1. What Color Is Hijiki While Alive in the Sea?
Hijiki is a brown alga belonging to the family Sargassaceae [1]. As the name suggests, it is brown while alive in the sea [2]. It turns jet black only after being harvested, dried in the sun, washed, steamed, and dried again [2]. It is distributed along the coasts of Japan, South Korea, and China [7]. If you only know it from the bag, it is hard to imagine what it looks like in the water. So, where in the sea does this brown seaweed grow?
2. Where Does Hijiki Grow in the Sea?
Hijiki grows near the low tide line on rocky reefs that face relatively strong waves [1]. The low tide line is the height of the sea surface when the tide is lowest; this area is usually underwater but exposed to air only during low tide. It is distributed across a wide range of rocky shores from southern Hokkaido to the southern tip of Kyushu, with the peak season considered to be March to May [2]. In the coastal areas of Kaifu District, Tokushima Prefecture, hijiki communities form in the intertidal zone, and harvesting and processing are done by beach in spring [3].
Harvesting is often done during the low tide during a spring tide, collecting seaweed bodies that have grown to about 1 m (about 3 ft) [1]. It is thought that on days when the tide recedes significantly, people can walk to areas that are usually underwater. So, does hijiki not grow back after it is harvested?
3. How Does Hijiki Grow from Summer Buds Through Autumn and Winter?
Generally, the life cycle of hijiki is explained as follows: new buds emerge in summer, begin to grow in autumn, and reach their maximum length and maturity in spring [4]. However, a survey along the coast of Wakayama Prefecture (September 2011 to April 2012) found communities where young buds stayed flat and stuck to rocks, barely growing from autumn to early winter. This shape is called "rosette type," resembling the leaves of a dandelion spread flat on the ground [4]. Note that not all communities behave this way; some do grow in autumn.
The daily growth rate of the main branch varied by location and season, ranging from less than 1 mm to more than 4 mm [4]. If a pace of 4 mm per day continued for 30 days, it would be about 12 cm (about 4.7 in); if 1 mm, about 3 cm (about 1.2 in). This calculation compares the ends of the reported range, but it shows that even the same buds vary in how they grow. If growth is slow in winter, does it suddenly reach maximum size in spring? Let us look at surveys from other seas.
4. Does Hijiki Regrow from Its Base After Harvesting?
Hijiki is harvested by leaving the roots behind so it can grow again [2]. The part that looks like roots is actually a "holdfast" that attaches to rocks. In a survey on Suo-Oshima Island, many new buds emerged from the edge of the holdfast in late spring to summer [5]. It is said that hijiki communities are mainly maintained by this method of increasing from the holdfast, rather than from reproductive bodies like seeds [5]. Research in Kyushu and other areas reports that holdfasts are stronger than young plants formed from fertilized eggs under conditions of high summer temperatures or low nutrients, and can produce new buds even if only a fragment of about 1 mm remains [6].
Here, the sources disagree. The study on Suo-Oshima Island reported that the holdfasts in that location are annual and die off each year [5]. On the other hand, at harvesting sites, it is explained that if the roots are left, the plant will grow again [2]. Both descriptions agree on the point that "new buds come from the holdfast." How long they live may differ depending on the location and method of observation. If vegetative reproduction from holdfasts is so strong, raising seedlings from seeds by human hand seems easy. What actually happened?
5. What Happened in the Kagawa Hijiki Seedling Trial?
In a trial at the Kagawa Prefectural Fisheries Experimental Station, about 113,000 fertilized eggs (embryos, about 200 micrometers = 0.2 mm) were obtained from parent algae in June [7]. One week later, most had become germlings (about 0.35–0.4 mm), and about one month later, they had grown into initial leaves of 1.5–2 mm [7].
However, from summer to early autumn, other algae such as diatoms increased, which is believed to have hindered the hijiki's photosynthesis. After about one year of cultivation, only 35 seedlings were produced [7]. Dividing 35 by 113,000 gives about 0.03%, or roughly 1 in 3,200. However, this is the result of one trial and does not mean that artificial seedling production is impossible. The same report states that successful mass production has been achieved in other prefectures [7].
Despite this, why do people still try to make seedlings? Natural seedlings are often harvested along with the holdfast, raising concerns about the impact on natural resources. Therefore, more municipalities are working on producing artificial seedlings to ensure a stable supply [7]. How can we check this story in our home kitchen?
6. Which Parts of Hijiki Are Sold in Stores as Dried Hijiki?
In stores, the stem (main branch) part of hijiki is often called "naga-hijiki" (long hijiki), and the part using leaves and air bladders (float-like parts) is called "me-hijiki" (bud hijiki) [1]. Some sources, like those from Miyazaki Prefecture, call young algae in early spring "me-hijiki," so the names vary by region.
Try soaking dried hijiki in water and looking for stem-like parts, leaf-like parts, and small round bulges. Imagine how these parts were oriented in the water in the sea. This is one way to investigate. When using water, do this with an adult.
The sources from Miyazaki, Kagawa, and Wakayama Prefectures can be read on prefectural websites. They serve as an entry point for learning more about where, when, and how hijiki grows in the sea.
When using water, always do this with an adult.