The sanctuary has an estuarine ecosystem; the sanctuary is covered with mangrove forest on the major part with intermittent open mudflats.
Flora Char Kukri-Mukri mangrove ecosystem reveals the occurrence of 383 species belonging to 264 genera under 85 families of vascular plants, of which 48 species are mangroves and mangrove associates, and the rest of the 335 species are non-mangroves. The major plant groups, viz., pteridophytes are represented by 11 species under 10 genera and six families; gymnosperms by two species under two genera and two families; magnoliopsida (dicotyledonous) by 278 species belonging to 194 genera and 60 families; and liliopsida by the rest of the 92 species under 59 genera and 17 families. Fabaceae is the largest dicot family with 43 species, and Poaceae with 30 species is recorded as the largest monocot family. Among the life-form categories, 198 (51.7%) species are recorded as herbs, followed by 98 (25.59%), 45 (11.75%), and 42 (10.97%) species those are recognized as trees, shrubs, and climbers, respectively. The Mangrove species like Keora (
Sonneratia apetala), Soilla (
Sonneratia caseolaris), Baine (
Avicennia officinalis), Gewa (
Excoecaria agallocha), Hargoja (
Acanthus ilicifolius), Golpata (
Nypa fruticans) etc. are common.
Fauna Common mammals include
Fishing cat (
Felis viverrina) and
Oriental small-clawed otter (
Aonyx cinerea). Water fowl species of Bitterns, Herons, Egrets, kingfishers are very common. Eight species of Herons breed in the sanctuary. The Grey Pelican or
Spot-billed pelican which is included in the Near threatened species list of IUCN Red data Book is also found in this sanctuary. All three species of
Monitor lizard namely (
Varanus salvator),
Bengal monitor (
Varanus bengalensis) and
Yellow monitor (
Varanus flavescens) are also found in the sanctuary. ==Threats==