Mauritius is a volcanic island about 60 km by 45 km lying approximately 2400 km east of South Africa. Parts of the island are relatively high with a central plateau, at just under 600 m, separated from the coastal plains by mountains, some peaks of which reach 800 m. The unfortunate Dodo is only one of many endemic species which have become extinct since the arrival of the first Dutch sailors in the 17th century. Most of the evergreen tropical forest, which covered the island at that time, was cleared at an early stage for sugar cane production and today the only significant indigenous forest is in the mountains of the South West around the Black River Gorge. The mountains and central table land give rise to numerous rivers and streams many of which were systematically sampled in 1974 as part of the Austrian hydrobiological mission to the Seychelles, Comores and Mascarene archipelagos. Thirty sites were sampled and the results published by Starmühlner (1979, 1983).
A holiday in Mauritius in November 2004 gave me an opportunity to visit some of the rivers inspected by Starmühlner with a view to establishing if and to what extent the gastropod fauna had changed in the intervening thirty years. I was staying near Cap Malheureux in the North of the island and on my first excursion I followed the East coast as far south as Mahebourg. At the first stop, the Francoise River by the coast road, near Poste de Flacq, Neritina gagates Lamarck, 1822 was abundant on rocks in shallow water at the margins. It was also present in the River Poste de Flacq and this proved to be the most widespread gastropod in the lower reaches of the rivers. Further south in the Grand River Sud Est, below the coast road bridge, large eroded specimens of Septaria borbonica (Bory St Vincent, 1803) were common. At this point I was well above the limit of tidal influence and I may have found other species had I been able to check the lower reaches. In contrast, where the coast road crossed the mouth of the Champagne River estuarine conditions prevailed and juveniles of both N. gagates and Neritina mauriciae Lesson, 1831 (considered by Starmühlner to be a form of N auriculata Lamarck, 1816) were common on and under stones in the company of brackish water species. Similarly juveniles of both species occurred intertidally on the beach at Quatre Soeurs in the mouth of estuary of the Grand River Sud Est, but alongside wholly marine species such as Nerita albicilla Linné 1758 and Nerita aterrima Gmelin, 1791. This, of course, accords with the life cycle of tropical freshwater nerites which have veliger larvae which are washed downstream and out to sea where they complete a planktotrophic stage before settling in river mouths and working their way upstream (Bandel, 2001).
The sites referred to above had not been sampled by Starmühlner, the majority of his stations being in the south of Mauritius. At Mahebourg, in the Le Chaux River behind the museum, he recorded Clithon coronata (Leach, 1815), N. gagates, S. borbonica, Neritilia consimilis (Martens, 1879) and Thiara scabra (Müller, 1774). I was encouraged to find that, although Mahebourg was a large, bustling town, the river was nevertheless still healthy. Behind the museum the river is wide, shallow and fairly fast flowing with a rocky bed. N. gagates was common on the stones as were specimens of S. borbonica, although smaller than those I had found in the Grand River Sud Est. The tiny shells of N. consimilis occurred under some stones in quieter stretches of water where T. scabra was also common. I did not find C. coronata but adult N. mauriciae, not recorded by Starmühlner, were present.
On my second excursion I followed the West coast, south through the capital, Port Louis, and then as far as the River du Rempart West. Starmühlner sampled this river near the road from Trois Cavernes to Quatre Bornes, some 10 km upstream from the mouth, and found a good freshwater fauna which included Melanoides tuberculata (Müller, 1774), Lymnaea mauritiana Morelet, 1875, Physa borbonica Férussac (possibly a synonym of Physa acuta Draparnaud, 1805; see Brown, 1994: 248-9) and sporadic N. gagates. At the same station I found large numbers of Ampullaria sp. and some Viviparus sp. but not the species recorded by Starmühlner which had apparently been displaced by the alien gastropods. Further downstream a tributary flows through the ornamental water gardens of the Casela Bird Park, a small zoo and aviary. There a few N. gagates were present and a lot of Ampullaria sp. and it may well be that the Bird Park was the original source of the introduced gastropods. A few kilometers further south, in the Grand River Noire at the coast road bridge, in clear, slow moving water just above tidal influence, large specimens of N. gagates were abundant, covering the rocks in shallow water, N. consimilis occurred under stones and T. scabra and M. tuberculata were common on mud. I also found two specimens of C. coronata, the only place, as it turned out, where I observed this species. Starmühlner recorded C. coronata and the spineless form (f. despinosa) from only a few sites and those mainly in the South of Mauritius. Unfortunately there had been torrential rain the day before my trip to the South and the rivers were swollen and muddy which made sampling very difficult. A stop near the mouth of the River du Cap only produced a few N. gagates and at the River Jacolet I found this species again, with a few S. borbonica. The pool below Rochester Falls on the Savannes River contained C. coronata, N. gagates, N. consimilis, M. tuberculata and T. scabra when examined by Starmühlner. When I was there the falls were spectacular but in the raging brown torrent beneath I was unable to find any molluscs at all!
