30 November 2006

It was raining mussels and clams

"During a cloudburst hundreds of Anodonta anatina (L.) dropped on the pavement out of a rather strange black squall that suddenly appeared over Paderborn" and "in Chester (Pennsylvannia) masses of the tiny Gemma gemma dropped to the ground on June 6th 1869 during a heavy afternoon storm".
Kobelt (1897) cited in Dörge et al. (1999)

Dispersal mechanisms of land snails is one of the recurring subjects on this blog. Previous posts discussed dispersal by birds (here, here), the wind and continental drift, and also Charles Darwin's interest in the subject.

Whenever I find a small colony of a rare species, the first question that usually comes to mind is "How did these snails get here?" And, whenever I am thinking about the distribution patterns of species restricted to specific habitats, such as Gallandia annularis, a related question pops in mind: "How do these snails disperse between suitable habitats?"

A paper I read this week, by Dörge et al.<sup>1, provides a review of several natural and anthropogenic (associated with human activities) passive dispersal mechanisms for land snails. Passive dispersal takes place when snails do not migrate on their own (which would be active dispersal), but are transported by some external agency.

Floods are cited as one natural mechanism that disperses land snails along the floodplains of streams. The authors speculate that modifications of flood plains by humans and the lowering of the intensities of floods by the building of dams may have caused some species that may have relied on floods for dispersal to become rare. As an example, they cite Candidula unifasciata, a rare European species that was apparently once common on floodplains.

One dispersal mechanism that I wasn't aware of is that until methods were introduced to purify plants seeds, land snails were commonly transported mixed with seeds across country borders.

Despite the abundance of, especially anthropogenic, passive transport mechanisms for land snails, the authors, however, caution that "[i]t is unknown, how many of these single long-distance transport events actually lead or have led to the establishment of new populations...Many of the passive transport pathways provide dead ends, because the animals transported end up in unfavorable environments..."

One anthropogenic mechanism discussed in the paper attracted my attention. Land snails, for example, Helicella itala and Pupilla muscorum, have been found attached to the fur of sheep. In Germany, mostly during the 19th century millions of sheep were kept in migratory herds that were annually moved between winter, summer and autumn pastures. Therefore, it is speculated certain land snail species may have been dispersed by the migrating sheep herds.

When I was reading about sheep-mediated dispersal, I immediately thought of the goats that we frequently encounter on Turkish mountains. Although I don't think there are migratory herds of goats moved between vastly separated regions of Turkey, some local herds may cover significant distances over one mountain or mountain chains. And, if snails do attach to their bodies, they may have been responsible for the dispersal of some mountain species, such as Gallandia annularis. The next time we encounter goats in Turkey I will try to come up with a way to examine some of them for possible snails.

1. Dörge et al. 1999. The significance of passive transport for dispersal in terrestrial snails (Gastropoda, Pulmonata). Zeitschrift für Ökologie und Naturschutz 8:1-10.


monado said...

If sheep and goats can carry small snails, what about wildebeest? They have a huge annual migration trek around Africa. I'm not clear on the details, but they travelled in huge herds to excape drought and find forage, then returned. Along the way they forded rivers. None of the wildebeest want to be first in the water, because the crocodiles are waiting. The first ones are pushed in by pressure from behind. But eventually they all go, churning up the mud. Surely one or two must carry along snails to the next river?

ali bowman said...

can snail eggs be evaporated inot the sky like the pacipictaion.. i may have spelled that wrong. but my friend told me that they can. and when they get larger, the clouds cant hold them any more and they ewventually fall down when it rains.


An egg cannot evaporate, but a small egg can be blown up into the sky. However, I don't think snail eggs would survive for very long in the air. I suspect that they would lose their water quickly & consequently, the embryo inside would die.

Nevertheless, it's been claimed that very small snails can be transported by the wind.

Vilisics Ferenc said...

Very interesting post! As a matter of fact, I'm looking for the referred paper, can't find it anywhere. Could you send it to me, please?

I think you have my email address from earlier isopod issues. Thank you! Ferenc Vilisics