Jellyfish at Aquarium Pula

Aurelia is a cosmopolitan genus of scyphozoan jellyfish in which about 30 species have been described so far, although it is assumed that there are more. It belongs to the so-called cryptic species — species that are almost identical morphologically and can only be distinguished by molecular analysis. The moon jellyfish has a transparent body with four characteristic circles, or gonads, in the center of its bell. The species is distributed in temperate and warm seas and tolerates a wide range of salinity. In our exhibition, you can see Aurelia coerulea, the most common jellyfish in the Adriatic Sea, as well as moon jellyfish from different parts of the world.

The Compass Jellyfish can be recognised by its yellow to brown bell, which is marked with darker "V" shaped patterns that resemble a compass, hence its name. It is a fierce predator, armed with stinging cells on its 24 tentacles. Although most of the stinging cells are concentrated on the tentacles, they are also found in smaller numbers all over the surface of the jellyfish. It is considered one of the most dangerous species in the Adriatic (second only to the sea nettle according to literature) and can cause severe burns on the skin, especially in sensitive individuals.

Compass jellyfish are most commonly found in the upper layers of the water column, rarely descending below 30 metres. 

They are protandric hermaphrodites, meaning that all individuals start as males and later change sex to become females. This sequential hermaphroditism, with staggered maturation of the sex cells, helps to avoid self-fertilisation.

It is an endemic species of the Mediterranean and easily recognisable. It has a characteristic bell shape, resembling a "fried egg", and its tentacles end in white or purple balls. The appearance of juveniles is common in July, and by the end of August, they have matured and reached a bell diameter of around 40 cm. Interestingly, it plays an important ecological role by providing shelter for the juveniles of certain fish species. Young amberjacks, horse mackerels, and bogues, together with their close relatives, often find safety from predators in the tentacles of the Mediterranean jellyfish.

Symbiotic microalgae, zooxanthellae, are found in its tissue. Through photosynthesis, they produce nutrients, so this species obtains part of its energy from sunlight and has a reduced need to capture prey. It can be assumed that for this reason it does not have many stinging cells on its body and is therefore considered harmless to humans. Upon contact with the skin, only very sensitive individuals may feel a mild stinging sensation.

The mauve stinger is recognizable by its bell-shaped umbrella, pink to purple in color, covered with wart-like protrusions. The entire surface of the bell and its long, thin tentacles are covered with powerful stinging cells, which is why it is considered one of the most dangerous jellyfish species in the Adriatic Sea. Contact with the skin can cause intense pain, redness, swelling, and even systemic reactions in sensitive individuals. The name noctiluca means “night light,” as this species can exhibit bioluminescence.

It is a strong predator that feeds on zooplankton, fish larvae, and other small marine organisms. Unlike many other jellyfish, it has no polyp stage; its entire life cycle takes place in the pelagic phase (in the open sea). It reproduces sexually, and from the fertilized egg develops a larva called a planula, which directly transforms into a young jellyfish.

It is most often found in the surface layer of the sea, but it can also occur at greater depths, depending on temperature and sea currents. It appears seasonally, often in large aggregations that are carried toward the coast by marine currents.

The barrel jellyfish is one of the larger scyphozoan jellyfish of the Adriatic Sea, with a bell diameter of about 40 cm, although it can grow to more than 100 cm. Unlike other scyphozoans such as the moon jelly (Aurelia sp.) or the sea nettle (Chrysaora sp.), the barrel jellyfish has a very unusual feeding mechanism. Instead of a central mouth, it takes in food using eight specialized oral arms covered with millimetre-sized “mouths.”

In recent years, the barrel jellyfish has become one of the most common coastal scyphozoan species. Global warming, human impact, overfishing, and the acidification of seas and oceans are among the causes of its increasingly frequent occurrence. It is not completely harmless to humans, because upon contact with the skin, especially under stress, it releases mucus containing a large number of nematocysts (stinging cells), which may cause a mild rash or skin irritation. Interestingly, people also use this jellyfish as food due to its high collagen and protein content.

