Jellyfish-Like Creatures: Siphonophores and Hydrozoans

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Siphonophores and Hydrozoans by Haeckel, 1904

There are a number of animals that look like jellyfish but are actually something else. Chief among these are ctenophores (comb jellies). There are also some worms, snails and squid that look like jellyfish. The main difference between Cnidaria and ctenophores is that the former has a sting and the latter doesn't. Some hydrozoans are mistaken for jellyfish. They are colonies of polyps that often have an attached polyps stage and free-swimming medusa stage. Some are exist as medusae their entire lives. [Source: Richard Conniff, National Geographic, June 2000; Jack and Anne Rudlow, Smithsonian]

Then there’s the siphonophores, a kind of hydrozoan. The Portuguese man-of-war is one of these. Stephanie Pappas wrote in Live Science:Siphonophores operate like a single organism, but they're actually colonies of individual, asexually reproducing organisms that take on different roles within the larger whole. "People struggle to understand siphonophores at all," said Steven Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute who studies these oddities as well as other gelatinous creatures. Researchers in Australia once observed siphonophores up to 150 feet (45 meters) long. [Source: Stephanie Pappas, Live Science, April 20, 2022]

Haddock told Live Science that his personal favorite siphonophore is Erenna sirena, which uses red bioluminescent lures to attract prey. Another gelatinous favorite for Haddock is the bloody-belly comb jelly (Lampocteis), a deep-sea ctenophore. Ctenophores don't sting like jellyfish do; rather, they sport sticky cells to entrap prey. The eerily named bloody-belly comb jelly is bold red and propels itself through the depths with tiny beating cellular projections called cilia, which seem to sparkle as light hits them.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase; Encyclopedia of Life; Smithsonian Oceans Portal ; Monterey Bay Aquarium ; MarineBio

Comb Jellies (Ctenophora)

Comb jelly

Ctenophores (pronounced TEEN-oh-fours) are predators commonly known as comb jellies that travels through the ocean in search of food. Though they resemble jellyfish, they are distinctly different creatures that propel themselves through water using cilia (short vibrating or moving hairlike structure) instead of tentacles. Ctenophora make up a phylum of marine invertebrates, that inhabit marine waters worldwide. The phylum includes seven orders, with over 200 species, which are biradially symmetrical (divided into equal parts, but only in two planes) and acoelomate (animal without a body cavities) and resemble cnidarians (jellyfish). [Source: CBS News, Animal Diversity Web (ADW)]

Ctenophores use groups of cilia (commonly referred to as "combs") for swimming , and they are the largest animals to swim with the help of cilia. Depending on the species, adult ctenophores range from a few millimeters (fractions of an inch to 1.5 meter (5 feet) in size. According to some scientists only 100 to 150 species have been validated, and possibly another 25 have not been fully described and named. The name "ctenophora" means "comb-bearing", from the Greek word meaning "comb" and the Greek suffix meaning "carrying".

Comb jellies are among — if not THE — world’s oldest animals. Despite their soft, gelatinous bodies, fossils thought to represent ctenophores appear in fossils dating as far back as the early Cambrian, about 525 million years ago. The position of the ctenophores in the "tree of life" has long been debated by scholars.


Hydrozoans make up a subgroup of cnidarians with approximately 3700 species. Cnidarians include jellyfish, coral and sea anemones. Hydrozoa are distinguished from other cnidarian groups by: 1) their complex life cycle, 2) the growth of medusae from buds rather than strobilae or from metamorphosis, 3) the presence of a velum inside the bell of the medusa, and 4) the production of gametes from ectodermal, rather than endodermal, tissue. [Source: George Hammond, Animal Diversity Web (ADW)

Limnomedusae is an order of hydrozoans with biphasic life histories: medusae with ecto-endodermal statocysts and with gonads alongside their radial canals.

Hydrozoa is a diverse group with a variety of life cycles, growth forms, and specialized structures. Like many cnidarians, hydrozoans have both polyp and medusa stages in their life cycle. Some well-known examples of hydrozoans are 1) Portuguese man o' war (Physalia physalis); 2) By-the-wind sailors (Velella velella), 3) freshwater jellies (Craspedacusta sowerbyi), 4) "air ferns" (Sertularia argentea), 5) fire coral (Milleporidae) and 6) pink-hearted hydroids (Tubularia).

Hydrozoans are small, predatory animals. Some solitary and some colonial. Most inhabit marine environments. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony.

