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Acta Oecologica 19 (3) (1998) 309-321/ O Elsevier, Paris
Biodiversity patterns of the marine benthic fauna on the Atlantic coast of tropical Africa in relation to hydroclimatic conditions and paleogeographic events
Pierre Le Lœuff '*, Rudo von Cosel
Çmire Orstom de Brest, BP 70, 29280 Plouzané, Frunce. Laboratoire de biologie des iwertébrk marins et malacologie du Muséum national d'histoire nalurelle, 55 me Buffon, 75005 Puns, Fraiace. ;I Correspoiidiiig author
Received September 17,1996; revised February 24,1997; accepted September 1,1997
Abstract - Five different hydroclimatic regions are recognized in the tropical eastern Atlantic: the northern alternance region (Cape Blanc - Cape Verga), the atypical tropical region (Cape Palmas - border BeninlNigeiia), the southern alternance region (Cape Lopez - Cape Frio), all with periodical upwelling of colder water, and two intercalated typical tropical regions with warm water and reduced salinity. The faunal rich- ness in the regions with upwelling is higher than in the typical tiopical regions because many benthic species avoid warm and reduced salinity water, Faunistic exchange and affinity are greater between the upwelling zones and the bordering temperate zones. The cold regions are also more similar in faunal composition. Benthic communities in both tropical and temperate eastern Atlantic are not fundamentally different. Species diver- sity of benthic invertebrates in tropical West Africa is about the same order of magnitude as in Europe and the Mediterranean. Hydroclimatic con- ditions (upwelling, salinity reduction and internal waves with drastic temperature changes) and absence of coral reef formations do not favour the establishment and thriving of a warm stenohaline and stenotherm fauna in West Africa. Paleozoogeographic events, such as the formation of a large Euro-West African tropical province during the Miocene after the breakup of the Tethys, repeated climate deteriorations during Pliocene and Pleistocene with reductions of the tropical zone, sea level changes and large-scale extinction of warm-tropical species, are also factors responsible for the current low biodiversity in the tropical eastern Atlantic. Some species survived in two major relict pockets in Senegal and southern Angola with particular ecological conditions. O Elsevier, Paris
Tropical West Africa / marine benthic fauna / biodiversity / hydroclimate / paleozoogeography
1. INTRODUCTION and by Cosel [14] in a study on the zoogeography of bivalve molluscs of the tropical Atlantic. The first biological studies of the marine fauna of The purpose of this study is to extend and confirm the Atlantic coast of Africa have revealed the existence these observations by correlating distribution patterns of drastic faunistic changes at Cape Blanc in Mauri- of benthic species belonging to the best known zoo- tania (20'59' N) and Cape Frio in Northem Namibia, logical groups with the recognized hydroclimatic divi- near the Angolan border (18'30' S), that were corre- sions. lated with biogeographic barriers [30, 34, 37, 47, 67, 68, 69, 781. The tropical West African region is con- 2. HYDROCLIMATIC DIVISIONS ALONG THE sidered today to be situated between these two limits; WEST AFRICAN COAST it is characterized by a specific marine fauna. Within this part of the West African coast, hydroclimatology On the West African coast, cold waters from upwell- is quite complex, with zones of periodical upwelling ings are permanently present in the north of Cape of cooler water, but now well known [4, 12,48,49,61, Blanc (Canary region) and in the south of Cape Frio 70, 79, 921. Regional hydroclimatic divisions can thus (Benguela region). Between these points lies a tropical be identified. The importance of these hydroclimatic zone delimited by two thermal fronts with steep gra- divisions within the trop_ :al zone for the marine fauna dients that are often associated with salinity changes.
