The species richness and abundance of Gastropods and Bivalves
has decreased in post tsunami. For example, at the site 1, before
tsunami, the species richness of Gastropod consisted in 11 species
with density of 52 ind/m2 and they decreased to only 9 species
with density of 25 ind/m2 after tsunami. Before tsunami the
Cerithiidae had high abundance; however the species composition
was dominated by Potamididae after tsunami. Macintosh
et al (2002) reported that Potamididae was one of Gastropods
families which was more representative of the younger plantation,
assumed as they graze the young mangrove leaves like
Littorinidae and therefore, Macintosh et al (2002) suggested using
this family as bio-indicator of ecological changes. Nevertheless,
Asthon et al (2003) stated that young sapling leaves probably
was a better food source for mangrove Gastropods evidenced
by his observation that Gastropods was positively correlated
with sapling species number.
Decreasing in species richness and abundance post tsunami
probably due to changing in community structure of mangroves,
where the density of mangrove community in pre tsunami was
high than in post tsunami. The tsunami catastrophe destroyed
mangrove vegetation and also disrupted community structure
of biotic that lives in it. In general, the C-organic content before
tsunami was also higher than after tsunami at site 1; the
C-organic content before tsunami was 1.06% and it decreased
to 0.8% after tsunami.