A blue pincher emerging from its burrow in the rhizome mat. The green at the edge is likely MPB, which might collect in this relatively protected area.
While they are not the most attractive of crabs, nor of interest to any fishery, Thalamita crenata, or the "blue pincher" is numerically the most dominant crab in the fishpond. I am interested in it because it seems to have no problem living in concert with the toughest invaders. Blue pinchers can be found burrowing in the mangrove rhizome mat, or feeding within the Gracilaria canopy. They are an excellent example of how structure-forming species (mangrove and Gracilaria) can influence community structure, and also a lesson: species can be ecosystem engineers not just by increasing habitat complexity by building structure, but also by decreasing it.
Two blue pincher crabs mating at the ocean break.
Generally, invasive species that increase habitat complexity also increase abundance and/or diversity in invaded areas. Unlike invasive pigs (Sus scrofa) and other terrestrial invaders which decrease habitat complexity in native forests, organisms like limu and burrowing crabs create more diverse habitat, which can make room for more native or alien species. Knowledge of these community effects can be helpful for predicting responses to new invasions, but the scales of invasion and community interaction are important in determining what these responses are (Crooks 2002). Additionally, the life histories of the involved species can also affect community responses (e.g., the snail Littorina littorea transforms muddy habitat into what is essentially a rocky shore, but native species which favor rocky habitat do quite well with the modification). In a place like Hawaii where the preservation of native diversity is a high priority, knowing the effects of certain invaders on the ecosystem is a powerful conservation tool.
Crooks, J. (2002). Characterizing ecosystem-level consequences of biological invasions: the role of ecosystem engineers Oikos, 97 (2), 153-166 DOI: 10.1034/j.1600-0706.2002.970201.x
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