live in the sea.
It’s the case of salmon whose ancestors and children need freshwater, but adults need saltwater.
And it’s the case of eels whose ancestors and children need saltwater, but adults need freshwater.
In general it happens because there’s room in the adult ecosystem for a new species and this species from the child ecosystem randomly get an adaptation for it.
It would need new mutations for the children to also be adapted to the new environment, and it often doesn’t happen. Some cases of it happening is when amniotes evolved from amphibians, freshwater fish evolved from saltwater fish, and land arthropods evolved from sea arthropods.
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