North Pacific Giant Octopus (Enteroctopus dofleini)
pol: Ośmiornica olbrzymia, niem: Pazifischer Riesenkrake

This is the largest species of octopus in the world. The reach of its eight arms is as much as 10 meters, and it can weigh up to 90 kg. This predatory cephalopod, which means literally 'head-footed', inhabits the waters of the North Pacific, where it can be found from the shallows to depths of 750 meters along the continental shelves and slopes of the Pacific Rim from Southern California to Japan. The body of the octopus comprises the head and eight arms. It has an internal or highly reduced calcareous skeleton, a digestive gland, a mantle with a cavity and gills for respiration. This unique predator not only thrills with its bizarre looks, it amazes with its supreme intelligence! The key to this is its brain. The cephalopods have the most highly developed brain of all the invertebrates. The cephalopod brain, which is located in a cartilagenous skull, is similar to that of humans in that it comprises several highly specialized lobes that are responsible for various activities. Octopuses are also quick learners, as studies at other oceanographic institutions indicate. For example, within a week they learn to differentiate shapes and colors that they could never have encountered in the depths of the oceans, and they are able to remember them until the ends of their lives. The octapus's favorite meal is crab, but at the Gdynia Aquarium its main dish is frozen shrimp. Octopuses are very clever predators, which is why their aquariums have to be secure; their body structure allows them to squeeze through the smallest of spaces. They prefer water temperatures of 6° to 7°C. This cold water animal is quite sensitive, which is why the Gdynia Aquarium will spare no effort to ensure that Karolina has every possible comfort. This means that the 'no flash photography' rule is enforced strictly. Octopuses are also able change color in the blink of an eye, and in the wild this gives them a lot of room to maneuver, for example, when hunting. These animals also hunt their favorite food, crabs, using bait. Since crabs are scavengers, octopuses will catch a fish, place it in front of the crab's hiding place, and then catch the crab as it emerges to take the bait. If an octopus is being chased, it will release a dark secretion known as ink from its ink sac as a defense mechanism. They can also confuse enemies effectively by changing color rapidly. Octopus reproduction is also interesting. The male puts on a show of colors for his intended female. Then he uses a specialized arm called a hectocotylus to insert spermatophores (a packet of sperm) into the female's mantle cavity. After the eggs are fertilized, the female deposits several thousands of them, and then takes very good care of them until they hatch. Then she dies. Thus, the octopus, just like all cephalopods, including the cuttlefish and squids, reproduce only once in their lifetimes.

Zebra shark (Stegostoma fasciatum)
pol: rekin brodaty, niem: der Zebrahai

The zebra shark can grow to a maximum length of 3.6 meters, but its average size is 2.3 meters. Although this shark inhabits waters at depths ranging from 0 to 60 meters, it is most often encountered at depths of 5-30 meters near coastal coral reefs. Sometimes they enter the mouths of coastal rivers. These sharks are not very active during the day, but they come alive at night. They live a mostly solitary life, except seasonally when they gather in large groups of as many as thirty or forty. Their main prey includes molluscs and sometimes they will eat crustaceans, small fish, and aquatic snakes. If the zebra shark is provoked, it will bite, but this happens only rarely, and in popular tourist areas dives are organized to feed these sharks. This shark is oviparous, which means that the females release thirty or more encapsulated fertilized eggs that attach to the bottom of the ocean with sticky strands. Young zebra sharks hatch from these at a length of between 20 to 30 cm. Until young sharks reach a length of 50 to 90 cm in length, there are a different color from adults. Their bodies are ringed with yellowish stripes which is probably why they are know as zebra sharks. These markings are designed to make it difficult for predators to identify single youngsters in groups. Adult zebra shark markings are dark spots against a gray background. In the wild, zebra sharks live for 25 to 30 years.

Blacktip reef shark (Carcharhinus melanopterus)
pol: Żarłacz rafowy czarnopłetwy, niem: der Schwarzspitzenriffhai

The blacktip reef shark grows to a length of 2 meters and lives for about 25 years. The black markings that ring this shark's first dorsal fin and its caudal, or tail, fins make it instantly recognizable. It has relatively large eyes and a short, rounded snout. These sharks must move constantly to breathe and keep afloat! They feed on fish, crustaceans, and cephalopods. They inhabit mainly the tropical waters of the central Pacific Ocean and those off the coast of East Africa at depths of 20 to 75 meters. They are also seen in estuaries and even in the Mediterranean Sea, which they reach through the Suez Canal. While the blacktip reef shark does not have the world's best reputation, it is not as aggressive as the famous man-eating film star, the great white shark. This shark has earned its bad repute mainly from biting people wading near reefs, and by sometimes taking bites from surfers riding the waves of shallow coastal waters.