1- €ïas bëeñ-ñoETgy Intbs and Le Loeuff [52] in their The fronts are marked by a close set of surface iso- , work on the distribution If West African polychaetes, theiins between 23 and 27 'C. In the north, the current
Fonds Documentaire ORSTQ 310 P. Le Lœuff, R. von Cosel regime (Canary Current) seems to favour some fau- upwelling disperse along the Gulf of Guinea coast, nistic exchanges in spite of the thermic barrier; but this from Ivory Coast to Benin, causing the formation of tfoes not appear to be the case in the south where the two additional thermic fronts [61]. The first front, well direction of the Benguela Current south of Cape Frio marked, occurs at Cape Palmas on the border between moves away from the continent in the direction of St. Liberia and Ivory Coast, whereas the second, more Helena Island [l, 851. As a result, the meroplancton of diffuse, occurs at the border between Benin and species capable of establishing themselves in the trop- Nigeria. Between these fronts, the so-called central ical zone is transported offshore. atypical tropical region takes place. Thus one can distinguish two types of hydroclimatic regions within The two thermic fronts move seasonally to the south the tropical zone: contrasted regions and uniform (Cape Blanc Front) and north (Cape Frio Front) to rel- regions ($gum I). The provinces with contrasted atively fixed positions [4, 581 that delimit the regions hydroclimate include the northern alternance region dominated by strong seasonal contrasts, called the (Mauritania, Senegal, Gambia, Guinea-Bissau, alternance regions. In northern winter, the thermic extreme north of Guinea), the southern alternance fronts reach the southmost points of Cape Verga region (Gabon, Congo, Angola) and the central atyp- (Guinea) in the north and Cape Frio in the south. In ical tropical region (Ivory Coast, Ghana, Togo, Benin). northern summer, they ascend to the northmost points The latter region forms a climatic enclave surrounded of Cape Blanc and Cape Lopez (Gabon). During this by the two typical tropical regions, the northern, or season (July to September), cold waters from rather western region (greater southern part of Guinea,
WW 100 00 100 E
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ZOON
100
00
100 S
Figure 1. Hydroclimatic regions on the coast of tropical West Africa (2-6). External regions: 1. Atlantic coast of Northwest Africa and Europe, Mediterranean; 7. Namibia and Atlantic coast of South Africa. The zones of permanent and periodic upwelling are hatched.
Acta Oecologica J
West African marine benthic fauna biodiversity 311
Sierra Leone, Liberia) and the southern, or rather [39], ophiurids [64], asterids [lo, 11,72,73] and echi- eastern region (Nigeria, Cameroon, Fernando Poo noids [8, 711. However, literature on holothurians is [Bioko], Equatorial Guinea, extreme north of Gabon). very dispersed across the end of the 19th century to the These typical tropical regions are characterized by late 80s (see [9]).The same situation exists for poly- waters with little seasonal temperature variation but chaete annelids, although a number of important fau- important salinity fluctuations near the coast. They are nistic studies was published, notably Augener [2], subject to the largest precipitations of the West African Fauvel [25-271, Fauvel and Rullier [28-321, Tebble coast. The thermic characteristics of each region [87], Kirkegaad [54, 551, Rullier [81-831, Zibrowius (figure 2) are taken from Hastenrath and Lamb [46]. It [95], Intès and Le Loeuff [50,51]. Gastropod molluscs should be noted that the Cape Verde Islands are in the were not included in this study owing to the frequent northern alternance region (see also [13]), whereas the uncertainties of locations and the numerous doubtful islands of Annobon, São Tomé and Principe, of which or obviously incorrect identifications in the literature. at least the first two are affected by equatorial The West African marine gastropods are in need of a upwelling, are included in the southern alternance thorough revision. region. Most of the systematic works used for this study are based on biological material collected either by ocea- 3. MATERIAL AND METHODS nographic expeditions along the coast of West Africa I (Belgian expedition on the coasts of the southern Atlantic; Danish expedition of the RN 'Atlantide' all All benthic invertebrates identified in our study