Aquarium, I+R+D

Jellyfish: feeding, reproduction, breeding, and care

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By Milthon Lujan

Marine jellyfish. Photo by Florian Olivo on Unsplash
Marine jellyfish. Photo by Florian Olivo on Unsplash

Jellyfish are fascinating marine creatures that have existed for millions of years. With their gelatinous bodies and floating tentacles, they are often considered one of the most beautiful creatures in the ocean and have sparked interest in their breeding in aquariums. However, they can also be quite dangerous, as some jellyfish have stings that can cause intense pain and even death.

In recent decades, jellyfish blooms have become common, and due to climate change, it is anticipated that these events will increase. These aquatic organisms are infamous for their negative effects on human economic activities such as beach tourism, fishing, aquaculture, and power plants (Purcell et al., 2007). This has led scientists to develop computer applications to identify jellyfish species.

Despite the bad reputation of jellyfish, various studies have shown their potential as food, animal feed ingredients, the use of toxins in medicine, or the use of collagen for wound healing.

There are around 50 species of jellyfish that are edible. In China, jellyfish have been commercially exploited as a significant food source for over a thousand years (Hsieh et al., 2001), and currently, their consumption is being promoted in European countries such as Italy (Bleve et al., 2019).

This scenario has encouraged the increase in captures of natural jellyfish populations; however, experimentation with jellyfish aquaculture for food and ornamental purposes is also underway. In this article, we will learn everything about jellyfish, from their biology to their behavior and their impact on the environment.

Characteristics of Jellyfish

Common names: jellyfish, sea jellies, jellies
Scientific name: Cnidaria
Subphylum: Medusozoa
Classes: Cubozoa, Hydrozoa, Scyphozoa, Staurozoa

What are jellyfish like?

Jellyfish have a gelatinous body composed of 95% water. They are radial animals, meaning they have a symmetrical body with equal parts extending from a central axis. The body is bell-shaped, hanging a tubular manubrium with the mouth at the lower end, sometimes extended by long tentacles loaded with stinging cells called cnidocytes, used to inject venom into prey.

The size of sea jellies ranges from one millimeter in height and diameter of the bell to about 2 meters in height and diameter of the bell.

The smallest jellyfish belong to the genera Staurocladia and Eleutheria, with bell discs ranging from 0.5 mm to a few millimeters in diameter. On the other hand, the lion’s mane jellyfish, Cyanea capillata, has been cited as the largest jellyfish in the world, and possibly the longest animal, with tentacles that can extend up to 36.5 meters long.

The largest jellyfish in the world "Lion's Mane" Cyanea capillata. Soure: W. Carter
The largest jellyfish in the world “Lion’s Mane” Cyanea capillata. Soure: W. Carter

On the other hand, the giant jellyfish Nomura’s jellyfish (Nemopilema nomurai), found in the waters of Japan, Korea, and China, is another contender for the “largest jellyfish” in terms of diameter and weight. This sea jellies can reach a bell diameter of 2 meters and weigh around 200 kg.

Do jellyfish have brains?

Jellyfish do not have brains as we know them. However, they have radially distributed nervous systems that adapt to their unique body plan (Katsuki and Greenspan, 2013), consisting of approximately 10,000 neurons (Dajose, 2022), allowing them to respond to environmental stimuli.

Albert (2011) reports that the central nervous system of the moon jellyfish Aurelia sp. coordinates motor responses, such as sensory feedback, creating numerous adaptive behaviors critical for the survival of these species.

Where do jellyfish live?

Jellyfish are found in all the world’s oceans. They are most commonly found in warm waters but can also inhabit colder waters. Some sea jellies, like the moon jellyfish, live in surface waters, while others, like the barrel jellyfish, live in deep waters.

According to researchers, true jellyfish species of the class Scyphozoa are exclusively marine, but some hydrozoans with a similar appearance live in freshwater.

Jellyfish move through the water by pulsing rhythmic contractions of their entire body; they take in water, which enters their gastrovascular cavity, and expel it, using it as a “propeller.”

Some jellyfish species live near the coast, but the most abundant ones are concentrated in open sea, about 10 or 20 miles offshore.

In their natural habitat, sea jellies are frequently consumed by a wide range of marine predators, including fish, birds, turtles, and various invertebrates, including octopuses, sea cucumbers, crabs, and amphipods (Hays et al., 2018).

What do jellyfish eat?

