Body Size, Aerial Dispersal, and Origin of the Pacific Land Snail Fauna

Abstract

Vagvolgyi, J. (Department of Biological Sciences, City University of New York, Staten Island, N.Y. 10301) 1976. Body size, aerial dispersal and origin of the Pacific land snail fauna. Syst. Zool. 24:465-488.-Minute genera, whose shell measures less than 100 mm, form 60.0%o of the Pacific land snail fauna. In contrast to this, the minute class only represents 27.1%o of the continental fauna. The mean generic size in the Pacific fauna is correspondingly lower: 11.9 mm vs. 20.6. The difference between the 2 groups is statistically highly significant. Similarly to the Pacific, the eastern Pacific and eastern Atlantic insular land snail faunas and the wide-spread fauna also are dominated by the minute genera. From these observations it is concluded that minute body size is advantageous long distance dispersal, both overseas and overland, in the land snails. This in turn indicates that the mechanism involved must be aerial, since only in this type of dispersal are small size and light weight advantageous. The minute species also are, as a rule, more abundant than the large ones, which constitutes a further advantage. Support to the hypothesis is provided by the facts that land snails have been recovered from the plumage of birds; that recently formed volcanic islands have been colonized predominantly by minute land snails, and that mineral particles considerably larger and heavier than the minute land snails have been collected at high altitudes by airplanes. Alternative hypotheses are considered and rejected on the grounds that their tenets are unproven, unnecessary or erroneous; however, it is emphasized that the relationship between body size and area size needs further analysis. The characteristics of insular land snail faunas are summarized as follows: predominance of minute genera; high frequency of primitive, orthurethran genera and families; resemblance to the fauna of the nearest continent; low degree of endemism on the family level, increasingly higher, on the genus and species level, relatively recent origin, through aerial immigration. [Biogeography; body size; aerial dispersal; Pacific land snail fauna.] The aim of this paper is to present indirect, nevertheless strong, evidence in support of the hypothesis that the land snail fauna of the Pacific islands originated through overseas dispersal, primarily through the air. This accounts the fact that minute genera, measuring less than 10 and often only 2-4 mm as adults, predominate in the Pacific fauna. This hypothesis represents an ecological approach to the problem, contrasting to the historical approach of the alternative hypotheses. The hypothesis of overseas dispersal has been supported, in one form or another, by a long line of distinguished malacologists and biogeographers such as Kew (1893), Stearns (1893), Dall (1896), Hedley (1900), Guppy (1906, 1917), Dammerman (1928, 1948), Gulick (1932), Clench (1938), Darlington (1938, 1957, 1965), Gressitt (1963), Rees (1965), Carlquist (1965, 1966, 1972), and Peake (1969). Whereas most of the earlier authors thought that rafting was the most important method in overseas dispersal, the more recent ones considered the winds the main agents, especially where the minute species were concerned. Gulick, to whom particular credit is hereby given, lucidly stated this: for the most part only those snails will colonize in distant islands that are small enough wind transportation as adults (1932:414). He referred to these minute species as molluscan faunule or pulmonate micro-fauna. But it was recognized that probably no single method is exclusive to any particular group of molluscs (Peake, 1969:303). From a theoretical point of view, aerial dispersal seems the best method of overseas dispersal. Because, as Miller (1966) summarized it, it is more rapid than rafting so that the air-borne colonists reach their