were along the West African coast; expeditions of the FUV recorded from tropical West Africa between Cape 'Calypso' to the Gulf of Guinea and the Cape Verde Blanc in the north, and Cape Frio (or southern Angola) Islands; expedition of the American R/V 'Pillsbury' in the south, from the continental shelf and slope down to the Gulf of Guinea), or during other cruises in more to 200m. The studied species belong to the best or less limited parts of the coast on board smaller known zoological groups, that include species for research vessels of marine research institutes during which the distributional gaps are not likely due to sam- both colonial and independence periods. The most pling deficiencies. Some of these were subjects of sys- important of these institutes are Ifan (Institut Fonda- tematic and biogeographic surveys across West Africa: mental d'Afrique Noire) which worked in Mauritania, gorgonians [43, 441, brachyurans [65], bivalve mol- Senegal and Guinea, Wafri (West African Fisheries luscs [15, 741. Biological studies were also conducted Research Institute) which covered the shelf of Sierra on other benthic invertebrates, namely pagurid crusta- Leone, Ghana and Nigeria, and Orstom (now Institut ceans [35, 36, 381, and echinoderms, such as crinoids Franpis de Recherche pour le Développement en Coopération) working in the waters of Mauritania, Ta C Senegal, Guinea, Ivory Coast, Cameroon and the Congo. The samples were taken by a multitude of 28 - sampling gears, mostly different types of bottom grabs and dredges, as well as beam trawls and otter trawls of various sizes rigged up either for shrimp or fish, 26 - without counting littoral collecting intertidally or by scuba diving. The total number of sampling stations is estimated in the thousands. The data drawn from these 24 - samplings can be considered reliable concerning loca- tions and taxonomic identifications; however, their heterogeneity does not allow any quantitative studies, 22 - and we had to limit ourselves to the presence or absence of a species in a given region. 20 In total, the distribution of 1 449 benthic species was examined and determined as precisely as possible. The species were distributed as follows: 44 gorgones, 606
18 polychaetes, 170 brachyurid crustaceans, 59 pagurid Id crustaceans, 380 bivalve molluscs, 190 echinoderms. The presence or absence of each species in the five JFMAMJJASOND regions within the tropical zone was noted, as was Figure 2. Mean monthly surface water temperatures by hydroclimatic their distribution in the north from Cape Blanc to regions of the tropical Atlantic coast of Africa (seeJSgureI legend for Noiway and the Mediterranean, and in the south from details). Cape Frio to Cape Agulhas.
Vol. 19 (3) 1998 3i2 P. Le Lœuff, R. von Cose1
Data on the distribution of each species were plotted the tropical zone; and v) fraction of species present on graphs as follows: i) distribution pattern of species north of Cape Blanc or south of Cape Frio, also found avoiding the two typical tropical regions and the atyp- in other regions of the tropical zone (in both cases - ical tropical region; ii) distribution pattern of species the total fauna is considered, and in the first case only, avoiding the two typical tropical region only; iii) each of the zoological groups). number of species by region for all species and then Finally, a correspondence analysis was run to deter- separately for each zoological group; iv) number of mine the relationship between regions, based on their those species in each group present north or south of faunal composition. The data in the analysis are the
eastern Atlantic tropical eastern Atlantic eastern Atlantic north of northern 1 western 1 atypical I eastern 1 southern south of cape alternance I typical I troplcal I typlcal I alternance cape Frio Blanc reglan lropical regiod reglan troplcal reglord reglan I I I I I I I I I l I I I I I l l I I l I l I l 1 l ! I 1 I I I I I I l I l I I 1 ! l I I I I I I l I I l I l I 1 l I I I ! I 2 I l I 1 I I 17 l I I l I I I l 26 1 I I l I I l I I l I l I I l l I 88 I I I I I I l l I l I l l l I 1 I I I I 18 l l I I l l I I I l l I l I l I 95 l I I l - I l l l I 437 I l I % species: I (30
Figure 3. Distribution patterns of species avoiding the atypical tropical region (4) and the two typical tropical regions (3, 5). Strip's width is proportional to the number of species indicated on the right.
