See all media
- Key People:
- Oka Asajirō
- Related Topics:
- Portuguese man-of-war
- lion’s mane jellyfish
- freshwater jellyfish
See all related content →
Jan. 12, 2024, 3:28 AM ET (USA Today)
jellyfish, any planktonic marine member of the class Scyphozoa (phylum Cnidaria), a group of invertebrate animals composed of about 200 described species, or of the class Cubozoa (approximately 20 species). The term is also frequently applied to certain other cnidarians (such as members of the class Hydrozoa) that have a medusoid (bell- or saucer-shaped) body form, as, for example, the hydromedusae and the siphonophores (including the Portuguese man-of-war). Unrelated forms such as comb jellies (phylum Ctenophora) and salps (phylum Chordata) are also referred to as jellyfish. Scyphozoan jellyfish can be divided into two types, those that are free-swimming medusae and those that are sessile (i.e., stem animals that are attached to seaweed and other objects by a stalk). The sessile polyplike forms constitute the order Stauromedusae.
Free-swimming scyphozoan jellyfish occur in all oceans and include the familiar disk-shaped animals that are often found drifting along the shoreline. Most live for only a few weeks, but some are known to survive a year or longer. The bodies of most range in size from about 2 to 40 cm (1 to 16 inches) in diameter; some species are considerably larger, however, with diameters of up to 2 metres (6.6 feet). Scyphozoan medusae consist of almost 99 percent water as a result of the composition of the jelly that forms the bulk in nearly all species. Most feed on copepods, fish larvae, and other small animals that they catch in their tentacles, which have stinging cells (nematocysts). Some, however, simply suspension feed, extracting minute animals and algae (phytoplankton) from the water. Like all cnidarians, their bodies are made up of two cellular layers, the ectoderm and the endoderm, between which lies the gelatinous mesoglea. In jellyfish the transparent mesoglea layer is quite thick.Britannica QuizAnimal Factoids
The life cycle of free-swimming scyphozoan jellyfish typically consists of three stages. A sessile polyp (scyphistoma) stage asexually buds off young medusae from its upper end, with each such ephyra growing into an adult. The adults are either male or female, but in some species they change their sex as they age. In many species, normal fusion of egg and sperm results in an embryo that is brooded in the gut of the adult until it becomes a ciliated planula larva, but in some this development takes place in the sea. After the planula larva leaves its parent, it lives for a time in the plankton and eventually attaches to a rock or other solid surface, where it grows into a new scyphistoma. Such a life cycle characterizes the order Semaeostomeae, which contains some 50 species of mainly coastal-water jellyfish, several of which have very wide geographic ranges. Included among these are members of the genera Aurelia and Chrysaora and the big red jellyfish, Tiburonia granrojo (subfamily Tiburoniinae), one of only three species of jellyfish that lack tentacles.
The order Coronatae includes about 30 species of mostly deep-sea jellyfish, often maroon in colour. A deep circular groove delimits the central part of the bell-shaped body from the periphery, which is divided into broad flaps, or lappets. The marginal tentacles are large and solid. Some species are known to have a scyphistoma stage, but the life cycle of most of the forms has yet to be described. The coronate jellyfish are the most primitive of the present-day scyphozoans and are thought to be descended directly from the fossil form Conulata, which flourished between about 180 and 600 million years ago. Some of the known sessile stages form branched colonies, which were once separately identified under the name Stephanoscyphus.
The order Rhizostomeae includes some 80 described species. In these jellyfish the frilly projections (oral arms) that extend down from the underside of the body are fused, obliterating the mouth and forming a spongy area used in filter feeding. Marginal tentacles are lacking, and the gelatinous bell is firm and warty. In species whose life cycles are known, there is a typical benthic (bottom-dwelling) scyphistoma stage. Most members of the order are vigorous swimmers. Species of Cassiopea, the upside-down jellyfish, however, swim infrequently and sit inverted in tropical shallows, exposing their photosynthetic symbiotic algae to sunlight. The group Rhizostomeae is found mainly in shallow tropical to subtropical seas in the Indo-Pacific region, but members of the genus Rhizostoma, also called football jellyfish, often inhabit cooler waters, and Cotylorhiza is common in the Mediterranean.
