Why do so few insect species live in the ocean? Scientists think they know

Scientists from Tokyo Metropolitan University have recently proposed an intriguing hypothesis to explain the rarity of insects in marine environments, a question that has puzzled scientists for decades. 

The research team, led by Professor Tsunaki Asano, has attributed this phenomenon to the evolution of a unique chemical mechanism that hardens insects’ shells, which gives them an advantage on land but a disadvantage in the sea.

Insects have conquered the land, but not the seas

Insects are among the most successful organisms on Earth, accounting for the majority of biomass of all terrestrial animals and playing a significant role in the global ecosystem. 

Despite their abundance on land, however, insects are strikingly rare in marine environments. This perplexing disparity has left scientists searching for answers.

Recent advances in molecular phylogenetics have revealed that insects and crustaceans belong to the same family, Pancrustacea. 

Insects branched off from their crustacean ancestors and adapted to life on land, but both groups share a common feature: an exoskeleton composed of a wax layer and a hard cuticle (see image here). 

In previous work, Asano’s team discovered that insects evolved a unique gene responsible for creating an enzyme called multicopper oxidase-2 (MCO2), which allows them to harden their cuticles using oxygen.

MCO2 mediates a reaction in which molecular oxygen oxidizes compounds known as catecholamines in the cuticle, transforming them into agents that bind and harden the surface. This is distinct from crustaceans, who rely on calcium from seawater to harden their cuticles. 

What the researchers learned

The researchers argue that the terrestrial environment, with its abundant oxygen, is more favorable for insects, whereas the ocean, with its relative lack of oxygen and presence of better-adapted organisms, is less hospitable.

But the benefits of MCO2 for insects don’t end with shell hardening. The MCO2 pathway produces a cuticle that is not only protective but also lightweight. 

The team postulates that this lightweight material enabled insects to climb plants, glide, and eventually fly, allowing them to migrate and occupy previously unoccupied ecological niches. This diversification and expansion is a significant factor in the insects’ incredible success on land. 

In contrast, crustacean shells are much denser, with a strong correlation between density and the degree of calcification.

While insects are not the only arthropods to have adapted to life on land, the characteristics of their cuticles are key to their success in terrestrial environments. 

The researchers believe that MCO2 is a defining feature of insects, even going so far as to say, “no MCO2, no insects.” This groundbreaking work offers a fresh perspective on the role that cuticle hardening might play in insect evolution and the transition from aquatic to terrestrial habitats.

More about insects

Insects are a diverse and fascinating group of organisms, belonging to the class Insecta within the phylum Arthropoda. 

They are the most abundant and diverse group of animals on Earth, with over a million described species and estimates suggesting that millions more are yet to be discovered. Insects can be found in virtually every habitat, from the highest mountains to the deepest caves, and they play a critical role in many ecosystems as pollinators, decomposers, and food sources for other organisms.

Some key characteristics of insects include:

Body structure

Insects typically have a segmented body divided into three main parts: the head, thorax, and abdomen. The head contains sensory organs, such as compound eyes and antennae, as well as mouthparts adapted for feeding. The thorax bears three pairs of legs and, in many species, two pairs of wings. The abdomen houses the digestive, reproductive, and excretory systems.

Exoskeleton

Insects have a hard, protective exoskeleton made of a material called chitin. The exoskeleton provides support, protection from predators and harsh environments, and helps prevent water loss.

Life cycle

Insects undergo either complete or incomplete metamorphosis during their life cycle. Complete metamorphosis consists of four stages: egg, larva, pupa, and adult, as seen in butterflies, beetles, and flies. Incomplete metamorphosis involves three stages: egg, nymph, and adult, which occurs in insects like grasshoppers and true bugs.

Reproduction

Most insects reproduce sexually, with males and females producing sperm and eggs, respectively. Some species, however, are capable of asexual reproduction through processes such as parthenogenesis.

Respiration

Insects breathe through a series of tiny tubes called tracheae that connect to the outside of their body through small openings called spiracles. Oxygen diffuses through these tubes and reaches the cells directly, while carbon dioxide is expelled through the same system.

Communication and behavior

Insects exhibit a wide range of behaviors, including communication through chemical signals (pheromones), visual displays, and auditory signals. Many insects are social, with some, like ants and bees, forming complex colonies with division of labor and intricate communication systems.

Diet

Insects have diverse feeding habits, ranging from herbivores that feed on plant material to carnivores that prey on other insects or animals. Some insects, like butterflies and bees, consume nectar from flowers, while others, such as mosquitoes and ticks, feed on the blood of their hosts.

Economic and ecological importance

Insects have significant ecological and economic impacts. They are essential for pollination of many plants, including agricultural crops, and they help recycle nutrients by breaking down dead organic matter. Insects also serve as a vital food source for numerous other species. However, some insects are pests, causing damage to crops, transmitting diseases, and invading homes.

Conservation

Many insect populations are declining due to factors such as habitat loss, pesticide use, and climate change. It is crucial to protect insect biodiversity, as they play a vital role in maintaining healthy ecosystems and providing essential services to humans.

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