More I get to know what crabs do for mangroves, the more I find it amusing, sometimes even hilarious. These little guys (gals) probably have no idea how they are helping mangrove ecosystems while “minding their own business” and that is what make them the ecosystem engineers of mangroves!

Crabs are the major macrofauna, ecologically engineering the mangroves through digging burrows. Two major species whose names keep coming up are Sesermids(Grapsidae) and fiddler crabs(Ocypodidae); the former being ubiquitous to any mangroves in the Indo-West pacific. A major part of their diet consist of leaf litter. Without considering their contribution any calculation of trophic detritus web and detritus export from mangroves will be incomplete. But what is it that these species do that make them so important?!

Neosarmatium smithi
Family: Sesarmidae

Crabs dig burrows in the sediment to seek refuge from predation and environmental extremes which the mangrove is renowned for! Basically they open up windows of opportunities through burrows for both physical and chemical processes and interaction between groundwater, substrate and nutrient dynamics.

Burrows not only are their shelters but affect sediment topography and biogeochemistry by modifying particle size distribution, drainage, redox conditions and organic matter as well as nutrient availability1,2 and food storage3,4,5. When talking about their food, these herbivores literally retain, bury, macerate and ingest litter and macroalgal mats6,7,8. 

These process not only prevent nutrient loss but further encourage decomposition. Crabs literally increase nutrient use efficiency by removing up to 75% litter from forest floor. Thus they play critical role in nutrient cycling by influencing nutrient efficiency and availability to other mangrove flora, fauna and microbes. However as ecosystem effects of crabs are caused by non-trophic activities, they have earned the reputation of the “ecosystem engineer”. They literally alter physical structure, transport condition and substance chemistry9. 

smithi_leaf_bur
Family: Sesarmidae

However, in the Indo-pacific region, predatory behavior of crabs on mangrove propagules has also been observed as a threat for mangrove flora. The problem is severe more for managed mangroves (in Malaysia); in fact, predation was found responsible for complete failure of A. marina in northeastern Australia (Smith 1992). It demands similar investigation for Sundarbans mangroves as well.

nevertheless, ecoengineering role of crabs has a lot to discuss and yet more to investigate into. I intend to do the former in near future and the later, well, am sure the scientists are not giving it any rest 😉

Ciao.

 

References:
1.Mouton, E.C., Felder, D.L., 1996. Burrow distributions and population estimates for the fiddler crabs Uca spinicarpa and Uca longisignalis in a Gulf of Mexico salt marsh. Estuaries 19, 51–6.
2. Botto, F., Iribarne, O., 2000. Contrasting effect of two burrowing carbs (Chasmagnathus granulata and Uca uruguayensis) on sediment composition and transport in estuarine environments. Estuar. Coast. Shelf Sci. 51, 141–151.
3. Giddins, R.L., Lucas, J.S., Neilson, M.J., Richards, G.N., 1986. Feeding ecology of the mangrove crab Neosarmatium smithi(Crustacea, Decapoda, Sesarmidae). Mar. Ecol., Prog. Ser. 33, 147–155.
4. Warren, J.H., 1990. The use of open burrows to estimate abundances of intertidal estuarine crabs. Aust. J. Ecol. 15, 277–280.
5. Dittmann, S., 1996. Effects of macrobenthic burrows on infaunal communities in tropical tidal flats. Mar. Ecol., Prog. Ser. 134, 119–130.
6. Emmerson, W.D., McGwynne, L.E., 1992. Feeding and assimilation of mangrove leaves by the crab Sesarma meinerti Deman in relation to leaf-litter production in Mgazana, a warm temperate southern African mangrove swamp. J. Exp. Mar. Biol. Ecol. 157,41–53.7. Kristensen, E., Alongi, D.M., 2006. Control by fiddler crabs (Uca vocans) and plant roots (Avicennia marina) on carbon, iron and sulfur biogeochemistry in mangrove sediment. Limnol. Oceanogr.51, 1557–1571.
8. Lee, S.Y., 1997. Potential trophic importance of the faecal material of the mangrove sesarmine crab Sesarma messa. Mar. Ecol., Prog.Ser. 159, 275–284.
9. Kristensen, E., 2008. Mangrove crabs as ecosystem engineers; with emphasis on sediment processes. Journal of Sea Research, 59(1-2), 30–43. http://doi.org/10.1016/j.seares.2007.05.004
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