Akwarium Gdyńskie

Order: Perciformes

Family: Carangidae

Selene vomer

This fish inhabits the marine and brackish waters along the west Atlantic coast from Maine to Uruguay, including the Gulf of Mexico and Bermuda. It occurs in coastal waters with a hard or sandy bottom, at a depth of 1 to 53 meters. Juveniles prefer estuary areas or swim freely along sandy beaches. Selene is a pelagic fish, it actively swims in the depths of the water, but it can also be associated with the bottom. It can live in large shoals, in smaller groups or in pairs. It grows up to 50 cm in length, although the average length is 35 cm and weigh is over 2 kg. Due to its body shape and color, it is called a moon fish. Its body is tall and short, strongly flattened to the sides. The head profile is very steep and straight, angular at the top. The mouth is set low with the lower jaw sticking out and the eyes are high. Due to the location of its eyes, it keeps looking down when it is swimming in search of food. The body appears to be devoid of scales as they are very small, thin, transparent and embedded in the skin of the fish. The dorsal and anal fins have a characteristic appearance, in which the second ray is much longer than the others and gives the fin the shape of a scythe. Coloration is fairly uniform, usually silvery with a bluish tinge on the back. Sometimes it can also be aquamarine, turquoise or orange, and the edges are pale green-yellow. The juvenile has a few weak vertical stripes that disappear as it grows, and its first two fin rays are elongated and look like threads. Selene is a migratory predator that feeds on small benthic invertebrates, shrimps, small crabs and fish. It is a dioecious animal that is relatively poorly studied and little is known about its early developmental stages. The female lays eggs which are then fertilized by the male. After external fertilization, pelagic larvae hatch from the eggs. Selene does not look after offspring. It is not widely caught, but its meat tastes great. It is much more often wanted as an aquarium fish due to its interesting shape and attractive appearance.

How to effectively camouflage yourself in water?

Avoiding predators in the open ocean is difficult as there are no objects to hide behind. Then the silver surface of the fish plays an important role in the camouflage. The stealth system bends polarized sunlight on the animal. There are guanine crystals in the skin of selene, which have a high refractive index and reflect them by adjusting the light scattering to the angle of its incidence. Guanine crystals are arranged in the skin so as to coincide with the main axes of the fish, then regardless of the position of the animal in the water column, they will reflect the sun’s rays. This will make the fish invisible to predators. The skin of the fish is also silver for the human eye, but when we look at it under a microscope at higher magnification, it turns out to be a multicolored mosaic of yellow, orange, green and bluish-violet colors.

Order: Beryciformes

Family: Holocentridae

Myripristis murdjan

This species occurs in the tropical Indo-Pacific waters from the Red Sea and East Africa through Indonesia and Papua New Guinea to Samoa, north to the Marshall Islands, Taiwan and Ryukyu Islands and south to the east and west coasts of Australia. It inhabits coral reefs, both seas and lagoons. It is a fish associated with the bottom living near caves or rock crevices, where it can hide during the day. Most often it occurs to a depth of 10 meters, but it can also be found at a depth of 50 meters.

The Pinecone soldierfish can grow up to a maximum length of 60 cm, but the vast majority have a length of 18 to 25 cm. Its hallmark is red body color, large scales and large bulging eyes. The color may vary slightly between individuals from red to dark red with a dark band running from the top of the gill cover to the base of the pectoral fin, and the edges of the scales are reddish brown. The body is covered with chenoidal (comb-like) scales similar to a rectangle with comb-shaped teeth on the back edge. The scales have a rough surface, and there are also sharp protrusions on the body of the fish. Above the pupil is a large black spot, while below a smaller spot, which looks like a black vertical band passing through the center of the eye. The first pectoral fin on the spines and is usually folded along the body of the fish. It has a red color, like the other fins, but others have additional white leading rays. The second dorsal fin is soft and in shape and location corresponds to the anal fin.

The Pinecone soldierfish is active at night, then it feeds mainly on plankton, such as crab larvae. There are no obvious differences between males and females. It is oviparous, fertilization is external, and eggs and planktonic larvae. Juveniles reach sexual maturity after reaching an appropriate body length, which is 11 cm for females and 7 cm for males.

Due to the poor quality of the meat, they are not caught for consumption, but their appearance makes them interesting enough to be caught as aquarium fish.

Order: Characiformes

Family: Characidae

Astyanax mexicanus

It is a species of fish with an unclear taxonomic system. In literature on fishkeeping it can go to under different names. These discrepancies arose because the same fish was described as a separate species or subspecies. It was treated as a subspecies of A. fasciatus, but is now a separate species. It inhabits inland waters of Mexico and Guatemala and two US states – Texas and New Mexico. Its range in the southern US is expanding due to its use as bait. Prefers rocky and sandy bottoms of streams and small and large rivers.  The form inhabiting underwater caves is found Only in Mexico.

