1. Where Does Nori Hide During the Summer?
Nori has two body forms, and they look nothing alike. The leafy form that people eat grows in the sea, mainly in the cooler months. The other form is a tiny web of threads, called the conchocelis stage, that you can see well only with a microscope. According to Japan's Ministry of the Environment, these threads bore into shells and spend the summer there. When the water starts to cool, they produce spores, and each spore can sprout and grow into a new leafy plant [1].
A Japanese nori industry website describes the same thing from the farmer's side. Farmers grow the threads on oyster shells. The shells start out white, small black spots appear, and in about three months the shells turn almost completely black. The threads feed on the shell's limy part (calcium carbonate) and, like other seaweeds, take in nutrients such as nitrogen and phosphorus [2].
So nori is not missing in summer. It is changing into a form that only a microscope shows well. That raises a question: how did anyone discover this?
2. Who Discovered the Summer Stage of Nori?
The key person was Kathleen Drew-Baker, a British scientist at the University of Manchester. Working on the laver Porphyra umbilicalis, she showed that its life history includes a shell-boring conchocelis phase. Her discovery is credited with sparking the growth of nori farming in Japan [3]. Japan's Ministry of the Environment dates the finding to 1949 and says a Japanese researcher, Dr. Kuroki, worked out the full life cycle in 1952 [1].
Why did it matter so much? Before this, nobody knew where the seed came from in summer, so it was hard for farmers to plan. Once the shell stage was known, nori could be seeded on purpose. The Irish seaweed information site says modern nori farming no longer depends on the whims of nature and has become more stable, because the shell-borne stage is now largely raised on shore [3].
The same site says that Japanese nori farmers put up a statue of her overlooking the sea at Kumamoto and hold a small ceremony there each year [3]. Next question: with the shell stage in hand, how do farmers get the seed onto nets?
3. How Do Farmers Put Nori Seed on Nets?
In the cooling water of mid-September to early October, the threads in the shells release spores, which are the seed. Getting them onto a net is called seeding [2]. The nori industry website says this is the first hard step in nori making, because the spores should be spread neither too thickly nor too thinly.
On land, farmers put shells into a big tank and turn a net on a water wheel over it, so the spores stick to the net. A seeded net can be kept in a refrigerator until the sea cools to about 23 °C (73 °F). At sea, farmers hang bags of shells under stacks of nets, and the spores come out at sunrise [2]. A farm net is about 18 m by 1.5 m (59 ft by 5 ft) [2].
After that the young nori grows on the nets in the sea, and the next job is gathering it.
4. How Is Nori Harvested From the Sea?
The nori industry website says harvesting starts in late October in early areas and early December in late areas, and goes on until April or May of the next year. A net can be harvested again about two weeks later, and farmers repeat this cycle about four times before swapping in a frozen net saved earlier [2].
A harvesting boat, called a "diving boat," slides under the net. A machine on it uses a spinning wire fitted with small blades to cut the nori, and one net takes only about 1 to 3 minutes [2]. Because the seaweed keeps growing after cutting, one net can give several harvests. These numbers come from a trade site and vary by place and year.
Most farmed nori today comes from one species. Japan's Ministry of the Environment says 95 to 99 percent of farmed nori is susabi-nori (Porphyra yezoensis), chosen because it resists disease and has a good color and gloss [1]. The cut seaweed then goes to a factory.
5. How Does Cut Seaweed Become a Flat Sheet?
The sheet is not just dried leaves laid flat. The factory washes the seaweed in seawater to remove diatoms (tiny algae that cling to it), then chops it into small pieces and washes it with fresh water. The chopped nori floats in water and is spread over a bamboo-like screen, much like making paper by hand. Sponges take out water, then hot air at 40 to 80 °C (104 to 176 °F) dries it for about 2 to 3 hours, down to about 10 percent water. Then the sheet is peeled off [2].
A full sheet is usually 21 cm by 19 cm (about 8.3 by 7.5 in). Japan standardized this size across the country between 1965 and 1974, and before that sizes varied by region [4].
That is dried nori, but it is not yet the roasted sheet most people buy.
6. What Turns Dried Nori Into Roasted Nori?
The dried sheets bought at auction still hold 10 to 12 percent water. They are dried again to about 2 percent, then roasted, seasoned and cut before being sold [2]. The extra drying matters because a sheet with less water is crisp and keeps better.
A Japanese nori promotion group credits roasted nori to Tanaka Magosaemon, who ran a shop called Miuraya in the Omori area of what is now Tokyo. It says he was selling it in glass bottles in 1844 [5]. That comes from one trade-group history, so it is best read as "credited to," not as a settled fact.
Put it all together: summer in shells, autumn seeding, winter harvest, chopped and spread, dried, roasted. You can see a trace of that path at home.
7. How Can You See the Traces in a Nori Sheet at Home?
If you have a nori sheet, touch both sides. One side is smooth and one is rough. The nori industry website says the smooth side is the front and the rough side is the back [4]. It does not say why, so the link to the screen is only a guess. Then measure the sheet with a ruler. A full sheet should be about 21 cm by 19 cm, though some products are pre-cut smaller.
For more, the Ministry of the Environment page and the nori trade pages below show diagrams of the life cycle and factory steps. Times and sizes differ by region, so local fishing-cooperative pages are worth comparing.