by Matthew Cobb
One of my favourite questions relating to evolution is ‘Why are there no insects in the sea?’ Arthropods came onto the land around 380 MY ago, and crustaceans and insects separated soon afterwards, probably because of that ecological shift. More or less, you get crustaceans in the sea, and insects on land. So why didn’t the insects go back into the sea? It’s very hard to be certain of the answer to this – doing an experiment would be pretty tricky, after all. But we can get towards what might be the answer by thinking about some of the possible answers we might give:
– Insects can’t live in water. Although no insect species lives its whole life-cycle in water without access to air, many insect species pass their nymphal stage in freshwater, breathing with gills. Mayflies (more on this in an upcoming post) and dragonflies are two obvious examples. Last year Daniel Rubinoff, an entomologist at the University of Hawaii, discovered a number of moth species that have caterpillars (= larvae) that are equally at home on land or in freshwater. You can see a great video of one of these caterpillars moving between water and land here.
– Insects can’t cope physiologically with salt water. Not true. There are a large number of species of insect that have a larval stage that lives in brackish salt water, so living in the sea is not impossible. (Indeed, this fact shows that the real question should be ‘Why are there no insects that have their full life-cycle in the sea?’)
– The sea is full. I think this is probably the right answer – the niches that insects would occupy in the sea are already full. The insects’ cousins, the crustaceans, are already there. This is what ecologists call ‘competitive exclusion’. Any insect that started going back into the sea would either starve or be eaten, I reckon. Proving this, however, is tricky.
One implication of this is that evolution finds it difficult to go ‘backwards’ because life shapes and changes the ecosystem over time. When environmental conditions changed to make it possible for terrestrial life to evolve, crustacean-like arthropods moved onto the land and rapidly made a series of adaptations that led to their incredible success as insects (reducing the number of appendages, evolving first wings and then highly specialized larval stages, and so on). Retracing their steps back into the sea is no longer possible, for ecological reasons. Were something terrible to happen to some or all of the crustaceans, however, it seems pretty likely that those adaptable insects would be back in the sea in the blink of a geological eye.