18 March 2006

Critique of an old paper on Fossaria modicella

For the past 4 years or so, I have been monitoring the population structures of 2 colonies of Oxyloma retusa (family Succineidae) at a small local lake. Although O. retusa is a land snail, it always lives near the water, on mud and on cattails sometimes growing directly out of the water. In that sense, it comes close to being amphibious. The data I presented at the 2004 annual meeting of the American Malacological Society1 showed (if you believe my interpretation) that the snails were very short lived and that there were 2 generations per year. I still have not published my data, because I don’t quite know how to interpret the data in an evolutionarily meaningful way.

Earlier this week I was reading a paper that mentioned that the aquatic/amphibious pulmonate snail Fossaria modicella had 2 generations per year and cited a 1935 paper by Harley Van Cleave2 as the source of that information. Naturally, I got excited. I went to the library, got a copy of the paper and read it right away, hoping to extract from it some insight that would help me better understand my O. retusa data. However, I was to be disappointed.

In short, Van Cleave collected 10 samples (a total of 1793 individuals) of F. modicella in Indiana over 3 years, measured the shell length of each. From these data, he concluded that there were “two distinct broods per year, a spring generation in March and a summer generation in July or August.” Almost exactly the same situation as what I have found to be the case in O. retusa.

But, there are 3 problems with Van Cleave’s study. First, he collected live snails and does not appear to have returned them to the study site after he measured them. On some occasions, he removed relatively large numbers of live snails, for example, 178 on 2 April 1934, followed by 229 on 22 April. This is potentially problematic, because, unless the total population is so large that a few hundreds of specimens is a negligible quantity, one can not rule out the possibility that removal of large numbers of live snails could be affecting the growth and reproduction of the remaining snails. (To avoid this problem, I have always returned the live Oxyloma to my collection sites within a day.) Second, collections were done not to obtain a meaningful spread of the 10 samples over a year, but at somewhat haphazardly picked dates (during family vacations). As a result, 3 collections were done in April, 2 in October, but none in May or September. Furthermore, more than a month elapsed between the sampling dates in July and August when, according to the author, a new generation was emerging. Third, without first demonstrating that the annual generation cycle was temporally the same each year, Van Cleave pooled and analyzed the samples from 3 years together.

Here are his data plotted. To generate smoother curves, I decreased the number of size classes from 15 to 8.


The bottom line is that from this graph, and taking into account the problems with the study listed above, I am not convinced that F. modicella had 2 generations per year during the study period. What I see is that the generation that survives the winter matures during the summer. Their offspring (the snails less than about 3 mm in length) survive the winter to start the cycle over again next spring. So, there is only one generation per year.

I will keep working on my Oxyloma data.

1. Abstracts of the 2004 AMS meeting. Mine is on p. 63.
2. Van Cleave, H.J. 1935. The seasonal life history of an amphibious snail, Fossaria modicella, living on sandstone cliffs. Ecology 16:101-108.

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