Giant grouper (Epinephelus lanceolatus)
pol: Strzępiel goliat, itajara goliat, Brindle bass, niem: der Riesenzackenbarsch

This species is from the family Serranidae, and is very similar to the granik groupers, as which it was once classified. It can grow to a length of 2.7 meters and while the maximum published weight is 400 kg, there are reports that some specimens weigh as much as 600 kg. This is one of the largest Osteichthyes, or bony fish. The sunfish (Mola mola), which looks like a giant fish head, is of a similar length (3.3 meters), but it is much heavier at 2.3 tons. The giant grouper, or brindle bass, can be found in and around the coral reefs in the coastal waters of the Indo-Pacific, except in the Persian Gulf, a fact that remains unexplained. They can eat a variety of food including crabs and spiny lobsters or even young sharks and sea turtles. These fish have powerful jaws armed with large fangs, and they have sharp spines on their gill rakers. Adult specimens can pose a risk to divers or the shipwrecked. The giant grouper usually hides in underwater caves or among shipwrecks. Despite its size, the giant grouper is not an important commercial fish because of its low abundance; it is, however, a prized trophy fish among sport fishers.

Australian marbled catshark (Atelomycterus macleayi)
pol: Rekin marmurkowy, niem: Weißspitzen-Riffhai

This small shark grows to a length of 60 cm. It inhabits the shallow coastal waters of Australia to depths of 4 meters, where it feeds on molluscs, crustaceans, and small fish. This shark is a benthic creature, which means that you can find it lying on the sand or among rocks. The Australian marbled catshark is one of the world's smallest shark species, and it is endemic to Australian waters.

Dwarf crocodile (Ostaeolaemus tetraspis)
pol: Krokodyl krótkopyski, niem: das Stumpfkrokodil

The dwarf crocodile males reach a maximum length of 180 cm, while the females are smaller. The nostrils of this crocodile have an inner bony septum that divides each into two parts. There are ossifications, or bony growths, on the upper eyelids. The iris of this crocodile is brown, in contrast to the yellow-green irises of other species. Its back is dark brown with round, black spots. Young dwarf crocodiles are light brown in color with broad, black transverse rings. The color of the belly is yellowish with many black spots, and the head is adorned with yellow patterns. The length of this crocodile's stunted snout is the same as its width at the base. The flat back teeth are larger than the others, and they are surrounded by thickened rings that are an adaptation for crushing through crab and mollusc shells. The dwarf crocodile lives in streams, small rivers, and ponds in dense rainforests, and it is only rarely sighted in large water basins that are free of heavy vegetation. Its preferred habitats are shady, swamp forests or small, muddy forest ponds that are often far away from big rivers or large, bodies of standing water. This crocodile is known to inhabit brackish waters in mangrove forests, water-filled holes left by uprooted trees, and even large puddles. This animal is also known to venture into areas away from water. In the wild, the dwarf crocodile diet includes various species of small animals especially earth worms, crustaceans, fish, and reptiles. It is also the only crocodile species known to eat fruit, which is strange since biochemical analysis indicates that this species cannot digest vegetable protein. During the rainy season its diet is mostly fish, but in the other seasons it prefers crustaceans. Since this animal is nocturnal, it spends most of its days in burrows that it digs itself. Sometimes the entrance to their burrows are under the water surface. The crocodile has also been observed to spend its days hidden in small basins under tree roots. The mating period begins with the rainy season in late May and early June. The female builds her mound-shaped nest with bits of vegetation, and then she lays about ten eggs in it. Incubation lasts from 85 to 105 days. After hatching, the young crocodiles measure about 28 cm in length. The female tends her nest and then hatchlings for a period of time. The dwarf crocodile range of occurrence is in West and Central Africa in the countries of Angola, Benin, Burkina Faso, Cameroon, the Central African Republic, Congo, Côte d'Ivoire , the Democratic Republic of Congo, Equatorial Guinea, Gabon, The Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Mali, Nigeria, Senegal, Sierre Leone, and Togo. It is estimated that the wild population numbers from 25,000 to 100,000 individuals.

Giant arapaima(Arapaima gigas)
pol: Arapaima, niem: der Arapaima, Paiche

This fish is also known as pirarucu or paiche. It belongs to the family Osteoglossidae, which means "bony tongue". It occurs in the Amazon and Orinoco. It is one of the largest freshwater fish in the world. Although individuals as large as 4.5 meters long weighing 275 kg were once common, today fish longer than 2 meters and weighing more than 100 kg are rare because of overfishing. This is a protected species, and it is listed in Appendix II of the CITES Convention. This fish can also breathe atmospheric air thanks to its richly vascular swim bladder that is connected to its esophagus. This allows the arapaima to inhabit waters that have a very low oxygen concentration, and it can burrow into the sand periodically. This fish emerges from the water every 10 to 15 minutes, which makes it easy to catch. It hunts its prey by jumping out from hiding places, and its diet commonly comprises fish, crustaceans, snails, and earthworms. Less common dietary items include birds, amphibians, reptiles, and small mammals. The females become sexually mature at the age of five years at a length of about 160 cm. Between February and April, when water levels are low, the arapaima builds nests that are usually 50 cm wide and 15 cm deep. The female deposits some 40,000 to 50,000 eggs, and the young fish hatch from May to August when the water level is high. The males can protect the young by holding them in their mouths. The females protect the males and the young by swimming around them and chasing off potential predators. The tongue of the arapaima is used in South America as a remedy for intestinal parasites. The tongue is dried and mixed with ground guarana bark and then drunk in a liquid suspension.

