Fishes, shell fishes and crustaceans :
Major constituents of this group in the mangrove environment of India are 105 species of fishes, 20 species of shell fished and more than 225 species of crustaceans. Among these, commercially important are Meretrix sp., Crassostrea sp., Penaeus sp., Scylla serrata and Mugil cephalis
Many crustaceans in the mangroves make burrows which are used for refuge, the feeding, as a source of water or for establishing a territory necessary for mating. Some may filter water through their burrows, feeding on suspended detritus and plankton while others may breed there. These burrows play and important role in the mangroves, aerating, draining and turning the dense waterlogged soil - a direct benefit to the plants which in turn give them shelter.
There is a limit to how many burrows can be dug in any one area. It seems that when there are too many, homeless crabs may try to take over occupied ones. Some fiddler crabs and ghost crabs have been observed filing in the burrows of their neighbors to maintain their territories.
Scylla serrata, the large edible swimming crab, inhabits the muddy bottom of mangrove estuaries, as well as
coastal brackish water. Thalassina anomala, the mud lobster is also found along estuaries and tidal rivers.
They build long tunneling burrows that can reach up to four meters in length and can be recognized by
spectacular turrets of earth rising to two meters in height above the surface.