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 float on our planet’s surface.

Well, pretty much 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 proposed an intriguing hypothesis about this, claiming to have discovered a “simple explanation for a long standing question”.

They suggest one enzyme which helps insects to harden their intestines, named multicopper oxidase-2 (MCO2), which is why it is rarely found in the marine environment but works well on land.

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

Now, Asano and his colleagues explain in a published journal how it puts various organisms in the oceans at a disadvantage but helps them. This is mainly due to the abundance of chemicals in each environment and the lightness of the insects’ exoskeletons.

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

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

Modern molecular phylogenetic perspectives reveal that insects and shellfish (which live mainly in the oceans) belong to the same clade, called crustaceans.

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

Although insects diverged from their crustacean ancestors and evolved terrestrial lifestyles, both persist exoskeletons Made of wax and steel Cuticle carbohydrate called chitin.

This epidermis is a protective layer that lines the surface of the body, keeping moisture inside the epidermis and germs out, just like our skin. More than just a pretty bag, it also protects the body from external mechanical forces and helps maintain body shape and movement, 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 envelopes 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 colleagues say 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 shelters and feeds many of the most adapted species.

To the benefit of the insects, their shell becomes harder and drier through the MCO2 pathway, creating a protective biomaterial while remaining somewhat light like a feather. This is a stark distinction from crustaceans, whose shell is denser due to a direct proportion between shell density and level of calcification, which spoils life in the air.

Insects may have evolved their ability to climb plants, hover, and eventually fly through 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 notes, “No MCO2, No Bugs.”

Explaining in more detail the insect specificity, Asano and his team point out: “Other arthropods, including closest insect relatives, and non-insect hexapods such as springtail and two-pronged bristle-tail, lack MCO2 genes.”

The researchers note that insects aren’t the only arthropods that have adapted to life on land, so MCO2 is not a necessary condition for successfully leaving your ocean and settling on land.

But the unique way the insects’ scales are made provides a wealth of insights into how they evolved to survive in the terrestrial environment.

“If insects had not acquired an MCO2-mediated system, insect development and success would have been dramatically different from what we currently observe,” the team said. And he concludes.

“We hope to have further discussion of insect evolution and landing based on this view.”

Review posted on Physiological entomology.