Man As an Agent of Crustacean passive dispersal via useful plants-exemplified by Ostracoda Ospiti Esteri of the Italian ricefields ecosystem-and implications arising therefrom

The eggs of many crustaceans are minute and resistant to desiccation. These properties favour their accidental dispersal and parthenogenesis ensures survival in the newly occupied niches. Agencies by which such dispersal is achieved have been long ensconced in the literature. The role of man as an agent of crustacean dispersal via the spread of useful plants, however, has been relatively neglected hitherto but the example provided by ostracode ospiti esteri of the Italian ricefields ecosystem indicates that this neglect is unwarranted. Italian historical and archaeobotanical records concerning the introductions of numerous other useful plants make it obvious that the dispersal potential of this agency cannot be restricted to rice only. When modern Ostracoda assemblages from Europe are compared against fossil ones, their dissimilarity suggests a high proportion of introduced species in the modern biota. The implications of this are discussed. Students of crustacean biogeography must come to terms with several aspects of crustacean physiology that can affect distribution patterns. One such aspect is the ability of numerous crustaceans inhabiting terrestrial aquatic ecosystems to reproduce parthenogenetically. The widespread dispersal of many taxa, even in tercontinentally, has been linked frequently to this phenomenon which is known as geographic parthenogenesis. We stress two particular facets of geographic parthenogenesis: firstly, that parthenogenetic populations tend to form the dispersal fringe of the taxa concerned; secondly, that where amphigonic populations are also known for the taxa under study they tend to be confined to the zoogeographic regions where these taxa are native (McKenzie, 1971a). Some possible exceptions will be discussed later, but these serve only to accentuate rather than invalidate our ideas. Another germane aspect of crustacean physiology is the ability of many taxa to produce desiccation-resistant eggs; and linked to this is the fact that the eggs of most crustaceans that inhabit terrestrial aquatic ecosystems are very small — for Ostracoda they rarely exceed 50-µm diameter. The combination of resistance to desiccation and minute size makes it unsurprising that passive dispersal is the rule for these Crustacea, as confirmed by a vast literature that is recycled regularly by reviewers. Our purpose in this paper is to stress the hitherto little emphasised role of man as an indirect agent of crustacean passive dispersal via the spread of useful plants. Our exemplars are Ostracoda of the ricefields ecosystem, specifically some species introduced into the major Italian rice-growing region of Lombardy-Padana (Fig. 1). We also cite records (historical as well as archaeoethnobotanical) of the introduction into Italy of rice and many other useful plants via which desiccation-resistant ostracode eggs might have been incidentally dispersed. Our discussion of this more general hypothesis will conclude with reference to comparisons between fossil and modern European ostracode assemblages.