Zoogeography of Marine Gastropod in the Southern Caribbean. a New Look
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Caribbean Journal of Science, Vol. 31, No. 1-2, 104-121, 1995 Copyright 1995 College of Arts and Sciences University of Puerto Rico, Mayaguez Zoogeography of Marine Gastropod in the Southern Caribbean: A New Look at Provinciality JUAN M. DIAZ Instituto de Investigaciones Marinas de Punta de Betin -lNVEMAR-, Apartado 1016, Santa Martar Colombia ABSTRACT . – Recorded occurrences of 266 species of higher Gastropoda were arranged in nine subareas along the coastal areas of southern Central America and northern South America. The value of different gastropod families as zoogeographic indicators and the degree of faunal “singularity” of subareas were inferred respectively from the mean value of the index of “Breadth of Geographic Range” (BGR) of species involved. BGR was lower among families exhibiting predominantly direct development. About 43% of species are widely distributed in the tropical Western Atlantic, whereas approximately 40% are endemic to the southern Caribbean – enough to consider this area as a separate zoogeographic province. Northern Venezuela, Santa Marta, and the Leeward Islands are the subareas richest in species, whereas the subarea between the Orinoco delta and Surinam is the most depauperate. The number of species occurring in a given subarea is more closely related to environmental heterogeneity than to the shelf extent of the subarea. A similarity level of 50% distinguishes five zoogeographic areas within the lower Caribbean, two of them as transitional to other tropical Western Atlantic Provinces and the other three are proposed as subprov- inces. Trade wind-induced upwelling along the coasts of northern Colombia and Venezuela on the one hand, and zoogeographic links of the present molluscan fauna to the Eastern Pacific on the other, are the main factors explaining the present distribution patterns of marine gastropod in the southern Caribbean. RESUMEN. – Los registros de 266 especies de gasteropodos superiors fueron agrupados en 9 subareas a lo largo de las costas del Caribe entre el sur de Centroamerica y el norte de Suramerica. El valor de las diversas familias de gasteropodos como indicadores zoogeograficos y el grado de "singularidad" faunistica de las subareas fue deducido a partir del valor promedio del indite de “Amplitud del Rango Geografico” (ARG) de las especies incluidas. El ARG fue menor en las familias con desarrollo larval directo. El 43% de las especies estan ampliamente distribuido en el Atlantico Occidental tropical y el 39.8% son endemicas del sur del Caribe – suficientes para considerar esa area como una provincia zoogeografica aparte. Venezuela, Santa Marta y las Islas de Sotavento son las subareas con mayor numero de especies, mientras que la subarea comprendida entre el delta del Orinoco y Surinam es la mas pobre en especies. El numero de especies en una determinada subarea esta mas relacionado con el grado de heterogeneidad ambiental que con las dimensioned de la misma subarea. Un nivel de similaridad del 50% define cinco areas zoogeograficas en el sur del Caribe, dos de ellas consideradas zonas de transition hacia otras provincial del Atlantico Occidental tropical y otras tres propuestas como subprovincias. La surgencia inducida por los vientos alisios en las costas septentrionales de Colombia y Venezuela, y las relaciones zoogeograficas de la ma- lacofauna actual con la del Pacifico Oriental, son los factores que mejor explican los esquemas actuales de distribution de los gasteropodos marines en el sur del Caribe. INTRODUCTION stated that better knowledge of the fauna The coastal and shelf areas of southern of the lower Caribbean could result in di- Central America and northern South viding the region into zoogeographical America as far as the Orinoco delta are subregions. Meyer (1973) and Moore (1974) known as the southern Caribbean marine first referred to the coexistence of wide- region. This area was frequently disre- ranging Caribbean species with remark- garded from biotic and biogeographic ably high numbers of poorly known en- studies, and its fauna considered as “typ- demic elements along the shores of north- ical” West Indian-Caribbean (Houbrick, ernmost South America. More recently, 1968; Bayer et al., 1970; Briggs, 1974), al- based on molluscan distributions, Cosel though Rehder (1962) and Work (1969) (1976, 1982, 1986), Petuch (1976, 1981, 104 SOUTHERN CARIBBEAN GASTROPOD 105 1982a, b, 1987, 1990), Gibson-Smith and regarded or misnamed in species lists. This Gibson-Smith (1979), and Diaz and Getting criterion was also used to exclude several (1988) provided further evidence that cer- species belonging to the families Colum- tain areas in the southern Caribbean ex- bellidae, Vexillidae, and Marginellidae. hibit a somewhat “anomalous” faunal The species chosen inhabit exclusively composition. Many endemic molluscan coastal habitats and the upper shelf zones species have been described in the last two to 100 m, for at least part of their range. decades from the southern Caribbean (e.g., Deeper occurring species have been often Bayer, 1971; Altena, 1975; Petuch, 1979, recorded only from a few localities. 