Received 26 August 1992; in revised form 6 October 1992; accepted 6 October 1992. Key words: Rotifera, Niger River, floodplain lakes, Nigeria, zoogeography.
Hydrobiologia 250: 63-7 1, 1993. O 1993 Kluwer Academic Publishers. Printed in Belgium.
Rotifera of some lakes in the floodplain of the River Niger (Imo State, Nigeria) II. Faunal composition and diversity
'
Hendrik Segers ', Chukwuemekanim S. ~ w a d i a r o & Henri J . Dumont 1 Institute of Animal Ecology, University of Ghent, Ledeganckstraat 35, B-9000 Ghent, Belgium; 2~ydrobiology& Fisheries Unit, Dept. of Zoology, University of Port Harcourt, P.M.B. 5223, Port Harcourt, Nigeria Received 26 August 1992; in revised form 6 October 1992; accepted 6 October 1992
Key words: Rotifera, Niger River, floodplain lakes, Nigeria, zoogeography
Abstract The rotifer fauna of thirteen freshwater habitats in the lower Niger delta is commented upon. A total of 207 species of monogonont rotifers is recorded. The most diverse genus is Lecane with 59 species. Ten species appear to be endemic to the lower Niger delta, another 3 are restricted to Central and/or West Africa. Eleven taxa had ranges encompassing the Oriental and African regions, and six occur both in America and the study region. Three of the latter may represent recent introductions to the African continent. Two lakes (Iyi-Efi and Oguta), with 136 and 124 species on record, rank among the richest rotifer environments ever studied. It is hypothesized that (sub)tropical floodplains are the world's richest habitats for rotifers.
Introduction The lower Niger delta in Nigeria, West Africa, is endowed with innumerable shallow lakes, swamps and floodplains, rich in macrophytes, and alternatively flooded and disconnected through channels linking the lakes to arms of the river Niger. The monsoonal regime of the Niger catchment, causing strong fluctuations in waterlevel in the deltaic region, is also responsible for the enormous aquatic productivity of the delta. This is seen in the high biomass, and in the remarkably high faunal (and presumably floral) diversity of the region. We illustrate this in the present contribution by
a study of a major component of the zooplankton, the Rotifera. Material and methods Samples were collected form thirteen localities in the lower delta of the Niger River (see Fig. 1 in Segers (1992a), Table l), using a standard plankton net (mesh size 50 pm), and fixed in formaldehyde (4%). Most samples originate from littoral habitats. The region was visited on two occasions during the rainy season (June and August/September 1991) and once during the dry season (February 1992) by scientists from the Ecology Institute of the Ghent University and
Table 1. List of localities sampled. Locality
Collecting date June 1991 (J)
Aug-Sept 1991 (S)
February 1992 (F)
Oguta lake - littoral in mouth Awbana river - littoral in mouth Njaba river - swamp in mouth Utu river - littoral on south bank near Gogo Nwakuche - littoral at confluence with Orashi river Akika lake Apariri lake Ibina lake Iyi-Efi lake Nnemmiri lake Amaya river at Osu Imo river at Obigho Orashi river near Osu near Oguta lake Niger river at Ani-Eze ( + swamp) Ricefield swamp near Akiri flow station Ikwegbu forest pond
from the Hydrobiology and Fisheries Unit of the University of Port Harcourt, Nigeria. Six lakes, four rivers (two permanent, two temporary ones), a ricefield, and a forest pond were sampled. The most intensely studied localities were Oguta and Iyi-Efi lakes, of which several zones were sampled during all visits. All habitats are freshwater, nutrient-poor, and acid.
The rotifer taxocoenosis found consisted mainly of cosmopolitan (70%) or pantropical (15 %) species. However, a number of species with limited or remarkable ranges were also found. Afrotropical and paleotropical components of the fauna can be recognised, as well as a group of species recorded from Africa and America. Afrotropical species
Results and discussion A list of the rotifers found is presented in Table 2. In total, 227 taxa belonging to 207 species of monogonont Rotifera were found, 14 of which were new to science (Segers, 1993; Segers et al., 1992). Of the 227 taxa, 41 are recorded from Africa for the first time. Representatives of the genera Lecane (28.5 % ), Trichocerca (12.6 % ) and Lepadella (1 1.6% ) were most numerous, demonstrating the dominance of these genera in littoral habitats.
