1. Where Does the Unwanted Mud Go After a Loach Sucks It In?
Imagine seeing a loach with its head buried in the mud at the bottom of a rice paddy or irrigation channel. Its mouth points downward. The "Rice Paddy Living Things Map" from Miyagi Prefecture explains that the loach sucks up mud and sand from the bottom through its downward-facing mouth, like a vacuum [1]. However, the mud is not the food. The food consists of organic matter and tiny insects inside the mud, while the unwanted mud and sand are expelled through the gills [1]. Materials from the Ministry of Land, Infrastructure, Transport and Tourism also describe loaches as omnivorous fish that eat bottom-dwelling animals in the mud, such as red worms [3]. Red worms are the larvae of midges, a type of fly. This process is a bit like playing in a sandbox and using a sieve to keep only the large grains. But for loaches, the separation happens through the gills rather than a mesh screen. They gather small food mixed in the mud by taking the mud into their mouths. So, how do they find the food in the mud?
2. What Do the 10 Whiskers Around a Loach's Mouth Do?
Loaches have a total of 10 whiskers (5 pairs) around their mouths [2]. When moving through mud, these whiskers line up in front of the face like thin antennae. What are these whiskers doing? Some research on fish whiskers states that they contain receptors, such as taste buds (clusters of taste-sensing cells) [9]. The loach’s whiskers may help it find food using taste and smell as clues. The downward-facing mouth, the 10 whiskers, and the gills that expel mud all fit well with a lifestyle of searching for food at the muddy bottom. So, what is actually found inside their stomachs?
3. What Foods Were Found Inside Loaches' Stomachs?
Studies of stomach contents are conducted all over the world. In Japan, a study examined the food of 3,023 loaches (body length 25–149 mm) collected from agricultural channels in the Watarase River system between January 2005 and January 2006 [4]. In July, when these loaches coexisted with another species called the pond loach, their main foods were ostracods (seed shrimp), aquatic insect larvae, and fairy shrimp [4]. Small loaches under 30 mm in length mostly ate ostracods, while those over 30 mm also ate aquatic insect larvae and fairy shrimp [4]. In Idaho, USA, where loaches are an introduced species, researchers examined the stomachs of 237 loaches. 158 were empty. 42 contained only the bait used in the traps, and only 37 contained natural food [5]. Calculating this, about 16% (roughly 1 in 6) had natural food in their stomachs. This ratio applies to the 237 fish examined. Among the fish with natural food, larger invertebrates were the main component, with midge larvae being the most common. Organic debris in the mud (detritus) is also considered an important food source [5]. In both Japan and the USA, small creatures from the muddy bottom were the main diet. So, do loaches just eat whatever they find?
4. Do Loaches Pick Slow-Moving Prey, and Does Their Diet Change with the Seasons?
A study in northwestern Italy examined 1,000 loaches collected in 5 samples from agricultural channels fed by spring water (June 2024 to February 2025; stomach contents were analyzed for 993 fish) [6]. From a Japanese perspective, this is a result from a population that has spread overseas. The loaches preferred slow-moving bottom creatures, such as soft-bodied mollusks and fly larvae like midges. They tended to avoid fast-moving creatures like mayflies and caddisflies [6]. The diet also changed by season. In winter, detritus from the mud increased significantly, while in summer, the variety of food widened [6]. This study found that loaches chose many slow-moving creatures from those living on the bottom. However, this is the result of one population in one channel. We cannot definitively say it is the same in Japanese rice paddies. So, in the rice paddy, is the loach always the one doing the eating?
5. Why Are Loaches Both Predators and Prey in Rice Paddies?
Loaches do better in muddy, still areas than in sandy, flowing ones, and they have long had a deep relationship with rice paddies [2]. The Nagano Prefectural Fisheries Experiment Station describes "ascending loaches" that move from rivers to paddies in spring and summer, and "descending loaches" that move from paddies to rivers in autumn, which were actively caught [2]. A public relations magazine from the Ministry of Agriculture, Forestry and Fisheries states that loaches improve water quality by eating leftovers from other creatures and aquatic plants [7]. This is an explanation from the magazine, not the result of a study measuring water quality. The same magazine also writes that loach eggs and fry (young fish) become food for creatures like diving beetles and dragonfly nymphs [7]. A survey of 19 aquaculture ponds in Ishikawa Prefecture and aquarium experiments suggested that fry about 18 mm long are eaten by backswimmers, dragonfly larvae (including *Potamonyx* and *Anax*), and American crayfish [8]. Loaches suck up mud to eat small creatures, but when they are small, they are eaten by other creatures. Loaches are in the middle of the food web in the rice paddy.
6. How Can You Check a Loach's Mouth and Whiskers Yourself?
Use fish encyclopedias or photos at home to count how many whiskers a loach has and which direction its mouth faces. Compare this with the mouth direction and whiskers of other fish. If there is a safe place nearby to observe, such as an aquarium, you can see how they bring their mouths close to the bottom sand or mud. When going to the water’s edge, go with an adult. Wet places are slippery, so do not get too close. Do not catch creatures from rice paddies or channels without permission, as they have owners or managers. For more information, the Miyagi Prefecture "Rice Paddy Living Things Map" and the Nagano Prefectural Fisheries Experiment Station’s "Fish of Shinshu" introduce loaches and their life in rice paddies.