# The Sensory Equipment of Diving Lice, a Host Ecology-Based Comparative Study

## TL;DR

This study explores how seal lice, which live on seals that dive deep into the ocean, sense their environment using their antennae.

## Contribution

The study identifies eight types of antennal structures in seal lice and reveals species-specific adaptations, particularly in deep-diving species.

## Key findings

- Eight morphotypes of antennal sensilla were identified across five seal lice species.
- Southern elephant lice have thick sensilla squamiformia, unlike other species with thin sensilla chaetica.
- Variation in sensilla morphology and distribution suggests functional adaptations to host diving behavior.

## Abstract

Seal lice are a highly unusual group because they are permanent and obligate ectoparasites of amphibian mammals that can spend most of the year in the open ocean, diving to depths of kilometers below the surface. Although some of these insects’ adaptations to survive the extreme conditions of the deep ocean are beginning to be understood, how they sense their environment remains unexplored. Unlike human lice and most insects, seal lice have no eyes and rely only on antennae to sense their environment. Our study uses scanning electron microscopy to investigate the morphology and possible function of antennal sensilla in five seal lice species that parasitize hosts with different diving habits. Eight morphotypes of structures were identified, six of which are sensilla, with differences in external morphology and distribution in different species. The most striking difference observed across species concerns antennal mechanoreceptors. In most cases, these are thin sensilla chaetica, but in the southern elephant louse, i.e., the deepest divers, they seem to have changed and become thick and strong sensilla squamiformia.

Seal lice (Anoplura) parasitize amphibious hosts, such as pinnipeds, and are uniquely adapted to an oceanic environment. As obligate, permanent ectoparasites feed on the blood of their hosts and are completely dependent on them. While studies have begun to explore general diving adaptations, research into seal lice’s sensory biology remains limited. In contrast to the vast majority of insects, including human lice, seal lice are devoid of eyes and depend on antennal sensory reception. This study aims to describe the morphology and putative function of antennal sensilla in five seal lice species: Antarctophthirus microchir, A. carlinii, A. lobodontis, A. ogmorhini, and Lepidophthirus macrorhini, which parasitize the South American sea lion, Weddell seal, crabeater seal, leopard seal, and southern elephant seal, respectively. The antennal structures of each species were analyzed using scanning electron microscopy, and eight morphotypes were identified: spine, cuticular lobe, sensilla squamiformia, sensilla chaetica, sensilla basiconica I and II, tuft organs, and pore organs. The morphology of sensilla and their distribution on the antennal flagellum exhibited variability among genera and species. For instance, the southern elephant louse (Lepidophthirus macrorhini) is characterized by the presence of sensilla squamiformia, while Antarctophthirus spp. are distinguished by sensilla chaetica.