Ten newly described species are perhaps endemic to the lakes studied (Squatinella lunata, Dipleuchlanis omata, Euchlanis semicarinata, Itura deridderae, Lecane dumonti, L. nigeriensis, L. nwadiaroi, L. simonneae, L. stichoclysta, L. sylviae: Segers, 1993). Testudinella kostei De Ridder is west African (Senegal: De Ridder, 1983, and Nigeria), Lepadella berzinsi (see Segers, 1993) and Trichocerca hollaerti De Smet (ZaYre: De Smet (1990) and Nigeria) occur in central and west Africa. Thus, thirteen African species (6.3 %) fig
Table 2. List of Monogonont Rotifera in Nigerian samples. Codes: see Table 1. (* taxa, new to Africa). Anuraeopsis coelata (De Beauchamp): 5j, 6f Anuraeopsisfisa (Gosse): 4f, 5j, s, f, 12f Ascomorpha ovalis (Bergendal): If Asplanchna sieboldi (Leydig): 5j, 10s Asplanchna brightwelli (Gosse): 10j Asplanchna tropica Koste & Tobias: 2f Brachionus angularis Gosse: 2f, 5j, s, f, 10s * f. chelonis Ahlstrom: 2f, 12f Brachionus bidentatus (Anderson) f: testudinarius Jakubski: 1Of Brachionus calycz$orus Pallas: 10j Brachionus caudatus Barrois & Daday: 3f, 5f, 6j, f, 10j Brachionusfalcatus Zacharias: 2f, 4f, 5j, 6j, 9j, 10j, s Brachionus sericus Rousselet: 3f, 5j, f, 12f Brachionus quadridentatus Hermann: 2j, f, 3f, 5j, s, f, 8j, 10j, s f. cluniorbicularis (Skorikov): 5j, f, 6j, 10s f: mirabilis (Daday): 5s Brachionus urceolaris 0 . F. Mtiller: 5j, 10s Cephalodella catellina ( 0 . F . Miiller): 10s Cephalodella forJicula (Ehrenberg): 2j, f, 5j, s Cephalodella gibba (Ehrenberg): 2f, 5s, f, 9s, 10s * Cephalodella mira Myers: If, 5s Cephalodella mueronata Myers: If Collotheca campanulata (Dobie): If Colurella colurus (Ehrenberg): If * Colurella hindenburgi Steinecke: 5s Colurella obtusa (Gosse): lj, 2j, f, 3f, 5s, 7j Colurella sulcata (Stenroos): If * Colurella tessellata (Glascott): If Colurella uncinata ( 0 . F . Mtiller) f. bicuspidata (Ehrenberg): ls,f, 2j,f, 3f,4f, 5s,f, 10s Conochilus hippocrepis (Schrank): Is, 5s Cupelopagis vorax (Leidy): 5j, s, f Dicranophorus caudatus (Ehrenberg): 10s Dicranophorus claviger (Hauer): 10s Dicranophorus epicharis Harring & Myers: Is, f, 2f, 3f, 4f, 5s, 10s Dicranophorus grandis Harring: If, 3f * Dicranophorus cf: halbachi Koste: 5j, s, f * Dicranophorus luetkeni (Bergendahl): 3f, 5s * Dicranophorus robustus Harring & Myers: l j * Dipleuchlanis ornata Segers: If, 4f Dipleuchlanis propatula (Gosse): Is, f, 2f, 5j, s, 7j, 8j, 10j, s Epiphanes clavulata (Ehrenberg): 9j, 10s, 12f Epiphanes macroura (Barrois & Daday): 6j, 10j Euchlanis dilatata Ehrenberg: If, 2f, 9j, 5s, f, 10s, l l j Euchlanis incisa Carlin: lj, f, 3f, 5s, 6j, 8j, 10j, s Euchlanis meneta Myers: lj, f, 5s, f * Euchlanis semicarinata Segers: lj, f, 2j, f, 3f, 4f, 5s, f, 9s, 10s Euchlanis triquetra Ehrenberg: lj, f, 2j, f, 3f, 5j, s, f, 9j, l l j Filinia longiseta (Ehrenberg): 5j, f, 12f Filinia opoliensis (Zacharias): 2j, 5j, 6j, 9j, 10j, s, f
