Biogeographic considerations
The water temperature is thought to be the principal limiting factor of the majority of shallow water species (Stuardo, 1964; Hall, 1964; Vannucci, 1964; Barnes & Hughes, 1988; Levinton, 1995), and it has been the basis of all discussion about the provincial limits in the southwestern Atlantic (Ekman, 1953; Boltovskoy, 1964; Stuardo, 1964; Vannucci op. cit.; Semenov, 1978; Coelho & Santos, 1980; Palacio, 1980; Absalão, 1989).
Ekman (1953) and Hall (1964) defined tropical region using 20°C and 18°C as minimal temperatures reached during the coldest months of the year. On the other hand, Hall (1964), working exclusively with molluscs, determined the marine shallow water climates by the number of consecutive days or months that shallow sea water is at required temperatures for reproduction and early growth. The Tropical Province, for example, is defined by the group of species that live at 20°C water temperature for six or more months, with no months cooler than 18°C.
According to Carcelles (1944), concerning the mollusc distribution, the Tropical (or Caribbean) Province has its northern limit at Cape Hatteras (35°15'N) and a southern limit at Cape Santa Marta Grande (28°28'S). Ekman (1953), based on reef formations and water temperature of 20°C in the coldest months, suggests the Rio de Janeiro region as the limit between the temperate and tropical faunas. Vannucci (1964) points out that mangroves should be used as indicators of the tropical fauna limit. There is no doubt that the Cabo Frio latitude 23°S represents the northern and southern boundary for many species, but the existence of mangrove swamps as far south as Santa Catarina, as well as a considerable number of species belonging to the Caribbean fauna, show that this limit is much broader than Ekman (op. cit.) and others had suggested.
Cabo Frio is suggested by Briggs (1974) to be the limit between the Brazilian and the Patagonian Provinces using 10% of endemism as the criteria for province definition. Briggs (op. cit.), consider the Brazilian Province as the region from the Orinoco River to Cabo Frio.
Palacio (1980), in his southwestern Atlantic zoogeographical review, do not recognize the Brazilian Province as a valid province and proposed the establishment of a zoogeographic province, the Paulista Province, based in many studies of the fauna, flora, oceanic circulation patterns, morphology, and geology of the Brazilian coast. This province would have as its northern limit the 23°C isotherm between southern Espírito Santo and Northern Rio de Janeiro, and the Southern boundary between Rio Grande do Sul and Uruguay (Fig. 1). This zone is characterized by having a significant portion of endemic elements, and some adjacent fauna members constituting a broad transition zone between tropical and temperate water species, having the gradual disappearance of the tropical fauna and the beginning of the patagonic fauna based on a north-south gradient. The distribution of the gastropods provides some insights for a better comprehension the southwestern Atlantic zoogeographical patterns.
The geographic distribution patterns of the gastropods species from the Margarita Shelf (10°54'N), in the Venezuelan Caribbean (Carvajal & Capelo, 1993), showed a great majority of species belonging exclusively to the Tropical Province (65.3%), with a considerable amount of species common to the Tropical and Paulista Provinces (31.9%), and a very low number for the Tropical, Paulista, and Patagonic Provinces (2.7%) (Fig. 2).
In the Marataízes region (21°10'S) (Floeter, 1996), the great proportion of tropical species as well as the new occurrences of warm water species found, reinforce the affinity between the malacofauna of the region with tropical waters, demonstrating the importance of the Brazilian Current in the maintenance of this biogeographic structure (Absalão, 1989) (Fig. 2). Marataízes is on the boundary between the Tropical Province and the Paulista Province. This region is almost entirely covered with wide shallow (10 – 25 m) calcareous algal banks, mainly of the family Corallinaceae. These extensive banks create various microhabitats of great structural and ecological complexity, similar to coral reefs. In fact, this benthic community is characterized by a high specific richness, a great variety of taxonomic groups, and many biological forms (Hong, 1982; Barnes & Hughes, 1988).
A substantial difference, however, was found between the proportion of species common to both tropical and Paulista regions when we compare the Marataízes region – 21°10'S (64.9%) and Margarita Shelf – 10°40'N in the Venezuelan Caribbean (31.9%). In addition, the fact that Marataízes represents the southern limit for many species (22.1% of the total), suggests that the Marataízes region is located at the beginning of a transition zone. Young (1995) working with barnacle distribution patterns, also defines the beginning of subtropical zone on latitude 21°S.
In Ubatuba (23°30'S) (Pires-Vanin, 1989), there is an important increase in the species percentages with cold water affinities – belonging to the Paulista and Patagonic Provinces – (43.2%) compared to Marataízes (2.3%) and Campos Basin – 22°30'S – (15.3%) (Miyaji, 1995); Nevertheless, there is still an important contribution of warm water species (16.2%) (Fig. 2).
In Rio Grande (32°S) on the southern boundary of the Paulista Province, temperate water species constitutes the great majority of the species (69.2%) (Absalão, 1986), showing not only an impressive latitudinal gradient in relation with Ubatuba, but also characterizing the Paulista Province as a transitional zone (Fig. 2). The circulation in Rio Grande shows a clear seasonal pattern, in which the interaction between the wind driven transport and geostrophic circulation over the shelf produce a net flow directed southward and offshore during summer – warm waters, and northward and onshore during winter – Sub-Antarctic waters (Lima et al., 1996).
The Cabo Frio region (23°S) does not seem to be a great natural barrier. Neves (1994), studying the Rio de Janeiro continental shelf micromolluscs, did not find remarkable differences between the northern and southern fauna of this region.
