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Earshell Slug (Testacella haliotidea)

Family: Testacella Family (Testacellidae)

IUCN Conservation Status: Least Concern

All slugs are believed to have descended from shelled snail-like ancestors, and are thought to have lost their tough external shells in exchange for greater manoeuvrability and flexibility. However, a handful of species of slugs still retain small external shells, and the Earshell Slug (which possesses a tiny abalone-shaped shell on the "tail tip" at the end of its body) is one such species. That being said, it is unclear what purpose, if any, such a tiny shell serves; it is plausible that it is a vestigial characteristic that has no function, although it has been suggested that it may serve some purpose in allowing the slug to retain moisture when exposed to sunlight, or that it may provide some minor protection from predators (notably, when threatened Earshell Slugs will retract their tentacles and "curl" their large bodies up into a small compact oval, in which state they may be slightly better protected.) Native to much of western Europe and also present in New Zealand, Australia and North America as an invasive species, Earshell Slugs are believed to be fairly common throughout their range but are very rarely seen due to their unusual lifestyle; they are specialised predators that feed exclusively on earthworms, and as such spend the vast majority of their lives underground or amongst dense leaf litter searching for prey, coming to the surface only when soil compaction caused by heavy rain forces them to do so. While the subterranean lifestyle of this species means that much of its lifecycle is a mystery, it is known that (like most terrestrial gastropods,) members of this species are hermaphrodites (producing both sperm and ova,) and based on the anatomy of their reproductive organs it is possible that they are able to reproduce asexually by fertilizing their own eggs.

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Image Source: https://www.inaturalist.org/taxa/232389-Testacella-haliotidea