Before explaining what sturgeon is
What are fish?
Fish is a general term for a taxonomic group of vertebrates excluding tetrapods (four-legged animals）. Most of the “fish” we imagine are teleosts but, with a few exceptions, this definition is as ambiguous as the one above. Exceptions include lampreys, sharks, rays, coelacanths, and lungfish. The sturgeon and bowfin, which are called ancient fish, do not belong to the teleosts. Let me explain this in a little more detail.
What are vertebrates?
Animals that have a brain and centrally manage neural information include mollusks, arthropods, and vertebrates. On the other hand, animals in which neurotransmission pathways are scattered throughout the body include cnidarians and echinoderms, such as sea urchins and starfish[A1] . Mollusks and arthropods have transmission pathways on the ventral side, whereas chordates have transmission pathways bundled on the dorsal side. The notochord is connective tissue (supporting tissue, derived from the mesoderm) that replaces the spine.
It has the function of inducing neural tubes (which later develop into the spinal cord) during vertebrate development. It normally degenerates later in the process of development (it is replaced by the spine). However, the spine is underdeveloped in cyclostomata and sturgeons, and the notochord even in adults.
The evolution of organisms
All organisms were born from a common ancestor and differentiated at some point to evolve separately. Organisms are believed to have originated 3.8 billion years ago, suggesting that our common ancestor was born then. Most existing organisms are thought to have differentiated in the Cambrian period (approximately 500 million years ago) at the beginning of the Phanerozoic era, which is because the Precambrian (Paleozoic) geological records (i.e., fossils) have not shown any multicellular organisms that appear to be our ancestors. In addition, the “Phanerozoic era” is an era in which the existence of organisms can be clearly confirmed. The periodization of the Phanerozoic era, including the present time, is divided at the points when the biota (type and number) changed significantly, based on information from fossils. A major change in the biota indicates that a great extinction event of organisms occurred immediately prior, followed by the rapid evolution of the surviving organisms. A great extinction event occurred immediately before the Cambrian period (although there is no evidence), and biological evolution is thought to have occurred rapidly to fill the void. Since then, many great extinction events have repeated to the present day.
The evolutionary lineage of organisms
Here, we consider the evolutionary lineage of vertebrates. The spine is the organ through which the spinal cord and notochord pass in the spinal column bone, and lamprey is considered to be the most primitive vertebrate. However, it does not have the same vertebrate ancestor as the existing hagfish, and a vertebrae-bearing animal (an ancestor of the vertebrates), which does not exist today, is thought to have been born somewhere in the evolutionary history, differentiating into lamprey and other vertebrates. To find out whether lamprey is a species close to the common ancestor of the vertebrates (whether it has reached the present without much differentiation), it is presumed to be closer to its ancestral species if it has more points of agreement than existing related invertebrate species (for example, Amphioxus of Cephalochordata). Methods of comparing the characteristics of body morphology as well as DNA have been used for this purpose. The remaining lamprey-like biological fossils will provide important evidence for exploring common ancestors and periods of differentiation, allowing us to trace how it differentiated from its ancestral species to the present species.
The bones of primitive vertebrates were cartilage. The existing cartilaginous fishes, including sharks and rays, have differentiated into ○× as they retained cartilage. Bones have been calcified (calcium phosphate) somewhere in the evolutionary history of vertebrates to evolve into those organisms with hard bones (③). From there, the diversity of vertebrates to this day was born. At what point did these differentiations occur? I would like to give a time axis. The most direct evidence comes from fossil records. However, fossil records are often recorded only in special places and at special times, such as when there is the sudden accumulation of volcanic deposits in a certain place. Thus, sufficient evidence cannot be obtained in most cases. There is also a method in which the number of differentiations is calculated by DNA analysis and the time required for it is estimated (molecular clock). The period of differentiation is inferred by combining multiple pieces of circumstantial evidence.
Evolution of fish
Cartilaginous fish and bony fish
Let’s add a rough timeline to get an overview of the evolutionary history of fish. The most primitive vertebrates that exist today are the lamprey and hagfish. Their bone is soft cartilage, and they are characterized by having no jaw. The jawless fish are believed to have flourished in the sea approximately 400 million years ago (the Silurian period to the early Devonian period). Fish with jaws similar to modern chimaera were born in the middle Devonian period. Then, they evolved while the bones in their bodies remained soft in the Carboniferous period approximately 350 million years ago, and the cartilaginous fish, including sharks and rays, prospered. At the same time when the sharks prospered, bony fish evolved by calcifying bones to acquire a hard skeleton.
Lobe-finned fish and ray-finned fish
The diversification of the bony fish occurred in the Permian period (approximately 299 million to 251 million years ago). Although bony fish are described as “fish” with hard bones, they include four-legged animals (tetrapods). Because the four legs were evolved from fish fins, there are animals with fleshy fins that are called lobe-finned fish. Coelacanths and lungfish are primitive lobe-finned fish that exist today. These primitive lobe-finned fish prospered in the late Permian period (260 million years ago). Some lobe-finned fish that survived the mass extinction event at the boundary between the Permian period and Triassic period (PT boundary) differentiated into reptiles and mammals. Reptiles prospered on land in the Mesozoic era. Among the bony fish that survived the PT boundary, there are also the ray-finned fish with ray-shaped fins, which are the typical “fish” we think of today.
