Why are insects so rarely found in marine habitats? Scientists may have the answer

One possible explanation for the mystery of why such a discrepancy exists is at the heart of a recently-published review from scientists at Japan’s Tokyo Metropolitan University.

Scientists propose that, through evolution, insects developed a special chemical process that helps them harden their shells, utilizing molecular oxygen and an enzyme dubbed multicopper oxidase-2 (MCO2).

While this process helps them to survive on land, this same enzyme makes the insects less suitable for aquatic environments. 

The hypothesis comes from a team of researchers led by the Biologist Tsunaki Asano. The scientists suggest that molecular oxygen, abundantly present in the atmosphere, is used by the enzyme MCO2 to facilitate the growth of lightweight cuticles, the insects' hard protective exoskeletons.

The study finds that using this mechanism to evolve lightweight cuticles is what allowed insects to climb plants, fly, and thrive in terrestrial spaces, enabling them to make the transition to land.

In contrast, marine-based crustaceans use calcium ions, which are abundant in seaware, to form and harden their cuticles. The accumulation of calcium ions leads to an increase in weight, which makes this mechanism unsuited for life on land.

Interesting Engineering (IE) reached out to Professor Asano for more insight on this research.  

The following exchange has been lightly edited for clarity and flow.

Interesting Engineering: What are the evolutionary advantages for terrestrial insects to develop hardened cuticles?

For this question, Professor Tsunaki pointed us to the following passage in the study:

Tsunaki/study: The cuticle covers the surface of the body as a physical barrier to prevent dehydration and infection by pathogens. The cuticle also serves as an exoskeleton for the mechanical support of the body shape and movements and as a protective armor against mechanical loads.

The presence of cuticles might have been the factor for the earlier terrestrialisation of arthropods, compared to the later terrestrialization of tetrapods or other invertebrates.

IE: Since crustaceans can harden their cuticles using calcium from seawater, why haven't insects evolved to do that?

For this question, Professor Tsunaki pointed us to the following passages in the study:

Tsunaki/study: In marine environments, crustaceans harden and stabilize their cuticles primarily by incorporating the abundant calcium in seawater. The calcium ions that are taken from the external environment (or from the internal organs for calcium storage) accumulate in the cuticle as calcium carbonate or calcium phosphate.

In a terrestrial setting, however, calcium supply is limited compared to marine environments.

In contrast to crustaceans, insects do not rely on calcium accumulation but instead, harden and stabilize their cuticles mainly through covalent crosslink formation (termed tanning or sclerotization). The crosslink reactions are mediated by a three-domain multicopper oxidase (3dMCO).

Molecular phylogenetic analyses have demonstrated that the gene for 3dMCO functioning in cuticle formation (termed “multicopper oxidase-2 (MCO2)” or “laccase2”) is insect-specific.

MCO2-mediated reactions are accompanied by the consumption of molecular oxygen that acts as an acceptor for substrate-derived electrons. The MCO2-mediated cuticle formation involving the consumption of molecular oxygen might be an adaptive trait that evolved in association with the water-to-land transition.

Compared to crustaceans, insects might have expanded their distribution in terrestrial niches more freely, irrespective of the restriction in availability of calcium ions.

IE: Proportionally, how many insects are living in the water compared to those on land?

Tsunaki: I think the presence of the cuticle could be advantageous because it can be a barrier to protect [the organism] from dehydration and maintenance of homeostasis, and also can be a protective armor. 

About the number of marine insects, I do not have an exact number, but the sea skater, sea beetles (similar to diving beetles), marine midges or marine crane fly, and seal lice are (I think) regarded as insects that have colonized the sea environments more deeply than other insects, but many reviews say that the number of marine insects species is much less than those of insects in terrestrial environments.  

IE: What is next for your research? Is there a possibility of isolating the MCO2 enzyme for applications useful to humans?

I do not know about practical applications because I belong to the faculty of science, but not to those of agriculture or industrial tech. The next step may be a study on insects and crustacea in environments between fresh water and marine (brackish water).

We want to know whether insects are more diverse in brackish water compared to marine habitats (and also whether insects are rarer compared to those in freshwater).

Such information will support our theory that the content of calcium can be a factor in determining which one of the insects and crustaceans are more dominant in water where oxygen levels are very low compared to the atmosphere. 

Read the study “Eco-evolutionary implications for a possible contribution of cuticle hardening system in insect evolution and terrestrialization,” published in the journal Physiological Entomology.

Abstract 

Previously we proposed one aspect of how insects could adapt to terrestrial environments during their evolution. The hypothesis is based on a theory that insects have evolved an insect-specific system for cuticle formation mediated by an insect-specific enzyme. This enzyme, multicopper oxidase-2 (MCO2), catalyzes the oxidation of catecholamines using molecular oxygen as the acceptor of electrons from the substrates. A potential advantage of this MCO2-mediated system is the utilization of molecular oxygen abundant in the atmosphere, which is different from the case in crustaceans (close relatives of insects) that utilize calcium ions. Accumulation of calcium ions leads to an increase in weight, but the lightweight cuticle without calcification might have been a critical factor enabling insects to evolve flight first in the history of Metazoa. Our theory also provides a simple explanation to a long-standing question of why insects are so rare in marine environments. In previous reviews, we have mainly focused on the details of the biochemical processes occurring in arthropod cuticles, but here only the essence of our theory is extracted and briefly summarized with newly added information.