1. How Clams Move Water Through Their Gills and Two Siphons
Clams are bivalves that live in water and use two tubes to move water in and out. These are called the inhalant and exhalant siphons. The tubes are not the mouth or anus themselves. Instead, the water drawn in passes through the gills, where particles of dirt and food are filtered out [2].
The feeding method changes with water temperature. The Japanese river clam lives in brackish water, a mix of seawater and fresh water. When the water temperature drops below about 5°C, the clam burrows into the mud or sand at the bottom and hibernates. When the temperature is higher, it extends its siphons above the bottom and filters plant plankton and other particles from the surrounding water [1].
The amount of water filtered varies with water temperature. One report states that a single clam filters about 0.2 liters of water per hour at 20°C, and about 0.4 liters at 25°C. These values change depending on the water temperature and the size of the clam, so they are not fixed amounts for every situation [1]. But is everything the clam takes in actually eaten?
2. The Three Types of Material Clams Expel, Starting With Uneaten Pseudofeces
A study of Japanese river clams in Lake Shinji found that what the clams expel is divided into three types. The first is "pseudofeces," which comes out of the inhalant siphon. The other two are "undigested feces" and "digested feces," which come out of the exhalant siphon. The order of release is pseudofeces, then undigested feces, then digested feces [3]. Pseudofeces are not feces in the normal sense because they are expelled before entering the digestive tract.
Pseudofeces contain undigested algae, sand, and small organic matter like dead organisms (detritus). In experiments, this material began to appear within a few minutes. It was brown, crumbly, and contained many sand grains. Intact pennate diatoms (long, thin algae) were found inside [3].
Digested feces, which are brown and thread-like, were usually released 4 to 9 hours after the experiment began. Most of this release happened at night. During the day, the clams often released nothing. This observation was made on clams from the west bank of Lake Shinji in 2016 [3]. The clam returns uneaten material in minutes, but releases digested material hours later. So, what does the clam actually eat?
3. Clams Eat Algae From the Bottom, Not Just From the Water
When researchers examined the waste, they found that on every experimental day, the waste contained large amounts of benthic pennate diatoms. These are algae that live on the bottom. This suggests that Japanese river clams do not just eat plant plankton floating in the water, but also take in algae from the bottom [3].
The types of food available change depending on the location. In brackish water areas, there are multiple food sources, including organic matter carried by rivers, organic matter from seaweed, and plant plankton in the water. A study by Shimane University used stable carbon isotope ratios to analyze clams collected from Lake Shinji and Lake Nakaumi. The results showed a wide range of values, and the food sources varied by location. This may reflect differences in how the clams use food resources in different places [5]. If there are multiple food sources, does digestion differ depending on the type of algae?
4. How Clams Digest Cyanobacteria, Green Algae, and Diatoms
In an experiment with Japanese river clams from Lake Shinji, researchers fed them three types of algae one at a time: cyanobacteria, green algae, and diatoms. It is thought that the clams could digest all three types. Green algae were taken into the midgut gland deep in the digestive tract as intact cells and then digested. For diatoms, only the contents of the cells were taken in. Diatoms became digested feces the fastest among the three types. It is inferred that diatoms are digested quickly and are easy to utilize [3].
However, if the algae are too large, they may be expelled unchanged. When large diatoms (genus Coscinodiscus) with a shell diameter of about 180 micrometers were fed to the clams, some clams expelled the shells completely intact in their undigested feces. This result was from an indoor experiment at 20°C using one type of algae at a time [3]. If some algae are easier to digest, what is the best food for the clam?
5. How Three Types of Clams Clear Cloudy Water
At an event at the Ibaraki Prefectural Kasumigaura Environmental Science Center, researchers compared three types of clams: the freshwater mussel (dobugai), the Japanese river clam, and the Manila clam. They used equal weights of each clam and placed them in 1 liter of water clouded with Chlorella (a green alga). They compared how clean the water became. The Japanese river clam cleaned the water the best [2].
It is important to note how to read this result. The comparison was based on equal weight, not one clam per species. The materials state that about 12 Japanese river clams are equivalent to 1 freshwater mussel. Also, the results could change if conditions like water temperature, pH, salinity, or food type are different [2]. So, which algae is the best food?
6. Which Algae Is the Best Food for Clams, Judged by Fat Content
The Shimane Prefectural Institute of Public Health and Environmental Science fed cyanobacteria, green algae, and diatoms to Japanese river clams from Lake Shinji. They compared the amount of fatty acids in the clams. For clams of all sizes, those fed diatoms had the highest total fatty acid content, while those fed cyanobacteria had the lowest. Therefore, diatoms were considered the best food [4].
Diatoms and green algae contain many polyunsaturated fatty acids, a type of fatty acid needed by the body. Cyanobacteria contain very little of these. Young clams and medium-sized clams may be more affected by the type of food. It is thought that because they are weak at producing these fatty acids themselves, they grow slowly if they only eat plankton that does not contain them. Adult clams can produce them from other fatty acids. However, this is still at the stage of inference and has not been proven as a cause-and-effect relationship in the lake [4].
7. How to Watch Clams While Removing Sand Before Cooking
If you are removing sand from clams bought at the supermarket before cooking, try following the instructions on the package or bag. In a quiet place, look to see if two tubes are extending from the clams in the water. They may not always appear. After a few hours, check the bottom of the container to see if sand or brown thread-like objects have fallen out. In the Lake Shinji experiments, digested feces were mostly released at night, but this may not be the same in a home container [3].
To learn more, you can look at the original reports. The observation of feeding is in the report by Shuji Otani for the Shimane Prefecture Lake Shinji Conservation and Restoration Council. The fatty acid study is from the Shimane Prefectural Institute of Public Health and Environmental Science. The food resource study is from Shimane University. You can find the original texts with diagrams and photos in the source list.
Safety note: Observations of clams before eating should be done on the premise that they are cooked thoroughly.