Ecology of the intertidal crab Dotilla intermedia from tsunami-impacted beaches in Thailand

Abstract

Crabs of the genus Dotilla are ecologically important members of intertidal sandy shore
communities. Exposed sandy shores represent one of the main habitat types along the coast of the
Laem Son National Park in Thailand, and Dotilla sp. is the dominant macrofaunal species on
these beaches, occurring in immense numbers. Despite their importance as a key member of the
faunal community on these beaches, little is known about the ecology of Dotilla crabs in the
Laem Son. Taxonomic investigations identified the Dotilla crabs present on the exposed oceanic
beaches in the Laem Son to be D. intermedia. This represents the first time that D. intermedia has
been recorded from Thailand. Dotilla intermedia inhabits a very well defined zone on the sandy
beaches, and the factors underlying these zonation patterns were examined. The gradient of the
beach was correlated to the height at which the boundaries of the Dotilla zone occurred, with
physical factors associated with the beach gradient driving the distribution of D. intermedia on
the beach. The upper limit of the Dotilla zone was controlled by the total water content of the
sediment, with D. intermedia absent from areas with less than 15% total water content. Tidal
influences defined the lower boundary of the Dotilla zone, with crabs requiring an area with a
minimum exposure time between tidal immersions of 4-5 hours to feed on the sediment. Within
the Dotilla zone, size segregation was observed; larger crabs occurred higher on the shore, and
small crabs lower down. The sandy shores of the Laem Son were heavily impacted by the
tsunami of 26th December 2004, which effectively destroyed the populations of D. intermedia on
the beaches. However, by April 2005 D. intermedia was present again on the beaches. A
temporal population genetic study was undertaken to investigate the impact of extinction and
recolonisation on the genetic variation of a population. Genetic variation in mtDNA markers was
found to decrease over time, matching the predictions of mathematical models concerning the
effect of bottlenecking events on genetic diversity within populations. The impact of the tsunami
on D. intermedia is discussed further in light of the ecological and molecular data produced in
this thesis.

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