Sea jellies are carnivorous and feed on small fish, plankton, and other marine animals. They use their tentacles to catch their prey and then inject them with venom to kill them. Jellyfish then digest their prey in their gastrovascular cavity, which is a stomach bag located in the center of their body.

Brodeur et al. (2002) report that the jellyfish Chrysaora melanaster in the Bering Sea mainly feeds on pelagic crustaceans: euphausiids, copepods, and amphipods, although they also consumed other jellyfish and juvenile fish. On the other hand, Nagata and Morandini (2018) worked with the South American sea jellies Lychnorhiza lucerna, determining that calanoid copepods were the most abundant prey, followed by cyclopoids and poecilostomatoids and veligers of bivalves.

In terms of feeding rate, Nagata and Morandini (2018) estimated that between 110 to 102,871 copepods were ingested daily by jellyfish (5-30 cm in diameter), indicating that L. lucerna has one of the highest feeding rates among Scyphomedusae.

Currently, you can find a significant offer of specialized food for feeding your jellyfish in stores or online; the choice of one type of food over another depends on the species, size, and particular needs of each species.

How do jellyfish reproduce?

Most jellyfish species have separate sexes.

During sexual reproduction, males release sperm into the water. These swim through the water until they penetrate the gastric cavity (stomach) of females and then reach the female gonads where the eggs are.

In other jellyfish species, both sexes release sperm and eggs into the water where they meet and fertilize.

From the egg, a larva called a pelagic planula is released. When it finds a suitable substrate, it attaches and transforms into an asexual polyp. The polyps produce sexually mature jellyfish that complete the cycle.

Coastal sea jellies species have a polyp phase in their life cycle that settles on the seabed.

Jellyfish life cycle. Source: Zina Deretsky, National Science Foundation
Jellyfish life cycle. Source: Zina Deretsky, National Science Foundation

Are jellyfish immortal?

According to the American Museum of Natural History (2015), sea jellies have an extraordinary survival ability. In response to physical damage or even hunger, they transform into a polyp. In a process resembling immortality, the colony of reborn polyps eventually sprouts and releases jellyfish that are genetically identical to the injured adult.

Green et al. (2018) highlight that plastics should be adopted as substrates to be colonized by jellyfish polyps, and cooling with the addition of iodine can improve moon jellyfish reproduction protocols.

Jellyfish Consumption

Some jellyfish are edible and considered a delicacy in certain cultures. However, it is important to be cautious when eating jellyfish, as some can be poisonous.

Semi-dried jellyfish represent a multimillion-dollar fishing industry in Asia, where they are considered a delicacy, and in the U.S., the cannonball jellyfish (Stomolophus meleagris) has gained interest as food due to consumer demand from Asia (Hsieh et al., 2001). Additionally, Duarte et al. (2021) report that 35 different sea jellies species are suitable for human consumption, with most belonging to the Rhizostomae order, which is the most consumed species.

In this regard, Bleve et al. (2019) believe that Rhizostoma pulmo jellyfish, captured off the Italian coast, may represent a new food source in Europe. Torri et al. (2020) studied Italian consumers and concluded that individuals more inclined to accept jellyfish as food are young, familiar with the marine environment, with a high level of education, or students and frequent travelers.

One of the main concerns with sea jellies consumption is safety. Bonaccorsi et al. (2020) studied microbiological, chemical, physical, and allergenic risks associated with jellyfish consumption, concluding that studies report a good level of health safety for jellyfish consumption in terms of allergic and microbiological risks, and no evidence of physical risks was found.

According to Hsieh et al. (2001), traditional processing methods for sea jellies involve the use of a mixture of salt and alum to reduce water content, lower pH, and firm up the texture.

Jellyfish Fishing and Aquaculture

The main source of sea jellies is fishing; however, interest in jellyfish aquaculture is increasing for use as food, raw materials for the industry, or the trade of marine ornamentals.

Bleve et al. (2019) mention that the main producers of jellyfish for human consumption are China, Indonesia, and Malaysia, and that new species from the U.S., Mexico, Australia, and India are being exploited and used in food preparations for export to Asia.

Similarly, Brotz et al. (2017) report that jellyfish fisheries are being experimented with, with varying degrees of success, in numerous countries in North America, Central America, and South America to meet the demand in the Asian market.

On the other hand, jellyfish aquaculture began in China due to its economic importance, and biological studies have focused on Rhopilema esculentum (You et al., 2007).