Silver arowana (Osteoglossum bicirrhosum)
pol: Arowana srebrna, niem: der Arowana

Like the arapaima, the arowana belongs to the family Osteoglossidae. It occurs in the Amazon and its basin. This fish grows to 150 cm in length and can live for 20 years. It hunts at the water surface (note the placement of its snout). An aggressive fish, it eats fish, large insects, bats, and even snakes. When in search of food, it is able to leap out of the water and grab prey such as birds. It can inhabit waters with lowered oxygen levels. Its reproduction is interesting: the male carries the fertilized eggs in its mouth for about 60 days. The eggs are about 16 mm in diameter, and the little fish that hatch after incubation are about 5 to 8 cm long.

Australian spotted jellyfish(Phyllorhiza punctata)
pol: Meduza , niem: die Gepunktete Wurzelmundqualle

This jellyfish grows to a diameter of 50 cm, and it lives in waters to depths of 3 meters. It inhabits mainly the tropical and sub-tropical waters of the Indo-Pacific, but it also occurs near the Hawaiian Islands, and the tropical Atlantic waters of the Gulf of Mexico and the Caribbean Sea. This jellyfish is easy to identify by the white spots on its body. They usually live in large groups, and they feed on the zooplankton that they filter from the water. A single jellyfish is capable of filtering up to 50,000 liters of water a day. In areas where commercial fisheries are important, this organism is considered to be a pest since one of the main components of the plankton it consumes is the eggs of exploited fish species.

Lookdown, Silver Moonfish (Selene vomer)
pol: Selena, niem: der Bodengucker

The lookdown, or silver moonfish, is a member of the Carangidae family. The family name in Polish is ostrobokowatych, which refers to the rows of sharp scales that occur along the lateral line of the body. This fish is silver in color with a streamlined, laterally-compressed body, that can arch dramatically. It has two spikes in front of its anal fin. One of the peculiarities of this species is that adults have two dorsal fins. The lookdown can reach sizes of up to 50 cm in length. Its range of distribution includes the coastal waters of the warm Atlantic. It is often sighted near bridges, dock piers, or beaches. It feeds on small fish and invertebrates. The lookdown often forms shoals. Seleny często tworzą ławice.

Horseshoe crab (Limulus polyphemus)
pol: Skrzypłocz, niem: die Pfeilschwanzkrebse

The horseshoe crab is known as a 'living fossil' because very similar forms are found in sediments that are 350 million years old. An interesting fact about this creature is that its blood is blue. It is also sterile and has antibiotic properties, which is why it is used in the production of bacteria detection tests. These tests are used in many fields, and one application is in the aerospace industry where they are used for on-the-spot sterility checks of components that are taken aboard space shuttle missions.

The horseshoe crab, which reaches sizes of 60 cm, are carnivorous and prefer inhabiting coastal waters. They are found in Atlantic waters along the coasts of North and Central America and the southeast coasts of the Pacific.

Northern pike (Esox lucius)
pol: Szczupak pospolity, niem: Europäischer Hecht

In a change of exhibits, on September 1 the northern pike moved in with the European eel. This freshwater predator inhabits both running and standing waters and the brackish waters of the Baltic Sea. It can reach a length of 1.5 meters and a weight of about 10 kg. Individuals weighing more than 15 to 20 kg are rare. The pike has a laterally compressed, streamlined body and an elongated head with narrowing jaws and a broad, flattened snout. The very sharp teeth on the lower jaw are directed inwards, while the upper jaw is toothless. Northern pike coloring depends on the environmental condition in which they live. Young fish are usually light green, brown, or silver with dark patches on their sides that sometimes merge into lateral streaks. Older fish have dark green backs with lighter colored sides speckled with yellow spots and white bellies. The paired fins are usually a light red or buff color, while the unpaired fins have dark lateral stripes. Although pike is among the most predatory of fish, it is not normally an aggressive hunter. It prefers lurking among submerged vegetation in wait of prey that swims past. Larger pike will actively chase shoals of fish, such as herring, in the pelagic zones of large lakes or the Baltic Sea. Pike will often choose their prey from among fish that are abundant in its ecosystem and that are big enough 'to have been worth the effort'. Cannibalism is noted with frequency among pike. Spawning takes place in early spring in March and April in overgrown shallows, old river beds, and flooded meadows. The minute eggs attach to underwater vegetation, and then hatch after incubating for 12 to 15 days. The numbers of pike in Polish waters has decreased notably in recent years mainly because the management strategies for this fishery are unsustainable (catches are not restricted by either minimum sizes or closed seasons), poaching is a problem, and angling catches are very high.