Trophic ecology of crabs from Pakistan: evidence from stable isotopes and fatty acid biomarkers and biogeochemistry of mangrove forest in the active Indus deltaic creeks and their adjacent coastal waters of the Northwest Arabian Sea
Waqar Ahmed^1*, Ying Wu², Samina Kidwai¹, Jing Zhang²

1. National Institute of Oceanography, Pakistan
2. State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, P.R. China *waqar25@gmail.com

The study links the nutrients, organic matter, and primary productivity to the trophodynamics of crabs in the typical mangrove fauna of the Indus River, Indus Delta, its adjacent coastal waters of the Northwest Arabian Sea. The samples were collected in the winter (northeast monsoon), summer (southwest monsoon), and inter-monsoon periods. The fatty acids (FA) and Stable Isotopes (SI) were used to characterize the diversification and feeding behavior of crabs from the Indus Delta and adjacent coastal waters. Fatty acid C18:3n‒3, a mangrove detritus marker, was found in very low concentration in all the crab samples. This suggests that the mangroves were not the main food source of the crabs from the mudflats/coastal waters. The mangrove-derived material contributed only to the food web in the form of the exportation of organic matter (OM) to the adjacent nearshore waters. Diatoms and dinoflagellates were the major primary food source for the crabs, Poly Unsaturated Fatty Acids (PUFAs) were the main fatty acids found in the tissues of the crabs, and the major contributing fatty acids were C20:5n‒3 and C22:6n‒3. In the crabs, the saturated fatty acids (C16:0 and C18:0), which are ubiquitous biomarkers for phytoplankton in marine environments, were the main food source in the Indus Delta and adjacent areas. The presence of C20:5n‒3 and C22:6n‒3 was found in high concentrations in all the crustacean samples, showing that the diatoms and dinoflagellates were the main food source for crabs (crustaceans), which is significant to the fishery catch from the Indus Delta. PUFAs (C18:2n‒6, C18:3n‒3, and C18:3n‒6) were in low percentages, and this could be due to the labile nature, grazing of zooplankton, and bacterial degradation. The depleted isotopic values from the species from the Indus River as compared to creeks, coastal waters, and offshore suggested that there was a variety of food available to the crustaceans, and they were opportunistic feeders.
This is a joint follow-up study between NIO Pakistan and SKLEC, P.R. China, to study the diet preferences of crabs and shrimps in detail because of their fishery interest and ecological position in a mangrove ecosystem.