Lo et al. (2008) evaluated changes in a coastal lagoon in Taiwan after the removal of oyster farming rafts to determine why jellyfish disappeared. They concluded that aquaculture rafts increase sea jellies populations through three probable mechanisms: rafts provide substrate and shade for larval settlement and polyp colony formation, and rafts restrict water exchange in the lagoon.

Duarte et al. (2021) describe the state of the art of jellyfish farming systems at different life stages.

Uses of Jellyfish

In recent years, sea jellies have become a source of some inputs for the food, nutraceutical, pharmaceutical, and other industries.

Khong et al. (2016) evaluated the nutraceutical potential of three commercial jellyfish species: Acromitus hardenbergi, Rhopilema hispidum, and Rhopilema esculentum. The researchers concluded that A. hardenbergi exhibited higher amino acid and collagen content.

Cheng et al. (2017) isolated collagen from jellyfish (Rhopilema esculentum) and assessed its hemostatic property. According to the researchers’ results, jellyfish collagen sponges exhibited higher water absorption rates than medical gauzes, and they concluded that sea jellies collagen sponge is a suitable candidate for use as a hemostatic material and for wound healing applications.

Similar results were found by Felix et al. (2019), who concluded that collagen peptides derived from Rhopilema esculentum jellyfish can accelerate the healing process and may benefit clinical dressings in the future.

On the other hand, Rastian et al. (2018) concluded that Type I collagen extracted from the sea jellies Catostylus mosaicus in the Persian Gulf is an alternative to mammalian Type I collagen for applications in biomaterials.

Nisa et al. (2021) report that jellyfish venoms have valuable pharmacological and biological aspects. Meanwhile, Duarte et al. (2021) describe that jellyfish are also being used as inputs for artificial feeds for chickens and pigs.

Jellyfish Stings – Poisonous Jellyfish

Jellyfish have tentacles formed by stinging cells, or nematocysts, which they use to capture prey and as a defense mechanism. These cells contain a capsule with a toxic (poisonous) filament.

The sting of cnidarians is a serious health problem for humans in areas where extremely venomous jellyfish are common, such as in temperate and tropical zones, particularly along various Pacific coasts (Mariottini and Pane, 2010).

Depending on the species, sea jellies stings can be painful but do not represent a medical emergency. Upon contact with prey, the filaments are ejected and inject venom. The toxicity of jellyfish stings varies by species.

Portuguese Man o’ War (Physalia physalis)

The Portuguese Man o’ War (Physalia physalis), also known as the Portuguese man-of-war, is considered the most dangerous along the coasts of Spain and Portugal.

The venom of the Portuguese Man o’ War has neurotoxic, cytotoxic, and cardiotoxic properties, which can lead to neurogenic shock in some situations. The Portuguese Man o’ War is typical in temperate waters of the Atlantic but can occasionally be observed in Mediterranean waters (Marco, 2021).

Luminous Jellyfish (Pelagia noctiluca)

Blooms of the luminous jellyfish (Pelagia noctiluca) are most frequent in the Mediterranean Sea. This jellyfish can reach a diameter of 20 cm and is characterized by a reddish-pink color.

Pelagia noctiluca is the jellyfish responsible for mortality in fish farming facilities in the Mediterranean Sea (Bosch-Belmar et al., 2017).

Fried Egg Jellyfish (Cotylorhiza tuberculata)

Cotylorhiza tuberculata, also known as the “fried egg” jellyfish due to its peculiar shape and an umbrella that can reach 40 centimeters in diameter (AQUAE Foundation).

Irukandji Jellyfish (Carukia barnesi)

The Irukandji jellyfish (Carukia barnesi) is small and highly venomous, inhabiting the waters of northern Australia; however, it has also been reported in Wales (United Kingdom).

The Irukandji measures only 1.5 cm, but its tentacles can reach over 80 cm in length. The stings of jellyfish from the Carukiidae family, including C. barnesi, Malo kingi, and M. maximus, generate a set of signs and symptoms known as the Irukandji syndrome.

The main symptoms of Irukandji syndrome include a powerful burning sensation on the skin with unbearable cramps in the arms and legs, intense pain in the back and kidney area, followed by headaches, nausea, sweating, and vomiting.

