The World of the Jellyfish Esther Luengo
Index 1 Introduction 2 Terminology 3 Anatomy 3.1 Nervous system 3.2 Vision 3.3 Size 4 Taxonomy 5 Life history and behavior 5.1 Phases 5.2 Reproduction 5.3 Brooding 5.4 Lifespan 5.5 Movement 6 Relation to humans 6.1 Fisheries 6.2 As food 6.3 Biotechnology 6.4 Aquariums 6.5 Toxicity 6.6 Treatment 6.7 Hazards
Introduction Jellyfish or jellies are the major non-polyp form of individuals of the phylum Cnidaria. They are typified as free-swimming marine animals consisting of a gelatinous umbrella-shaped bell and trailing tentacles. The bell can pulsate for locomotion, while stinging tentacles can be used to capture prey. Jellyfish are found in every ocean, from the surface to the deep sea. Scyphozoans are exclusively marine, but some Hydrozoans live in freshwater. Large, often colorful, jellyfish are common in coastal zones worldwide. Jellyfish have roamed the seas for at least 500 million years, and possibly 700 million years or more, making them the oldest multi-organ animal.
Terminology The English popular name jellyfish has been in use since 1796. It has traditionally also been applied to other animals sharing a superficial resemblance, for example ctenophores (members from another phylum of common, gelatinous and generally transparent or translucent, free-swimming planktonic carnivores now known as comb jellies) were included as “jellyfishes”. Even some scientists include the phylum ctenophora when they are referring to jellyfish. Other scientists prefer to use the more allencompassing term gelatinous zooplankton, when referring to these, together with other soft-bodied animals in the water column. As jellyfish are not vertebrates, let alone true fish, the word jellyfish is considered by some to be a misnomer. Public aquariums may use the terms jellies or sea jellies instead. Indeed, it may be said that the term “jellies” has become more popular than “jellyfish”. In scientific literature, “jelly” and “jellyfish” are often used interchangeably. Some sources may use the term “jelly” to refer to organisms in this taxon, as “jellyfish” may be considered inappropriate. Many textbooks and sources refer to only scyphozoa as “true jellyfish”. A group of jellyfish is sometimes called a bloom or a swarm.“Bloom” is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected. Another collective name for a group of jellyfish is a smack, although this term is not commonly used by scientists who study jellyfish. Jellyfish are “bloomy” by nature of their life cycles, being produced by their benthic polyps usually in the spring when sunshine and plankton increase, so they appear rather suddenly and often in large numbers, even when an ecosystem is in balance. Using “swarm” usually implies some kind of active ability to stay together, which a few species such as Aurelia, the moon jelly, demonstrate. Medusa jellyfish may be classified as scyphomedusae (“true” jellyfish), stauromedusae (stalked jellyfish), cubomedusae (box jellyfish), or hydromedusae, according to which clade their species belongs. The term medusa was coined by Linnaeus in 1752, alluding to the tentacled head of Medusa in Greek mythology. This term refers exclusively to the non-polyp life-stage which occurs in many cnidarians, which is typified by a large pulsating gelatinous bell with long trailing tentacles. All medusa-producing species belong to the sub-phylum Medusozoa. In biology, a medusa (plural: medusae) is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below. The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella; the mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum). The digestive cavity consists of the gastrovascular cavity and radiating canals which extend toward the margin; these canals may be simple or branching, and vary in number from few to many. The margin of the disk bears sensory organs and tentacles as its said. German biologist Ernst Haeckel popularized medusae through his vivid illustrations, particularly in Kunstformen der Natur.
Anatomy Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. The manubrium is a stalk-like structure hanging down from the centre of the underside, with the mouth at its tip. This opens into the gastrovascular cavity, where digestion takes place and nutrients are absorbed. It is joined to the radial canals which extend to the margin of the bell. Jellyfish do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement, but can use their hydrostatic skeleton to navigate through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are mostly passive. Depending on the species, the body contains between 95 and 98% water. Most of the umbrella mass is a gelatinous material — the jelly — called mesoglea which is surrounded by two layers of protective skin. The top layer is called the epidermis, and the inner layer is referred to as gastrodermis, which lines the gut. Jellyfish employ a loose network of nerves, located in the epidermis, which is called a “nerve net”. Although traditionally thought not to have a central nervous system, nerve net concentration and ganglion-like structures could be considered to constitute one in most species.A jellyfish detects various stimuli including the touch of other animals via this nerve net, which then transmits impulses both throughout the nerve net and around a circular nerve ring, through the rhopalial lappet, located at the rim of the jellyfish body, to other nerve cells. Some jellyfish have ocelli: light-sensitive organs that do not form images but which can detect light and are used to determine up from down, responding to sunlight shining on the water’s surface. These are generally pigment spot ocelli, which have some cells pigmented. Certain species of jellyfish, such as the box jellyfish, have been revealed to have more advanced vision than their counterparts. The box jellyfish has twenty-four eyes, two of which are capable of seeing color, and four parallel information processing areas or rhopalia that act in competition, supposedly making it one of the few creatures to have a 360-degree view of its environment.
The eyes are suspended on stalks with heavy crystals on one end, acting like a gyroscope to orient the eyes skyward. They look upward to navigate from roots in mangrove swamps to the open lagoon and back, watching for the mangrove canopy, where they feed. Jellyfish range from about one millimeter in bell height and diameter to nearly two meters in bell height and diameter; the tentacles and mouth parts usually extend beyond this bell dimension. The smallest jellyfish are the peculiar creeping jellyfish in the genera Staurocladia and Eleutheria, which have bell disks from 0.5mm to a few mm diameter, with short tentacles that extend out beyond this, on which these tiny jellyfish crawl around on seaweed or the bottoms of rocky pools. Many of these tiny creeping jellyfish cannot be seen in the field without a hand lens or microscope; they can reproduce asexually by splitting in half. This process is called fission. Other very small jellyfish, which have bells about 1mm, are the hydromedusae of many species that have just been released from their parent polyps; some of these live only a few minutes before shedding their gametes in the plankton and then dying, while others will grow in the plankton for weeks or months. The hydromedusae Cladonema radiatum and Cladonema californicum are also very small, living for months, yet never growing beyond a few mm in bell height and diameter. Another small species of jellyfish is the Australian Irukandji, which is more or less about the size of a fingernail.