A characteristic feature of the form living in darkness is the lack of eyes and loss of body pigmentation. Lack of sense of sight is compensated by sharp smell and hearing. The body length of an adult fish does not exceed 12 cm, on average it grows up to 7.5 cm.

Commonly, the fish has a gray body, but the cave form has a pink and white body. Its natural diet consists of crustaceans, insects and ringworms, while the cave form feeds on invertebrates, which it finds with a strongly sharpened sense of smell. The Blind cave tetra uses a sensitive lateral line to detect small changes in water pressure around it, enabling it to navigate and get food. When the animal is placed in a new environment containing objects that it does not recognize, it begins to swim at a higher speed. In this way, it increases the lateral line stimulation enabling it to obtain information about the environment faster.

It is known that this fish creates a detailed spatial map of its surroundings, which it remembers.

It is a dioecious animal, very fertile. The female can lay up to 1000 eggs. Dimorphism is evident in the appearance of the anal fin. In the male it has a slightly curved edge, while in the female it is straight. In caves, the fry develop eyes in the early stages of life, but they disappear at a later stage of development. Individuals with eyes can interbreed with those without eyes giving fertile offspring. This makes this species an excellent model organism for studying convergent and parallel evolution, regressive evolution in cave animals, and the genetic basis of regressive traits. The standard variety living near the surface is not popular among aquarists, unlike the cave form. Initially, all aquarium specimens had been obtained from the natural environment, but this fish breeds perfectly in captivity, which is why its main exporter is now Asia. Interestingly, these fish begin to develop a silvery gloss on their sides, which may be the beginning of artificial adaptation to breeding outside the underground environment.

A new diabetes medicine?

Due to the natural environmental conditions of this fish, it had to develop a number of metabolic adaptations to survive. In caves, it is difficult to have constant access to food, which is why it is important to accumulate reserves in the form of large amounts of fat and resistance to hunger. This fish shows symptoms typical of diabetes and non-alcoholic fatty liver disease, but there are no obvious health problems. The discovery of the genetic basis for adaptation to an extreme environment may lead to the development of new treatments for diabetes and other diseases. Scientists are focusing on changes in fish metabolism that allow them to go through very long periods without food. Large “supplies” of food occur most likely once a year during the flooding of caves. The Blind cave tetra feed then intensively and increase body fat, afterwards it uses stored fat as the next meal. To be able to increase fatness, allowing them to survive during a period of lack of food, they become insulin resistant. Biologists have identified a genetic mutation that is responsible for this resistance. Nevertheless, it will take a long time before the discovery of scientists will bring solutions for patients.

Order: Tetraodontiformes

Family: Tetraodontidae

Arothron meleagris

This species is widespread in tropical Indo-Pacific regions from East Africa east to Mexico and Ecuador, north to Ryukyu Islands and Ogasawara Islands south to Lord Howe and Easter Islands, as well as Christmas Island. It is most often found around oceanic islands. It is closely related to coral reefs, where it occurs in large numbers. It is less common on the rocky reefs of the Eastern Pacific. Occurs at depths from 1 to 73 m, however, it can usually be found at a depth of 3 to 24 m.

A mature individual can reach a maximum length of 50 cm and prefers a lonely lifestyle. This species has a strong, direct impact on coral communities. Due to its nutritional predisposition, it may limit the distribution and number of predatory corals. It feeds mainly on coral tops and to a lesser extent algae, bryozoans, foraminifers, tunicates, sponges, molluscs and detritus (dead organic particles). The massive four teeth resembling a beak support feeding.

Like other pufferfishes, its body is oval, almost round in cross-section, and the scales are transformed into small spikes resembling thick sandpaper. This species shows variable color, occurring in black and yellow forms. The dark variety is speckled with white spots, while the yellow variety has an almost uniform yellow coloration with individual black spots scattered throughout the body. It is a gonochoric, oviparous species.

This fish has developed a successful strategy of defense against predators. First of all, in case of danger, it fills its body with water, increasing its volume considerably, additionally tiny spikes covering its body become visible. Increasing body size is supposed to discourage the predator from attacking. Moreover, it produces neurotoxins, tetrodotoxin and saxitoxin. The black species variety is more toxic, with the highest concentration of toxins found in the liver, while the yellow variety is less toxic and accumulates toxins in the mucosa. The degree of toxicity also depends on the geographical area and season. Due to its toxicity, it is not caught for consumption, but it is used as a decorative aquarium fish. It can get high prices of up to USD 200 per individual, while individuals from the yellow variety are more valuable and can cost up to USD 500.

How do pufferfish swim?

Due to the shape of its body, this fish cannot move at high speed, also the movement of the tail is hindered. When moving at a low speed, the fish changes its angle with its head higher by about 10 ° and moves the dorsal, anal and pectoral fins. The caudal fin does not make any movements that affect the animal’s speed, it only acts as a rudder. Body shape is similar to shape at rest.When the animal begins to move faster, the anterior abdominal part flattens, the mouth is wide enough to see the teeth and the caudal fin is activated.