Why are there so few insects in the ocean?

Tokyo, Japan – Scientists from Tokyo Metropolitan University have put forward a hypothesis as to why insects are so rare in marine environments. They previously showed that insects developed a unique chemical mechanism to harden their shells that uses molecular oxygen and an enzyme called multicopper oxidase-2 (MCO2). Now, they argue that this gives them a disadvantage at sea, while it gives them an advantage that helps them on land, placing MCO2 at the heart of insect eco-evolution.

Insects are some of the most successful organisms on the planet. They are said to make up the most biomass of all land animals and have a significant impact on global ecosystems. However, their abundance is matched by their surprising rarity at sea. Very few insects call the ocean home, even though their biological ancestors originate there. This is a science mystery that surrounds it, which scientists have been trying to answer for years.

Now, researchers from Tokyo Metropolitan University led by Assistant Professor Tsunaki Asano have proposed a solution based on evolutionary genetics. Recent molecular phylogenetics has taught us that crustaceans and insects are part of the same family, Pancrustacea, and that insects are a branch that left the sea and adapted to land. They share an important feature, an exoskeleton consisting of a waxy layer and a hard cuticle. In previous work, the same team showed that as insects adapted to a terrestrial environment, they developed a unique gene that creates an enzyme called multicopper oxidase-2 (MCO2) that helps them harden their cuticles using oxygen. MCO2 mediates reactions in which oxygen molecules oxidize compounds called catecholamines in the cuticle, turning them into substances that bind and harden the surface. This is different from crustaceans which harden their cuticles using calcium from seawater. The team’s claim is that this makes the soil much more suitable for insects due to the abundance of oxygen. The ocean is now a harsh environment due to a lack of oxygen and many better adapted organisms.

But it’s not just the ocean that is no longer hospitable to insects. Hardening and drying of the cuticle via the MCO2 pathway results in a biomaterial that is not only protective, but also lightweight. They postulated that this may be why insects acquired the ability to climb plants, glide, and eventually fly. This allows them to migrate and occupy previously vacant niches in ecosystems, a powerful driving force that has led to their great numbers. Again, this is in contrast to crustaceans, whose shells are much denser, with a strong correlation between density and calcification.

Of course, insects are not the only soil-adapted arthropods, so it is clear that MCO2 is not strictly necessary for success in “terrestrial niches”. However, the nature of the insect cuticle speaks volumes about their success in a terrestrial environment. In fact, the team believes that MCO2 may be characteristic of insects: “no MCO2, no insects.” Their work promises to shed a whole new light on the role that cuticle ossification may have played in insect and terrestrial evolution.