1987, 1990; Diaz and Velasquez, 1986; Jong Recent works including lists of gastro- and Coomans, 1988). pod species from the countries of this re- The purpose of this paper is to document gion and additional records, plus species the faunal heterogeneity along the south- inventories from various localities were ern Caribbean coasts, and to test the some- used to develop a composite list of species what intuitively claimed existence of mol- that occur in the region (see Appendix). luscan faunules or “anomalously” com- Emphasis was placed on revisionary works posed areas, and its significance as zoo- such as Vokes and Houart (1986) and Vink geographic areas. The analysis is based on and Cosel (1985) rather than more general the recorded occurrences of species from works. Especial caution was devoted to se- selected families of prosobranch gastro- lecting the species described recently by pod that can be considered zoogeographic Petuch (1987, 1990), as some of them “indicators.” The origin of the present bio- (mainly in the families Olividae and Con- geogeographic relationships of molluscan idae) were described on the basis of subtle faunas in the southern Caribbean are dis- shell characters of single specimens falling cussed according to the emerging ideas. into the variation range of known taxa. Only records where a species was specifi- M ETHODS cally listed from an area were used; distri- Selection of Taxa. —Marine molluscs, par- bution maps suggesting that the range of ticularly gastropod, have been tradition- a species should include one of the areas ally used for determining paleobiogeo- examined, but not specifying a locality, graphic and zoogeographic patterns (e.g., were disregarded. Coomans, 1962; Hall, 1964; Woodring, 1974; Subareas. —Although the term “Southern Petuch, 1982a, b; Kohn, 1990). Many neo- Caribbean” refers conventionally to the gastropods exhibit a non planktonic de- area south of latitude 15°N to the conti- velopmental mode or have lecithotrophic nental coasts of Central and South America larvae with very low dispersal capacity (cf. (from the Honduras-Nicaragua boundary Radwin and Chamberlain, 1973), so they are to Trinidad), for the purposes of this paper often very restricted in their habitat pref- the “Southern Caribbean” embraces the erences. The durable shells of most pro- shores and shelf areas along the continen- sobranch gastropod preserve well as fos- tal coasts of Central and South America, sils and allow direct access to the paleo- from northern Costa Rica to Surinam- biogeographical record. Furthermore, gas- French Guiana, including the Leeward An- tropod are common collecting objects, thus tilles off Venezuela (Aruba, Bonaire, Cu- systematic lists or species inventories are racao, Margarita, and other smaller Vene- available for many areas. zuelan islands). Nicaragua and Trinidad Twenty-two families of higher caeno- have been disregarded, because data on gastropods were selected. All but two, Cy- their molluscan faunas are very scarce. praeidae and Ficidae, belong to the Neo- However, since the molluscan fauna of the gastropoda. Some families, such as Nassar- area between the Orinoco delta and Suri- idae and Turridae, were not included be- nam has been adequately documented, it cause most of their members are too small has been included for this analysis. (less than 5 mm) and taxonomically very According to broad environmental fea- controversial, so that they are often dis- tures and the availability of faunal infor- 106 J. M. DIAZ FIG.1. The nine subareas selected for the analysis. A, Aruba; C, southern half of the Colombian Caribbean coast; G, Goajira and Paraguana peninsulas; LI, Leeward Islands; M, Santa Marta; O, Orinoco delta-Surinam; P, Panama; R, Costa Rica. mation, the region was partitioned into which can be recognized on the basis of nine subareas (Fig. 1). This paper compares both ecological and faunal features. An the molluscan composition of these sub- outline of the environmental features of areas. The data available from Costa Rica each subarea is presented in Table 1. and Panama does not allow the creation of Comparative Analysis.— I used Czechan- more subareas there, since most of the lit- ovsky’s presence-absence index of similar- erature or specimen records list simply ity, which is widely used to determine af- “Panama” or “Costa Rica” as collecting finities among biotic assemblages, and is sites. Aruba (A) has been singled out from also employed to compare species overlap the remainder Leeward Islands because “its between geographic areas (e.g., Crovello, marine fauna is to certain degree distinct 1981; Wells, 1990). The formula is: from that of Curacao and Bonaire” (Jong 2c x 100 and Coomans, 1988). Furthermore, the sea S= floor around these islands drops away rap- a+b' idly to a depth of almost 1000 m, whereas where a is the number of species in subarea the maximum depth between Aruba and A, b the number in subarea B, and c the the Venezuelan coast does not exceed 135 number of species in common.