Have come across swarms of megalopa in Jodiya creek (district Jamnagar-Gujarat state) in the month of May. In experiments with rearing of larval stages of P merguiensis, rearing them at higher temperature than the normal, has cut short the rearing time & the same could be beneficial in your problem.
Probably the feed of Skeletonema sps. could help as the first feed.There are reports that the phytoplankton is found in brackish water zones along Kerala coast soon after monsoon. First instar of Artemia parthenogenitica which has 180 micron size could help as the second feed,which will look after the fatty acid requirements of the crab. Macerated egg custard is another feed which can be tried.
please refer to the publication of Associate Prof. Dr. Mhd Ikhwanuddin. he doing about the mud crab culture. maybe u can get some info from his journal. search for ikhwanuddin, M.