Have some animals evolved to stop reproduction?

Deep sea fish

Searching for food under difficult conditions

The goals of deep-sea fish are clear: to eat, not to be eaten and to produce offspring. The way there is very different for the individual species. Huge mouths, telescopic eyes or light rods on the head: what makes the fish appear like monsters is in reality the perfect adaptation to the barren and dark expanse of the deep sea.

In the barren sea, it is important to be able to eat as many different types of food as possible. The pelican eel, for example, is well prepared for particularly large prey. He can open his mouth extremely wide because the jaws are only connected by an elastic membrane. It is believed that it ambushes its prey.

But there are also active hunters in the deep sea, such as the Schnepfenaal. Many fish migrate upwards at night because there is more food in the higher water layers. During the day they sink back into the depths, because here they cannot be easily found by predators.

Some species have long barbels on which there are luminous organs. The females of the frogfish carry such a bait organ called a fishing rod on their foreheads. They attract prey, which take the small, luminous dots for food (for example bioluminescent shrimp). When hunting, the light organ that sits on the front of the fishing rod is hung directly over the open mouth.

A sensitive tactile organ sits on the tip of the fishing rod, with the help of which the fish immediately notices when prey is approaching. When that happens, he folds his fishing rod up and stops the oxygen supply to the light so that it goes out. The prey continues to swim disoriented - directly into the angler's wide-open mouth.

The black dragonfish has developed another strategy for hunting at great depths, where prey can no longer be seen visually: In contrast to most other deep-sea animals, it is able to see red light and, with the help of a filter, also produce it itself. With its luminous organ, it has a kind of searchlight with which it can recognize its prey, but cannot be seen itself.

Most other deep-sea dwellers can only perceive blue light, if at all, because the blue part of the sunlight penetrates the deepest.

Camouflage in the twilight

In the twilight zone (from about 200 to 1000 meters depth), where there is little sunlight, some fish have developed a special camouflage technique. With the help of light organs on their belly side, hatchet fish and lantern fish adapt perfectly to the twilight so that their body contours seem to disappear completely. Viewed from below, they are invisible to predators.

In addition, the fish have shiny silvery bodies, which they also camouflage. The color pigments of the hatchet fish darken at night, presumably because the silver color would be more visible in the dark. Studies on lantern fish have shown that they can adapt the light intensity of their luminous organs to the environment.

During the day, most of the fish in the twilight zone are rather inactive and hang motionless horizontally in the water - perfectly camouflaged, of course. With increasing depth, the fish tend to get darker and their luminous organs smaller, because here there is no more light falling from above to which they would have to adapt.

Perception in the dark

There is hardly any light in the deep sea, at the latest from a depth of 1000 meters it is pitch black. Fish that live in the twilight zone usually have well-trained, large eyes. It is assumed that the grenade perch can still perceive sunlight even at depths of 1000 meters. The eyes of the inhabitants of greater depths are usually smaller and receded.

A counterexample is the grenadier fish: It lives in the area of ​​the deep sea trenches (3000 to 6000 meters depth) and still has well-trained eyes, probably to be able to recognize deep-sea inhabitants who glow bioluminescent.

Some species, such as the high-gazer, have telescopic eyes that curve outwards. With them, he can see binocularly, which means that the fields of vision of both eyes overlap. This enables him to better estimate distances and recognize movements.

In many deep-sea fish, the inner ears are well developed. Some species communicate using drum sounds. Grenadier fish, for example, attract their females.

Searching for a partner in endless spaces

Finding mates is a problem that all deep-sea fish struggle with. The density of individuals is low, it is dark and many species can only move relatively slowly.

Recognizing one another using the light organs is also difficult, simply because there are so many bioluminescent animals in the deep sea. One tactic is therefore to identify potential partners using sex pheromones, i.e. special odorous substances.

Another tactic has prevailed among frogfish: the formation of dwarf males. The males have no fishing rod and are tiny. They weigh just over one percent of the weight that the corresponding female brings on the scales. On the other hand, they have relatively large eyes, well-developed olfactory organs and are good swimmers. So they are well equipped to look for a female.

If one is found, the male bites his partner. Their bloodstreams grow together so that the male is also fed by the female. The main advantage of this technique is that the energy required to find a partner only needs to be used once. In addition, the male needs little food because of his small size - a great advantage when there is a lack of food in the deep sea.

The stilt fish have developed an even more energy-saving method of reproduction. With their elongated pelvic fins, they can stand on the ocean floor. They stand against the current and wait for food that flows towards them in the form of plankton. You don't have to worry about finding a partner: stilt fish are hermaphrodites and can produce eggs and sperm at the same time, i.e. fertilize themselves.

Although there are some hermaphrodites among the deep-sea inhabitants, their sexual organs usually do not mature at the same time. This ensures that two individuals are still necessary for reproduction and that genetic material is mixed.

Apparently, however, the stilt fish live so widely that there is a risk of not finding a partner at all. The possibility of self-fertilization ensures that the survival of the species is still guaranteed. It is one of many tricks to survive in the extreme deep sea habitat.