Table 2. (Continued) -
Floscularia ringens (Linnaeus): 5s Hexarthra intermedia Wiszniewski: 5s * Ztura deridderae Segers: If, 5s Keratella americana Carlin: lj, 5j, s, 6j, 10s f: hispida Koste: 2f Keratella cochlearis (Gosse): 2j, 5j, f, 10f f. pachyacantha Thomasson: 1s Keratella javana Hauer: If, 5s, 9s Keratella lenzi Hauer: 5s, 10s Keratella serrulata (Ehrenberg): 5j Keratella tecta (Gosse): 10s, f Keratella tropica (Apstein): 5s, f, 6j, 10j, s Lecane aculeata (Jakubski): 2j, f, 3f, 5s, f * Lecane agilis (Bryce): If, 2j Lecane arcuata (Bryce): Is, 5s Lecane arcula Harring: 2f, 5f Lecane bifurca (Bryce): 2j Lecane bulla (Gosse): lj, s, f, 2j, f, 3f, 4f, 5j, s, f, 6f, 7j, 9j, s, 10s, f Lecane clara (Bryce): If, 2f, 3f, 5s Lecane climacois Harring & Myers: If Lecane closterocerca (Schmarda): Is, f, 2j, f, 3f, 4f, 5j, s, f Lecane crepida Harring: 2f, 5f Lecane curvicornis Murray: Is, 2j, f, 3f, 4f, 5j, s, 7j, 9j, 10j, s, 12f 1: nitida Murray: If Lecane decipiens (Murray): Is, 5s * Lecane doryssa Harring: Is, f, 2f, 3f, 4f, 5s, f, 6f * Lecane dumonti Segers: If * Lecane elegans Harring: 5s Lecane elsa Hauer: Is, f, 2j, 5j, 7j, 8j, 9j Lecane$exilis (Gosse): Is, f, 2f, 3f, 5f Lecanefurcata (Murray): lj, f, 2j, f, 3f, 5f, 6f, 10s f. elachis (Harring & Myers): 3f, 5j unnamed f.: Is, f, 4f, 5s, 9s Lecane hamata (Stokes): lj, s, f, 2j, f, 3f, 4f, 5j, s, f, 7j, 10j, s Lecane hornemanni (Ehrenberg): If, 2j, f, 3f, 5f Lecane inermis (Bryce): 2j, f, 5j Lecane inopinata Harring & Myers: 2f * Lecane lateralis Sharma: lj, 2f, 5s, f, 8j, 10s Lecane leontina (Turner): lj, s, f, 2j, f, 5j, s, f, 7j, 8j, 9j, 10j, s, f Lecane ludwigii (Eckstein): If, 2j, f, 3f, 5s, f, 7j, 9s f: ercodes Harring: If, 2f, 5f unnamed f.: If, 5f Lecane luna ( 0 . F . Mtiller): Is, f, 2f, 5j, s, f, 10s Lecane lunaris (Ehrenberg): Is, f, 2j, f, 3f, 5s, f, 10s f: crenata (Harring): If, 7j 1: constricta (Murray): If Lecane monostyla (Daday): 2j, 5j, s, 10s Lecane myersi Segers: If Lecane nana (Murray): 2f * Lecane nigeriensis Segers: 5s
Table 2. (Continued)
Table 2 . (Continued)
* Lecane nwadiaroi Segers: 5f Lecane obtusa (Murray): If, 2f, 3f, 4f, 5j, s, f * Lecane ohioensis (Herrick): If, 5j, f unnamed f .: 5s, f Lecane papuana (Murray): 2f, 4f, 5s, 10s Lecane paxiana Hauer: 2j, 5j Lecane perplexa (Ahlstrom): 10s Lecane pertica Harring & Myers: lj, s, f Lecane pusilla Harring: If, 3f, 5s Lecane pyriformis (Daday): lj, 2j, f, 5s, f, 9s Lecane quadridentata (Ehrenberg): lj, f, 2j, f, 3f, 5j, s, f * Lecane rhytida Harring & Myers: 5s Lecane ruttneri Hauer: If * Lecane serrata Hauer: 5s Lecane signifera (Jennings)f. ploenensis Voigt: Is, f, 2j, f, 3f, 5j, s, f, 7j, 10s, f * Lecane simonneae Segers: Is, f, 2f, 3f Lecane stenroosi (Meissner): 5s f. lineata Wulfert: 7j Lecane stichaea Harring: lj, s, f, 2f, 