Acta Oecologica West African marine benthic fauna biodiversity 313 different observed distribution patterns, in total 94 example, in the matrix, the information on the first dis- (theorically 127 distribution patterns are possible), the tribution pattern offigure 3 is expressed by the line of most characteristic of which are illustrated on graphs values: 12, 12, O, O, O, 12, 12. (as explained before). This analysis is based on the presence-absence of the species in a defined region. In 4. RESULTS the analysed matrix (94 x 7), for a fixed pattern where n species were found (line i), the value O was used in 1) Among all observed distribution patterns, two cells that correspond to the regions where the n species groups are particularly interesting. are absent, and the value n in the columns corresponds A first group (figure 3) includes all the species that to the regions where the n species are present. For avoid the coast from Cape Verga to Cape Lopez where
6 7 tropical eastern Atlantic eastern Atlantic north of south of cape Frio
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1 I l 22 I
I I
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I I I 3 I l 10 l I I l 24 1 I l I I l l I I I I I I I I 44 I I - I l I I I I 236 I I I I I I l I I l l I (16 % species: l l I I I I I I I
Figure 4. Distribution patfems of species always present in the atypical tropical region (4) and avoiding the two typical tropical regions (3,5). Strip's width is proportional to the number of species indicated on the right.
Vol. 19 (3) 1998 314 P. Le Lœuff. R. von Cose1 the waters are often (atypical tropics) or permanently A second group Vgure 4) including a substantial (typical tropics) warm, stratified and brackish near the number of species (16 % of the total), is not recorded shore. Some of these species can be found in the north in the two typical tropical regions but is always present and south, with a rather large disjunction in their dis- in the atypical tropical region. Their distribution area tribution area; others remain restricted to the northern is characterized either by one or two disjunctions. or southern zones. These species represent 30 % of the Other species are restricted to the atypical tropical total fauna. zone.
lw goreones (n = 44) 100ID 1polvchaetes (n=606) 80 I 60 60
40 40 20 PO O I) ..... 1234567 123456,
"1 hrachvurids (n = 170) 80
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1449) 70 (n = 60 50 40 30 20
10 O 2 4 7 1 5 6
loopaprids (n=59) no~~ 1 60
40 20 O 1234567
'Oo bivalves (n=380) 'Oo echinoderms (n = 190) 10 I 7 ."I 4060 60
40
20 20 o o ..... 1234567 1234667
the Figure 5. Faunistic richness: number of tropicà1 West African species (percentage) present in each region. Graphs for each group and total of treated species (black coldest, white: warmest).
Acta Oecologica West African marine benthic fauna biodiversity 315
is found in the northern alternance region next to Cape Blanc, but high percentages were also found in the atypical tropical region which shares nearly 60 % of the species living outside the tropical zone. Even the southern alternance region shares species with the area north of Cape Blanc in spite of the distance between the two. Each zoological group separately treated reveals more or less the same pattern. The same result (figure 8) was found when talcing into account the species living in the tropical zone and the littorals of Namibia and South Africa, the only dif- North Cape Blanc region (1) ference being that nearly 70 % of these species are also found north of Cape Blanc. The data on this area are scarce; the benthos of the Namibian coast is the least known in the world. 4) In the correspondence analysis of the faunal composition of the different regions (figure 9), the first two axis explain together nearly 50 % of the variance (27.4 and 22.0 % respectively). The obtained ordina- tion on axis l (figure 9b) is similar to that obtained in the faunistic richness study; it opposes along a gra- South Cape Frio region (7) dient the regions with permanent or seasonal cold water to those with constantly warm water. More pre- cisely, on axis 1, the northern and southern regions Figure 6. Faunistic exchange by zoological group, north and south of outside the tropical zone (1 and 7) are very close to the studied area: number of species (percentage) found on the Atlantic each other. This is also the case of the central atypical coast of Africa and present north of Cape Blanc (black, region I; 17 tropical region (4) and the southern alternance gorgones, 341 polychaetes, 61 brachyurids, 18 pagurids, 130 bivalves, region (6). 