Long-lost relatives: Joining the dots in snail superfamilies

Long-lost relatives: Joining the dots in snail superfamilies

A new DNA study by our researchers Rodrigo Salvador and Lara Shepherd has revealed an unexpected land snail family across the Pacific.

Dot snails are a very diverse group, estimated to have over two thousand species. In the jargon of biological classification, they are known as the Superfamily Punctoidea. They are distributed worldwide, but up to now, how each branch of this superfamily was related to one another was poorly understood.

A snail shell from three angles on a black background
An example of punctoid land snail from NZ, Phrixgnathus murdochi Suter, 1894, Hokianga Harbour, New Zealand. CC BY-NC-ND 4.0. Te Papa (M.088067)

So we set out to understand those relationships using DNA. We contacted museums all around the world to gather specimens for our study, which has just been published in the open-access journal Zoosystematics and Evolution.

Ancestry and DNA

Using DNA sequences we can produce an evolutionary tree showing the relationships among each of the seven families that make up Superfamily Punctoidea.

The first thing we discovered was that all these families were actually split into three very distinct groups, not directly related to one another, as you can see in the image below.

A horizontal family tree chart with black, green and purple text.
Evolutionary tree with representatives of major land snail and slug families. The Punctoidea were split into three distinct groups, marked in different colours. From Salvador et al. (2020).

The group marked in red font in the image above contains the families Discidae (disc snails and tiger snails) and Oreohelicidae. The latter is restricted to North America and the former can be found in North America and Europe. This sort of geographic combination is called “Laurasian” (as opposed to the southern supercontinent Gondwana). Because this group didn’t have a name, we called it Superfamily Discoidea. It is more closely related to the helicoid snails, the group that contains the common garden snail.

The second group, in purple, is made up solely of family Helicodiscidae, also restricted to North America and Europe. However, we could not be sure if it would belong to an existing superfamily or if it would be a superfamily of its own.

The final group, in green, is the “true” Punctoidea (dot snails), made up of the families Endodontidae, Charopidae, Punctidae, and Cystopeltidae. This is a Gondwanan group, although a few Punctidae have reached the northern hemisphere. Both Punctidae and Charopidae, are widespread families and are particularly diverse in New Zealand, with hundreds of species.

Here’s the full evolutionary tree with all species studied:

A horizontal family tree chart with black, green, blue and red text.
Evolutionary tree showing superfamilies Discoidea and Punctoidea, and family Helicodiscidae. From Salvador et al. (2020).

Finding new cousins on the family tree

The most interesting finding of our study was family Cystopeltidae. This family was previously thought to include only the Australian slug Cystopelta, but we have shown that it also contains additional Australian species, as well as an entire South American branch of snails.

A grey-green slug on a tree branch with a brown fern in the background
Cystopelta purpurea, from Australia, 2019. Photo by Reiner Richter / iNaturalist (CC BY-NC 4.0)
a snail with a spiral shell sitting on dirt
The South American genus Lilloiconcha, despite superficially similar to New Zealand snails, was just discovered to be part of the new family Cystopeltidae. Photo from Salvador et al. (2018).

This was an unexpected result and so we will investigate this new family further. We want to test whether some of the New Zealand species of Charopidae actually belong within Cystopeltidae or if the latter group is truly restricted to Australia and South America.

1 Comment

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    I never knew snails could be so interesting! Thanks Rodrigo and Lara.

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