17 November 2005

Some questions on terrestrial mollusks

What is the highest altitude at which live land snails have been collected? Land snails live at many high altitude mountains, including the European Alps and the North American Rocky Mountains. I wouldn't be surprised to find land snails at any altitude provided that the location is free of ice and snow for at least 2-3 months a year.

Are there any sinistral slug species? Despite the external apparence of a slug that may give the first impression that its body is symmetric, all slugs species that I know of have their breathing holes and genital openings on the right sides of their bodies. In that respect, they are like dextral snails. And just as one occasionally encounters a sinistral specimen of a snail species that is normally dextral, one may occasionally find a sinistral slug that has its breathing hole and genital opening on the left side of its body1. Moreover, there are many snail species that are normally sinistral. Are there also normally sinistral slug species?

Most (all?) sinistral snail species have shells that are taller than they are wide. Slugs have evolved from snails. Although some slug species have completely lost their shells, many species still have a small external or internal vestigial shell. There are also so-called semi-slugs, snails with shells that are much smaller than their bodies and into which they cannot withdraw. The shells of semi-slugs are always wide, not tall. This suggests that slugs have descended from snails with wide shells. And since sinistral snail species with wide shells are rare (absent?), this may be the reason why there are no normally sinistral slug species, or if there are any, why they are rare.

What is the smallest land snail species? The smallest North American land snails are probably Punctum minutissimum and Guppya sterkii. Pilsbry gave the diameters of shells of P. minutissimum as 1.5 mm or less and those of G. sterkii as 1.2 mm. The largest G. sterkii shells I have measured were 1.3 mm wide. The smallest European land snails may be Punctum pygmaeum and Truncatellina claustralis. The shells of the latter species are 1.5-1.8 mm long. Vagvolgyi2 noted that "Some species of the genus Ptychodon reach only 1 mm as adults, Charopa 1.2, Pronesopupa 1.3". Back in August, I had a post on miniaturization in animals and Tim Pearce commented that some adult diplommatinid land snails could be as small as 0.7 mm long. But I think this information, which came from a 1938 publication, needs to be confirmed with more recent work.

Who discovered that pulmonate snails are hermaphrodites? All pulmonate snails are hermaphrodites. I have written about the American naturalist Thomas Say, who, in an 1818 paper, gave dimensions for “male” and “female” pulmonate snails. Twelve years later, in his American conchology3, Say wrote that the snails in the then catchall genus Helix were hermaphrodites. It seems that sometime during the first decades of the 19th century it became established that pulmonate snails were hermaphrodites. I have done some searching and asked around, but haven’t found out who first figured out that pulmonates were hermaphrodites. Was it Lamarck?

Are live land snails or their live eggs being transported over long distances by the winds? There are numerous records of live land snails that have been found on migrating birds, but in 1965 Rees4 wrote that "Direct evidence for the transport of minute snails [by hurricanes and tornadoes] is not yet forthcoming". Ten years later, Vagvolgyi2 speculated that very small land snails can be transported by air currents. But I am not aware of a published record of live land snails, their eggs, or even empty shells, that have been collected high enough above the ground to eliminate local wind-blown transport.

1. Reise et al. 2002. A sinistral specimen of the terrestrial slug Arion lusitanicus. Malakologische Abhandlungen 20:247-252.
2. Vagvolgyi, J. 1975. Body size, aerial dispersal & origin of the Pacific land snail fauna. Systematic Zoology 24:465-488.
3. Say, T. 1830. American conchology: or, Descriptions of the shells of North America. New Harmony, Indiana.
4. Rees, W.J. 1965. The aerial dispersal of mollusca. Proc. Malac. Soc. London 36:269-282.


Tim Pearce said...

Normally sinistral snails tend to be taller, but some wider snails are also sinistral, for example many Bradybaenidae in E Asia. Regarding species that normally have both dextral and sinistral forms (e.g., Achatinellidae, Partulidae), most (all?) are the taller forms, and Asami and others have hypothesized that is because tall snails tend to mate one on top of the other with heads the same direction, which would make mating (finding the hole) between different handedness (=chirality)snails easier, while flatter snails tend to mate with right sides of their heads together (Oreohelicidae is an exception - having flat shells but mating one on top), which makes mating between different chirality more difficult.

pascal said...

I was reading a newsletter from Yellowstone NP regarding a comment one of the naturalists (rangers?) wrote about snails getting into the fur of large mammals when they bed down for the night.

Some snails lay eggs on leaves and the leaves can blow around, while I suspect snails aestivating on wood that floats to a new island is a good source too.

How they move upstream into new continental areas, that's an answer I would really like to have...

Angus said...

I agree that there do not seem to be any sinistral slug species, at least based on my limited experience.

However, I think that the more likely explanation for the lack of sinistral slugs comes from their mating position: most slugs that I know of mate reciprocally in a face-to-face position, just like most low-spired snails. It is not so much the shell shape that constrains the evolution of chiral variants, but the mating position. Unfortunately, as mating position, shell shape, and the chances of being “the wrong way round” (and whether they have love darts or not, incidentally) are all highly correlated, then it is difficult to be absolutely certain.

As you say, if there are any sinistral slug species out there, then they will probably be closely related to high-spired species of snail. There are definitely some candidates – e.g. Bulimulus guadalupensis and the semi-slug Gaeotis nigrolineata from Puerto Rico (both dextral though).



I agree that the reason why there may be no sinistral slug species is because they mate face-to-face.

I am not familiar with Bulimulus guadalupensis & Gaeotis nigrolineata. I have to look them up. Thanks for the info.