64 echinoderms, total 63 1 species ) and south of Cape Frio (white, region 7; O gorgone, 211 polychaets, 7 brachyurids, 2 pagurids, 19 The southern external region beyond Cape Frio (7) bivalves, 12 echinoderms, total 25 1 species). is far from the others on axis 2 (figure 9a), which reflects the difficulty for tropical species to cross this The invertebrates living only in the typical tropical zoogeographical barrier and, consequently, the few regions are not numerous (2 %) and the same species faunistic exchanges with the tropical zone. Lower on is never found in both regions, west or east. axis 2 are the regions situated in the southern and the central areas of the tropical zone (6, 4, 5, 3), and 2) Species richness, an expression of biodiversity, is finally the northern regions (1, 2). Therefore, the sig- reflected in the percentage of the total number of spe- nification of axis 2 seems to be related to the geo- cies found in each region. It is clearly higher where graphical distribution of the different regions and it upwellings occur (northern and southern zone of alter- underlines the faunistic similarities between regions nance; atypical tropical zone). On the contrary, in the close to each other. Finally, figure 9a reveals the much inner Gulf of Guinea, from Nigeria to Equatorial higher affinity of the fauna in the northern external Guinea, species riclmess is lowest. These observations zone (1) compared to the tropical fauna than that of the apply for the whole fauna and for each zoological southern external zone (7). group identified (fisure 5). It is interesting to note that, in this analysis, the dis- 3) The number of benthic species occurring in both tribution patterns of species with the highest contribu- the tropical zone and in the region(s) north and/or south tion values to axis 1 and 2 all fit into the patterns of it, is distinctly higher in the north than in the south, shown onfigures 3 and 4, representing species found for all species together and for each zoological group (figure 6). This number can be considered as an expres- only in upwelling regions; the total of these species (46 % of all studied species) grouped in 27 different sion of the faunistic exchange at Cape Blanc or Cape distribution patterns contribute 55.4 % to axis 1 and Frio; the polychaetes are more prone than other groups to cross the northern and southern thermic barriers. 69.9 % to axis 2. In total, 631 species were encountered in both the 5. DISCUSSION tropical environment and the area north of Cape Blanc; however, the number of species varies across regions The results of this study suggest that there is a rela- in the tropical zone (figure 7).The highest percentage tion between distribution patterns of benthic inverte-
Vol. 19 (3) 1998 316 P. Le Lœuff, R. von Cose1 brates populations and hydroclimatic conditions on the The fauna-rich regions of the tropical Atlantic coast ?oast of tropical West Africa. of Africa with seasonal upwelling, contrast with those
100 100 BO 80
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'~~O 12
6 1 2 3 4 5 1O0
BO
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40 20
O 1234567 1O0 100
80 80
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O O 1234567 12 34567
Figure 7, Faunistic affinity of the regions of tropical West Africa with those located north of Cape Blanc (region 1): fraction of species number present beyond Capeblanc and found in one or more of the studied regions (17 gorgones, 341 polychaetes, 61 brachyurids, 18 pagurids, 130 bivalves, 64 echinoderms: tntal: 631 nnecies).
AC~AOecdogicn 4 West African marine benthic fauna biodiversity 317
O0 1 90 ao
70 60 50 40 30 20 10 O
1 23456 7
Figure 8. Faunistic affinity of the regions of tropical West Africa with thoselocated south of Cape Frio (region 7): fraction of species number present beyoud Cape Frio and found ín one or more of the studied regions (211 polychaetes, 7 brachyurids, 2 pagurids, 19 bivalves, 12 echinoderms; total: 251 species).
