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POPULATION ECOLOGY OF HARVESTED GASTROPOD MOLLUSCS IN MANGROVES IN THE WAKATOBI MARINE NATIONAL PARK
Introduction
Throughout the world, one of the dominant members of the mangrove fauna is the potamidid mud-whelk, locally called mangrove snails or horn shells. These achieve heights of up to 190 mm and are throughout the Malaysian/Philippino/Indonesian region harvested for food, for medicinal use and, in India, as a source of lime. In some areas of Indonesia – for unknown reasons – cerithiid mudcreepers seem to take the place of the potamidid Cerithidea: such has been described for a mangal at Batu Gong, near Kendari, Sulawesi Tenggara, and was determined to be the case on P. Hoga and sites visited on P. Kaledupa in the Tukang Besi Islands. Other potamidid genera such as Terebralia and Telescopium are however common in Sulawesi Tenggara.
Objectives
This study endeavoured to characterise the local mangroves and their associated deposit-feeding gastropod mollusc fauna, and to investigate whether the dominant potamidids and cerithiids are at carrying capacity. An ultimate aim of the project is to determine whether the harvest of these species is being conducted at a sustainable level.
Methods
Population estimates were obtained within 0.0625 m2 quadrats either thrown randomly or placed in a line along a linear transect across any inhabited zone. Material within the quadrats was also used to provide information on population size structure via measurement of shell heights by means of vernier calipers. Quantification of the more easily measured defaecation rate was used - as in previous studies of hydrobiid mudsnails - as the measure of feeding rate. Faecal production was assessed in the field on 25 replicate individuals by removing them from the sediment and placing them individually for 1 hour in a small plastic pot containing a little sea water. After the 1 hour the snails were removed, the containers were sealed and then transported by boat to the Marine Research Centre, Hoga, for later counting under a binocular microscope. Each batch of test animals were selected so as to represent a given field density of the two most common gastropod species, the potamidid Terebralia sulcata and/or an unidentified cerithiid (unknown because of the local unavailability of keys to this group), separately or in sympatry. The study was undertaken during July and August 2001 with the aid of my family and Operation Wallacea volunteers.
Analysis
Futher and more detailed analysis will be carried out on my computer on return to Cambridge using ANOVA and other relevant statistical techniques.
Preliminary results
The floristically rich Hoga mangal is dominated by Rhizophora stylosa, Sonneratia alba, Osbornia octodonta and Ceriops tagal, although four other species are also present (Xylocarpus moluccensis, Scyphiphora hydrophyllacea, Bruguiera gymnorrhiza and Avicennia marina), together with the mangrove-associate Pemphis acidula. The low diversity mangal studied at Ambeua, Kaledupa, is dominated by Avicennia officinalis (most Kaledupan mangals, however, are dominated by Rhizophora stylosa and Bruguiera gymnorrhiza- these are relatively inaccessible). Both mangals supported a diverse benthic fauna of ocypodid crabs (fiddler crabs, Uca spp., and sentinel crabs, Macrophthalmus convexus & M. definitus) and periophthalmid mudskippers as well as the gastropods of this report.
The abundance of both mud-whelk species is generally low in both mangals – some 20-35 individuals . m-2 – but the two are locally abundant on mudflats within the Osbornia zone of the Hoga mangal (at 1230, 46’, 22" E; 50, 27’, 34" S): the cerithiid there attaining a mean density of 153 (SE 17) . m-2, and T. sulcata of 75 (SE 7) . m-2. Terebralia palustris occurs in local abundance in the same area, as does Telescopium telescopium more rarely. Undisturbed populations of T. sulcata attain heights of >60 mm. At Ambeua (at 1230, 45’, 22" E; 50, 29’, 52" S) few individuals larger than 45 mm height could be found, and on Hoga very few were larger than 40 mm; however, away from centres of human habitation in the Rhizophora mangals on Kaledupa it does indeed achieve 60 mm. Equivalently, T. palustris normally achieves 120 mm – and in several areas 190 mm – but on Hoga all were less than 78 mm: all were also in an atypical habitat, pools rather than on the mud surface. These could well be consequences of human harvesting.
Although the cerithiid and T. sulcata occurred together in 43% of a series of 150 quadrats within the Osbornia zone, their numbers appear strongly to be inversely correlated. Only 2 of the 150 samples contained significant numbers of both, in those cases because the quadrats happened to straddle interfaces between abutting aggregations of the two species. Otherwise, quadrats with 0-10 cerithiids contained an average of 6 T. sulcata, whilst those with 11 or more cerithiids contained only an average of less than 2 of the Terebralia; and those with 0-5 T. sulcata contained an average of 13 cerithiids, whilst those with 6 or more Terebralia contained an average of less than 2 cerithiids. This suggested significant interaction which was investigated as described below. Numbers of Terebralia palustris and the cerithiid were also inversely correlated where they co-occurred, although time did not permit this to be followed up.
No evidence of intraspecific competition for food was found in either T. sulcata or the cerithiid at field density. Cerithiid defaecation was effectively constant at 76 pellets in the test hour over a population density range of 96-944 . m-2 and likewise that of T. sulcata at 176 pellets in the test hour from a range of 32 to 656 . m-2 – the upper limits both being well in excess of normal field abundance. At the very highest densities of the cerithiid that could be located within the mangal mudflats (1248 & 1600 . m-2), however, faecal output did fall by one half to one third. There was also a very marked interspecific effect of T. sulcata on the cerithiid. Even 16 T. sulcata . m-2 decreased average cerithiid presumed feeding by 30%, increasing to a 50% reduction when 100 of the potential competitor are present. The effect of the cerithiid on T. sulcata was less easy to determine because of the scarcity of significant numbers of both within the same immediate area. One such test was carried out but the results require computer analysis before conclusions can be drawn. T. sulcata may therefore have an effect on populations of the cerithiid, but what controls Terebralia numbers remains uncertain.
Reports
Production of a final report and of a paper for external publication require computer analysis of the data, identification of the cerithiid species, and access to the existing literature, of which the middle one is outside my control. I am hopeful, however, of doing both by the end of December 2001. Richard Barnes, University of Cambridge.