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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All of evolution is a game of cat and mouse between a species and the various things in the environment that want to kill it. If something has even a marginal benefit to survival, it will likely become a dominant trait given enough time. The adaptation doesn’t have to be _perfect_, just _better_.
So, in both cases, some ancestor of the modern animal decide to lay its eggs in a non-traditional location due to a quirk of genetics and those offspring were better able to survive to adulthood.
Turtles breath air, and the embryos in the eggs must get their air through the membrane of the egg which they wouldn’t be able to under water. Also, the right temperatures for the eggs don’t exist in the water.
Salmon are incapable of living in salt water when they are first born, so the eggs must be laid in fresh water. Going back to the same place they spawned makes sense: it’s a known quantity capable of supporting salmon eggs.
So the general reason behind both is that these are creatures that have adapted to live in environments as adults that they could not survive in as newborns, so it is necessary to return to the place (or a similar place) to that in which they themselves were born.
Baby turtles would drown if they were born in the ocean. I don’t think anything more really needs to be said about that one.
I don’t know about fish, but turtles lay their eggs in sand so the embryos can breath. Sea turtles have to come up for air every so often.
Good answers so far but there could be one additional reason: By sheer happenstance, the hatching of turtle eggs on a beach creates a kind of selection pressure that wouldn’t otherwise be created in a “safer” environment(Let’s assume that there were no environmental problems like temperature and O2 for laying eggs under the ocean floor, which are the main answers to this question).
So in simpler words: Making your hatchlings run a predator filled obstacle course offers more selection pressure, which leads to more robust turtles
For salmon, the rivers that they spawn in have fewer predators and more cover than the ocean, which allows the young salmon to mature some more safely. When they get to a larger size, the ocean has more food than the rivers.
Salmon are anadromous fish, they live in the sea and migrate into fresh water to spawn; Catadromous fish like Seabass or Eels do the opposite – as adult fish they live in fresh water and migrate into salt water to spawn. Now you know.
Salmon are so badass.. I just recently learned about ‘zombie salmon’. When they’re swimming back to their spawning grounds and expend so much energy swimming upstream that their bodies literally start rotting away and decomposing 😳. Metal af lol
Good answers, but there is even more cool reasons in regards to eel migration. Eels are an old species that originated around where Indonesia is today. Because of continental drift waterways were opened westward. Some eels moved to live there (in future Europe and America) and continued to travel to “Indonesia” for spawning, but even more drift closed the gap towards the Pacific and the Atlantic eel branch was established (some in N America and some in Europe). Initially both groups had similar distance to their new spawning ground – the Sargasso sea, but as continental drift continued westward, the American eel can chill, while the European eels today have to travel 5000-6000 km to their spawning grounds. Also this separation resulted in the speciation of two distinct species.
Easily digestible [source](https://worldoceanreview.com/en/wor-5/coastal-dynamics/on-the-origin-and-demise-of-coasts/evolution-of-the-eel-a-matter-of-continental-drift/)
Turtles are within the order or reptiles. Reptiles eggs cannot be turned as the umbilical can be detached from the yolk and the embryo will die.
Thus the eggs need to be layer in a stable surface in an environment with stable warm temperatures.
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