where a stratified hydrological structure is permanent, with a superficial layer of warm, turbid water that can undergo important salinity decreases during the rainy -1 1 season. This was shown in the analysis of richness and of the faunal composition, both considered as expres- Figure 9. Faunistic distance between the regions. Correspondence sions o€ biodiversity. analysis based on presence-absence of the species in each region. Our results suggest that the factor determining the a) regions projected on the plane of axis 1 (27.4 % of total variance) degree of affinity between the tropical regions and the and 2 (22.0 % of total variance); b) regions ordered according to their external regions is not the geographic distance values along axis 1. between them but the presence of seasonal upwelling of colder waters. The faunal exchange is more impor- tant in the north than in the south because the current faunas in non disturbed muddy bottoms off system is much more favourable to the dispersion of Spitzbergen (arctic), in the North Sea (cold temperate) species at Cape Blanc, The essential element of this and on the coast of Java (tropical) by Kendall and faunal exchange are the polychaetes, of which many Aschan [53] seems to show that there are no essential are thought to be cosmopolitan, ubiquitous and greatly differences in biodiversity between these three sites). tolerant of environmental conditions variation. In the tropical West African realm, faunistic richness It is currently admitted that marine tropical ecosys- is not associated with the presence of wann water tems characterized by stable hydrological conditions masses which are supposed to favour the growth of have high specific and bionomic biodiversity, as in, for coral reefs. But the region is completely devoid of example, the Indo-Pacific region. Guille et al. [45] large coral reef around which a major fraction of the listed 233 species of echinoderms in the New Cale- marine faunistic richness is concentrated (for informa- donia lagoon and Sérène [84] reported 280 species of tion on West African reef corals, see [58]). The brachyurids in the super-family of Xantlioidea, from absence of coral reef formations is possibly due to the Western Indian Ocean and Red Sea. More gene- both the upwelled waters with lower temperature in rally, in a study on the biogeography of recent marine some regions and the periodical strong salinity bivalves of the world, Flessa and Jablonski [33] decreases in other areas. So the tropical African showed the existence of a latitudinal diversity gradient Atlantic littoral is not fundamentally different from with an increase in species and genus numbers from that of the whole eastern Atlantic, as noted by Le the cold latitudes towards the equator. Loeuff and htès [60]:"The same bathymetric stratifi- The higher bionomic and faunistic richness in the cation of the fauna is found nearly everywhere, so are tropical regions is mostly located on hard bottoms sur- similar sedimentary conditions due to the terrigeneous rounding coral reef environments (a study of benthic input in the sea on the one hand, and to the degradation
Vol. 19 (3) 1998 I 318 P. Le Lœuff, R. von Cose1 of ancient bioconstructions on the other hand. Under high productivity, their halieutic richness and the these conditions, whatever the latitude, we find the existence of important fish nuryries. An evaluation of same general types of communities". Furthermore, the annual production (mt/km-) of demersal species comparing species richness data from West Africa suggests values of 2.0 for Senegal to Guinea, 0.7 for with the temperate European marine fauna in the same Liberia, 1.2 for Ivory Coast to Benin, 0.6 in the Bight zoological groups (without taking deep water species of Biafra, 0.9 for Gabon and 2.4 for the Congo [7]. In into account), we find values in the same order of the tropical zone of West Africa there exists a positive magnitude. For both French coasts, Fauve1 123, 241 relationship between biodiversity and productivity, at describes 550 polychaete annelids; Zariquiey Alvarez least for the benthic and demersal systems. [94] in the Iberian Peninsula counts 31 pagurids and So the present regional hydroclimatic conditions are 112 brachyures; the guide to the bivalves of the British a first explanation for the poor marine biodiversity in Isles [88] reports 184 species; and finally, 190 echi- the West African tropical zone. Another important ele- noderms are listed in the European fauna of this group ment is paleozoogeography. by Koehler [56, 571, 103 in Italy [89]. During the Tertiary, the different oceanic basins Observations in the tropical West African littoral were formed as a consequence of the separation of the suggest that, where hydrological structures remain African and South American continents and the stable and warm temperatures prevail, biodiversity is joining of Africa with Eurasia. The geography of the lower than on the coasts under the influence of peri- tropical ocean was modified: the pan-tropical Tethys odical upwellings. The Guinean fauna has a dominant disappeared at the beginning of the Miocene and was component with affinity for cooler water. succeeded by three major tropical zoogeographic The warm (20-30 m thick) 'Guinean waters' [3] of