3f, 5s, 9s, 10s * Lecane stichaeoides Hauer: If, 2f, 3f, 4f, 5j, s, f, 10s * Lecane stichoclysta Segers: If Lecane subtilis Harring & Myers: If, 5s * Lecane sylviae Segers: 5j, s Lecane sympoda Hauer: Is, 2f * Lecane kluchor Tarnogradskif. syngenes Hauer: Is, 5s, 9s Lecane tenuiseta Harring: If, 5j, s * Lecane thienemanni Hauer: 3f, 5s Lecane undulata Hauer: If, 2f, 5s Lecane ungulata (Gosse): lj, s, f, 2j, f, 3f, 4f, 5j, s, f, 8j, 9j, s, lOj, s, l l j Lecane unguitata (Fadeew): Is, f, 2f, 5s, f, 7j, 9s, 10j, s Lepadella acuminata (Ehrenberg): lj, s, f, 2f, 5s, f, 6f Lepadella amphitropis Harring: l j Lepadella apsicora Myers: Is, f, 2f, 5s, f * Lepadella arabica Segers & Dumont: If * Lepadella benjamina Harring: 5s Lepadella berzinsi Segers: Is, f, 2j, f, 3f, 5j, s, f, 7j Lepadella costatoides Segers: lj, s, f, 2j, f, 3f, 5j, s, f, 9s Lepadella cristata (Rousselet): Is, f Lepadella cryphaea Harring: lj, s, f, 2j, f, 5s, f Lepadella discoidea Segers: Is, f, 5s, f Lepadella dactyliseta (Stenroos): If, 2f, 3f, 5s Lepadella ehrenberghi (Perty): 2j, f, 5s, f Lepadella heterodactyla Fadeew: 1s, f Lepadella heterostyla (Murray): If, 2f, 3f, 5s, f Lepadella latusinus (Hilgendorf): 2f, 5f * Lepadella minoruoides Koste & Robertson: If Lepadella ovalis ( 0 . F . Muller): lj, s, f, 2f, 5s,12f Lepadella patella ( 0 . F. Miiller): If, 5s, f Lepadella quadricarinata (Stenroos): If Lepadella quinquecostata (Lucks): 1s Lepadella rhomboides (Gosse): Is, f, 2f, 5s, f
Lepadella triba Myers: If Lepadella triptera Ehrenberg f. alata (Myers): 5f f. rhomboidula (Bryce): If, 3f Lepadella vandenbrandei Gillard: Is, f, 3f, 5s, f Lindia torulosa Dujardin: If, 2f, 5f Lophochlaris oxysternon (Gosse): 5s, 10j, s * Macrochaetus altamirai (Arevalo): 7j Macrochaetus collinsi (Gosse): Is, f, 2f, 5f Macrochaetus multispinosus Myers: If Macrochaetus sericus (Thorpe): 2f Manfredium eudactylotum (Gosse): 4j, f, 5j, s, f, 9s * Metadiaschiza trigona (Rousselet): 4j, f, 5s * Monommata grandis Tessin: Is, f, 2j, 3f, 4f, 5j, s, f, 6f, 9s Mytilina compressa (Gosse): 5f, 10s Mytilina macrocera (Jennings): 8j, 9j Mytilina trigona (Gosse): If Mytilina ventralis (Ehrenberg): Is, f, 2f, 5s, f, 9s f. macracantha (Gosse): lj, f, 2j, f, 3f, 4f, 5j, s, f, 8j, 9s, 10f * Mytilina unguipes (Lucks): 5s, 10s Notommata copeus Ehrenberg: If, 2f Notommata cyrtopus Gosse: 2f, 5s Notommata glyphura Wulfert: If Notommata pachyura (Gosse): 553, 10j f. triangulata (Kirkman): 5j, 1l j Paracolurella aemula (Myers): If Platyias leloupi Gillard: 2j, 3f, 4f, 5j, s, 6j, 7j, 8j, 9j, 10j, s, l l j Platyias patulus (0.F . Milller): Is, f, 2j, 3f, 4f, 5j, s, f, 7j, 8j, 9j, lOj, s, l l j Platyias quadricornis (Ehrenberg): lj, s, f, 2j, f, 3f, 5j, s, f, 6j, 7j, 8j, 9j, lOj, s Ploesoma lenticulare Herrick: 5s Ploesoma truncata (levander): Is, f, 5s, 9j Polyarthra dolichoptera Idelson: 