20 June 2008

Snails of the sea shore: neither aquatic nor terrestrial

The question of which snails may be considered to be "terrestrial" is continuing to occupy my mind. Some previous thoughts were in this post.

Only the species that spend their entire lives in the sea or on land may unambiguously be referred to as marine and terrestrial gastropods, respectively. The trouble is with those species that live in the transition zone between the sea and the land and which, because of their idiosyncratic lifestyles, cannot be labeled unambiguously as either marine or terrestrial; even the often used description of them as amphibious is not quite applicable to many species that avoid entering the sea.

A casual literature search has come up with several instances of glaring inconsistencies in the labeling of coastal snails as either marine or terrestrial. Here are some examples.

The genus Truncatella

—Pilsbry included Truncatella in his Land Mollusca of North America (1948).
—Abbott had in his American Seashells (1974) some of the same species that Pilsbry had considered to be land snails.
—In 1989, the same species were now in Abbott's Compendium of Landshells.
Truncatella subcylindrica was not in Kerney & Cameron's A Field Guide to the Land Snails of Britain and North-west Europe.
—Poppe & Goto (1991) included T. subcylindrica in their European Seashells, while noting that the species was actually more terrestrial than marine.
—Anderson (J. Conchology 38:607, 2005) included T. subcylindrica in his list of non-marine mollusca of Britain and Ireland.

The family Ellobiidae

—Pilsbry did not include the ellobiids, except the truly terrestrial Carychium*, in his Land Mollusca of North America (1948).
—Abbott had them in his American Seashells (1974).
—Some of the same species were later in Abbott's Compendium of Landshells (1989).

*Now placed in its own family the Carychiidae.


Kevin Bonham said...

I encounter the same thing in our local literature, with onchidiids, amphibolids, ellobiids, truncatellids, hydrococcids, pomatiopsids and some hydrobiids having appeared as both "marine" and "non-marine" in lists published in the last 30 years. Some kind of intermediate term is needed for this mob, although of course there is not one intermediate borderline sea/shore lifestyle but many. I have seen "marginal marine" used (although some of the suspects are more defensibly "marginal terrestrial"); your "semi-terrestrial" seems as good as any.

Either "marine" or "non-marine" seems valid for many of these species in some defined contexts but not in others. As for "seashells", as seashells are generally shells found while wandering the seashore, it seems reasonable (for the sake of assisting the beachcombing public to identify their finds) to include fringe species that can be found the same way, even if they don't actually live in the sea as such.

But as for "land snail" I reckon that anything that follows the tide in and out while avoiding inundation, hangs around the high tide mark getting splashed by waves or very high tides now and then, or lays its eggs in the sea is not a diehard "land snail", and as a worker on "land snails" I don't consider any of the local representatives of the families I mentioned above to be among my primary responsibilities. However, I'm not familiar with the groups that lay their eggs in the sea, so if there is one of them that does this but then spends most of its life cycle well away from the water I could revise that opinion. Also it may be that there are species in these families elsewhere that I would regard as truly "terrestrial". It's easier to demarcate "land snails" from (whatever you want to call the fringe molluscs) in some areas than others.


Kevin: Thanks for the detailed comments. I have also started using in my personal notes "coastal-terrrestrial" to refer to these snails.

Kevin Bonham said...

There's a bunch of snails I refer to as "coast snails", but they're a slightly different matter - I don't know how common this sort of habitat preference is worldwide but we have some species (punctids mostly) that are completely terrestrial, but always or usually occur along the coast (for instance under shrubbery on rocks above high tide, or on sand dunes). Some of these aren't known to ever occur more than a few hundred metres inland.