provinces (Indo-West Pacific, eastern Atlantic and the superficial mixed layer undergo a reduction in tropical American province), within which, further salinity during the rainy season in the atypical and faunal differentiation took place. This process ended even more so in the typical tropical regions. On the with the uplifting of the Isthmus of Panama by the continental shelf of Cameroon, the surface water mid-Pliocene, about 3.1 million years ago, and thus salinity is < 18 PSU in December and < 28 PSU in the division of the tropical American province into the March [16]. In Guinea, the salinity during the rainy eastern Pacific and western Atlantic provinces. season is < 10 PSU at depths < 10 m, as far as 10 nau- tical miles offshore [ 191, the Guinean continental During the Neogene at the end of the Tertiary, a shelf, with an average of about 80 nautical miles, large tropical eastern Atlantic faunal province (or being the broadest shelf of all West Africa. The pres- Euro-West African province) stretched out from the ence of brackish water reduces the faunal richness north of France southward to Angola [5, 62, 631. This because it precludes the recruitment of many marine is proved by an affinity at the generic and subgeneric stenohaline species, especially echinoderms. During a level of the recent West African mollusc fauna with survey of the continental shelf populations of Ivory that of the Miocene of the Aquitanian Basin and espe- Coast, Le Loeuff and Intès [60] recorded 1 300 ben- cially that of the Pliocene of Northern Italy [14, 151. thic invertebrates whereas surveys in the Ebrie lagoon Particularly species of the Italian Pliocene, or very revealed no more than 130 species, 50 of which are close descendants, are found within the northern and marine species that invade the lagoon only during the southern zones of alternance on often very short dry season [41, 42, 931. coastal strips with special hydrological conditions known as 'endemic pockets', 'relict pockets' or Another recently discovered hydrologic phenom- 'refuges' [go]. The term 'refuge' illustrates that, in enon on the Gulf of Guinea coasts [75, 771 was these mostly small areas, species originating in the observed on the shelf bottom around the thermocline Tertiary (Pliocene, Miocene) have survived until today that corresponds to the near-shore circalittoral, at with no or very slight alterations because the ecolo- depths of 25-65 m [60]. On these bottoms, the benthic gical conditions have not changed or remained within organisms are forced to undergo daily thermic adjust- the tolerances of these species. The most important ments (2-4 "C at 35 m up to 5-7 "C at 65 m depth) to refuge in tropical West Africa is situated in the cope with the effects of short-period waves; and, in southern alternance zone (region 6) on the coast of Ivory Coast, it was noted that the benthic fauna is very southern Angola between Lobito and Moçâmedes. It is poor at depths of 60-70 m [59,60]. This phenomenon characterized by a nutrient-rich, clear and agitated occurs in Ivory Coast during the warm season but it is water, which reaches 20 "C during the cold season. likely present in the typical tropical regions where it Another refuge with similar conditions exists in the must be a perturbation factor for the warm-stenotherm northern alternance zone (region 2) on the 'Petite species. Côte' of Senegal between the Cape Verde Peninsula The upwelling regions, especially the northern and and Joal-Fadiouth. The vast upper continental shelf southern alternance region, are well known for their (10-30 m depth) with clean sandy bottom off Casa-
Acta Oecologica West African marine benthic fauna biodiversity 319 mance also shows features of a relict pocket. A prob- the present benthic fauna is not significantly higher able refuge is situated on the coast of Ghana, in the than that of the European marine fauna with which it atypical tropical zone (region 4), from where a few has strong affinities. This explains why the greatest quite unusual endemic mollusca have recently been faunal richness exists where cooler waters periodically collected. Similar relict pockets in the tropical western invade the continental shelf. Atlantic are well known and, like their West African counterparts, characterized by high productivity (see e.g. [76, 911). Acknowledgements
During the Pliocene, the large tropical eastern S. Atlantic province became smaller and smaller as a We sincerely thank Gofas, P. Lozouet (both MNHN) and M. consequence of climate deterioration and finally its Labelle (Ifreiner) for critically reviewing initial draft of this manuscript limits were about the same as today. A large part of the and offering constructive advice. original tethyan fauna disappeared and was replaced by a more temperate fauna, an event which did not happen in the western Atlantic [22]. This is a reason REFERENCES for the prevalent affinity of the West African fauna with that of the European Atlantic and Mediterranean. [l] Arnaud E, Arnaud P.M., Intès A., Le Loeuff P., Transport d'invertébrés benthiques entre l'Afrique du Sud et Sainte-Hélène During the Pleistocene, abrupt and repeated climate par les lamin$res (Phaeophyceae), Bull. Mus. nat. 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