2f, 12f Polyarthra major Burckhardt: If, 12f Polyarthra vulgaris Carlin: lj, s, f, 6f, l l j Ptygura elsteri Koste f. thailandis Koste: 2f Scaridium bostjani Daems & Dumont: 2f, 3f, 5s, f, 9s Scaridium longicaudum ( 0 . F. Milller): lj, s, f, 2j, f, 5j, s, f, 10s * Squatinella lamellaris (Mtiller): 2f, 5s, f Squatinella lunata Segers: 2f, 3f, 5f Synchaeta pectinata Ehrenberg: lj, s, f Taphrocampa selenura Gosse: 2f, 5s, f Testudinella brevicaudata Yamamoto: 3f Testudinella emarginula (Stenroos): Is, f, 2j, f, 3f, 5j, s, f, 7j, 9s Testudinella kostei De Kidder: If Testudinella mucronata (Gosse): 9j Testudinella parva (Ternetz): Is, f Testudinella patina (Herman): lj, s, 2j, 3f, 4f, 5j, s, f, 7j, 8j, 9j, lOj, s, 1l j f . dendradena De Beaucharnp: 9j Testudinella tridentata Smirnov: Is, f, 2j, 5f Trichocerca bicristata (Gosse) f. mucosa (Stokes): Is, f, 3f, 5f
Table 2. (Continued) Trichocerca bidens (Lucks): lj, s, f, 3f 5s, f Trichocerca chattoni (de Beauchamp): Is, 9j Trichocerca collaris (Rousse1et)f: maior Koste: If Trichocerca cylindrica (Imhof): 5j, 6f Trichocerca dixonnutalli (Jennings): Sj Trichocerca elongata (Gosse): lj, s, f, 2f, 5s, f Trichocerca hollaerti De Smet: If, 2f Trichocerca iernis (Gosse): Is, f Trichocerca insignis (Herrick): 2j Trichocerca insulana (Hauer): If * Trichocerca jenningsi Voigt: 2j * Trichocerca kostei Segers: If Trichocerca macera (Gosse): If, 2f, 3f, 5f Trichocerca musculus (Hauer): If Trichocerca myersi (Hauer): Is, f, Ss, f Trichocerca porcellus (Gosse): Sf Trichocerca pusilla (Jennings): 5j * Trichocerca relicta Donner: If Trichocerca similis (Wierzejski): Is, f, 3f, 4f, 5j, s, f, 6j, 10f * f: grandis Hauer: If Trichocerca simoneae De Smet: Sj, 9s Trichocerca tigris (Miiller): If, 2j, f, 3f, 5j, s, 7j * Trichocerca tropis Hauer: If, 2f Trichocerca uncinata (Voigt): 3f Trichocerca weberi (Jennings): Is, f, 2j, f Trichotria tetractis (Ehrenberg): If, 2j, f, 3f, 5s, f, 9s f. caudata (Lucks): If
* Trichocerca braziliensis (Murray):
ure in our list. For Lecane, this fraction even amounts to 10%. An additional number of species (several Lecane: e.g., Figs 1-4, and Trichocerca) were encountered in numbers too low to warrant a description. The proportion of endemics agrees well with that of Madagascar (ca 5 %: Segers, 1992) and that of the Upper Lualaba (De Ridder, 1981) whence ca 8 % endemics were recorded (Dumont, 1983), although the validity of some is dubious (e.g. Vanoyella globosa Evens 1947: contracted Notommata sp.?). It contrasts with the degree of endemicity recorded from Africa as a whole (0.5 % : Dumont, 1982). However, when compared to South America (Brandorf et al., 1987: 11%) and Australia (Shiel & Koste, 1983: 15%), it is low, mainly because of the absence of endemicity in the genera Brachionus and Keratella, typical for the Australian and American faunas.
