The Porphyra life history is complex. To grow Porphyra (known in the trade by its Japanese common name, "nori") commercially (there are nori aquaculture industries in many places, especially Japan, Korea and China but also in the USA), one needs to know and be able to control this life history.

[Note: The arcane terminology applied to the sexual process in Porphyra and other red algae dates to the 19th century, at a time when sperm that didn't swim couldn't be called sperm, and eggs that weren't round couldn't be called eggs. The reader, unfortunately, will find few informative texts on red algae that see past this problem, and therefore the $5 words are used in the description that follows.]

The conchocelis phase is diploid. Under specific conditions of light quantity, light quality, daylength and temperature (the permissive conditions differ between species, and sometimes between strains of a species), the filaments form swollen branches ("conchosporangia") in which the cells, though still diploid, develop the plastid features of blade-phase cells (stellate plastid with conspicuous pyrenoid). These branches protrude from the substrate and eventually release their contents as individual wallless cells ("conchospores").

In some species, different environmental conditions will stimulate the asexual reproduction of the conchocelis via naked "monospores" produced at the tips of normal-width branches.

Meiosis takes place in each conchospore after wall secretion and during initial rhizoid production, and usually the four meiotic products survive. Hence, the blades are haploid and chimeric. In some species, monospores produced at thallus margins reproduce the blades asexually. Blades arising from monospores are, of course, not chimeric.

Specific conditions of light quality and quantity, daylength and temperature stimulate the production of gametes. Male gametes ("spermatia") are produced in packets at the blade margins and are released by dissolution of the margin. Female gametes ("carpogonia") are formed some ways back from the margin. A receptive surface ("trichogyne") protrudes from each carpogonium and through the surrounding matrix, to which spermatia attach and effect fertilization. The zygote then divides to form a packet of diploid cells (the "carposporangium"). Diploid "carpospores" are released from the carposporangium by dissolution of the blade margin. Germination of carpospores, to form diploid conchocelis filaments, is not dependent on the presence of solid calcium carbonate, but in nature, apparently, only those carpospores that do germinate on, and subsequently penetrate, this substrate avoid getting eaten by snails and other small marine grazers.

A few species of Porphyra undergo the morphological phase alternation without gamete fusion or meiosis. An additional few species reproduce, as blades, only asexually by monospores.

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