Jellyfish Breeding in Aquariums

Many public marine aquariums worldwide feature sea jellies specimens. According to Duarte et al. (2021), more than 50 jellyfish species are exhibited in many zoos and aquariums worldwide. In recent decades, there has been a growing demand for jellyfish in the marine ornamental trade due to their attractive species. The commercial potential of jellyfish is expanding.

Choosing the Right Jellyfish for Your Aquarium

Not all jellyfish are suitable as pets. Some sea jellies are too large for home aquariums, while others are too delicate and challenging to care for.

Additionally, jellyfish do not adapt well to confined spaces because they rely on currents to move from one place to another. However, if you want to keep a jellyfish as a pet, you must provide it with the necessary environmental conditions it needs to survive.

Types of jellyfish suitable as pets:

  • Moon Jellyfish: They are the most popular sea jellies for aquariums. They are relatively easy to care for and are quite tolerant of changes in water quality.
  • Barril Jellyfish: Small and peaceful sea jellies that are ideal for community aquariums.
  • Crystal Jellyfish: Transparent sea jellies that are very beautiful to watch.

Moon jellyfish are the most marketed for aquariums; they can live in tropical climates and small spaces. Moon sea jellies do not require very strong currents.

Specialists recommend using cylindrical-shaped fish tanks, which can be purchased online from some specialized stores. Additionally, you should modify the water flow and filtration system. Jellyfish can get trapped in pumps and filtration equipment, and aeration should be avoided because any air bubble will be trapped under the jellyfish’s bell, causing holes in the body.

If you are into design, Ballesteros et al.’s studio (2022) describes the design and operation of a modified kreisel aquarium that allows the growth of the early planktonic life stages of sea jellies in both closed and continuous flow systems.

Aquarium for the growth of the first life stages of jellyfish. Source: Ballesteros et al., (2022); Front. Mar. Sci.
Aquarium for the growth of the first life stages of jellyfish. Source: Ballesteros et al., (2022); Front. Mar. Sci.

If you are thinking about having a jellyfish as a pet, it is important to research thoroughly to ensure that you can provide proper care.

Jellyfish Tank Setup

Tank Size and Shape:

  • Size: The tank should have at least 20 gallons for a small jellyfish and 50 gallons for a large sea jellies.
  • Shape: Round tanks are ideal for jellyfish, as they give them space to swim without hitting the walls.

Necessary Equipment:

  • Filtration System: An efficient filtration system is essential to maintain water quality.
  • Water Flow: Jellyfish need water that moves constantly through the tank to help them breathe and swim.
  • Lighting: Jellyfish need light to thrive.

Jellyfish Care Tips

Jellyfish are delicate creatures and require special care. Here are some tips to help you care for your jellyfish:

  • Feed your sea jellies a regular diet of small fish, plankton, or other live foods.
  • Keep the tank water clean and free from debris.
  • Regularly monitor the temperature and salinity of the tank water.
  • Avoid exposing your jellyfish to sudden changes in temperature or salinity.
  • Handle your sea jellies with care.

Bioluminescent Jellyfish

Many jellyfish species can produce light, an ability known as bioluminescence.

Bioluminescence is the light produced by a living organism and is commonly emitted by marine life, with photoproteins being the main contributors to bioluminescence (Sharifian et al., 2018). Jellyfish have been used in research because the green fluorescent protein used by some species causes bioluminescence that has been adapted as a fluorescent marker for genes inserted into other cells or organisms.

Studies of the bioluminescence mechanisms of sea jellies have been confined to the species Aequorea victoria; Xia et al. (2002) described the luminescence system in Aequorea macrodactyla, which is a common jellyfish species found on the east coast of the China Sea.

The jellyfish bioluminescence system consists of proteins present in some tissues that react chemically to produce blue or green light in response to a stimulus.

Sea jellies use their bioluminescence to warn predators of their toxicity.

Conclusion

Jellyfish are fascinating creatures that play an important role in the marine ecosystem. They are significant predators of small fish and plankton and are also a source of food for larger animals, such as sea turtles and whales, and even for humans. Despite the bad reputation sea jellies have and the negative impacts on industries like fishing and aquaculture, some researchers report that they are among the foods to be consumed in the future.

Although significant advances have been made in understanding the biology of jellyfish, we cannot yet say that their cultivation cycle is closed.

Beyond the use of jellyfish for food in Asia, this aquatic organism also shows potential for use in the nutraceutical and pharmaceutical industries.

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