How Did Seal Lice Turn into the Only Truly Marine Insects?

Abstract

Insects are the most evolutionarily and ecologically successful group of living animals, being present in almost all possible mainland habitats; however, they are virtually absent in the ocean, which constitutes more than 99% of the Earth’s biosphere. Only a few insect species can be found in the sea but they remain at the surface, in salt marshes, estuaries, or shallow waters. Remarkably, a group of 13 species manages to endure long immersion periods in the open sea, as well as deep dives, i.e., seal lice. Sucking lice (Phthiraptera: Anoplura) are ectoparasites of mammals, living while attached to the hosts’ skin, into their fur, or among their hairs. Among them, the family Echinophthiriidae is peculiar because it infests amphibious hosts, such as pinnipeds and otters, who make deep dives and spend from weeks to months in the open sea. During the evolutionary transition of pinnipeds from land to the ocean, echinophthiriid lice had to manage the gradual change to an amphibian lifestyle along with their hosts, some of which may spend more than 80% of the time submerged and performing extreme dives, some beyond 2000 m under the surface. These obligate and permanent ectoparasites have adapted to cope with hypoxia, high salinity, low temperature, and, in particular, conditions of huge hydrostatic pressures. We will discuss some of these adaptations allowing seal lice to cope with their hosts’ amphibious habits and how they can help us understand why insects are so rare in the ocean.

Eco‐evolutionary implications for a possible contribution of cuticle hardening system in insect evolution and terrestrialisation

“…Considering the short life span of the adults, their dispersion seems to be passive (hitch‐hiking on sea turtles, algal mats, and flotsam) (Huang et al, 2014). Recently, researchers have debated that seal lice are the only truly marine insects since they can survive in deep seawater (as deep as 2000 m), with a system of gas exchange through the thin cuticle on their body surface and not as part of the tracheal system (Leonardi et al, 2022), although their reproduction and transmission occur only when their hosts are on land. It has been considered that insects are not capable of overcoming physiological and physical constraints associated with living in the sea (Cheng, 2009; Cheng & Mishra, 2022).…”

Tell me who you go with and I will tell you what you do: Antarctic seal lice behave differently

“…sea lions, seals, and the walrus, are infested by a particular group of sucking lice, the echinophthirids (Anoplura: Echinophthiriidae) (Kim 1971; Leonardi et al 2021). These lice are peculiar by their behavioral, ecological, morphological, and/or physiological adaptations to the amphibious life of their hosts (Kim 1971;Murray 1976; Leonardi et al 2021). Furthermore, they are among the few insects that have been able to adapt to the marine environment (Leonardi et al 2021).…”

“…These lice are peculiar by their behavioral, ecological, morphological, and/or physiological adaptations to the amphibious life of their hosts (Kim 1971;Murray 1976; Leonardi et al 2021). Furthermore, they are among the few insects that have been able to adapt to the marine environment (Leonardi et al 2021). They are permanent and obligate ectoparasites throughout their whole life cycle (Fig.…”

“…The long evolutionary history they share with their hosts has exposed them to selective pressures. As a result, they show speci c adaptations that allow them to tolerate long and deep immersion periods (Leonardi et al 2021). However, as terrestrial insects, their reproduction remains tied to the land.…”

Attachment performance of the ectoparasitic seal louse Echinophthirius horridus

“…During their evolution, echinophthiriids had to adapt to a challenging new environment, when the ancestors of their recent host returned from land to sea in the Miocene [7][8][9] . Thereby, these ancestrally terrestrial insects adapted gradually to challenging physical conditions in extremely changing environments with high salinity, fluctuating temperatures, hypoxia and exceptionally high hydrostatic pressure 10 . The adaptation to this new way of life impacted morphology, reproduction, and distribution of the parasites 11,12 .…”