Porphyra is the name for a genus of red algae (a.k.a. sea vegetable, seaweed) that grows in intertidal to shallow subtidal areas of coastal waters around the world. “Laver”, “nori”, and “kim” are all common names for Porphyra. In nature, healthy Porphyra appears purple-red in color and has a sheet-like morphology (often called a “blade”);however, Porphyra can look green and even pale yellow when conditions are not favorable. Porphyra is normally found attached to rocks on rocky shores, but some species can grow on other algae as epiphytes. In terms of its seasonality, individual Porphyra species often have a distinct period during the year in which they are present; however, there are different species that occur in every season of the year. It can be difficult to tell the different species apart on the shore because of their simple morphology. In many cases, correct identification requires inferences made from its position in the intertidal zone, time of year, the use of a microscope, and even genetic testing.

Porphyra Kützing (Phylum: Rhodophyta, Class: Bangiophyceae, Order: Bangiales, Family: Bangiaceae) forms a polyphyletic clade with Bangia in the family Bangiaceae. Porphyra represents over 115 different species with new species being described frequently.
The biphasic life history of Porphyra includes a foliose gametophytic stage (blade) and a uniseriate, filamentous sporophytic stage (conchocelis). The blade can be either one or two cell layers thick, and each cell contains one or two stellate-shaped plastids. Blade morphology ranges from long and thin to round and very ruffled (lanceolate to pseudoumbilicate). This morphological variation can be highly diverse even within a species. Blades can reach up to one meter in length; however, individuals are often not longer than 30 cm.

Reproductive development is either hermaphroditic (monoecious) or with separate male and female plants (dioecious). Individuals can also produce asexual spores or reproduction can even be exclusively asexual. Asexual reproduction occurs through archeospores, neutral spores, endospores, and agamospores (see Glossary). In hermaphroditic thalli, sexual differentiation of cells occurs either in sectors of the blade or diffusely in the blade; this varies by individual species. Fertilization occurs when a non-flagellated spermatium fertilizes a single-celled female gamete held within a blade. Mitotic amplification of the zygote subsequently occurs through lateral and periclinal divisions; the number of cell divisions can be used to identify the species in some cases. Release of the resulting (diploid) zygotospores normally occurs at the margin of the blade with a distal bias. Zygotospores germinate into the conchocelis stage of the life history.

The conchocelis stage of Porphyra has unipolar germination from a zygotospore (formerly carpospore); it bores into and grows within calcareous materials (such as oyster shells). The conchocelis consists of microscopic branching filaments that contain pit plugs between cells. During reproductive maturation of this stage, multiple conchosporangia emerge from the shell. Conchospores are released and go through meiosis during bipolar germination, which gives rise to the haploid blade. Some spores (“monospores”) that fail to go through meiosis regenerate the conchocelis phase.