My research interests concern the Taxonomy, Biogeography and Reproductive Biology of gastropods, of Israel and in general.
Taxonomy, Biogeography and Conservation
Israel is exceptionally rich in terrestrial gastropods. Many of my studies, accordingly, deal with taxonomic revisions. This effort culminated in a book, the first ever on the land snails of Israel, in which all terrestrial gastropod species of Israel are described and illustrated, including detailed distribution maps. Some distribution patterns bear evidence of an ancient fauna that once reached Israel from the north and that today remains in a few tiny, isolated pockets in the Negev and Sinai.
The database of these taxonomic-distributional studies served to develop a new bio-geographic methodology for modeling faunal responses to climatic gradients (with R. Kadmon). The integration of a modern computerized technology with multivariate techniques is suitable for the identification and interpretation of bio-geographic patterns at the level of an entire fauna, because it enables to simultaneously analyze the distributions of several species. This opens new possibilities for bio-geographic analysis, since it becomes possible to identify and interpret patterns of bio-geographic variation at levels of an entire fauna.
This database also served for the development of a new conservation methodology, unique in that it is group-orientated. Again using land snails as a case study, I developed (with U. Safriel) a priority scale for conservation, based on three groups of criteria: 1. Range-related vulnerability. (Species with narrow global distribution get higher conservation priority than those with broad distributions; within a country, higher priority goes to species with very narrow ranges). 2. The genus. (High conservation priority goes to species of a genus with few species; within a country, priority goes to a species of a local genus). 3. Pragmatic considerations. (Unusual features should increase conservation priority). For each species, priority within each of these criteria is expressed in numerical values and the higher the total score, the higher its conservation priority. This priority scale is simple, straightforward, makes full use of museum material, can be applied within a short time, updated by the addition of new records and adjusted to fit current developments in taxonomy. Most criteria are objective scientific records, and are exposed, as such, to standard scientific criticism.
Freshwater gastropods, as they are abundant in fossil assemblages, bare direct evidence of the faunal history of a region. I carried out several taxonomic revisions of freshwater genera of the Levant. Faunal origins of the Levant include northern elements coming from a vast faunal province that began evolving 7-8 million years ago, with faunal connections between the Levant-Anatolia and Spain-Morocco, yet circumventing mainland Italy and France. Tropical elements may have reached the Levant directly, from Africa or the Orient; or first they may have reached Anatolia, from where they eventually spread southwards. At first the Euphrates, Orontes and Jordan formed a common faunal pool but by 2 million years ago the fauna of the Jordan had diverged from that of the Orontes. Some 780,000 years ago the fauna was exceptionally diversified with many new, northern elements and considerable endemism. Many of the mid-Pleistocene species did not survive to recent times. Modern distribution patterns reflect an ancient system of rivers that drained from east of the Jordan westwards to the Mediterranean Sea, across the Jordan Valley of today.
Noteworthy, among Modern species of freshwater gastropods, hybrids were found. Also fossil hybrids were found, at 780 000 year old and 1.4 Million years old sites of the Rift Valley. This is the earliest direct evidence of hybridization among mollusks (among all animal groups?) in nature, that is still going on today in the same region and aquatic system, among the same species. Classic evolutionary theory argues that hybrids are unfit, and therefore doomed to become extinct. The fossil hybrids of the Rift Valley pose a gap between this classic evolutionary theory on the one hand, and reality of nature on the other.
These taxonomic studies of fossil freshwater gastropods also form the basis for the development of a new, general taxonomic methodology. I have recently introduced (with S. Yitzhaki) a methodology recently developed in Economics, to enable the classification of fossil material into recent taxa. The methodology is advantageous in that it does not pose the requirement of a normal distribution. It differs from the intuitive, heuristic approach in that it offers orderly, quantitative tools to explore among several characters and decide on the best discriminator. Moreover it provides a measure of the robustness of classification. This methodology is represented in formal terms and also in informal terms, through the fossil freshwater gastropods of the Jordan Valley.
Longevity. To reveal common patterns underlying modes of reproduction I surveyed the literature for data on the longevity of Mollusks of marine, freshwater and terrestrial habitats. Life spans of mollusks range from two months to two hundred years. A short-lived mode of life is correlated with lack of an external shell, or possession of an external shell that is semitransparent. This correlation may be explained in adaptive terms, in that shell absence may affect age-specific mortality via growth rates, or shell-less mollusks may utilize transient food resources. However, the ubiquity of this correlation favors a non-adaptive explanation: shell and longevity co-vary, so that an initial, adaptive change in shell engenders a secondary, automatic change in the life span. Thus, the short lifespan of many mollusks may be a by-product of selection on the shell rather than an independently selected trait.
Hermaphroditism. To reveal further common patterns underlying modes of reproduction I surveyed the literature for data on hermaphroditism among various groups of Mollusks. About 40% of the 5600 mollusk genera are either simultaneous or sequential hermaphrodites. The ubiquitous occurrence of hermaphroditism throughout the Euthyneurans, coupled with this group’s widespread dispersal in marine, terrestrial and freshwater habitats and its wide range of population densities, suggests that hermaphroditism is not an adaptive response to selective forces of the environment and it may well be a phylogenetic constraint. A few stylommatophorans have broken this constraint by possessing genitalia in which the male system is absent or extremely reduced. In theory this is advantageous in that these species save the cost of developing two sets of reproductive apparatus. In reality there is no record of aphally occurring throughout an entire species, as one may expect if this saving of cost were so overwhelmingly advantageous. There also is no record of individuals developing only the male system, as one may expect if there was any evolutionary trend from hermaphroditism toward gonochorism. Hermaphroditism beyond the Euthyneura is widespread among mollusks that live in close and permanent intimacy with live marine invertebrates, whether as parasites, commensals or predators.
Parthenogenesis. Among animals sexual reproduction is the most abundant reproductive strategy and many theories attempt to explain its abundance. The Red Queen hypothesis argues that sexual reproduction will be favored when virulent parasites exert a time-lagged, negative frequency-dependent selection on their hosts. I investigated this hypothesis in an aquatic snail, in which both sexual and parthenogenetic individuals exist in natural populations in Israel, and some populations are heavily infested with trematodes. Male frequency is not correlated with the frequency of individuals infected by trematodes, and these results do not support the Red Queen hypothesis. In samples with high male frequency the number of embryos is low, perhaps representing a trade-off between mating and egg production costs. It is sometimes argued that parthenogenetic species are doomed to an early extinction. However both parthenogenetic and sexually reproducing species have been living in the Jordan Valley during the past 2 million years, under rapidly shifting physical and biotic conditions. This suggests that parthenogenetic species are not necessarily doomed to an earlier extinction than sexual species.