Among animals there are many species known for their unpretentiousness to environmental conditions. Basically, these are relatively primitive organisms: for example, Baikal tardigrades are microscopic invertebrates that, in the dried state, can survive for months in oxygen-free, unsuitable gas environments, and even in a vacuum. But among the representatives of vertebrates with a high level of metabolism there are record holders, such as the famous naked mole rat - a small burrowing rodent that can do without oxygen for 18 minutes! Nevertheless, until recently, ocean fish in such "tricks" were not noticed. But recent observations by marine biologists suggest something else
Oxygen is one of the foundations of animal life: it participates in the processes of the breakdown of organic substances, as a result of which the energy necessary to maintain the vital functions of the body is released. But in 2015, biologists discovered large schools of fish in the Gulf of California at depths where oxygen is almost absent. For observation and shooting, they used a remotely controlled underwater robot equipped with sensors, inter alia, measuring the concentration of oxygen dissolved in water. It turned out that two types of fish - black brotulavariety conger eel (Cherublemma emmelas), and californian big-headed shark (Cephalurus cephalus) live near the seabed, resembling a bare lunar landscape, where the oxygen content in the water is extremely low.
The partial pressure of oxygen in the areas inhabited by them is only 0.1–0.3 kPa, while the average critical oxygen threshold for fish is approximately 5 kPa, i.e. ten times higher! And these fish do not just live in these places, but even prefer them: at least in the neighboring areas, more saturated with oxygen, they were not found.
Species of fish that can survive for a long time hypoxia (lack of oxygen) have been known before. For example, the well-known and well-studied common crucianliving in stagnant bodies of water covered with ice in winter. In cold weather, crucian carp fall into a stupor, at which their metabolism slows down significantly, and they winter successfully under the ice cover, despite the fact that the oxygen content in the water drops by spring. However, in such cases, we are usually talking about a temporary stay in hypoxic conditions. But the environment where deep-sea California fish live is fairly stable.
To date, scientists identify three strategies for adapting to hypoxia: reducing oxygen demand, improving methods for its absorption and storage, and also actively using anaerobic (oxygen-free) biochemical pathways. How exactly our "extremes" adapted is not yet entirely clear. Apparently, they reduced their energy requirements by reducing body size (individuals of the California big-headed shark reach a length of only 8–40 cm) and a less developed skeleton, and large gills more effectively capture oxygen. It is possible that they also have a high affinity for hemoglobin for oxygen and a slowed metabolism, but this assumption has not yet been verified.
Similar studies of the tolerance range and the limits of adaptation for different species are relevant in connection with the possible dramatic changes in climatic and other living conditions on the planet. And they are also of certain interest in the light of future flights into space and, no matter how fantastic it sounds, life itself in extraterrestrial environments.
Tadpole shark or candy shark
This unusual predator is the only species of its kind Cephalurus (large-headed cat sharks). The area of the large-headed California shark is located in the east of the Pacific Ocean, off the coast of California (USA) - from the Gulf of California to New Caledonia. The species is deep-sea and poorly understood. Most often, these predators can be found at a depth of 150-1000 m (individual specimens were caught in the range of 155-937 m).
It should be noted that the existence of undescribed species of large-headed sharks is expected in the coastal areas of Panama, Chile and Peru, since from these places there have been reports of the capture of individual fish species that look similar to the California big-headed cat shark.
This is a very small species of sharks, even among representatives of the feline family - the largest one that has been in the hands of researchers reached a length of 28 cm. At birth, these crumbs do not exceed decimeter length. They become sexually mature with a length of 19 (males) - 24 cm (females).
As the name of the species implies, this shark is the owner of a relatively large and wide head. Thanks to this appearance, the big-headed cat shark is sometimes called the "tadpole shark" - so much the shape of its body resembles a frog fry.
The body is thin, cylindrical and slightly expanded in the region of the gill apparatus. The space between the spiracles and the fifth branchial fissure is elongated, reaching 60% of the entire length of the head. This indicates that the gill apparatus of this fish is very effective, due to which the large-headed cat shark is able to live in water with a low oxygen content. Five pairs of gill slits have an arched bend towards the head.
The snout is short, squeezed from above, rounded horizontally.
The skin of this shark is thin and very soft, thanks to weakly calcined and small plactoid flakes. The eyes are horizontally elongated, followed by spiracles. The supraorbital (brow) ridge is absent, the infraorbital is small. Triangular nasal valves, well separated from each other, lacking mouth. In the corners of the mouth labial folds (grooves) stand out.
Dorsal fins about the same size, offset to the tail of the body. The pectoral fins are relatively large, with rounded apices. Their length is almost two times the width. Caudal fin with underdeveloped lower lobe.
Body color can vary from taupe to dark brown. There are no patterns, lines and spots on the body and fins, but the edges of the fins are usually slightly lighter than the general background. The eyes are greenish.
The diet of this bottom predator is based on small crustaceans (shrimps, crabs, crustaceans), as well as juvenile bony fish.
Propagate by egg-born - the female carries eggs in her body, like a living incubator, during the entire development of the embryo, which receives food from the yolk sac. The eggs are thin-walled capsules, and are placed one in each of the two sections of the oviduct.
The breeding cycle is annual.
California big-headed cat sharks rarely get caught in fishing tackle - their small size and deep-sea lifestyle allow them to escape from the pursuit of fishers.
The conservation status of the species is not defined (DD) due to a lack of data on the state of the population.
For humans, this small deep-sea shark is even less dangerous than freshwater perch, since it does not have spines on its body.
California big-headed cat shark, cat lollipop shark, cat tadpole shark, Lollipop catshark.
Binomial Name: Cephalurus cephalus (C. H. Gilbert, 1892).
The literal translation of the binomial epithet of this fish, which has Greek roots, is “head + tail”, which emphasizes the unusual shape of its body.