Cabo Frio itself is specially permeable to tropical species, having on the southern coast of Rio de Janeiro, 40.2% of species common to this fauna (Absalão, 1989) and also in Santos, SP, Vannucci (1964) found the same value. Although it functions as an ecological "filter" quite impermeable to patagonic species (Vannucci, op. cit.; Absalão, op. cit.; Mello, 1993).
Another important characteristic of the Cabo Frio area is the gradual disappearance of the calcareous bottom, due to the increase of terrigenous sediments in the South of this region (Mabesoone, 1971; Kempf, 1972).
As a result, the distribution of tropical species, related to calcareous beds would have its southern limit at Cabo Frio, not directly related to climatic zones, which traditionally limits the faunal provinces (Miyaji, 1995). This fact helps to explain the decrease of the species richness below Cabo Frio (23°S) (see Table 1), since calcareous banks are highly diverse environments.
The endemism rate of some distinctive groups: Gastropods – in the Campos Basin (22°30'S) and in Ubatuba (23°30'S), the endemic species of constitute respectively 12.9% (Miyaji, 1995) and 10.8% (Pires-Vanin, 1989). Errant polychaetes – the data provided by Lana (1987) and Paiva (1990 apud Miyaji, 1995) shows a high endemism rate (27%) for the of Paraná State coast. Reef fishes – there is a steadily growing evidence that a sizable portion of the Brazilian reef ichthyofauna is endemic to the western South Atlantic and many supposedly Caribbean reef fish species recorded for the Brazilian coast proved later to be undescribed endemic species (Moura et al., 1999) – supply elements for the establishment of the Paulista Province, as proposed by Palacio (1980). Additional taxonomic studies on the referred groups will be required to clarify some of these issues.
However, great differences between different places within the Paulista Province (Fig. 2), and the large proportion of species occurring from the tropical to patagonic regions (16.2% in Ubatuba, and 23.1% in Rio Grande) show the transitional character of the region (Fig. 4), as well as its biogeographic heterogeneity, agreeing with Stuardo (1964), Vannucci (1964), and Tommasi (1985). Moreover, the low overall molluscan endemism rate found by Palacio (1980) (4.27% exclusive of the supposed Paulista Province), and Rios (1994) in his extensive work (5.14% of endemism for Gastropoda), did not show sufficient consistency (at least in terms of molluscs) to support the individualization of this region as a biogeographic unit (Absalão, 1989; Melo, 1985).
Richness Patterns
The species richness tends to increase with decreasing latitude. It is common knowledge that the tropics are richer in life than colder regions (Ekman, 1953; Barnes & Hughes, 1988; Stevens, 1989; Levinton, 1995).
The best known richness gradient is the increase from high to low latitudes in continental shelf benthos (Levinton, op. cit.). The prosobranch gastropods, for example, show a strong latitudinal decrease in species richness from tropical regions, where primary production is continuous throughout the year, to the colder regions where this production shows seasonal pulses (Barnes & Hughes, op. cit.).
In the Brazilian coast there is a latitudinal decrease in the number of coral (Leão et al., 1988) and reef fish species (Moura et al., 1999).
The most commonly proposed factors responsible for this global pattern are: greater habitat diversity (environmental heterogeneity) in the tropics, the trophic stability, the continuous primary production and the diversity associated with complex three-dimensional architecture (of coral reefs or algae beds at low latitudes), the impact of predation in maintaining diversity and a greater physical oceanographic stability. This pattern is found when we plot the gastropod richness results obtained in works done along the western Atlantic (Fig. 3 and Table 1). Although different collecting methods were used (the comparison in Table 1 should be done very carefully, due to methodological problems such as: standardization of the sampling methods, mesh size, replication, and the great variety of size and depth in the sampling areas) the general trend is clear.
Observing the distribution of species along the southwestern Atlantic (Fig. 4), we find another interesting pattern: an increase of the proportion of species with greater latitudinal ranges (occurring from the tropics to Patagonia) from lower to higher latitudes. The proportion in Campos Basin were similar to Marataízes resulting in a slight deviation from the general trend.
Recently, Stevens (1989) proposed that there is an inversely proportional ecological connection between the correlation of geographical-range size and latitude (Fig. 4) and the correlation between species richness and latitude (Fig. 3). This is a phenomenon Stevens called Rappoport's rule. According to Stevens (op. cit.) this rule suggests that tropical organisms have narrower environmental tolerances than temperate or polar organisms.
The broad tolerance of individuals from high latitudes has lead to wider latitudinal extent in the geographical range of high-latitude species than of lower-latitude species. Although our results followed this pattern, there are many authors that do not agree with it.
Rohde (1996) and Rohde & Heap (1996) have shown that there are always exceptions to this correlation and that tropical animals have not been shown to have consistently narrower latitudinal ranges from higher latitudes. Thus, it cannot explain latitudinal gradients in species diversity.
Colwell & Hurtt (1994) have shown that a spurious Rappoport effect can be the result of a sampling bias, arising from a decline in the per-species sampling effort in high-diversity regions, even when the total sampling effort is equal.
Hopefully, the present study on gastropods biogeography will stimulate further interest and research in this area.
Acknowledgments — We would like to thank Paulo Márcio Costa, Ricardo Absalão, Brian Canas, Cíntia Myaji, Paulo Y. G. Sumida, Rosebel C. Nalesso and Samuel Snedaker for the helpful comments and revision; Gilberto Dias and Fabio B. Pitombo for helping in the samplings. This research was supported by a CNPq grant (No 120907/95-0).
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