Bacteria, elemental sulfur and stable carbon isotope rations in the mangrove mud clam Anodontia Edentula (Linne, 1758) / by Ma. Junemie Hazel L. Lebata.

Thesis (Master of Science in Fisheries Biology) University of the Philippines in the Visayas

Includes bibliographical references.

LEBATA, MA. JUNEMIE HAZEL L. University of the Philippines in the Visayas, November 1998. Bacteria, elemental sulfur and stable carbon isotope ratios in the mangrove mud clam Anodontia edentula (Linné, 1758). Thesis Adviser: Dr. Jose A. Ingles

Different sizes of the mangrove mud clam Anodontia edentula were collected from the mangroves in Brgy. San Roque in Estancia, Iloilo and examined for the presence of gut through dissection and microscopic observation. The presence of bacteria in the mantle, gill and foot tissues was also examined through bacterial culture and scanning electron microscopy (SEM). Elemental sulfur content of the mantle, gill and foot tissues and total sulfur content of the sediment were analyzed; and stable carbon isotope ratios of the clams, sediment, detritus and plankton in the study area were determined. These experiments were conducted to establish the possibility of clam-bacteria symbiosis in this species.

Dissection and histological sections of juvenile and adult mangrove clams showed the presence of a very simple gut, unlike the well-defined organs of the digestive system of other bivalve species that have no symbionts.

A decreasing bacterial count (CFU/g tissue) with increasing size of clam was observed only in plates containing gill isolates. In contrast, no bacterial colonies were observed in the mantle and foot isolates. The results paralleled the observations in
other lucinids that bacteria are confined only in the gills.

Moreover, as the clam grows it becomes less dependent on the bacteria. The morphology of colonies was the same in all plates suggesting only one type of colony. Bacteria observed were gram-negative, colorless and spherical to rod shaped. SEM also revealed only one type of spherical bacteria both in cultured plates and in gill bacteriocytes.

Sulfur analysis showed highly significant (p<0.0001) amounts of elemental sulfur in the gills (1.38-1907.20 pmoles/g FW) compared with the quantities observed in the mantle (0.51-6.41 pmoles/g FW). Elemental sulfur was absent from the foot tissues.

This finding supports the aforementioned presence of bacterial symbionts only in the gills. Furthermore, results showed a significantly (p<0.0001) decreasing elemental sulfur from the newly collected clams compared to those maintained in mangrove mud and stocked in seawater only which were analyzed 3 weeks later indicating that stored elemental sulfur is being utilized by the bacteria in the absence of sulfide.

Moreover, smaller clams (40.1-50.0 mm SL) had significantly higher elemental sulfur content than bigger ones (50.1-60.0 mm SL) supporting the inverse relationship between number of gill bacteria and clam size. Total sulfur content of mangrove mud in situ was higher than those used as substrate in the experiment; and was not significantly different from initial to final readings in the latter. This shows that mangrove mud in situ is linked to a steady sulfur source.

Stable C isotope ratios showed that the clams do not obtain their carbon from plankton, sediment nor detritus and may thus be solely dependent on bacterial carbon. This was shown by the more negative 613C ratios of carbon fixed during bacterial chemoautotrophy compared to carbon fixed photosynthetically.

Submitted to the University Library 08/04/2020 T-5947