Islands, Report Stygobiont Crustacea Malacostraca Geologically Antillean
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Bijdragen tot de Dierkunde, 52 (2): 191-199 — 1982 Amsterdam Expeditions to the West Indian Islands, Report 18. Stygobiont Crustacea Malacostraca from geologically older and younger Antillean Islands: a biogeographic analysis by Jan H. Stock Institute of Taxonomie Zoology, University of Amsterdam, P.O. Box 20125, 1000 HC Amsterdam, The Netherlands Summary close coherence with geological and palaeogeo- data. Area-species graphs for stygobiont Crustacea Malacostraca of graphic seven islands in the southern Caribbean have been In is made compared. the present paper an attempt to It that the “constants” C and of these appears z graphs are of of islands evaluate the impact age certain on influenced by the geological time elapsed since the island’s stygofaunal diversity (the "age" being the time emergence. In older islands the values for C and z are higher than in islands. The values for z of and that island's younger younger elapsed since the emergence above sea older islands are much higher (0.79-0.97) than usually level). To this end, seven islands of similar obtained in literature for terrestrial animals (0.20-0.40). This and with similar climates in the be the limited faculties may explained by very dispersal of morphology K-strategists, such as stygobiont Malacostraca. southern Caribbean have been compared. Résumé On des des METHODS compare graphiques aréal-espèces pour Crustacés Malacostracés stygobiontes provenant de sept îles de la Mer des Caraïbes. have been taken in three Les «constantes» C et z dans ces graphiques Stygofaunal samples semblent influencées le écoulé être par temps géologique ways: depuis l’émergence de l’île respective. Dans le cas des îles The method. A hole is plus anciennes les valeurs aussi bien (1) Karaman-Chappuis sont plus élevées, pour C dans le des îles récentes. Les valeurs in loose alluvia. The que pour z, que cas dug inflowing groundwater des îles, trouvées pour z anciennes ou récentes, sont de and fine is ladled out passed through a net (mesh élevées beaucoup plus (0,79-0,97) en comparaison avec celles 0.05 mm). mentionnées dans la littérature pour des animaux terrestres Ceci The Bou-Rouch method. A hollow iron (0,20-0,40). s’explique probablement par les facultés (2) de très limitées des tels les dispersion stratégistes-K, que probe is driven in the substrate until it reaches the Malacostracés stygobiontes. groundwater table and the groundwater is then means of a Norton INTRODUCTION pumped up by hand-pump. (3) The Cvetkov method. A vertical plankton net the of During past 10 years (1973-1982) a team (mesh 0.05 mm) provided with a back-stroke biologists of the University of Amsterdam, occa- piston is lowered and lifted in drill holes, shafts, sionally assisted by scientists of other universities, and wells. have studied the fauna of groundwater biotas Methods (2) and (3) have been described in (cave waters, wells, springs, hyporheic and inter- more detail by Bou (1975). stitial environments) in the West Indies. Over 50 The samples are preserved in 4% neutralized islands have been sampled in a more or less formalin, sorted later in the laboratory, and trans- and uniformous systematic way. ferred to 70% ethanol. The project aims at an analysis of the of validity Taxonomie of the animal analyses groups en- a number of biogeographic models, explaining the countered in the samples taken have been, and origin of the West Indian nonmarine in fauna, will in be, published the present series of Reports the Amsterdam on Expeditions to the West Indian *) Report 17 appeared in Bijdr. Dierk., 52 (1): 13-42 (1982). islands. 192 J. H. STOCK - ANTILLEAN BIOGEOGRAPHIC ANALYSIS INTRODUCTION TO THE REGRESSION oped endemic taxa (subspecies, species, and even MODEL of marine genera) stygobionts. In several cases, still in relatives (not necessarily ancestors) are In I a number of previous papers, have advanced existence. Elsewhere (Stock, 1981a) I have called the theory that sea-level fluctuations have been attention to the fact that such stygobionts are scenario of great significance for the evolutionary restricted to islands in which regression of the of biotas of many members groundwater (the relative sea level predominated over transgressive so-called stygobionts) of the West Indian islands Most islands of the Leeward movements. group, (Stock, 1977b, 1980a). In particular negative of the Windward and of the Greater An- group due relative changes of the sea level, either to tilles belong to this category. In the Bahamas, in- eustatic of local geological uplifting, or to drops cluding the Turks and Caicos islands, transgres- of intertidal or the water level, caused stranding than sion has been much more important regres- For ele- shallow-water communities. many faunal sion. These islands strikingly lack regression model such strandings will have been