Biogeography studies the geographic distribution of living species, analyzing its causes, limits, historical determinants and the affinities between different regions, which are delimited and influenced by the characteristics of both the environments and the organisms under consideration.
From a technical standpoint, the term fauna refers to the set of species documented within a given geographic area, ascertained through direct sampling or through the identification of well-localized specimens preserved in collections. This activity is carried out by researchers, specialists, or knowledgeable amateurs, who publish faunal lists in scientific papers or amateur journals. The completeness and reliability of such lists generally increase proportionally with the number of samplings: the greater the number of field observations, the higher the probability of documenting even the rarest species. Repeated records of a species from the same locality further strengthen confidence in both its presence and its correct identification (or at least it is presumed, trusting in the accuracy of the identifications). In some areas, such as New Caledonia or Hawaii, collecting activity has generated an exceptionally rich body of records; in others, including various sectors of the Pacific, available information remains scarce or almost entirely lacking.
The family Cypraeidae occupies a large portion of seas and oceans worldwide, with a strong concentration in tropical and subtropical regions, with distributional limits extending approximately between the 45th parallel north and the 45th parallel south Only a few extremes cases - such as N. spadicea, C. hesitata, and the Mediterranean species - reach latitudes of about 25° in both hemispheres.
In malacology, considerable emphasis is placed on the study of species distributions and the faunal connections existing among different regions of the globe. The subdivision of the submerged portions of the Earth’s surface is based on affinities among malacofaunal distributions, considering both the presence and the frequency of molluscan species in the various regions. According to the degree of affinity between assemblages, bioprovinces are delineated and hierarchically organized into provinces, subprovinces, and regions. Such a subdivision cannot be sharply defined: transitional zones always exist between adjacent sectors, where malacofaunal characteristics intermingle. Moreover, striking similarities are often observed between provinces that are currently separated or distant, but which may not have been so in the past.
The factors influencing and limiting the distribution of mollusks are mainly related to dispersal capability and the presence of limiting factors (barriers). More specifically:
The resulting distribution patterns are often recurrent, as external factors exert the same limiting and selective forces on different species. Thus, one may observe species with wide Indo-Pacific dispersal, species endemic to specific areas, and species with highly restricted, point-like distributions. Although the species represents the basic unit of analysis, such patterns are not always exclusive to the species level and may also involve other taxonomic ranks (for example, genera or subspecies).
Among the most relevant zoogeographic studies (animal biogeography) devoted to cowries are those by the Schilder family (Prodrome, 1938), who proposed a subdivision into geographic sectors that has served as an inspiration for the present work. Here, leaving aside climatology, substrate types, and marine zonation, attention is focused on the lists of species reliably recorded for each area and on recurrent distribution patterns.
The intertropical belt occupied by cowries can be subdivided into units characterized by relative homogeneity in faunal composition. For example, the coast of Tanzania, together with the islands of Mafia, Pemba, and Zanzibar, represents a unit within which all cowrie species may potentially occur, within the limits imposed by the specific ecological niches of each species.
It should be emphasized that such distributional uniformity is theoretical: many species occupy restricted habitats or highly specific microhabitats. Nevertheless, for the purposes of geographic assessment, each species should be considered theoretically present throughout the entire unit, even if certain bays or islands have not yet yielded specimens.