1. Why Does the Seabed Run Low on Oxygen in Summer?
In summer, the water near the sea surface is warmed by the sun, creating a large temperature difference with the cold water below. This makes it difficult for the upper and lower layers of water to mix, preventing oxygen from reaching the bottom [2]. At the same time, dead phytoplankton sink to the seabed. When these are decomposed, oxygen is used up. Because the water layers do not mix, the used oxygen is not replenished [2]. This creates a mass of water with low oxygen, called a hypoxic water mass [1].
Chiba Prefecture’s glossary states that the guideline for dissolved oxygen for fish and shellfish to live is at least 3 milligrams per liter, and for oxygen-using microorganisms to be active, it is at least 2 milligrams per liter [1]. However, these are rough guidelines and not strict boundaries for death for all living things. So, what do the creatures left in this water do?
2. How Do Mobile Animals Like Fish Escape Oxygen-Poor Water?
Living things that can move, such as fish, are said to migrate away from oxygen-poor water [4]. Strength against low-oxygen environments varies by group. Polychaete worms, bivalves, and cnidarians (like jellyfish and sea anemones) are relatively strong, while crustaceans (like shrimp and crabs) and fish are considered relatively weak [4][14]. Fish are weak against low oxygen but can swim, so they can escape. This combination makes sense. However, this is a general trend. It is not true that "all fish can escape" or "all clams must endure," as it depends on how low the oxygen is and how long it lasts. So, how do clams, which do not move much, endure in place?
3. How Does the Saruboh Clam Survive by Switching to Anaerobic Respiration?
A study reproduced the environment at the bottom of Lake Nakaumi, a brackish lake in western Japan, in a laboratory to examine a bivalve called the saruboh clam (Sarubō-gai, an ark shell) [5]. When dissolved oxygen fell below 2 milligrams per liter, the clams began anaerobic respiration, which does not use oxygen. In severe hypoxia of less than 0.61 milligrams per liter, their oxygen-using respiration stopped. In that case, they produce the energy source needed to live (ATP) solely through inefficient anaerobic respiration [5]. Because these clams move little, they switch mechanisms inside their bodies to endure. However, this is the result of a laboratory experiment on one species, the saruboh clam. It cannot be said that all bivalves in the sea respond the same way.
4. Why Did Quahog Clams Gain an Advantage When Predators Left the Cove?
In a field experiment in a shallow cove in the United States, predators such as starfish, fish, and crabs left areas with low oxygen. As a result, quahogs, a type of bivalve, benefited because they were less likely to be eaten [6]. On the other hand, other bivalves like soft-shell clams and mussels died in severe hypoxia [6]. Low-oxygen areas are not always dangerous for clams; for those that can endure, they can become places without natural enemies. However, this result applies to quahogs, not all bivalves.
5. How Does the Blue Blood of Shrimp and Crabs Work in Oxygen-Poor Seas?
Hemocyanin is a protein pigment containing copper. Two copper atoms bind to one oxygen molecule, and the color changes from colorless to blue when bound to oxygen [7]. It is found in mollusks like squid and octopus, and arthropods like shrimp and crabs, performing the function similar to hemoglobin in human blood [7]. Compared by the same amount of blood, the efficiency of carrying oxygen is said to be about one-fourth that of hemoglobin [8]. This is a general comparison; how living things actually use it is a different matter.
When exposed to low oxygen, the activity of hemocyanin genes increases in decapods (shrimp and crabs). In 24 hours, gene transcription increases by about 1.65 to 1.98 times, but this is not the multiplier for the amount of hemocyanin in the blood [9]. Furthermore, blue crabs changed the combination of hemocyanin subunits after 7 or 25 days of low oxygen, increasing their affinity for oxygen [9]. This is content from prior studies introduced in the paper's introduction. This can be read as an example of a group considered weak against low oxygen responding not only by behavior but also by adjustments inside the body.
6. How Many Low-Oxygen Dead Zones Exist, and What Is Being Done About Them?
According to a report published in 2008, 405 coastal hypoxic zones (dead zones) were recorded worldwide by 2007. This increased from 305 sites in 1995, having nearly doubled every 10 years since the 1960s [10][14]. These are figures from 2008, not the latest numbers. In the Black Sea, a large hypoxic zone nearly disappeared around 1991–2001, and living things returned [11]. Fertilizer use had decreased in the early 1990s. Reduced fertilizer is thought to be one reason, but it cannot be determined as the sole cause. Note that this is based on a summary of an introductory article, so we cannot assert that reduced fertilizer was the only cause of recovery.
In Japan, countermeasures are also progressing, such as filling depressions left by dredging with soil from construction or covering them with sand. A review in Kanagawa Prefecture evaluated that improving the seabed and filling depressions have a relatively high effect in suppressing anoxic water masses in areas where water movement is poor [13]. In Mikawa Bay, Aichi Prefecture and the Ministry of Land, Infrastructure, Transport and Tourism are repairing deep excavation sites and covering them with sand [15]. However, the size of hypoxic water masses in Tokyo Bay varies by year. It cannot be said that they are "continuously decreasing" or that hypoxia is resolved.
7. How Can You Investigate Low-Oxygen Survival Further on Your Own?
What you can do today: Write "flee," "endure," and "change blood" on paper. Think about where to place the fish, saruboh clam, quahog, and blue crab mentioned in this article. If a creature is hard to place, write down why. What you can do when going out: Look for shrimp, crabs, squid, and octopus at supermarkets or aquariums. In this article, they appeared as relatives with blue blood. You cannot see the blood color, so look at their body structure and movement to find how they differ from fish and clams. Do not touch the animals at the aquarium.
Official information: The Chiba Prefecture Fisheries Research Center’s water quality reports contain records of when hypoxic water masses were confirmed and when they disappeared for each year [3][12]. Comparing year by year, you might notice increases or decreases.