Porphyra species have complex life histories.
The blade that you see on the shore is the gametophyte phase.
In Porphyra umbilicalis the male and female gametangia are usually on separate blades. Sometimes they are on the same blade but separated.
The blade phase is just a part of the life history. There is a separate, microscopic phase which lives in shells and is called the conchocelis phase.
Look for pink patches on shells because these might be evidence of conchocelis.
Phase of the seaweed’s life history when it has filaments and lives inside an animal shell. This phase is only seen in the Bangiophycidae - the class of red algae to which Porphyra umbilicalis belongs.
The phase of a plant's life history when all the cells are haploid - they contain only a single set of chromosomes (half the number found in the adult). The haploid gametophyte is typically formed by division of a diploid (full chromosome set) sporophyte.