The fourth order, Stauromedusae, comprises some 30 described species of nonswimming, stalked jellies. These species occur chiefly in cooler waters. They are goblet-shaped and fixed by a basal stalk; the mouth is situated at the upper end. Ranging from 1 to 10 cm (0.4 to 4 inches) in diameter, the body has a tetradiate design and typically bears eight clusters of tentacles. Some species can detach and resettle. Stauromedusae usually feed on small marine animals and live for several years. Development is direct from a larva into an adult. The polyp stage is suppressed.
Are you a student? Get Britannica Premium for only 24.95 - a 67% discount!
The class Cubozoa contains two orders, Carybdeida and Chirodropida. Together, both orders comprise about 20 described species. Although some reach a diameter of 25 cm (10 inches), most range between 2 to 4 cm (1 to 2 inches). The jelly is rather spherical but squared off along the edges, giving rise to the common name of box jellies. The genera Chironex and Chiropsalmus, commonly called sea wasps, occur widely from Queensland northward to about Malaya. These forms have remarkably sophisticated eyes, and they are dangerously venomous; a moderate sting can cause death within a few minutes. In all the box jellies so far studied, the polyp stage produces but a single medusa. Through the process of budding, polyps emerge from a medusa or from another polyp. Essentially, a single planula larva may produce numerous, genetically identical medusae.
See also cnidarian; hydroid; medusa; Portuguese man-of-war.
The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Adam Augustyn.
Insights, advice, suggestions, feedback and comments from experts
I am an expert and enthusiast based assistant. I have access to a wide range of information and can provide assistance on various topics. I can help answer questions, provide information, and engage in detailed discussions. I have been trained on a diverse set of data sources and can provide accurate and reliable information.
Regarding the concept of jellyfish, I can provide information based on the search results you provided. Let's dive into the details!
Jellyfish: Overview and Classification
Jellyfish are planktonic marine animals belonging to the class Scyphozoa in the phylum Cnidaria. There are approximately 200 described species of jellyfish in the class Scyphozoa and about 20 species in the class Cubozoa.
The term "jellyfish" is also used to refer to certain other cnidarians, such as members of the class Hydrozoa, which have a medusoid (bell- or saucer-shaped) body form. This includes hydromedusae and siphonophores, including the Portuguese man-of-war.
Unrelated forms such as comb jellies (phylum Ctenophora) and salps (phylum Chordata) are also sometimes referred to as jellyfish.
Types of Jellyfish
Jellyfish in the class Scyphozoa can be divided into two types: free-swimming medusae and sessile forms. Free-swimming jellyfish are often found drifting along the shoreline and occur in all oceans. They have disk-shaped bodies and range in size from about 2 to 40 cm (1 to 16 inches) in diameter, with some species growing up to 2 meters (6.6 feet) in diameter.
Sessile forms of jellyfish, known as Stauromedusae, are stem animals that are attached to seaweed and other objects by a stalk. They are non-swimming and occur chiefly in cooler waters. Stauromedusae have goblet-shaped bodies, typically ranging from 1 to 10 cm (0.4 to 4 inches) in diameter, and bear eight clusters of tentacles.
Other types of jellyfish include those in the orders Coronatae and Rhizostomeae. Coronate jellyfish are mostly deep-sea jellyfish with maroon-colored bodies and large, solid marginal tentacles. Rhizostomeae jellyfish have fused frilly projections called oral arms and lack marginal tentacles. They are found mainly in shallow tropical to subtropical seas.
The class Cubozoa contains two orders, Carybdeida and Chirodropida. These jellyfish are commonly known as box jellies due to their squared-off shape along the edges. They have sophisticated eyes and are dangerously venomous. The polyp stage of box jellies produces a single medusa, and polyps can emerge through budding.
Life Cycle and Reproduction
The life cycle of free-swimming scyphozoan jellyfish typically consists of three stages: a sessile polyp (scyphistoma) stage, an asexual budding of young medusae from the polyp's upper end (ephyra stage), and the growth of ephyra into an adult medusa. The adults can be either male or female, and some species can change their sex as they age. Fertilization can occur through the fusion of egg and sperm, resulting in an embryo that is brooded in the gut of the adult or in the sea. After the planula larva leaves its parent, it lives in the plankton before eventually attaching to a solid surface and growing into a new scyphistoma.
Jellyfish are fascinating marine animals belonging to the class Scyphozoa and the class Cubozoa. They come in various shapes and sizes, and their life cycles involve different stages, including polyp and medusa forms. It's important to note that the information provided is based on the search results you provided, and additional research can provide more in-depth knowledge on the topic.
Let me know if there's anything else I can assist you with!