Gastropod Fauna of the Cameroonian Coasts

Helgol Mar Res (1999) 53:129–140 © Springer-Verlag and AWI 1999 ORIGINAL ARTICLE Klaus Bandel · Thorsten Kowalke Gastropod fauna of the Cameroonian coasts Received: 15 January 1999 / Accepted: 26 July 1999 Abstract Eighteen species of gastropods were encoun- flats become exposed. During high tide, most of the tered living near and within the large coastal swamps, mangrove is flooded up to the point where the influence mangrove forests, intertidal flats and the rocky shore of of salty water ends, and the flora is that of a freshwater the Cameroonian coast of the Atlantic Ocean. These re- regime. present members of the subclasses Neritimorpha, With the influence of brackish water, the number of Caenogastropoda, and Heterostropha. Within the Neriti- individuals of gastropod fauna increases as well as the morpha, representatives of the genera Nerita, Neritina, number of species, and changes in composition occur. and Neritilia could be distinguished by their radula Upstream of Douala harbour and on the flats that lead anatomy and ecology. Within the Caenogastropoda, rep- to the mangrove forest next to Douala airport the beach resentatives of the families Potamididae with Tympano- is covered with much driftwood and rubbish that lies on tonos and Planaxidae with Angiola are characterized by the landward side of the mangrove forest. Here, Me- their early ontogeny and ecology. The Pachymelaniidae lampus liberianus and Neritina rubricata are found as are recognized as an independent group and are intro- well as the Pachymelania fusca variety with granulated duced as a new family within the Cerithioidea. Littorini- sculpture that closely resembles Melanoides tubercu- morpha with Littorina, Assiminea and Potamopyrgus lata in shell shape. These three species move about on as well as Neogastropoda (Thais) and Heterostropha the moist surfaces even when not covered by water. In (Melampus and Onchidium) are described and compared the mangrove consisting predominantly of Rhizophora with representatives of the Caribbean and Indo-Pacific and Avicennia, the Pachymelania fusca population with province. varieties having granulated sculpture gradually grades into the population with varieties usually bearing spiral Key words Gastropods · Mangrove · Cameroonian coast keels. This displays individuals with a variety of orna- ments usually dominated by spiral keels in the later whorls. The individuals move about on the mud, feed- Introduction ing when water has run off the tidal flats, and they rest when they are covered by water. Where freshwater Between the slope of Mount Cameroon south of Limbe seeps from the ground the small variety of Neritina and the forest region at the mouth fo the Sanaga River, afra is common, usually having the shell covered by an almost uninterrupted zone of mangrove forests is de- mud. In areas with soft mud Tympanotonos fuscatus veloped, with mud flats at the mouths of the rivers with strong nodulation is found, and more rarely also Mungo near Tiko, Wouri in Douala and the Sanaga at Tympanotonos fuscatus variety radula with less nodula- Mouanko (Fig. 1). Mangrove and mud flats grade right tion appearing during production of the later ontogenet- into volcanic rock beach near Limbe and at Mabeta and ic shell. are protected by a continuous sand bar in front of the Pachymelania aurita prefers the sandy and muddy Sanaga delta, while the delta region opens to the sea off- flats in front of the mangrove, being dominant on sandy shore of Douala. During low tide, the mangrove becomes flats and less so on more soft muddy flats. Here, on mud- dry and usually rather wide mud flats and muddy sand dy grounds, Pachymelania fusca is more common, which usually has a shorter shell than its companion. Pachymelania aurita is not only larger than P. fusca, but K. Bandel · T. Kowalke (✉) Geologisch-Paläontologisches Institut und Museum, also active when covered by water during high tide as Bundesstrasse 55, D-20146 Hamburg, Germany well as when water has run off during low tide. P. fusca, e-mail: [email protected] in contrast, moves about only on the exposed mud flats 130 Fig. 1 Location map of ex- plored area on northern coast of Cameroon and rests when covered by the flood. P. aurita prefers er with the variety with fine granulated sculpture without water with salt concentrations closer to those of the open intermediate individuals. Thus, it appears in the field, as sea and plows through the sediment when covered by if both are separate species. Thais cf. fucus also lives on water, while it moves about on the surface of the bottom the rocky shore close to the mangrove area and close to when exposed at low tide. On first sight, Pachymelania river mouths. Where mud is less abundant in tidal pools, aurita closely resembles Tympanotonos fuscatus, even as in Limbe and north of it along the volcanic rock coast though the latter grows to a larger size and has a