Taxonomy Jellyfish belong to Medusozoa, the clade of cnidarians which excludes Anthozoa (e.g., corals and anemones). This suggests that the medusa form evolved after the polyps. The phylogenetics of this group are complex and evolving. The Medusozoa and Octocorallia are proposed as sister groups according to research published in 2012. That research also proposes coronate Scyphozoa and Cubozoa as a sister clade to Hydrozoa and discomedusan Scyphozoa, which are themselves sister groups. The hydroidolinans are a sister group to Limnomedusae, also called Trachylina. Semaeostomae is paraphyletic with Rhizostomeae. The class Storozoa was the earliest group of Medusozoa to diverge and the Limnomedusae were the earliest Hydrozoa to diverge. Scyphozoa are often called true jellyfish. They have tetra-radial symmetry. They have tentacles around the outer margin of the bowl-shaped bell, and oral arms around the mouth. Cubozoa (box jellyfish) have a box-shaped bell, and their velarium assists them to swim more quickly. Box jellyfish may be related more closely to "true jellyfish" than either are to hydrozoa. Hydrozoa may form medusa which resemble scyphozoans (but generally with a velum) and are distinguished by an absence of cells in the mesoglea. However, many hydrozoa species do not form medusa at all (such as hydra, which is hence not considered a jellyfish).
Staurozoa (stalked jellyfish) do not have a polyp stage, however the medusa is generally sessile, oriented upside down and with a stalk from the “bell” planted to the substrate. Until recently, staurozoa was classified within scyphozoa. Some other animals are frequently associated with or mistaken for medusa jellyfish. Siphonophorae are an order of hydrozoa which generally live as colonies (for example, freeswimming chains of repeated units, some units similar to polyps or to medusa). They are not considered medusa jellyfish. A well-known example is the Portuguese Man o’ War. Ctenophores (comb jellies) are a separate phylum from Cnidaria. Their method of propulsion is cilia paddles rather than a pulsating bell. Salps are transparent, gelatinous marine organisms that form pelagic colonies like siphonophores. Salps are chordates, and as such are actually more closely related to humans than they are to cnidarians and comb jellies. There are over 200 species of Scyphozoa, about 50 species of Staurozoa, about 20 species of Cubozoa, and in Hydrozoa there are about 1000–1500 species that produce medusae (and many more hydrozoa species that do not).
Life history and behaviour Jellyfish development occurs in multiple phases. Sperm fertilize eggs which develop into larval planulae, become polyps, bud into ephyrae and then transform into adult medusae. In some species, specimens may skip some phases. The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp. The polyp is generally a small stalk with a mouth that is ringed by upward-facing tentacles. The polyps resemble the closely related Cnidaria anthozoan (sea anemones and corals) polyps. The jellyfish polyp may be sessile, living on the bottom or on another substrate such as floats or boat hulls, or it may be free-floating or attached to tiny bits of free-living plankton or rarely, fish or other invertebrates. Polyps may be solitary or colonial. Polyp colonies form by strobilation, in which multiple polyps share a common stomach cavity. Most polyps are only millimeters in size. They feed continuously. The polyp stage may last for years. The next stage is the ephyra, which is a free-swimming precursor of the final adult stage. The ephyra then develops into a medusa. The medusa is the life stage that is typically identified as a jellyfish.
Relation to humans Fisheries have begun harvesting the American cannonball jellyfish, Stomolophus meleagris, along the southern Atlantic coast of the United States and in the Gulf of Mexico for export to Asia. Jellyfish are also harvested for their collagen, which can be used for a variety of applications including the treatment of rheumatoid arthritis. In some countries, such as Japan and Korea, jellyfish are known as a delicacy. “Dried jellyfish” has become increasingly popular throughout the world. The jellyfish is dried to prevent spoiling; if not dried they can spoil within a matter of hours. Once dried, they can be stored for weeks at a time. Only scyphozoan jellyfish belonging to the order Rhizostomeae are harvested for food; about 12 of the approximately 85 species. Most of the harvest takes place in southeast Asia. Rhizostomes, especially Rhopilema esculentum in China and Stomolophus meleagris (cannonball jellyfish) in the United States, are favored because of their larger and more rigid bodies and because their toxins are harmless to humans. Traditional processing methods, carried out by a Jellyfish Master, involve a 20- to 40-day multi-phase procedure in which after removing the gonads and mucous membranes, the umbrella and oral arms are treated with a mixture of table salt and alum, and compressed. Processing reduces liquefaction, odor, the growth of spoilage organisms, and makes the jellyfish drier and more acidic, producing a “crunchy and crispy texture.” Jellyfish prepared this way retain 7–10% of their original weight, and the processed product contains approximately 94% water and 6% protein. Freshly processed jellyfish has a white, creamy color and turns yellow or brown during prolonged storage. In China, processed jellyfish are desalted by soaking in water overnight and eaten cooked or raw. The dish is often served shredded with a dressing of oil, soy sauce, vinegar and sugar, or as a salad with vegetables. In Japan, cured jellyfish are rinsed, cut into strips and served with vinegar as an appetizer. Desalted, ready-to-eat products are also available.