Jellyfish: A Natural History

Jellyfish: A Natural History

by Lisa-ann Gershwin


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Jellyfish, with their undulating umbrella-shaped bells and sprawling tentacles, are as fascinating and beautiful as they are frightening and dangerous. They are found in every ocean at every depth, and they are the oldest multi-organed life form on the planet, having inhabited the ocean for more than five hundred million years. In many places they are also vastly increasing in number, and these population blooms may be an ominous indicator of the rising temperatures and toxicity of the world’s oceans.

Jellyfish presents these aquarium favorites in all their extraordinary and captivating beauty. Fifty unique species, from stalked jellyfish to black sea nettles, are presented in stunning color photographs along with the most current scientific information on their anatomy, history, distribution, position in the water, and environmental status. Foremost jellyfish expert Lisa-ann Gershwin provides an insightful look at the natural history and biology of each of these spellbinding creatures, while offering a timely take on their place in the rapidly changing and deteriorating condition of the oceans. Readers will learn about immortal jellyfish who live and die and live again as well as those who camouflage themselves amid sea grasses and shells, hiding in plain sight.
Approachably written and based in the latest science and ecology, this colorful book provides an authoritative guide to these ethereal marine wonders.

Product Details

ISBN-13: 9780226287676
Publisher: University of Chicago Press
Publication date: 06/07/2016
Pages: 224
Sales rank: 584,615
Product dimensions: 8.50(w) x 9.70(h) x 0.90(d)

About the Author

Lisa-ann Gershwin is director of the Australian Marine Stinger Advisory Services. She was awarded a Fulbright in 1998 for her studies on jellyfish blooms and evolution, and she has discovered over two hundred new species—including at least sixteen types of jellyfish that are highly dangerous, as well as a new species of dolphin. She is the author of Stung!: On Jellyfish Blooms and the Future of the Ocean.

Read an Excerpt


A Natural History

By Lisa-Ann Gershwin, David Price-Goodfellow, Amy K. Hughes

The University of Chicago Press

Copyright © 2016 Lisa-ann Gershwin
All rights reserved.
ISBN: 978-0-226-28770-6




Jellyfish, or sea jellies, as they are also known, are simple creatures. Many seem more like plants than animals, although they are animals. Their reproductive capacity can approach rates of exponential growth generally seen only in bacteria, and many of their structural features have no counterpart in more familiar creatures. This chapter provides an overview of the pieces and parts — and their functions — that make up the different groups in the splendidly weird world of jellyfish.

The group "jellyfish" is actually an unnatural collection of creatures from three quite different evolutionary lineages — the medusae and siphonophores, or Medusozoa; the ctenophores; and the salps and their kin — that all just happen to be transparent drifters. Even within these larger groups, smaller subsets often have radically different features. But although they are quite disparate, they share basic elements of their anatomy, which are reflected in their taxonomy, or identification and classification.

All jellyfish have a gelatinous body; a way to catch food, digest it, and get rid of waste; a way to defend themselves; a way to reproduce; and a way to get from point A to point B. But the ways they accomplish these basic life tasks are amazingly and exquisitely varied. These simple creatures are, in many respects, not actually all that simple.

The Three Lineages of Jellyfish

The most numerous jellyfish — and certainly the ones that are most familiar — are the medusae (singular, medusa) in the phylum Cnidaria (pronounced nye-DARE-ee-uh, with a silent C), which also contains the corals, sea anemones, and sea fans. Belonging to four classes within the subphylum Medusozoa (Scyphozoa, Cubozoa, Staurozoa, and Hydrozoa), the medusae are generally dome- or disk-shaped but occur in all sorts of unimaginable variations on that theme. They are built on a radially symmetrical body plan, somewhat resembling a perfectly apportioned pie cut into equal slices, each slice identical to the others. Most species are tetraradial, meaning they have four identical slices, while others have eight (octoradial), and a few have six (hexaradial).

