(Mollusca, Gastropoda Prosobranchia) of the Afro- Tropical Region
Total Page：16
File Type：pdf, Size：1020Kb
SOME REFLECTIONS, MAINLY BIOGEOGRAPH- ICAL, ON THE LAND OPERCULATES (MOLLUSCA, GASTROPODA PROSOBRANCHIA) OF THE AFRO- TROPICAL REGION by A. C. VAN BRUGGEN (Departmentof Systematicsand EvolutionaryBiology, Leiden University, P.O. Box 9517,2300 RA Leiden; The Netherlands) Opgedragenaan Prof. Dr. W. Vervoortbij zijn af- scheidals buitengewoonhoogleraar in de systematische dierkunde en directeur van het Rijksmuseumvan Natuurlijke Historie 'And nowwhat rests, but that we mayspend the time With statelytriumphs, mirthful comicshows, Such as befit thepleasure of the court? Sounddrums and trumpets!_farewell,sour annoy! For hereI hope,begins our lasting joy.' WilliamShakespeare, Third Part of KingHenry VI, Act V, SceneVII. SUMMARY The land operculates of the Afrotropical Region represent a noticeably small minority among the land snails. As regards distribution, these snails are absent from most of the desert and semidesert areas (South West Africa and areas bordering on the Sahara) with the exception of the Horn of Africa. The less than 70 known species are little diverse, representing 4 families and 9 genera, 1 family and 5 genera of which are endemic (almost all species are endemic). The continental taxa usually have small shells (major diameter not more than 32.0 mm, but generally much smaller), while island dwellers east of the continent reach considerably larger sizes (major diameter up to 55.5 mm). The terrestrial prosobranchs are strikingly diverse on these islands, particularly on Madagascar; some families on the islands are not known to occur on the mainland and are of Asiatic derivation. Conditions of island life may have contributed to factors such as increased diversity and size. Low diversity and small size on the continent may be due to competition with land pulmonates as suggested by CAIN(1978); whatever the reasons, the terrestrial operculates of the African continent in some respects are ab- errant from those found elsewhere in the world. INTRODUCTION Land molluscs exclusively belong to the class Gastropoda. The great majority of the terrestrial or air-breathing gastropods consists of the subclass Pulmonata, which group has literally conquered the earth. There are few places in the world that do not harbour at least a few 285 representatives of this very diverse group. The pulmonates have ap- peared comparatively late in the history of the earth and of the molluscs (first fossil record of pulmonates: Late Palaeozoic occurrence in North America and Europe, SOLEM, 1979). A small minority of the terrestrial molluscs is more directly related to marine forms. These are the land operculates (formerly called the Pneumonopoma), consisting of only ten families of air-breathing pro- sobranch snails belonging to two different orders, the Diotocardia or Archaeogastropoda (Neritacea: Hydrocenidae) and the Taenioglossa or Mesogastropoda (Cyclophoracea or Architaenioglossa: 5 families; Littorinacea: 2 families; Rissoacea: 2 families). Two more Rissoacean families, the Trunctatellidae and Assimineidae, are only partly ter- restrial. As a group the land operculates are about equally old as the pulmonates, although the major families only appear considerably later (Jurassic and Cretaceous). Generally the land operculates are consid- ered an ancient element among the terrestrial gastropods, but this is more due to the age of the prosobranch lineage to which they belong than to duration of existence on dry land. On the whole the terrestrial prosobranch families are widely dis- tributed although not by far as wide as the pulmonates. There are obviously definite limits to their dispersal. The Pneumonopoma are restricted to the warmer parts of the world and the extremities of the various continents are devoid of these snails (fig. 1 ) .Indeed, the Holarc- tic Region (the largest of the zoogeographical realms and also the one containing the largest amount of inhospitable area) is particularly poorly endowed as regards terrestrial operculates. The limits in the Northern Hemisphere are in North America at ca. 40°N (PILSBRY, 1948; BURCH, 1962), in Europe at ca. 53°N (KERNEY, 1976b; KERNEY & CAMERON, 1979), while the situation in Asia is slightly more complex. Here the limits are western Asia (Caspian area and Iran, LIKHAREV & RAMMEL' MEIER, 1962), the Himalaya range and the eastern margins of the continent as far north as Japan at ca. 40°N (KOBELT, 1902). In the Southern Hemisphere the situation is different. In South America there are no land operculates south of ca. 30°S (PARODIz, 1957 ; JAECKEL, 1969), in Africa these reach the southernmost tip of the continent at ca. 34°50'S (CONNOLLY, 1939), while in Australia these do not occur south of ca. 30°S (BURCH, 1976; BISHOP, 1981 ), but on the other hand appear to be fairly diverse in New Zealand (POWELL, 1976). Incidentally, in a comprehensive paper on the Tertiary non-marine molluscs of South America (PaRODiz, 1969) there is no mention at all of terrestrial oper- culates. All this probably implies that these snails originated somewhere on the warm coasts of tropical or subtropical seas and subsequently spread to their climatic limits all over the world, adapting themselves to .