1. Loaches Live in the Muddy Bottoms of Rice Fields and Canals
Loaches live in rice fields and canals, and they sometimes lay eggs in rice fields [1]. Their habitat is characterized by a muddy bottom. They are described as fish that live in freshwater areas with soft mud containing decaying plants [3].
In a survey of living things in rice fields conducted in 2002, loaches were the most frequently confirmed fish. The survey covered 335 districts nationwide. According to the Ministry of the Environment, loaches were found in 142 districts, at 308 locations, across 36 prefectures. This means they were present in about 42% of the districts surveyed [2]. They were particularly noticeable in small irrigation and drainage channels near rice fields. These numbers reflect the districts where loaches were confirmed in 2002 and do not directly show the current number or distribution of loaches.
The muddy bottom seems like a place where air might be hard to reach even underwater. So, how do loaches breathe in such an environment?
2. How Loaches Swallow Air and Breathe Through Their Intestines
Fish usually take in oxygen dissolved in water through their gills. Loaches do this too, but reports state that they also breathe through their intestines when the oxygen level in the water drops [4]. It is also said that their skin is used for air breathing [3].
According to research papers, the process of intestinal breathing works like this: the loach sucks in air from the water surface through its mouth and sends that gas to the rear part of its intestine. After exchanging oxygen there, it expels the used air out through the anus [4]. So, although we say they "drink" air, they send it to the intestine to exchange oxygen. Another paper explains that loaches use the intestine, skin, and barbels as air-breathing organs to maintain oxygen levels in their blood [3].
This way of breathing seems to support their life in places where water lacks oxygen or where water is running out. So, what do loaches do when the water dries up?
3. How Loaches Survive in Mud When the Water Decreases
Loaches prefer mud and are said to be able to spend the winter even in quite dry field soil [1]. There is a theory that their name comes from the characters for "earth" and "life," but this should be read as just one possible origin story.
An English resource on loach care and observation notes an example where loaches burrowed into mud during a long drought and survived for more than one month [5]. However, this is from a single introduction and is not a guaranteed number of days they can withstand drought in Japanese rice fields.
Thus, loaches seem to burrow into mud to get through periods with little water. So, during times when water is abundant, how do loaches use rice fields and canals differently?
4. How Loaches Lay Eggs in Rice Fields and Travel Between Canals
Loaches sometimes lay eggs in rice fields [1]. When a flooded rice field is connected to a canal, it becomes a place where fish can enter from the canal or exit back into it. The Ministry of Agriculture, Forestry and Fisheries states that it is effective to help loaches, catfish, crucian carp, and common carp move up from canals to rice fields and down from rice fields to canals using rice field fish ladders [6]. Conversely, this means these fish use both canals and rice fields to live.
During construction to improve rice fields, height differences can form between canals and fields, preventing fish from moving. What do people do in such cases?
5. How Fish Ladders and Steps Help Fish Move Between Canals and Rice Fields
According to the Ministry of Agriculture, Forestry and Fisheries, construction to improve fields can create drops between canals and rice fields, making it impossible for fish to move. Artificial channels called rice field fish ladders are installed in such places. They are designed to protect freshwater fish that use rice fields for spawning and growth, and are considered effective for the upstream and downstream movement of loaches, catfish, and crucian carp [6].
For a close relative, the Japanese loach (Hotoke-dojou), the Kyoto Prefecture Red Data Book notes that habitat destruction due to field and canal improvements often leads to the extinction of local populations [7]. This is not about the common loach itself. However, this example from a close relative shows that construction work on canals can affect the lives of fish living in them. Does the loach’s way of breathing also help people?
6. How Loach Intestinal Breathing Inspired Medical Research
A research team at Tokyo Medical and Dental University focused on intestinal breathing, as seen in loaches, and tested an "intestinal ventilation method" that sends oxygen to the intestine in animal experiments with mice and pigs. They published in the academic journal "Med" in May 2021 that in mice, blood oxygen levels increased and survival time extended in a low-oxygen environment [8].
This is the result of animal experiments, and the effect has not been proven in humans. However, it can be read as an example where the body’s mechanisms in fish living in low-oxygen water connected to medical ideas for humans. Rice fields and canals support the loach’s life, and fish ladders help them travel. How do these look in real places?
7. How to Look Safely for Fish Ladders and Fish in Nearby Canals
Rice fields and canals are mostly private property or important facilities used by farmers. Do not enter them; just look from the road. When near water, go with a parent or guardian and do not approach slippery places.
Points to observe: Is the bottom of the canal mud, sand, or concrete? Is the water flowing or stagnant? Is there a step where water enters the field from the canal? If you see a slanted water channel next to a step, it might be a fish ladder. If you see one, it is interesting to imagine which fish it helps.
The Ministry of Agriculture, Forestry and Fisheries page on rice field fish ladders and the Ministry of the Environment summary of the rice field survey are good starting points. Both are available in Japanese.