Cassiopea sp., known as the upside-down jellyfish, is recognizable because it lives attached to the seafloor with its bell facing downward and its tentacles pointing upward. Its body is flattened, and the oral tentacles are branched and contain numerous stinging cells. Its tissue hosts symbiotic algae (zooxanthellae) that produce nutrients through photosynthesis, so the jellyfish obtains part of its energy from sunlight. In addition to photosynthesis, it feeds on small plankton and organic particles from the water. The stinging cells are distributed along the tentacles and can irritate upon contact with skin.

It most commonly inhabits shallow, warm, and calm coastal waters, lagoons, and seagrass areas. It reproduces both sexually and asexually, alternating between polyp and medusa stages. Due to its lifestyle and association with symbiotic algae, it is important for studying symbiosis and ecological relationships in tropical marine ecosystems.

It naturally inhabits the waters around Australia and is one of the most common jellyfish along its eastern coast. The bell grows up to a maximum of 35 cm, and its color can range from white and blue to purple or brown. It has eight oral arms, which often host symbiotic copepods. A single jellyfish can support up to 600 copepods. Its tissue contains symbiotic microalgae, zooxanthellae, which produce nutrients through photosynthesis, so this species obtains part of its energy from sunlight and has a reduced need to capture prey.

Its stinging cells can cause mild skin irritation but it is not considered dangerous to humans.

Chrysaora colorata is a species of jellyfish that inhabits the eastern Pacific Ocean, especially along the coasts of California and Mexico, where it most commonly appears during the warmer months. It is recognizable by its transparent, bell-shaped umbrella, which can reach up to 70 centimeters in diameter, with dark purple or brownish stripes extending across it. Beneath the bell are eight oral arms and numerous long, thin tentacles, which can extend for several meters. The tentacles are covered with stinging cells (nematocysts) used for capturing prey and defense. It feeds on plankton, small fish, fish larvae, and other tiny marine organisms. Although its sting can cause pain, stinging, and redness on human skin, it is generally not dangerous, except for sensitive individuals.

Chrysaora colorata is a species of jellyfish that inhabits the eastern Pacific Ocean, especially along the coasts of California and Mexico, where it most commonly appears during the warmer months. It is recognizable by its transparent, bell-shaped umbrella, which can reach up to 70 centimeters in diameter, with dark purple or brownish stripes extending across it. Beneath the bell are eight oral arms and numerous long, thin tentacles, which can extend for several meters. The tentacles are covered with stinging cells (nematocysts) used for capturing prey and defense. It feeds on plankton, small fish, fish larvae, and other tiny marine organisms. Although its sting can cause pain, stinging, and redness on human skin, it is generally not dangerous, except for sensitive individuals.

It inhabits the northeastern Pacific Ocean, from the Gulf of Alaska to Baja California. During autumn and winter, it lives in shallow bays and inlets, while in spring and summer it migrates to deeper ocean waters. Its bell is golden-brown in color and can grow up to 30 cm in diameter, while its tentacles can reach lengths of up to 4.5 meters. It feeds on zooplankton, including other jellyfish species, and its natural predators include sea turtles, the ocean sunfish (Mola mola), and the northern fulmar (Fulmarus glacialis). Interestingly, the sting of this species is similar in intensity to a bee sting.

The South American sea nettle inhabits the southeastern Pacific Ocean and the southwestern Atlantic Ocean, particularly along the coasts of Chile and Argentina, where it is common in plankton-rich areas. It is recognizable by its large, bell-shaped umbrella, which can reach up to about 1 meter in diameter. Its color varies from white and yellowish to orange and brown, often with prominent radial markings. Beneath the bell are eight oral arms and numerous tentacles with stinging cells, which can be several meters long.

Its sting can be painful and cause redness, stinging, and skin irritation in humans, but it is generally not life-threatening. In the marine ecosystem, it plays an important role as a predator of planktonic organisms, and it also serves as prey for larger marine animals, contributing to the balance of coastal and open-water communities in South America.

