Brachyuran or short tailed crabs are the most important invertebrates associated with mangroves in the estuaries of Kerala. The movements of these crabs in mangroves disturb sediments at the bottom of the estuaries. This is an important ecosystem service.
Nearly twenty species of brachyuran crabs have been reported from Kerala so far. However, their diversity patterns in relation to environmental parameters and seasonal variations are unknown.
So, K P Apreshgi and Kurian Mathew Abraham, University of Kerala, set out to study spatial and temporal variations in mangrove crab diversity in the estuaries of Kerala. The duo collaborated with K V Aneesh Kumar, Sacred Heart College, Thevara, and Sileesh Mulaseeri, St Alberts College, Ernakulam.
They focused on five estuaries: Puthuvype, Ayiramthengu, Dhalavapuram, Pullichira and Kottaramthuruth. From January to December 2015, they collected crab samples every week in an area of one square metre of each estuary, either by hand or with ring-shaped crab traps.
A male specimen of the fiddler crab, Austruca annulipes. Image: Chinmayisk via Wikimedia
The researchers identified thirteen species of short-tailed crabs in the five estuaries. The fiddler crab, Austruca annulipes, was the most abundant species at all sites, except in Pullichira. Scylla olivacea and Scylla tranquebarica showed remarkable frequencies and abundances, while Pseudosesarma glabrum was rare.
The researchers calculated biodiversity indices such as taxonomic diversity and taxonomic distinctiveness. Four of the five estuaries showed similar crab diversity. The highest diversity was observed in Kottaramthuruth, while the Pullichira estuary had a significantly lower diversity.
The low diversity could be due to unique habitat characteristics. The shallow depths and the small islands or dunes there could decrease mangrove diversity. Habitat degradation and pollution could be other reasons. Estuaries with greater mangrove diversity had higher crab diversity.
The researchers then assessed key environmental parameters that influence the diversity of mangrove crabs. Varying salinity, dissolved oxygen, nutrients and substrate types influenced crab diversity. These parameters shaped community structure in the estuaries.
In the pre-monsoon season, crab diversity is generally higher. This is attributed to higher salinity, maximum dissolved oxygen and warmer temperatures. These conditions favour the survival and activity of the crabs.
‘Targeted conservation strategies are needed to maintain crab diversity,’ says K. P. Apreshgi, University of Kerala.
‘Our research shows the ecological dynamics of mangroves and crabs in the estuaries of Kerala,’ adds Kurian Mathew Abraham, his mentor.
Regional Studies in Marine Science 79 (2024) 103830;
DOI: 10.1016/j.rsma.2024.103830
Reported by Amit Kumar
Sathyabama University, Chennai
STEAMindiaReports 
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