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b

It is very likely to be Bostrychia, but the image does not show the detail needed to confirm it

Mangroves are generally associated with three estuarine algae such as Bostrychia, Catenella and Caloglossa. But in the above picture there is no clarity and it may not belongs to Bostrychia. try to take the photographs of tetrasporophytic and cystocarpic populations of above plant for proper idientification

Thank you