Ancient fish Sturgeon
Among the ray-finned fish, primitive sturgeons are classified as Chondrostei because their spinal cord and the spine that houses the notochord remain as cartilage. Ray-finned fish that have undergone further evolution to acquire hard bones throughout the body are called teleosts. Therefore, the sturgeons are called ancient fish as they are the most primitive group of ray-finned fish (a taxon to which many fish belong).
Finally, we’ve arrived at the classification of the sturgeons (Acipenseriformes). Although the sturgeons and sharks, which are cartilaginous fish, have a common ancestor, they have evolved separately since ancient times. Therefore, the sturgeons and sharks belong to different taxa. Now, let’s classify the sturgeons. The Acipenseriformes consist of two genera, the genus Acipenseridae and the genus Polyodontidae. The genus Acipenseridae consists of the members of the Acipenseridae family including Acipenser mikadoi, Huso dauricus (Kaluga), Acipenser ruthenus, and Huso huso (Beluga).
Classification and distribution of Sturgeons
Huso dauricus (Kaluga)
Huso huso (Beluga)
(Trivia：Bester is a crossbreed of Huso huso and Acipenser ruthenus. Because it reaches maturity as early as 7–8 years, its ovaries, which are the source of caviar, can be obtained sooner than those of other species.)
Acipenseridae・・・They inhabit the sea in the northern hemisphere, and some migrate up rivers during spawning, while others live their entire lives in freshwater.
Polyodontidae・・・They inhabit freshwater areas, as well as brackish water in rare cases, in China and the United States.
Many sturgeons inhabit the Russian coast. The Amur River, which flows into the Sea of Okhotsk, has Acipenser ruthenus. The Ob River, which flows into the Arctic Ocean, has Siberian sturgeon, and the Caspian Sea has Huso huso. Some Arctic sturgeon are landlocked and inhabit Mongolia. In China, there are sturgeon from the Yangtze River, Changjiang River, and Yellow River. Although sturgeon were widely distributed in Europe, all are on the verge of extinction. The main reason for the decrease in their populations is thought to be the decrease in the spawning grounds due to overfishing and river improvement works.
In Japan, there used to be upstream-migrating groups of stuegeons in the Niigata Prefecture, north of Fukushima Prefecture, the coastal areas of Hokkaido, Teshio River, and Ishikari River. However, they are now extinct as river improvement works have eliminated suitable pools for spawning.
Huso dauricus was discovered in the Ishikari River in 2004, and Acipenser mikadoi was discovered in Rausu in 2013. It is likely that the individuals inhabiting Russia have migrated to these rivers.
Body characteristics of sturgeons
Large tabular hard scales (tabular ganoid scale) are lined up on the sides of the body. There are five rows in total, two rows on each side (four rows on both sides) and one row on the back. Many of the fish that prospered in the Palaeozoic era (Devonian period and Carboniferous period) are thought to have had tabular ganoid scales. In addition to the existing ancient fish sturgeons, coelacanths and lungfish, which are called living fossils, have ganoid scales.
Stuergeon is called “butterfly shark” in Japanese because their scales look like butterflies flying in a row.
Sturgeons are slimy like teleosts.
The spinal cord and notochord run inside the spinal column, which is made of cartilage. The spine of the sturgeon is underdeveloped and is not able to fully function as a supporting organ. Therefore, the notochord, which normally degenerates during the developmental process, remains even in adulthood and functions as a major supporting organ in the sturgeon.
In addition to the sturgeon, adult lamprey and hagfish have a notochord. These fish also have underdeveloped vertebrae, and the notochord plays a major role as a supporting organ. This is a remnant of primitive vertebrates. The notochord of the sturgeon, which is over one meter in length, is one of the largest in existence.
Compared to the cerebellum of teleosts, that of the sturgeon is smaller and poorer. Considering that the cerebellum controls vision and movement, the size is consistent with its slow movement.
Feeding habits of Sturgeons
Eats mainly shellfish, polychaetes, crustaceans, and fish.
- There are two pairs of mustaches on the ventral anterior side of the
lower jaw. They are hard cartilaginous mustaches, with fine protrusions on the
surface of the mustaches. There are taste buds in the opening of the
protrusion, which senses chemical substances (odors) in the water. Hard
mustaches extend down in front of the mouth, making it suitable
for finding food in the mud and inhaling it by mouth. They are similar to the
mustaches of cod, which is a demersal fish.
- Adult fish have no teeth and swallow
food as if they are inhaling it.
The loop-shaped elongated stomach has an ampulla at the end of the stomach (the pyloric region) with a thick muscular layer. This muscle layer contracts and grinds food into a paste. It plays a role similar to that of the gizzards of birds.
Sturgeon Fishery around the world
The wild sturgeon fishing industry was active for caviar production until the 1980s. Overfishing of wild sturgeon drastically reduced catches. Correspondingly, sturgeon production by aquaculture has increased sharply since the 2,000s, and is now comparable to the old catches. According to the 2007 statistics, China accounts for 80% of the world’s sturgeon production. On the other hand, China accounts only for 5% of the world’s caviar production. In China, sturgeon production for meat is the main production, most of which is consumed in China. Caviar production is, of course, carried out in China as well, and has been increasing year by year.