On visits to sites further inland I found L. mauritiana and P. borbonica in the Black River Gorge below Macchabee Point. Viviparus sp. was in pools below the Tamarin Falls; it may be significant that the mouth of the Tamarin River is joined to that of the River du Rempart West where this species also occurrs. Streams in the Botanic Gardens of Pamplemousses still contained good numbers of M. tuberculata, T. scabra, L. mauritiana and P. borbonica, as recorded by Starmühlner, as well as N. gagates, not listed by him.
In conclusion, I was pleasantly surprised to find that the rivers which I inspected were quite healthy and that almost all the gastropod species recorded by Starmühlner were still present, at least at some locations in Mauritius, although not always where he had recorded them. Although I found C. coronata at only one site this may well have been due to adverse sampling conditions when I visited the South. N. gagates, on the other hand, had apparently extended its distribution somewhat since 1974. The only significant change which I detected in the gastropod fauna, namely in the River du Rempart West, had been caused not by pollution but by the introduction of alien species.
References:
BANDEL, K. 2001. The history of Theodoxus and Neritina connected with description and systematic evaluation of related Neritimorpha (Gastropoda). Mitteilungen aus dem Geologisch- Paläontologischen Institut der Universitat Hämburg, 85: 65-164
BRoWN, D 1994. Freshwater snails of Africa and their medical importance. Taylor & Francis, London.
STARMÜHLNER, F., 1979. Results of the Austrian Hydrobiological Mission, 1974, to the Seychelles-, Comores- and Mascarene Archipelagos. Annalen des Naturhistorischen Museums in Wien. 82: 621-742
STARMÜHLNER, F., 1983. Results of the Hydrobiological Mission, 1974 of the Zoological Institute of the University of Vienna. Annalen des Naturhistorischen Museums in Wien. 84 /B: 127-249
Figures (not all to the same scale):
I. Neritina gagates, a. apertural, b. abapatural views. Height 18 mm, width 17 mm. c. operculum outer side, d. inner side. Length 10.5 mm, width 5 mm. River Francoise, near Poste de Flacq.
2. Neritina gagates, juvenile showing the typical zigzag colour pattern which is obscured by the periostracum in the adults. a. apertural, b. abapatural views. Height 6 mm, width 6 mm. River Champagne by the coast road bridge.
3. Neritina mauriciae, a. apertural, b. abapatural views. Height 18.5 mm, width 15 mm. c. operculum outer side, d. inner side. Length 9.5 mm, width 6 mm. River Le Chaux, Mahebourg , behind the museum.
4. Clithon coronata, a. apertural, b. apical views. Height (excluding spines) 15 mm, width 13 mm. c. operculum outer side, d. inner side. Length 8 mm, width 5 mm. Grand River Noire by the coast road bridge.
5. Septaria borbonica, a. apertural, b. abapatural views. Length 21.5 mm, width 15.5 mm. c. & d. operculum. Width 7 mm. River Le Chaux, Mahebourg , behind the museum.
6. Neritilia consimilis, a. apertural, b. abapatural views. Height 3.5 mm, width 4 mm. c. operculum outer side, d. inner side. Length 2 mm, width 1.5 mm.. River Le Chaux, Mahebourg , behind the museum.
7. Pomacea sp. Height 34.5 mm, width 31 mm. River du Rempart West near road from Trois Cavernes to Quatre Bornes.
8. Melanoides tuberculata. Height 28.5 mm, width 9 mm. Grand River Noire by the coast road bridge.
9. Viviparus sp. Height 19.5 mm, width 14.5 mm. Tamarin River, a pool below the Tamarin Falls.
10. Physa borbonica. Height 11 mm, width 6.5 mm. Black River, Black River Gorge below Macchabee Point.
11. Thiara scabra. Height 14.5 mm, width 9 mm. River Le Chaux, Mahebourg, behind the museum.