The earliest hydrozoans may be from the Vendian (late Precambrian), more than 540 million years ago.Hydroid colonies are usually dioecious, which means they have separate sexes—all the polyps in each colony are either male or female, but not usually both sexes in the same colony. In some species, the reproductive polyps, known as gonozooids (or "gonotheca" in thecate hydrozoans) bud off asexually produced medusae. [Source: Wikipedia]

Hydrozoan Characteristics

Most hydrozoan species include both a polypoid and a medusoid stage in their lifecycles, although a number of them have only one or the other. The hydroid form is usually colonial, with multiple polyps connected by tubelike hydrocauli. The colonies are generally small, no more than a few centimeters across, but some in Siphonophorae can reach sizes of several meters. They may have a tree-like or fan-like appearance, depending on species. The polyps themselves are usually tiny. [Source: Wikipedia]

In any given colony, the majority of polyps are specialized for feeding. These have a more or less cylindrical body with a terminal mouth on a raised protuberance called the hypostome, surrounded by a number of tentacles. The polyp contains a central cavity, in which initial digestion takes place. Partially digested food may then be passed into the hydrocaulus for distribution around the colony and completion of the digestion process. Unlike some other cnidarian groups, the lining of the central cavity lacks stinging nematocysts, which are found only on the tentacles and outer surface.

Anatomy of a hydrozoan polyp. (A) A Hydra polyp is essentially a two-layered tube, with a ring of tentacles around the mouth opening at the tip of the hypostome. Asexual budding occurs on the lower half of the body column. Interstitial stem cells and nematoblasts are distributed evenly in the body column, below the tentacle ring and above the border of the peduncle, which is the stalk between the budding region and pedal disc. (B) The bilayered cellular organization of a Hydra polyp. Ectoderm and endoderm are separated by an acellular matrix called the mesogloea (gray). All epithelial cells in Hydra are myoepithelial, with myofibers on the basal side (red). In ectodermal epithelial cells (green), the fibers are oriented longitudinally, and in endodermal epithelial cells (pink) they are oriented circumferentially (ring muscle). Most interstitial cells and nematoblast clusters are located between ectodermal epithelial cells. Neurons are found in both the endoderm and ectoderm. Sensory neurons are located between epithelial cells and connect to ganglion neurons (purple), which are at the base of the epithelium on top of the myofibers and sometimes cross the mesogloea. Different types of gland cells, most of which are found in the endoderm, are intermingled between the epithelial cells. [Source: Ulrich Technau and Robert E Steele, “Evolutionary crossroads in developmental biology: Cnidaria”, March 2011, Researchgate]

All colonial hydrozoans also include some polyps specialized for reproduction. These lack tentacles and contain numerous buds from which the medusoid stage of the lifecycle is produced. The arrangement and type of these reproductive polyps varies considerably between different groups. In addition to these two basic types of polyps, a few colonial species have other specialized forms. In some, defensive polyps are found, armed with large numbers of stinging cells. In others, one polyp may develop as a large float, from which the other polyps hang down, allowing the colony to drift in open water instead of being anchored to a solid surface.[2]

The medusae of hydrozoans are smaller than those of typical jellyfish, ranging from 0.5 to 6 centimeters (0.20 to 2.36 inches) in diameter. Although most hydrozoans have a medusoid stage, this is not always free-living and in many species exists solely as a sexually reproducing bud on the surface of the hydroid colony. Sometimes, these medusoid buds may be so degenerated as to entirely lack tentacles or mouths, essentially consisting of an isolated sex organ.

The body consists of a dome-like umbrella ringed by tentacles. A tube-like structure hangs down from the centre of the umbrella and includes the mouth at its tip. Most hydrozoan medusae have just four tentacles, although a number of exceptions exist. Stinging cells are found on the tentacles and around the mouth. The mouth leads into a central stomach cavity. Four radial canals connect the stomach to an additional, circular canal running around the base of the bell, just above the tentacles. Striated muscle fibres also line the rim of the bell, allowing the animal to move along by alternately contracting and relaxing its body. An additional shelf of tissue lies just inside the rim, narrowing the aperture at the base of the umbrella, and thereby increasing the force of the expelled jet of water.

