water lice and “water skaters”

This article was co-written with Maria Soledad Leonardi (IBIOMAR – CONICET – Puerto Madryn, Argentina).

Insects are recognized as the living organisms with the greatest ecological and evolutionary success. Millions of species populate our planet, occupying all types of habitats, from the tropical forest to the Antarctic continent, including deserts and our urban areas. The only invertebrate animals capable of flight, they move through the air devouring the most diverse foods, such as our agricultural crops, our waste, other insects or even our blood.

This spectacular success is however only visible on dry land. In the marine environment things are very different and their presence is very rare. Specialists have always wondered about the reasons why insects are virtually absent from the oceanswhich represent approximately 99% of the biosphere (“life zones”) of our planet. Various hypotheses have been put forward but, for the moment, no clear and definitive answer has been found. One of the most common ideas is that the biological characteristics that have allowed them to be so successful on land, such as their strong and light skeleton or their simple respiratory system, would be incompatible with life offshore.

However, like any generalization, this one has some remarkable exceptions: five species of the genus Halobates (“water skaters”) which live on the surface of the ocean thousands of kilometers from the coast and thirteen species of lice which parasitize amphibious mammals, such as otters, seals and elephant seals, whose life occurs by alternating the terrestrial environment and the aquatic environment. It should be noted that these two groups of insects have little in common with each other, apart from their ability to evolve in the oceans.

The halobates, surfers of the oceans

THE water skaters surf the sea, thanks to a very hydrophobic body which makes them unsinkable. Thus, they feed, develop and reproduce very far from the nearest coastline, without ever entering the water. For them, gliding on water is equivalent to walking on dry land for other insects. We say “walk” because they do not have wings which would be of little use in a turbulent and windy environment.

It is difficult to imagine that we can study their biology in detail. Oceanographic expeditions are very expensive and it is complicated to find tiny insects (5 mm or less) in the vastness of the sea. The fact that they die quickly in captivity doesn’t make their study any easier either. Despite these constraints, Lanna Chengdistinguished researcher at the Scripps Institute of Oceanography at the University of California in San Diego (United States), has succeeded in deciphering fundamental aspects of their way of life. She came to the conclusion that the highly hydrophobic body and small size of halobates probably play a key role in their conquest of the open sea.

Staying on the surface of the water all the time means permanent exposure to UV light, which, as with us, can have a deleterious effect on insects. Without any shaded area to protect themselves, it is likely that these little surfing insects have also developed their own “sun protector” in the skin (cuticle).

Aquatic lice, a model of adaptation

Mammals evolved as land animals, but some groups among them returned to the marine environment, occupied by their ancestors long before. Whales, dolphins, orcas and manatees have taken this path back to the sea permanently.

Others, such as seals, otters, lions and elephant seals as well as walruses, have adopted a double life, alternating between land and water. Some of them, like elephant seals, spend most of the year in the ocean, thousands of kilometers from the coast, and only return to land to breed.

Before them, their carnivorous ancestors, completely terrestrial, already shared their lives with small insect tenants who clung to their skin and fur and lived by feeding on their blood: lice. This association between carnivores and blood-sucking lice did not disappear when the hosts gradually moved closer to the sea. On the contrary, during a process which lasted more than forty million years, the lice also evolved in parallel, adapting to aquatic life.

Given the close relationship between lice and their vertebrate hosts, as carnivores evolved into different species (otters, seals, walruses, elephant seals, etc.), lice also diverged, giving several species, each closely associated with a specific group of hosts.

Since lice are obligate ectoparasites throughout their life, they must remain constantly attached to the skin of their hosts; only the young leave them temporarily when they are born, to climb onto another host on which they will spend the rest of their lives. This dependence on the host forces them to accompany them when they go to sea. This is why certain species, such as sea lion or elephant seal lice, must spend most of the year in the ocean and undergo regular immersions to great depths.

The conditions of low temperature, high salinity, little available oxygen and above all, sometimes phenomenal hydrostatic pressures constitute an extreme environment for most organisms. However, lice are doing very well and will return with their hosts for the brief period of reproduction of both partners on dry land.

An Argentine team based in Patagonia (IBIOMAR-CONICET) in collaboration with the Research Institute of Insect Biology (CNRS-University of Tours) has dedicated itself for several years to deciphering how amphibious mammal lice manage to withstand the extreme conditions to which they are exposed during their hosts’ long stays on the high seas. As with water skaters, progress is slow, due to the difficulties associated with studying these lice. It is not possible to keep them in captivity for more than a few days. They must be collected during limited periods of the year, on specific protected sites, by taking them one by one with tweezers from the bodies of animals, which can exceed three tonnes and which are in the reproductive period.

Despite these strong constraints, we were able to learn that to survive, sea lice have surprising abilities. When the host enters the water, the lice reduce their metabolism to a minimum to reduce their dependence on oxygen, which they can obtain from the water, despite the fact that this gas is less available in water than in the air. Furthermore, they have proven to be perfectly capable of tolerating rapid changes in hydrostatic pressure, when the hosts, very fast swimmers, descend to depths that exceed 2,500 meters, where the pressure is equivalent to more than 250 atmospheres, and rise to the surface when the pressure reaches 1 atmosphere.

It is remarkable to note that halobates and lice are hardly different from any other insect. This observation forces us to rethink the reasons why insects are not more numerous in the marine environment; if skaters and lice were able to conquer the oceans, why not others?

This article is published as part of the Science Festival (which takes place from October 4 to 14, 2024), and of which The Conversation France is a partner. This new edition focuses on the theme “ocean of knowledge”. Find all the events in your region on the site Fetedelascience.fr.