Many insects live in fresh water, or close to the sea in salt marsh and beach habitats. Halobates are the closest thing we know of to a marine insect, but even they don’t actually live in the sea, just on it. Marine crustaceans, which have been in the sea for hundreds of millions of years, are well-adapted to these challenges. Unlike crustaceans, insects developed a system of oxygen intake through tiny holes in their bodies. Insects might be better off staying put where they are than trying to compete with marine crustaceans.