Figure 4. Density of mangrove species at each station in Kendari Bay. A = Soneratia alba, B = Rhizophora sp., C = Xylocarpus sp, and D = Avicennia sp.
CA joint plot
Su m bu 2
Sumbu 1 I
Figure 7. Relation of clam density and mangrove density in Kendari Bay
Figure 8. The relation of clam density and organic matter in Kendari Bay Discussion
Sediment type is an important factor for the existence of benthic communities (Van Hoey et al., 2004). This study showed that clams are found to be abundantin mangrovehabitat with substrate texture dominated by dust. Habitat with high dustwill provide stability to clamsfor digging during burial period (Lyimo et al., 2002) and it is also provide the clams to breathe in air during the protruding phase for a long time compared to clay and sand substrates. Substrate that supports respiration in air will provide higher clam density and play an important role in their distribution thanin substrates which inhibit its activity (Clemente and Ingole, 2011). In addition, this texture will make it easier to bind the organic matter needed by clam (Lopez & Levinton, 1987) compared to other textures. Through the mechanism of the pedal feeder, clams sweep all the food contained in the ground floor of mangroves. According to Box and William's (2000) substrate textures greatly determine bivalve density, which provides sediment stability and the presence of organic matter (food) for bivalves.
Ecologically, substrate size determines the density of some bivalves. P. erosa was also found at high density in fine mud substrate but not inhabitant of high sand content (Clemente & Ingole, 2011).
Preferences of certain substrates was also found in some species of freshwaterbivalves such as Dreissena polymorphawhichfavored of sand and clay fraction, whereas D. bugensis favored of clay fraction (Jones
& Ricciardi, 2005) and Corbicula flumineawas found abundant in the sand fraction (rough and medium sand) (Sousa et al., 2008). It is known that the substrate provides a strong attachment point in the denser surface hence making it as a suitable ecosystem for biological activity of the clams,thuscolonization of adult bivalves (Vaughn et al., 2008). However, the sediment is not always directly related to habitat preferences of theclams, given that some other environment quality variables also affect theirpresence.
For example, the large grains of sand content in sediment are associated with food availability, stronger currents associated with increasing phytoplankton abundance and small grain but slow current associated with benthic algae density (Snelgrove & Butman, 1994).
The highest density of Polymesoda sp. was found in thick mangrove areas of seedlings, saplings and trees. It is suggested that the thick mangrove area has a high productivity characterized by the high
organic materials that produce food for the organisms associated with it. In addition, clamsare taking plankton as food and filtering the detritus from mangrove leaves decomposition. The lower density of clams at mangrove habitatcan be caused by several factors: 1) lowdensity of mangrove only provides relativelysmall/limited food in the ground, 2) direct exposure to the sunlight making clams must adapt to high temperature in thin mangrove areas (Paschoal, 2015); and 3) over harvesting of clams by local communities contributed greatly to the decreasing of clams density, although the environmental quality of the area was favorable for its growth such as in in station III where low density of clams was observed although mangrove density was at medium level.
The clams densities were found to be higher in mangroves where Rhizophora sp. and S. albawere dominant. Although this finding is not in agreement with Skilleter and Warren (2000) who discoveredthe densities of Polymesoda sp. were higher in Avicenia sp. habitat but Capehart & Hackney (1989) stated that theunique root systemof Rhizophora sp. and S. alba, their rhizome and rootstock may provide a comfortable environment structure for the survival of Polymesoda sp. Some studies showed that Avicenia sp. had the ability to oxidize sulfides higher than other mangrove species (Lyimo et al., 2002; Thibodeau
& Nickerson, 1986); 2) Also, pneumatophores structure in Avicenia sp. makes the substrate denser to support the mechanism of resuspension filter and they also provide higher supply of organic materials.
Chapman, 1998; Skilleter & Warren , 2000). In contrast, in the present study, it was found that in station III where Avicenia sp. was dominant with lowerclams density. Therefore, it can be concluded that the low density of clamsin this station was not due to unfavorable environmental condition but because of the high harvesting in this area.
The clams density in the present study was relatively not much different with those of P. erosa (Clemente & Ingole, 2011), but was significantly different from P. caroliniana (Duobinis, et al., 1982).
Also, this study showed a much lower clamsdensity compared to freshwater clams from lakes and rivers (Table 1).
Table 1. Densities of clams species in various types of waters
Location Species Density
(ind/m2) Reference
Cermin Bay, Riau Anadara sp Pharus sp Geloina sp Pernaviridis
2.07 2.75 2.53 2.32
Suwondo, et al., 2012
Irupe Lake Mini Lake
Limno pernafortunei 2,765 981
Musin et al., 2015 Missisipi Estuary P. caroliniana 126 - 136 Duobinis et al., 1982
Chorao Island, India P. erosa 7 - 12 Clemente and Ingole, 2011
The high density of freshwater clams can be attributed to continuous distribution of organic materials as food for clams. This food is directly absorbed by freshwater clams through filter feeder mechanism resulting in higher productivity of freshwater clamscompared to marine and brackish water clams (Musin et al., 2015; Suwondo, et al., 2012).
Conclusions
Clams density in mangrove forest of Kendari Bay was found to be in medium category due to high harvesting by local people. Habitats preferences of clams in the present study were in high density of mangrove (seedlings and trees), with dominant dust fraction and high organic matter in the sediment.
Acknowledgements
The author is thankful to Muhammad Ashar, Nirwana, Sabaruddin, Jalil, and Nurmin who helped in sampling and data collection.
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