1. Japanese Sea Cucumbers Live in Shallow Rocky and Sandy Areas of the Sea
All sea cucumber relatives are marine animals. They do not live in rivers or in the diluted water of estuaries [1]. The Japanese sea cucumber, which is eaten as food, lives mainly on sandy-gravel bottoms and rocky reefs from the intertidal zone down to a depth of 40 m (about 131 ft). It is sometimes collected from depths of 100 to 300 m (328 to 984 ft) [5]. Its distribution ranges from Alaska, the Kuril Islands, Sakhalin, and the Korean Peninsula to Hokkaido, Kyushu, and Tanegashima in Kagoshima Prefecture [5]. Even within shallow seas, the environment differs greatly between rocky shores and muddy bays. Do the same sea cucumbers live in both places in the same way?
2. Red and Blue-Green Japanese Sea Cucumbers Prefer Different Habitats
Japanese sea cucumbers have color variations: reddish "Akagata," bluish-green "Aogata," and blackish "Kurogata." A guide from the Osaka Prefectural Research Institute for Environment, Agriculture, and Fisheries states that red types are common on rocky shores near the open sea, while blue and black types live on sandy-muddy bottoms [6]. Another source says red and black types are common on rocky reefs facing the open ocean, while blue types live on sandy-muddy bottoms in inner bays; however, the habitat for black types varies between sources [7]. It is best to compare two colors: red types prefer open-sea rocks, and blue types prefer inner-bay sand and mud. Whether these color types are separate species or variations of the same species also varies by source. This is a general tendency, not an absolute rule; red types can also be found in habitats favored by blue types. Shallow seas experience large seasonal temperature changes. How do sea cucumbers spend the summer heat?
3. In Summer, Japanese Sea Cucumbers Hide Under Rocks to Escape Warm Water
Shallow sea temperatures rise in summer. When the water temperature gets high, Japanese sea cucumbers hide under rocks or between obstacles on the seabed and stay still without eating to pass the summer. This is called summer dormancy [8][6]. The Japanese term contrasts with the word for winter hibernation. The temperature threshold for starting this dormancy varies by source. Kyoto Prefecture data states that activity stops when the water temperature reaches 18°C (64°F) or higher [8], while the Osaka guide says they wake up when the temperature drops to 23°C (73°F) or lower [6]. Since the starting and waking temperatures are different measures, these two numbers cannot be directly compared. Reports suggest that whether a sea cucumber enters summer dormancy differs by body color and size. For example, red types almost always enter summer dormancy, while small blue types may not [9]. This is a summary from an encyclopedia, so it should be read as a single study result rather than a universal fact. Where are the small sea cucumbers during this time?
4. Young Sea Cucumbers Are Thought to Live on Shallow Rocky Reefs
According to Iwate Prefecture data, juvenile sea cucumbers live mainly on shallow rocky reefs. As they grow, their habitat range is thought to expand to deeper places and sandy-muddy bottoms [10]. This suggests that shallow rocky areas are homes for adult sea cucumbers and also places where young ones grow up. However, the source describes this as something that is thought to happen, meaning the exact process of how they change locations as they grow is not fully understood. So far, we have discussed depths of only tens of meters. Do sea cucumbers live in much deeper seas?
5. Sea Cucumber Relatives Live at All Depths, Even in Deep Ocean Trenches
The ocean floor has long, narrow valleys called trenches. Sea cucumber relatives are found at almost all depths, from shallow seas down to trench bottoms at approximately 6,000 to 11,000 m (19,685 to 36,089 ft) [2]. A paper on the Kermadec Trench states that sea cucumber relatives are one of the groups with high species numbers in the ultra-deep sea [2]. Sea cucumbers of the genus Psychropotes have been found in the ultra-deep Pacific Ocean at depths of 6,135 to 7,290 m (20,128 to 23,917 ft) [2]. Compared to Mount Fuji’s height of 3,776 m (12,388 ft), this is about 1.6 to 1.9 times deeper. Note that this is a record for this genus, not the absolute deepest record for all sea cucumbers. An older report states that sea cucumbers make up about 90% of the biomass in the ultra-deep sea. However, this is considered biased because it comes from trawl results in specific spots where organic matter collects at the trench bottom, not the entire trench [3]. It is safe to understand that "sea cucumbers are very prominent at the trench bottom." How do these deep-sea sea cucumbers move?
6. Deep-Sea Sea Cucumbers: Some Walk on Legs and Some Swim
In the abyssal plains at depths of 1,000 to 6,000 m (3,280 to 19,685 ft), relatives of the sea pig (Scotoplanes) live. According to the Monterey Bay Aquarium Research Institute (MBARI), they stand on long tube feet like legs and lift their bodies above the soft mud to walk [11]. Meanwhile, some deep-sea species, such as the floating species called "yume namako" in Japanese, swim away from the seabed [1]. A British science magazine explains that relatives of Enypniastes are seen in seas worldwide from depths greater than 500 m (1,640 ft) to at least 6,000 m (19,685 ft). They flap fin-like membranes to swim up from the seabed. Their bodies are watery and buoyant, allowing them to swim with little energy. Their body length is up to about 25 cm (10 in) [12]. According to the English Wikipedia, sea cucumbers swimming near the seabed can rise up to about 1,000 m (3,280 ft) in the water column, and the genus Pelagothuria is said to be the only echinoderm that never lands on the seabed [4]. Since these figures come from secondary sources, they should be read as rough estimates. From under shallow rocks to the swimming deep sea, can you verify where sea cucumbers live?
7. How to Explore Sea Cucumber Habitats Yourself
If you see Japanese sea cucumbers or others at an aquarium, look at their body color and the tank environment. Are they on rocks or sand? Remembering that habitat preferences differ by color can be helpful. Always observe rocky shores with an adult. Do not turn over rocks, and watch your footing and the waves. To understand the depth range, divide the trench depths of 6,000 to 11,000 m (19,685 to 36,089 ft) by the height of a nearby mountain or Tokyo Skytree (634 m / 2,080 ft). If you read materials, the Wakkanai Fisheries Experiment Station guide [5], the Osaka guide [6], and the Kyoto Prefecture data [8] are available in Japanese. Comparing why the summer dormancy temperatures differ between sources is an interesting exercise.