As someone living in Japan, I associate flounders primarily with the sea. I wonder how people in other countries view them? In Japan, fishing gear for flounders is so popular that you can find it in any fishing tackle shop. They are also commonly seen in supermarkets as a food item, often prepared as simmered dishes. However, they are not completely absent from rivers, as some surveys suggest.
Primarily Marine Fish
It appears there are nearly 800 species within the order of flounders. Of these, 137 species have been recorded in Japan. Interestingly, 14 of these have been identified in rivers during national riverine surveys. This means that while the majority of flounder species are marine, the image isn’t entirely incorrect. However, it’s not right to say that rivers and flounders are unrelated; some species are indeed found in rivers.
Table1 List of Flounder Species Found in Japan’s National River Environmental Survey (2016-2020)
| No. | Species | Cycle | Origin | Hokkaido | Tohoku | Hokuriku | Kanto | Chubu | Kinki | Chugoku | Shikoku | Kyusyu |
| Pleuronectiformes | ||||||||||||
| Bothidae(Lefteye Flounders) | ||||||||||||
| 1 | Bothus pantherinus | E | Native | ○ | ||||||||
| Cynoglossidae(Tonguefishes) | ||||||||||||
| 2 | Paraplagusia japonica | E | Native | ○ | ○ | ○ | ○ | ○ | ○ | ○ | ||
| 3 | Cynoglossus abbreviatus | E | Native | ○ | ||||||||
| 4 | Cynoglossus lighti | E | Native | ○ | ||||||||
| Paralichthyidae(Largetooth Flounders) | ||||||||||||
| 5 | Paralichthys olivaceus | E | Native | ○ | ○ | ○ | ○ | ○ | ○ | ○ | ○ | ○ |
| 6 | Pseudorhombus pentophthalmus | E | Native | ○ | ○ | |||||||
| 7 | Pseudorhombus arsius | E | Native | ○ | ○ | |||||||
| Pleuronectidae(Righteye Flounder) | ||||||||||||
| 8 | Platichthys stellatus | E | Native | ○ | ○ | ○ | ○ | |||||
| 9 | Platichthys bicoloratus | E | Native | ○ | ○ | ○ | ○ | ○ | ○ | |||
| 10 | Limanda punctatissima | E | Native | ○ | ||||||||
| 11 | Pseudopleuronectes yokohamae | E | Native | ○ | ○ | ○ | ○ | |||||
| 12 | Pseudopleuronectes schrenki | E | Native | ○ | ||||||||
| 13 | Pseudopleuronectes obscurus | E | Native | ○ | ||||||||
| Soleidae(Soles) | ||||||||||||
| 14 | Heteromycteris japonica | E | Native | ○ | ||||||||
| Total | 5 | 5 | 5 | 5 | 3 | 4 | 3 | 5 | 6 |
※1: Classification of Life Cycles
・ Freshwater: Species that can spend their entire life in freshwater.
・ Migratory: Species that must migrate between freshwater and saltwater during their lifetime.
・ Euryhaline: Species that primarily inhabit brackish or saltwater, but may also venture into freshwater.
However, since there are species with intermediate characteristics, such species have been categorized at the author’s discretion.
※2: This list was compiled based on the publicly available data from “The National Survey for River and Riparian Environment” (https://www.nilim.go.jp/lab/fbg/ksnkankyo/).
Flounders Relying on River Habitats
Among the flounders in Japan, the Starry flounder (Platichthys stellatus) is likely the most dependent on rivers. They inhabit brackish areas like estuaries and are not commonly found in open seas. When I think of flounders, I imagine them as a dish, but seeing the Starry flounder (Platichthys stellatus) in the wild, it’s more about appreciating their remarkable adaptation to riverbed life than their tastiness.
Starry flounder (Platichthys stellatus)
Habitat of the Starry Flounder (Platichthys stellatus) is Biased Towards Northern Japan
Interestingly, rivers confirmed to have Starry flounders (Platichthys stellatus) are all north of the Kanto region. It’s fascinating how clear the distribution boundary is in the sea, which has no borders. Could the boundary be due to water temperature? Unlikely, as the division in sea temperature isn’t that distinct. It’s probably related to ocean currents. Indeed, the southern limit of the Starry flounder (Platichthys stellatus) corresponds with ocean current boundaries. On the Pacific side, it’s where the Japan Sea Current meets the Oyashio Current, and on the Sea of Japan side, it’s where the Tsushima and Liman currents collide.
Ocean Currents and Fish Distribution
Research shows that ocean currents significantly impact fish distribution. With recent climate change, stronger southern ocean currents have led to many fish species previously limited to the south expanding their range northward. For northern fish like the Starry flounder (Platichthys stellatus), their range might conversely shrink northward. This is an interesting point to consider.
Freshwater and Flounders
Returning to the topic, flounders are apparently found in seas worldwide. Looking at a map, it seems the seas are all interconnected. Outside of Japan, in North and South America, Southeast Asia, India, and Papua New Guinea, there are freshwater flounders. In New Guinea, viviparous species also exist. These flounders seem to have originated from the sea and adapted to freshwater environments. In Japan, species like gobies and sculpins have some freshwater-adapted varieties. Some species have adapted by producing larger eggs and more developed juveniles. It might be a similar case for flounders. However, so far, no species have evolved to complete their entire life cycle in freshwater. Observing rivers around the world on Google Earth, one can notice that Japanese rivers are generally rapid, which might explain why it’s challenging for fish to penetrate deeply into freshwater, or why there are fewer meandering sections and oxbow lakes where fish could be trapped in freshwater.
Conclusion
The freshwater adaptation of marine-origin species, like gobies and sculpins, is a widely discussed topic in academic papers. Flounders might also be suitable subjects for studying how fish adapt to freshwater.
In this article, we understand that:
- Flounders are primarily marine species.
- Globally, there are freshwater flounders.
- However, there are no completely freshwater flounders in Japan.
- The Starry flounder (Platichthys stellatus) is not a fully freshwater fish but depends on rivers for its habitat.
- Their distribution is biased towards the north, with ocean current collisions marking their southern limit.
That should cover the key points for now.
Until next time.
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