@ Seena, the most abundant are sundari (Heritiera fomes), gewa (Excoecaria agallocha), goran (Ceriops decandra) and keora (Sonneratia apetala) all of which occur prominently throughout the area.
Crabs are the most important invertebrate shredders in the mangrove environs throughout the world. Families Graspidae and Ocypodidae dominate besides other species. Even mangrove molluscs (snails like Terebralia palustris and slugs) help in shredding of mangrove leaf litter. This also applies to the Indian mangroves including Sundarbans, West Bengal.
Marine wood borers are the real shredders in the mangrove ecosystem. They can attack any type of wood that contact with the seawater. They are ubiquitous and you can see even in the deadwood or fallen wood of Mangroves. They belong to the family Teredinidae (Mollusca: Bivalvia) and Pholadidae even crustacean isopods do their jobs belong to the family Sphaeromatidae. Compare to the bacteria and fungi these wood borers attacks faster.
For this you may consult the books written by Prof. Abijit Mitra and Prof. Chaudhuri. To my knowledge on the land insect groups and graspid crabs and ocepode crabs and on the aquatic medium mud crabs and shrimp and prawns. You can also refer the last book published by ZSI.