Today I’m going to be talking about some cute little slugs, the Onchidioideans. These pulmonate, that is, lung-bearing, gastropods are ubiquitous across the Atlantic and Pacific coastlines, favoring the intertidal.

What makes so many gastropods so interesting is their torsion, the rotational steps they undergo during development resulting in a twisting of their viscera. Onchidiodeans however, are part of a small subsection of gastropods which undergo torsion, but then subsequently undergo detorsion, reverting back to a linear state. Additionally, another weird artifact present in many onchidiodeans is that a shell mantle, shell, and operculum are present during the larval state, and disappear into adulthood (Giese, 2012). This makes for a generally morphologically unique group, given that two key elements of gastropoda, a shell and torsion, are present, but in a reduced or reverted way.

Onchidella carpenteri
Gabe Schp

Additionally, there’s some interesting things happening with their habitat use. Onchidioideans can be found across a wide range of habitats, but some genera exhibit speciation depending on microhabitats. This being a form of sympatric speciation, where despite being in very similar habitats, a form of speciation occurs regardless. This results in different species being present in very close proximity, depending on differences as subtle as size of the substrate particles they live on. There’s also been some really interesting evolutionary history research into this group, showing that their ancestral divergence follows for example the divergence of a species of mangrove tree in which they live. And, despite being largely reliant on a marine environment, a few species have been found fully terrestrialised, as far as 100m above sea level (Goulding et al., 2022).

Peronia verruculata
© もふ

This just goes to show that despite having some clear understandings of what constitutes a group, or what prerequisites are involved for speciation, we can still be surprised when these expectations are subverted!

Giese, A. (2012). Reproduction of Marine Invertebrates V4: Molluscs: Gastropods and Cephalopods. Elsevier.

Goulding, T. C., Khalil, M., Tan, S. H., Cumming, R. A., & Dayrat, B. (2022). Global diversification and evolutionary history of onchidiid slugs (Gastropoda, Pulmonata). Molecular Phylogenetics and Evolution, 168, 107360. https://doi.org/10.1016/j.ympev.2021.107360