26 November 2006

An operculate snail far from the sea: Hendersonia occulta

Terrestriality, the ability to survive and reproduce on land, has evolved independently several times in gastropods (snails). The close relatives of any group of land snails often reveal clues to where their common ancestors lived. The group of land snails with the most number of species is the pulmonates. Among their close relatives are the snails in the family Ellobiidae that always live by and still reproduce in the sea. One ellobiid snail that I wrote about was Melampus bullaoides. The ellobiids' close association with the sea indicates that the ancestors of pulmonates were marine snails.

Another family of snails that evolved to become terrestrial independently of the pulmonate snails is the Helicinidae. Their present-day distribution includes the subtropical and tropical zones of North and South America, the Indopacific and Pacific islands and some parts of Asia and Australia. The only species that seems to have moved away from the subtropical and tropical zones is the North American Hendersonia occulta. According to Hubricht1, the present range of this species consists of 2 disjunct areas: one in southwestern Wisconsin and northeastern Iowa and the other in the southern Appalachians (Virginia, West Virginia and down to Tennessee.

Hocculta1
Hendersonia occulta crawling. Note its single pair of tentacles.

A close inspection of the external anatomy of Hendersonia occulta offers several clues to its evolutionary origins. Perhaps, the easiest to notice of these is its single pair of tentacles like those of marine snails. Another clue, which may require closer inspection with at least a magnifying lens, is the location of its eyes on its head and not on the tips of its tentacles. And finally, the 3rd clue, which is even harder to notice, is that this snail has an operculum that it uses to close its aperture. Unlike the thick and hard operculum of Pomatias elegans, a species representing yet another independently evolved lineage of terrestrial snails, the operculum of Hendersonia occulta is thin and has much less calcium carbonate. Also, Hendersonia occulta can withdraw deep into its shell, which is something Pomatias elegans cannot do, because its inflexible operculum gets stuck at its aperture.

Now, we know that the ancestors of Hendersonia occulta were marine snails. In fact, some of its close relatives still are.

Hocculta2
The arrow points at the edge of the operculum of Hendersonia occulta.

The terrestrial pulmonate snails, on the other hand, have all but lost their opercula during their evolution. This vestige from their marine ancestors lingers on in the Ellobiidae: the larvae of at least some species still have opercula, which they lose before turning into adults.


1. Hubricht, L. 1985. The distributions of the native land mollusks of eastern United States Fieldiana #24.

3 comments:

Snail said...

I love that shot of the helicinid crawling.

Following on from the opeculate Ellobiidae, I thought you might be interested to know that southern Australia has a small number of pulmonates that possess an operculum as adults.

Species of Salinator (Amphibolidae) live in large numbers in the mud of temperate mangrove thickets.

There's also the tiny freshwater snails Glacidorbis spp., which was formerly thought to be a prosobranch. Their distriburion in SW and SE Australia and S America has a bit of a Gondwanan feel about it.

AYDIN ÖRSTAN said...

Thanks for the info. That gives me another reason why I should visit Australia one day.

I knew there were some aquatic pulmonates that retained their opercula as adults. I suppose the mangrove species would also be considered to be "aquatic" if they reproduced in the sea. It becomes more of a matter of definition.

AYDIN ÖRSTAN said...

Joan Jass of Milwaukee Public Museum e-mailed this comment:

"Subsequent to his 1985 publication, Hubricht graciously agreed to determine and verify identifications for terrestrial gastropods from the Milwaukee Public Museum collection on loan, confirming and generating many records for Wisconsin (Jass 1986). With regard to the southeastern area of the state, this included the 1905 record of Chadwick (The Nautilus XIX No.5) for this species from Milwaukee County. With regard to northeastern Wisconsin, in 1950 Levi & Levi (The Nautilus 63 No.4) reported this species from 2 counties in that area.

Your photos greatly appreciated!"