Medusae have the same basic body plan as a coral polyp or sea anemone but upside down; whereas a polyp is basically a stomach opening upward and surrounded with tentacles, medusae are basically the same form facing downward. And, of course, corals and anemones are stuck to the sea bottom, whereas medusae drift in the water.

A subset of the cnidarian class Hydrozoa is a very peculiar offshoot lineage known as the siphonophores (sigh-FON-uh-fores). They come in three different body plans consisting of (1) a float and swimming bells, (2) a float but no swimming bells, or (3) swimming bells but no float (see "Siphonophore Life History" on here for more on these forms). Siphonophores are neither predominantly radial nor bilateral, although certain components may be one or the other. They are some of the most difficult of all organisms to identify because they are made up of parts that do not resemble one another or the animal as a whole.

The two other groups of jellyfish are the comb jellies, phylum Ctenophora (ten-OFF-uh-ruh), and the salps and their kin, also known as pelagic tunicates, of the phylum Chordata (kor-DAH-ta). The creatures of both groups are bilaterally symmetrical — like humans — meaning there is only one way to slice to get equal portions. But in other ways these two groups are quite distinct from each other. Salps may be thought of as a barrel encircled by muscle bands, whereas ctenophores come in a variety of shapes adorned with eight longitudinal rows of large cilia (the "combs" in the name comb jelly).

Despite their different body plans, symmetries, and shapes, and their other various peculiarities, the species we call jellyfish share numerous obvious features. They are pelagic, which means they live in the water column instead of on the seafloor, and they are planktonic, meaning that they drift and, with few exceptions, are unable to fight a current. Their bodies are gelatinous, or jellylike, which helps with buoyancy. And most of them are transparent, which is believed to be a defensive adaptation.

All the structures mentioned in this overview are elaborated on in the sections below. The different groups are treated in more detail in chapter two.


While we generally think of jellyfish as drifting organisms, in fact, benthic — or permanently bottom-dwelling — medusae and ctenophores exist, living entirely on the seafloor as adult, sexual forms. In the medusae, these benthic forms are trumpet-shaped creatures known as stauromedusae, which attach themselves to rocks or algae with adhesive glands at the base of a slender, stalk-shaped foot. In the ctenophores, the benthic forms are creeping flatworm-like creatures known as platyctenes.

Most of the stauromedusae and platyctenes (PLAT-ee-teens) are unable to swim, but have free-swimming larvae. Even though the adults look totally different and do not drift in the water column, they are still rightfully considered jellyfish because they evolved from jellyfish-like ancestors. While jellyfish in general tend to be poorly known, the stauromedusae and platyctenes are by far the least known of all.

Other benthic forms include the asexual (clonal) stage of the familiar drifting medusae. Hidden away as tiny polyps, they are stuck to rocks, shells, or algae on the seafloor, or the benthic zone. These polyp forms, which include the plantlike hydroids, are treated more thoroughly in our examination of jellyfish life history in chapter two.


Exquisitely beautiful creatures shaped like champagne flutes, stauromedusae usually have eight arms radiating out in a star pattern. Each arm ends with a tuft of short tentacles, and each tentacle in turn has a small ball on the end. These tentacles are packed with stinging cells and are used for food capture and defense. Between the arms, many species have special organs called anchors; the function of the anchors is unknown but possibly sensory. Like other medusae, stauromedusae are radially symmetrical. Most species appear externally to be octoradial (eight-parted), but internally they are tetraradial (four-parted), as are most of their more traditional drifting medusa counterparts. Stauromedusae come in a dazzling variety of colors and patterns that help camouflage them in the red or green algae among which they are often found living.

One of the most interesting things about stauromedusae is that they are essentially upside down of upside down — that is, whereas normal medusae have all the structural features of polyps but upside down, stauromedusae have flipped right back upward again. So while they appear to be "right side up," this orientation came about late in their evolution. Their ancestors are believed to be normal medusae, whose ancestors are believed to be normal polyps.