catastrophic, ments, but be at elements, may invaded, presumably a such in particular of course for sessile organisms, rather recent (post-Pleistocene) geological epoch, A of events as reef corals. testimony catastrophic by marine species, penetrating karst features that be found on by negative sea-level movements can drowned during a marine transgression. islands of the Calcareous Antilles and many **) In the West Indies, for instance, regression of the Leeward group***), in terraces of fossil evolution is demonstrated by an abundance of level. Other coral reefs above the actual sea orga- hadziid taxa of Gammaridae (Amphipoda) on stand chance nisms kind of to a were pre-adapted islands of the Antillean whereas many arc, no In of sea-level regressions. partic- surviving during hadziids have been found in the Bahamian archi- ular those animals that (a) have a burrowing or pelago, where stygobionts are much more rare but interstitial and lack pelagic stages, life, (b) in the Antilles. anyway than the have instead brood care or pass greater part Limiting ourselves to rising islands, it would of their larval life inside the eggshell, belong to be the of their interesting to compare diversity this During the slow process of regressive group. stygofauna with palaeogeographic data. If the animals could movements of the sea level, these major factor influencing the evolution of stygo- survive in the humid macro- and micro-interstitia bionts in the Antilles would be the repeated trans- of coral rubble, gravel or coarse sand, and evolve and regressions of the sea level during the Pleisto- vicariant into groundwater in- through processes number of cene, one would expect a comparable habitants. In the interstitial biotope, a very gradual in islands of different taxa geological age. marine shift in can be observed, from fully salinity If the other hand the Plio-Miocene on regres- brackish near the waterline, through higher up sions are the most important factor in the evolu- In of the shore, to almost fresh more inland. many tion of the West Indian stygofauna, the older arid the Antillean islands, having (nowadays) an islands must have a significantly more diverse inland — most climate, the salinity even of the than islands. groundwater biota the younger mesohaline groundwaters — stays at oligo- or do not levels, so that real fresh groundwaters THE ISLANDS exist. The scenario in which vicariant evolution of stranded (uplifted) populations plays an im- For testing the two alternatives brought forward in the colonization of portant role groundwaters, in the last lines of the preceding paragraph, we is called the model references, see regression (for in the have selected 7 islands (6 Leeward group Stock, 1980a). plus Barbados) from a total of some 50 islands islands in the West Indies have devel- Many in sampled during our fieldwork program 1973- The islands with each other 1982. selected agree in that they have been subject to tectonic uplift **) Used in the sense of Weyl, 1966. and have calcareous More- of 8. important deposits. ***) Used in the sense Wagenaar Hummelinck, 1981: BIJDRAGEN TOT DE DIERKUNDE, 52 (2) - 1982 193 the over, islands are their THE AGE OF THE comparable by geo- ISLANDS of the graphic position, altitude, area same mag- The Netherlands islands nitude, and semi-arid climate. Leeward (Aruba, Bo- called The six islands of the Leeward north Curaçao, often the A-B-C group, naire, islands) as shallow shoals on a volcanic basement of the South American mainland, are Aruba, Cu- emerged the late Miocene the Los La during (De Buisonjé, raçao, Bonaire, Roques group, Tor- 1974). and in tuga, La Blanquilla. The seventh island, Bar- Geotectonic uplifts the post-Miocene periods raised abrasion bados, is selected because it is the southernmost are evidenced by terraces, ex- raised coral calcareous island of the north-south branch of cavated surf lines, and reefs, presently found altitudes to 150 m above level. the Antillean arc (this branch consists mainly of at up sea from each volcanic, non-calcareous, islands ). The position of The islands are separated other by very the islands in this and water to 900 waters occur seven compared paper, deep (700 fms). Deep certain other localities mentioned in the text, is also between Curaçao and Bonaire and the main- shown in fig. 1. land. Aruba, however, is separated from the South American a much shal- Every single island, or group of coherent keys, Paraguana peninsula by the and from the lower straits over 100 is isolated from next group con- (slightly fms). tinent by deep water (400 fms and more), with To the A-B-C islands of the Netherlands Lee- the exception of Aruba (vide infra). ward group belong, in addition to the three larger the small A number of islands of Leeward group are islands, two very ones, Klein Curaçao, to do not the of and Klein excluded from our analysis because they east Curacao, Bonaire, to the fulfill all is not west of Bonaire.