larger of the base of Mount Cameroon, Angiola lineata is pres- apical angle. Upon closer inspection the round opercu- ent as well as a Petaloconchus and the tidal zone into the lum and the granular ornament of T. fuscatus are distinc- splash zone is settled by Littorina and Nerita. Littorina tive. T. fuscatus is usually missing from the more sandy scabra angulifera is usually present in the mangrove areas of the intertidal mud flat, but may be common on bushes and trees especially at the seaward margin of the more muddy areas. It may be covered by barnacles mangroves. when fully grown, while barnacles are rare on the other In the mouths of small rivers coming from the slopes gastropods of the mud flats. On the mud flats, shells of of Mount Cameroon there is usually a sand bar, which Tympanotonos and Pachymelania are commonly encoun- creates an estuarine pond with freshwater issuing into it tered in which the latest whorls have been cracked open. at the landward side and seawater entering it across the This illustrates that predation of the intertidal gastropods sand bar at highest tide on the seaward side. Here, Nerit- by fish and crabs is severe. The presence of spiral and ilia manoeli is common as well as Neritina afra. On granular enforcements of the shell in addition to a thick drowned logs beside these Assiminea is also found. Neri- shell wall in all species found here is thus explained. tina afra of the larger variety produces white egg cap- In the mangrove near Tiko to the north of Douala, as sules attached to rocks and pebbles at the end of these well as behind the sand bar in the Sanaga delta to the ponds, in that portion in which seawater no longer influ- south of Douala, the supratidal driftwood, in addition to ences salinity and freshwater conditions are present Neritina rubricata, Pachymelania fusca with fine granu- throughout. Sand on the inside of the beach bar that clos- lated sculpture and Melampus liberianus, is also settled es the estuarine ponds is the settling ground for young by Assiminea hessei. The lower sand flats in the Sanaga Tympanotonos and Pachymelania, even though these estuary have no Pachymelania aurita, but many Thais cf. usually do not grow to adulthood in these environments. fucus and Potamopyrgus ciliatus. On sandy steep beach- es a little further down from the uppermost tidal zone the colorful Neritina glabrata lives here among organic de- Materials and methods bris on wet sand. The Nypa mangrove at Limbe is settled by many Tym- During a field trip in February 1998, the coastal area of north-west panotonos fuscatus, all of which have the granulated Cameroon between Mount Cameroon and the Sanaga delta was in- sculpture, along with Neritina afra and Thais cf. fucus. vestigated with regard to the gastropod assemblages of the mangroves and estuaries as well as of the rocky shores. The gastro- On the mangrove flats near Tiko Tympanotonos fuscatus pods were studied within their habitats to explore their ecology. with coarse granulation may form monospecific popula- Several hundred shells of 18 species reflecting all developmental tions; at other places in the general area it occurs togeth- stages were collected in order to characterize the ontogeny and 131 conchological variability of representatives of the Neritimorpha, Neritina tiassalensis Binder, 1955 lives in streams of the Caenogastropoda and Heterostropha. Ivory Coast. It is also rather large (more than 20 mm in Juvenile shells were extracted from silty and muddy sediments by sieves. For investigation of the protoconchs (embryonic and diameter) and lacks the extended aperture. larval part of the shell), juvenile shells were mounted on stubs, sputter-coated with gold and documented by scanning electron mi- croscopy (CamScan). Radulae were extracted by dissolving the Subfamily Neritininae Rafinesque,1815 – buccal mass with potassium hydroxide solution and documented by SEM. Genus Neritina Lamarck, 1816 Voucher material is deposited at the Geologisch-Paläontologi- sches Institut und Museum (GPIuM), Hamburg. Species Neritina afra Sowerby, 1841. The shell of Neriti- na afra Soverby, 1841, measuring up to 14 mm in height and 13 mm in width, has a low though distinct spire and Results coarse teeth on the columellar plate (Brown 1980: Fig. 18d,e). Coloration is dark greenish-brown, but some ju- Subclass Neritimorpha Golikov and Starobogatov, 1975 – veniles have large yellowish patches. The columellar Superfamily Neritoidea Rafinesque, 1815 plate has a distinct faintly orange tinge. Individuals of this species are present in the upper tidal mud flats of the Family Neritidae Rafinesque, 1815 Nypa mangrove near Limbe as well as in the estuarine pools of the small rivers that issue into the sea at Mount Within the family Neritidae we found representatives of Cameroon between Limbe and Idenao. Freshly metamor- the genera Nerita, Neritina and Neritilia.

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