Paleotropical species Taxa with a distribution encompassing the Ethiopian and Oriental regions were found in a proportion of 5.3 %. These are: - Asplanchna tropica Koste & Tobias: Recorded from Nigeria (Donner & Adeniji (1977) sub. A. brightwelli var.; present study), Mali (Koste & Tobias, 1989) and East Kalimantan (Indonesian Borneo: Koste, 1988). - Keratella javana Hauer: Known from Australia, the Oriental and the Ethiopian regions (Pejler, 1977; Dumont, 1983). - Lecane lateralis Sharma (Fig. 7): So far only recorded in the literature from the type locality (Achipur, West Bengal, India: Sharma, 1978). The species has also been found in China (prov. Yunnan, between Dali and Lijang, 24 July 1989, leg. H. J. Dumont), India (Lake Kolleru, Andra Pradesh, 9 March 1974, leg. Y. Ranga Reddy), and now also in Nigeria. - Lecane serrata (Hauer) (Fig. 5): Described from Sumatra (Hauer, 1938). Koste (1988) found the species in E. Kalimantan. Uncertainty about its taxonomical position relative to L. decipiens exists. However, these two taxa may represent vicariants (see further). - Lepadella discoidea Segers, Lepadella vandenbrandei Gillard: see discussion in Segers (1993). These two species also occur in Australia. - Ptygura elsteri Koste f. thailandis Koste: Known from Thailand (Koste, 1975), Zaii-e (De Smet, 1989), and Nigeria. The taxon's status as a infrasubspecific form of a common south american species (Koste, 1972; 1978) is questionable. - Scandium bostjani Daems & Dumont: described from Nepal (Daems & Dumont, 1974), recorded from ZaTre (De Smet, 1990), and Nigeria. - Testudinella brevicaudata Yamamoto: known from Japan (Yamamoto, 1951), Mali and Senegal (Koste & Tobias, 1987), and Nigeria. - Trichocerca kostei Segers: see Segers (1993). - Trichocerca tropis Hauer: Known from India (near Madras: Hauer, 1937), and Nigeria. Furthermore, the species-pair Lecane nwadiaroi Segers (Nigeria) and L. blachei Berzins (from
68
Cambodia and E. Kalimantan, Indonesia (Koste, 1988), is an example of vicariance between the Oriental and Ethiopian regions.
