Colonization of a dynamic substrate: factors influencing recruitment of the wood-boring isopod, Sphaeroma terebrans, onto red mangrove (Rhizophora mangle) prop roots

The isopod Sphaeroma terebrans, which bores into the prop roots of the red mangrove, Rhizophora mangle, can cause death and subsequent breakage of the inhabited root and, debatably, may reduce the support system of the tree. We examined whether different characteristics of a root or its physical set ting, both of which may relate to habitat quality, influence the colonizati on of S. terebrans. A series of in situ experimental manipulations were con ducted over 2 years. The first experimental protocol examined colonization of intact root substratum. Free-hanging aerial prop roots which were previo usly unsubmerged were bent down and anchored underwater at high tide after which the colonization of S. terebrans was monitored. A second manipulation was performed to evaluate the relative roles of submergence depth and plan t anatomy in isopod colonization preference. Detached aerial prop roots wer e oriented either with the root tip directed downward, or upside down with the root tip directed upward but at a constant water depth. Additionally, d etached aerial prop roots were oriented so that their tips were in contact with the sediment surface in order to investigate whether colonization on g rounded roots was prevented by benthic predators. For all experiments, colo nization began within 24 hr juveniles were the predominant colonizers. The submergence depth of a prop root, a characteristic of a root's physical set ting, was consistently the strongest component correlated with the abundanc e of colonizing S. terebrans. The root-quality feature of root diameter was also found to play a significant role in colonization. Biotic influences, such as predation, however, did not influence recruitment of the isopod ont o a root. Additionally, the absence of burrowing on grounded roots appeared to be related to intrinsic plant features. Along a root there was a strong preference by S. terebrans to colonize the root tip region (i.e., youngest part of the root) especially when these roots were also the deepest. The r esults of our experimental manipulation using natural root substratum are t he first to identify physical setting as an important feature of habitat su itability for S. terebrans.