The marbled jellyfish can grow to 30cm in length and is typically transparent or pale white to ochre in colour. It is widespread along the east coast of South America and is one of the most common scyphozoans in the region. Its seasonal mass outbreaks are a major problem for fishermen as they not only reduce fish stocks but can also damage fishing equipment. Interestingly, the Lucerne jellyfish provides protection to pelagic crustaceans during their most vulnerable life stages. However, more research is needed to better understand these relationships within marine ecosystems.

Mastigias papua is an interesting tropical jellyfish recognizable by its spotted, golden-brown bell. It inhabits the warm waters of the Indo-Pacific, especially around Palau, Indonesia, and the Philippines, where it is often found in shallow, sunlit lagoons. Its bell can grow to 20–30 cm in diameter and is covered with small white or yellowish spots.

Unlike many other jellyfish, it does not have long, thin tentacles but shorter, branched oral arms that help it collect food. This species is characterized by a symbiosis with microscopic algae (zooxanthellae) living in its tissues. The algae produce nutrients through photosynthesis, which the jellyfish uses for energy, while in return they gain protection and access to sunlight. Because of this connection to sunlight, Mastigias papua often moves toward the surface during the day.

Its sting is very mild and generally harmless to humans. In some isolated marine lakes, such as the famous Jellyfish Lake in Palau, these jellyfish have adapted to life without natural predators and migrate daily following the movement of the sun.

It is naturally distributed in the waters around Australia and the Philippines. Its bell is brown with white spots and can reach up to 50 cm in diameter. It is an invasive species; by the mid-20th century, it had been recorded near Hawaii, in parts of the Atlantic, and in the Mediterranean. It is believed that its spread occurred via ballast water or polyps that attached to ships. Observations show that in areas where it is established, it can threaten shrimp populations.

Its tissues contain symbiotic microscopic algae (zooxanthellae) that produce nutrients through photosynthesis. In return, the jellyfish provides them with protection and access to sunlight, and part of its energy is obtained from this symbiosis.

Due to its high abundance, low toxicity, fast growth, and large size (up to 50 kg), the flame jellyfish became the second jellyfish species in the world to be farmed in the 1980s in China, while in Asia it has been traditionally consumed for over a thousand years. Jellyfish from the order Rhizostomeae are rich in collagen proteins and minerals, with a low fat and calorie content. Although more than 95% of their total mass is water, when dried, over 40% of the remaining dry matter is collagen. Studies have shown that collagen obtained from the flame jellyfish plays a very important role in medicine, as it accelerates wound healing.

Amakusa jellyfish is naturally distributed in the Indo-Pacific. Its stinging cells are very strong and can kill a fish in just a few seconds. However, jellyfish raised in captivity have weaker and less painful stings. Like other jellyfish, it reproduces both sexually and asexually, and interestingly, Amakusa reproduces asexually in several ways, including budding, cyst formation, fission, and strobilation.

Strobilation is the only form in which free-swimming larvae, called ephyrae, are produced; these ephyrae mature sexually after a few months. Unlike other jellyfish in the family Pelagiidae, Amakusa exhibits monodisk strobilation, meaning it produces only one ephyra during strobilation. However, it invests more energy in development, so this single ephyra is significantly larger and consequently has a higher chance of survival compared to ephyrae produced by polydisk scyphopolyps.

The American “cannonball” jellyfish is distributed in the western Atlantic and eastern Pacific Oceans. Its bell resembles a cannonball, which is how it got its name. So far, two species of the genus Stomolophus have been described and accepted, but several undescribed species are known that differ in pigmentation.

Some fish species and juvenile crabs form symbiotic relationships with this jellyfish, using it for protection and feeding on the zooplankton it captures. The exact nature of the relationship between the jellyfish and its symbionts is not yet fully understood, but it is believed that, depending on the species, it may involve commensalism (one party benefits without harming the other) or parasitism.

Hydromedusae (Hydrozoa) are not classified as scyphozoans (Scyphozoa) but belong to a separate class within the phylum Cnidaria. Depending on the species, their bell diameter ranges from 0.5 to 6 cm. Interestingly, unlike scyphozoans, hydromedusae do not have an ephyra stage; instead, they develop directly from the polyp into small medusae resembling adults.