Species occuring both in the study region and America
The following six species (2.9 % ) were previously known from America only: - Keratella americana Carlin: A species common in Central and South America, and in Antarctica. Illustrated records from the old world are lacking. - Lecane climacois Harring & Myers: See Segers (1993). - Lecane decipiens (Murray) (Fig. 6): This species is common in Central and South America. Illustrated records from the old world (Hungary: Nogrady (1962), Sri Lanka: Chengalath & Fernando (1973), the Philippines: Mamaril & Fernando (1978)) are misidentified L . hamata (Stokes). L . decipiens may be the new world vicariant of the old world L . serrata. - Lecane myersi Segers: Described from New Jersey (U.S.A.: Harring & Myers, 1926), later recorded from Brazil (Brandorff etal., 1982), Zimbabwe (Thomasson, 1965), and now Nigeria. - Lecane rhytida Harring & Myers: Common in America (Koste, 1978); no reliable illustrated records from the old world are known. - Lepadella minoruoides Koste & Robertson (Figs 8a-c): Known from Brasil (Koste & Robertson, 1983), and from Nigeria. A close relative of L. minorui Koste from Australia (Koste, 1981). Three of the above-mentioned species (Keratella americana, Lecane decipiens and L. rhytida) may be introductions. All are common American species, previously unrecorded or of doubtful1 presence in the old world. The intensive oil exploitation in the region is a possible explanation for their present appearence in our samples: propagules of these species may have been introduced during exchange of material or personal of oil companies. Similar examples are Kelicottia bostoniensis and K. longispina (see Pejler, 1977) and Brachionus variabilis (see Dumont, 1983).
Fig. 8a-c. Lepadella minoruoides Koste & Robertson. a: dorsal view, b: ventral view, c: lateral view. Fig. 9. Dicranophorus cf. halbachi Koste. trophi (epipharynx not figured).
L. myersi is better not considered in this aspect in view of its unresolved relation with the cosmopolitic L. galeata (Bryce); too few records of Lecane climacois or Lepadella minoruoides are available to permit a conclusion to be drawn on their distribution. A possible vicariance between Africa and America is represented by L. simonneae Segers, versus L . eutarsa Harring & Myers and L . robertsonae Segers (see Segers, 1993). A problematic case is that of Dicranophorus cf. halbachi Koste, known from Australia (Koste,
1981) and Paraguay (Koste, 1986). A close relative (if not the same species) is the insufficiently described Dicranophorus kostei Pourriot & Zoppi De Roa, from Venezuela (Pourriot & Zoppi De Roa, 1981). Our specimens (Fig. 9) cannot be placed with certainty in either species. The taxon is, for the time being, considered of pantropical occurence. The number of species per waterbody is given in Table 3. Although the number of species recorded will depend on the intensitivity of study, it is clear that the fauna of the lakes is much more diverse than that of the rivers or of the other habitats examined. Only in Nnemmiri Lake was the number of species found relatively low, but this was the only lake which lacked a macrophyte-rich littoral. The number of rnonogonont Rotifera, identified from Iyi-Efi and Oguta Lakes is extraordinarily high. These lakes have the most diverse rotifer taxocoenosis ever recorded: we here found 136, respectively 124 (40, resp. 46 Lecane), fully identified species (not taxa) of Monogonont Rotifera. The only habitats that might rival the diversity found in the Niger delta lakes are Australian Billabongs (Shiel& Koste, 1983; Hillman, 1987) and the South American Amazon and Pantanal wetTable 3. Rotifer diversity per locality.
Locality
Nr. of species (nr. of samples) A/S
F
Tot.
Oguta lake Akika lake Apariri lake Ibina lake Iyi-Efi lake Nnemmiri lake Amaya river Imo river Orashi river near Osu near Oguta lake Niger river ( + swamp) Ricefield swamps Ikwegbo forest pond Total
207 (37)
lands (e.g., BrandorfT et al., 1982). This richness of the rotifer taxocoenosis of the Nigerian floodplain lakes, however, also reflects our special focus on littoral habitas, the faunal importance of which is often ignored (Segers et al., 1991). From the foregoing, the hypothesis emerges that (sub)tropical floodplain systems, such as those of the Murray-Darling or Alligator Rivers (Australia), of the Amazon and Parana (South America), and of the Niger (Africa), may represent an optimum type of habitat for Rotifera. If so, several other yet unstudied floodplain regions should yield comparably complex associations. Examples of such are the Zaire, Zambezi and Limpopo rivers in Africa and the Indus, Ganges, Brahmaputra and Mekong rivers in Asia.
Acknowledgements This study is part of the EEC project 'Floodplain lake management of the Lower Niger delta in Nigeria', contract no. ST2.A.287.B.
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