The nervous system is unusually advanced for cnidarians. Two nerve rings lie close to the margin of the bell, and send fibres into the muscles and tentacles. The genus Sarsia has even been reported to possess organised ganglia. Numerous sense organs are closely associated with the nerve rings. Mostly these are simple sensory nerve endings, but they also include statocysts and primitive light-sensitive ocelli.


Siphonophorae by Haeckel, 1904

Siphonophorae is an order within Hydrozoa, which in turn is a class of marine organisms within the phylum Cnidaria. According to the World Register of Marine Species, the order contains 175 species thus far. Siphonophores are pelagic floaters made up of colonial polyps, divided in types with certain functions such as feeding, defense or reproduction. Most are predators or filter-feeders, though a few have symbiotic algae (zooxanthellae) sort of like coral. Like other hydrozoans, some siphonophores emit light to attract and attack prey. While many sea animals produce blue and green bioluminescence, a siphonophore in the genus Erenna is only the second known life form found to produce a red light (the first one being the scaleless dragonfish). The name Siphonophorae comes from Greek siphōn 'tube' and pherein 'to bear'. [Source: Animal Diversity Web (ADW)]

Helen Scales wrote in The Guardian: Siphonophores look like jellyfish, and they do belong to the same group of animals, but they build their bodies in a unique way — more like hundreds of tiny jellyfish stuck together. Yet, a siphonophore is a single organism. Rather than growing in a more conventional way into a body with organs that carry out different functions, siphonophores consist of individual parts called zooids. Some zooids are responsible for feeding, some for reproduction, and others move and steer the animal through the water. “They’re just an example of doing things a bit differently. They’re one and they’re many,” Nerida Wilson from the Western Australian Museum, told The Guardian., adding they do have “two parents” and are “products of sex.”[Source: Helen Scales, The Guardian, November 30, 2022]

Siphonophores are highly polymorphic and complex organisms. Although they may appear to be individual organisms, each individual is a colonial organism composed of medusoid and polypoid zooids that are morphologically and functionally specialized. Zooids are multicellular units that develop from a single fertilized egg and combine to create functional colonies able to reproduce, digest, float, maintain body positioning, and use jet propulsion to move. Although Siphonophora are generally considered to be the most specialized hydrozoans, some researchers claim that it is in fact the most primitive order, with the medusa and the polyp not fully differentiated. Additional support for this view comes from the observation that the regenerative powers of the man-of-war are poor, in contrast to most other jellyfish. [Source: Mindy B. Kurlansky, Animal Diversity Web (ADW); Wikipedia]

45-Meter-Long Deep-Sea Siphonophore — the World’s Longest Animal?

In 2020, scientists in submersible in waters about 600 meters (2,000 feet) in an underwater canyon off the coast of Western Australia, came across a long gelatinous creature arranged in a giant spiral — a 45-meter-long siphonophore arranged in a feeding spiral, trailing its deadly tentacles “It was like a rope on the horizon. You couldn’t miss it,”Nerida Wilson from the Western Australian Museum said. “It was so huge.” [Source: Helen Scales, The Guardian, November 30, 2022]

Helen Scales wrote in The Guardian: The deep-sea siphonophore the scientists spotted “is probably a new species from the genus Apolemia, a group that generally look like tangled feather boas. The spiral arrangement is known to be a feeding posture in these types of siphonophores. Numerous stinging tentacles create a wall of death in the water, trapping small prey, including crustaceans and fish.

Finding it was one of the chance encounters that are common in deep-sea research. The scientists’ aim was to study life on the deep seabed, and they just happened to come across this floating jelly while their submersible was on its two-hour transit back to the ship, the research vessel Falkor, then run by the Schmidt Ocean Institute. Time was already short because the dive had run over schedule, and so the submersible pilot, controlling it from the surface, could only spend a few moments with the animal. “We circled around, took some footage and a little sample of tissue,” Wilson says. “Then we just had to go on our merry way.”

“Based on a rough calculation from the submersible’s track, the spiral-shaped siphonophore is a candidate for the longest specimen ever encountered. At about 45 meters (150 feet), it could even be the longest animal ever to be measured, much longer than a blue whale. Reluctant to claim any world records just yet, Wilson is working with a specialist in photogrammetry to get a more accurate estimate of the siphonophore’s size. It is not an easy task to extract three-dimensional information from the video, because the siphonophore moved about in the wake of the submersible’s thrusters. “Normally with photogrammetry, you’re going back and forth over a fixed object,” Wilson says. “This is technically a bit more challenging.”

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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