Habitat creation for animals by teredinid bivalves in Indonesian mangrove ecosystems

A better understanding of the fundamental role large woody debris (LWD) plays within mangrove ecosystems may provide further insights into important ecological processes, such as wood degradation and biodiversity maintenance within mangrove forests.Though the volume of fallen wood in mangrove forests can be huge, little is known of the breakdown pathways and biodiversity maintenance of LWD in mangrove ecosystems. The degree of mangrove fauna dependent upon LWD and the need for such substratum in mangrove ecosystems may provide further insights in to the important role of woody biomass in these otherwise globally shrinking habitats due to forest harvesting. The breakdown, recycling and flux of nutrients from LWD within mangrove forests is maintained by biodegrading organisms in areas from terrestrial to marine habitats. The tidal inundation sets limits on the wood degrading communities within the mangrove forests of Sulawesi. This study presents details of the environmental and biological association of biodegrading organisms within the forests in the Wakatobi Marine Park (WMP), Sulawesi. Wood boring animals belonging to the family Teredinidae are the dominant biodegraders of LWD in the mid- to low intertidal areas of the mangrove forests. Teredinid attack greatly reduces the volume of LWD in the mid- to low intertidal areas of the forests. Within the forests, emersion time was the greatest influence of the distribution of the biodegrading organisms spanning from the supra-tidal down to the low intertidal. The response of Rhizophora stylosa prop-roots to physical damage and the activity of teredinids upon damaged prop-roots were investigated. With severe levels of root damage, the level of teredinid activity increases, resulting in root death and detrital input. However, when the roots were exposed to a superficial and moderate level of damage, an over-compensation of tissue re-growth was observed. LWD in the intertidal zone is often tunnelled by teredinids. The tunnels are blind-ending cylinders that taper to a small opening at the wood surface. However, larger openings appear when wood is heavily tunnelled and the surface is broken open. Teredinid death then leaves niches for cryptofauna. The greater the number of teredinid tunnels within LWD, the more diversity was found. Animals of particular interest were the dartfish, Parioglossus interruptus and the intertidal spider, Desis martensi found in the vacant teredinid tunnels. Desid spiders were abundant within the LWD and dartfish collected from within teredinid-attacked LWD were smaller than dartfish populations not within LWD. Desids and dartfish residing within the wood may benefit from the significantly lower temperatures within teredinid-attacked detritus compared to external air temperatures. Desis martensi has a life-history strategy centred on strong parental care, with lots of energy invested in to its young. Vulnerable stages of dartfish exploit the vacant teredinid tunnels. If it were not for the tunnels created by the teredinids the unusual behaviour adopted by dartfish and spiders would not be possible. Thus, many animals in mangrove forests of the WMP rely on LWD as a predation refuge enhanced by the teredinid tunnels within the LWD. A variety of different species were found inside teredinid attacked LWD, and the cryptic behaviour of the fauna ranged from breeding to predator avoidance. These findings indicate that in forests where wood is harvested, reduced availability of LWD will result in reduced biodiversity.