MOLUSKA

Istilah bioindikator digunakan untuk biota atau organisme berasosiasi yang memiliki respon terhadap asupan polutan melalui perubahan fungsi-fungsi vitalnya atau yang mengakumulasi polutan. Menurut sifatnya, bioindikator digolongkan menjadi dua kelompok:

1. Indikator akumulatif, yang menyimpan polutan di dalam tubuhnya tanpa ada perubahan nyata dalam sistem biologisnya;
2. Indikator responsif, yang bereaksi melalui perubahan sel atau gejala-gejala visual kerusakan (pada jaringan, organ, sistem faal) setelah mengalami paparan zat berbahaya dalam jumlah dan kurun waktu tertentu.

Lingkungan pesisir merupakan daerah pertemuan antara sistem terestrial dan oseanik, yang di dalamnya berlangsung interaksi kompleks, rumit, dan dinamis secara fisik, kimia, dan biologi. Dengan demikian, dampak (positif dan) negatif dari kegiatan di darat maupun laut dapat masuk ke lingkungan pesisir. Zat-zat beracun dan logam berbahaya yang berasal dari pencemaran laut maupun kesedimen yang lebih lanjut lagi, akan masuk jejaring makanan melalui remobilisasi oleh akar tanaman dan organisme bentik/pemakan bentik. Di lingkungan pesisir, moluska hidup sebagai hewan bentik (menetap di dasar perairan) yang memakan fitoplankton dan detritus melalui mekanisme penyaringan (filter feeding). Dengan demikian, moluska berperan sebagai mata rantai esensial dalam jejaring makanan lingkungan pesisir karena memiliki dualisme peran ekologis, yaitu sebagai herbivor (yagiatan-kegiatan di darat, seperti persawahan intensif, industri, dan buangan rumah tangga, selanjutnya digolongkan menjadi polutan organik (DDT, PAH, PCB, PCDD, PCDF) dan anorganik (Cd, Cu, Fe, Mn, Pb, Se, Zn). Sebagian besar dari polutan kemudian didepositkan di dalam ng menghubungkan produsen dengan konsumen selanjutnya, bahkan top predator) dan di saat bersamaan sebagai detritivor (memakan detritus). Selain berdasarkan pendekatan jejaring makanan, moluska merupakan indikator biologis yang ideal karena beberapa hal berikut ini:

• Moluska merupakan organisme yang tersebar luas dalam jumlah melimpah di lingkungan pesisir di seluruh dunia.
• Moluska, terutama yang hidup di lingkungan pesisir, memiliki rentang distribusi yang luas di dalam dan antar benua. Bahkan ada sejumlah spesies dan genus yang kosmopolitan (misalnyaMytilus M. edulis, M. galloprovincialis), sehingga memungkinkan untuk survei skala geografis (geographical large scale surveys). -
• Sejumlah spesies moluska merupakan spesies kunci (key species) dari fungsionalitas ekosistem pesisir, sehingga apabila

Selenium is deposited in sediments from reservoirs and irrigated lands and may enter aquatic food chains through deposition in the sediments and then be remobilized by rooted plants and benthic feeders (Lemly 1987). Corbicula fluminea, the Asiatic clam, is a filter feeding benthic bivalve that feeds on phytoplankton and detritus. Such animals may be some of the first to be affected by high levels of Se. Rusk (1991)noted that C. fluminea tissues generally had selenium levels above background along the lower Colorado River. She also noted that C. fluminea was an important food item for carnivorous birds and fish. It would be beneficial to managers if such an organism could be used as a bioindicator of the availability of contaminants to animals at higher trophic levels and of baseline levels within the system. Contaminant levels in bioindicator organisms can be more useful than records of concentrations in water because contaminant levels in biota reflect exposure over time and the magnitude of exposure. They also provide an indication of long-term effects on the ecosystem and possible effects on other taxa. Bioindicators are used to make hazard assessments (analysis of the potential exposure and effects from contaminants at a particular site) and for surveillance (routine monitoring of current and long-term trends in levels of exposure). They also can be used to measure effectiveness of remedial or management actions. Phillips (1977) suggests that indicator organisms for trace elements contamination should:

1. accumulate the pollutant without suffering mortality, 2. be sedentary,
2. be sedentary,
3. have a life span sufficiently long to allow for the sampling of more than one year class,
4. be abundant in the study region,
5. be large enough to allow adequate tissue samples for analysis,
6. be easy to sample and hardy enough to be maintained in the laboratory,
7. tolerate brackish water,
8. exhibit a high metal concentration factor,
9. have a simple correlation between the metal concentration of the organism and the average metal concentration in the surrounding water,
10. exhibit the same correlation between their metal content and that of the surrounding water for all locations studied under all biotic and abiotic conditions. C. fluminea fits the first seven of these criteria (Cherry et at. 1980, Rodgers et al. 1980, Graney et al. 1983).

C. fluminea also fulfills criteria 8 and 9 for Cd, Cu and possibly Zn (Graney et al. 1983). It fails to satisfy criterion 10 because substrate, pH and temperature effect cadmium uptake (Graney el al. 1984). However, the effects of violating criterion 10 can be minimized by documenting pH, temperature, and substrate at sites of collection. Bivalve mollusks have been used extensively for trace elements assessment (Phillips 1976). Tessier et al. (1984) investigated the relationships between partitioning of trace metals (Pb, Fe, Zn, Cu and Mn) in sediments and their accumulation in the tissues ofthe mollusk Elliptio complanata. Abaychi and Mustafa (1988) found a correlation between metal content in mollusks and metal content in particulate matter. Abaychi and Mustafa (1988) established that C. fluminea is capable of accumulating and eliminating trace elements in relation to their concentration in ambient water and concluded that C. fluminea is a suitable bioindicator for monitoring trace metal pollution. Doherty (1990) concurs that C. fluminea is a valid bioindicator of trace metal contamination and satisfies the criteria established by Phillips (1977). Johns et al. (1988) successfully used C. fluminea as an indicator of selenium distribution in San Francisco Bay.

PUSTAKA: McCaulou T, WJ Matter & OE Maughan. 1994. Corbiculae fluminea as a Bioindicator on The Lower Colorado River. University of Arizona. http://orion.cr.usgs.gov/dec_reports 115/report.html (9 of 42) [10/17/2000 1:17:16 PM]