Surprisingly few insects live in the ocean, and we may finally know why: ScienceAlert

Insects have been around for approximately 480 million years, which gives them plenty of time to crawl, crawl, burrow, and flutter all over the surface of our planet.

Well, almost everywhere. Surprisingly few species live in the oceans, and scientists have been trying to figure out why.

A research team from the US and Japan recently came up with an interesting hypothesis for this, claiming to have discovered a “simple explanation for a long standing question”.

They suggest that the enzyme is unique Helps insects harden their covers, named multicopper oxidase-2 (MCO2), is why it is so rare in marine environments but works so well on land.

Biologist Tsunaki Asano of Tokyo Metropolitan University, who led the team, has previously shown that insects have evolved a special mechanism to harden their tough outer layer that uses molecular oxygen and MCO2.

Now Asano and his colleagues explain in a published review how this puts various creatures at a disadvantage in the oceans but helps them out of it. It mainly comes down to the abundance of chemicals in each environment, and how light the insects’ exoskeletons are.

“The emergence of insects is an important event in the evolution of life on Earth,” the team writes, “and highlights a major adaptive expansion of organisms into a new terrestrial ecosystem.”

Insects are some of the most successful creatures on the planet, the largest group in the phylum Arthropoda, and they contribute the largest biomass of all land animals. They play an important role in maintaining the balance of life on Earth.

Recent insights from molecular genetics have revealed that insects and crustaceans (which live mostly in the oceans) belong to the same branch, called Pancreastaceae.

Phylogenetic tree showing the ecological phylogenetic relationship between Insecta and crustaceans. (Asano et al., Physiol. Intomol.2023)

Although insects have diverged from their crustacean ancestors and evolved terrestrial lifestyles, both remain exoskeletons Made of wax and steel A cuticle of carbohydrates called chitin.

This epidermis is a protective layer that lines the surface of the body, keeping moisture inside the epidermis and germs in, unlike our skin. More than just a pretty shell, it also protects the body from external mechanical forces and helps maintain the body’s shape and ability to move, acting as an external scaffolding.

However, while crustaceans primarily use calcium from seawater to harden their scales into shells, insects use molecular oxygen to transform their cuticles into durable coverings for their organs through the mediation of MCO2.

Crustaceans fortify their shells with calcium, while insects fortify their exoskeletons with oxygen, to match what is abundant in their habitats. (Tokyo Metropolitan University)

Asano and his co-workers argue that the presence of oxygen in the air makes the Earth more attractive to insects. The sea is now a harsh place for them because there is not enough oxygen, not to mention that it already houses and feeds many species that are better adapted.

To the benefit of the insects, their shells become harder and drier through the MCO2 pathway, creating a protective biomaterial while remaining as light as a feather. This is a striking distinction compared to crustaceans, whose shells are much denser due to a direct proportion between shell density and level of calcification, and which are not suitable for life in air.

Insects may have evolved their ability to climb plants, glide, and eventually fly thanks to MCO2 locomotion, allowing them to move more easily and fill previously unoccupied ecological niches.

The team believes that MCO2 may be what makes insects unique; As they say in their paper, “No MCO2, No Bugs.”

Explaining the insect’s specificity, Asano and his team point out: “Other arthropods including the insect’s closest relatives, non-insect hexapods such as the springtail and the two-pronged bristle-tail, do not possess genes for MCO2.”

The researchers note that insects aren’t the only arthropods that have adapted to life on land, so MCO2 isn’t a necessary condition for successfully moving from your ocean dwelling and establishing a home on dry land.

But the unique way insects’ cuticles are made provides a lot of information about how well they evolved to live in the Earth’s environment.

“If insects had not acquired an MCO2-mediated system, insect evolution and success could have been dramatically different from what we currently observe,” he concludes.

“We hope to conduct further discussions about the evolution and formation of insects on Earth based on this point of view.”

Review posted on Physiological Entomology.