Platyctenes are another enigmatic form. Like stauromedusae, adults are entirely benthic. They are often encountered by scuba divers, home aquarium enthusiasts, and public aquarium visitors, but rarely recognized for what they are. Resembling flatworms, they are essentially an oval-shaped thin film of tissue that glides over sponges and algae and among the spines of sea urchins. The feature that gives away their true ctenophore nature is the tentacles, resembling those of the related sea gooseberries in bearing numerous lateral filaments arranged in one direction, similar to the barbs along one side of a feather.

When first hatched, the larval stage of a platyctene looks a lot like a miniature sea gooseberry, and it even drifts in the water column in a similar manner. As it matures, it grows out of its planktonic stage and takes up residence on the sea bottom. One of the biggest challenges a young platyctene encounters is finding a suitable host on which to grow.

Curiously, whereas stauromedusae are essentially medusae turned downside up, so that the mouth faces toward the water's surface, platyctenes are essentially sea gooseberries or sea walnuts flattened in the extreme and turned upside down, with the mouth facing down against the sediment or their host. The body is extremely soft and almost amoeba-like. The upper surface bears numerous ephemeral, intermittently prominent, ciliated papillae, which are believed to serve a respiratory function. Occasionally, two "chimneys" appear, one near each of the two long ends of the body, from which the long feathery tentacles emerge. As with the stauromedusae, platyctenes come in striking colors and patterns, which help them camouflage against their host species.


The creatures of the two main phyla that contain the jellyfish — Cnidaria and Ctenophora — capture food and defend themselves with the help of two very different types of cellular organelles, or tiny structures made by cells. In the cnidarians these are stinging cells called nematocysts, and in ctenophores these are sticky cells called colloblasts. Both are microscopic.

Nematocysts: Stinging Cells

Stinging cells are found in all cnidarians. In fact, the phylum name Cnidaria comes from the Greek word knide, meaning "nettle." This is the primary character that unites such disparate creatures as stony corals, soft corals, sea anemones, sea fans, sea pansies, hydras, medusan jellyfish, and siphonophores.

Stinging cells are wondrous little structures. Each is essentially a double-walled keratinized capsule, with a harpoon coiled up inside and a trapdoor and hair trigger at one end. Because of the hair trigger, the harpoons may discharge with even slight mechanical stimulation. Nematocysts discharge the harpoon with an explosive force of 40,000 Gs, or 40,000 times the force of gravity; their discharge is among the fastest biological processes. Discharge occurs by eversion, or turning inside out, similar to the action when one peels off a rubber glove.

The shaft of the harpoon is hollow, like a hypodermic needle, and often perforated. The venom is contained inside the capsule both on the inside and outside of the harpoon, so that as the harpoon penetrates the skin, it may deliver venom three ways: by hypodermic injection through the tip, along the shaft through the perforations, and by the residue on the outside of the shaft. Strong spines, particularly near the base of the shaft, help anchor the harpoon into prey as it penetrates. The remainder of the shaft may be unarmed or may have three rows of smaller spines spiraling along its length.

Colloblasts: Sticky Cells

The colloblast of the ctenophores is a similarly wondrous structure to the cnidarian nematocyst, though totally different in form and function. Whereas a nematocyst can be thought of as a harpoon full of poison, a colloblast is more like a rope covered in honey. Nematocysts inject; colloblasts ensnare. Colloblasts contain no venom; they are used for food capture but not defense.

The colloblast consists of a bouquet-shaped structure called a collosphere, which bears adhesive granules supported by an axial (central) thread wrapped by a spiral filament. When stimulated, the spiral thread straightens, activating the colloblast, and the granules burst, releasing their glue. Ctenophores with tentacles or lobes have large numbers of colloblasts on these structures; colloblasts are also found on fine tentacles around the lips in some species. One species, Haeckelia rubra, lacks colloblasts but co-opts nematocysts from its cnidarian jellyfish prey for its own defense.

Jellyfish Stings

Because of their venomous nature, nematocysts have been quite well-studied, whereas colloblasts have not. Generally, different types of nematocysts are found in different areas of the jellyfish, such as the bell, tentacles, lips, and stomach. Many dozens of different types of nematocysts have been identified. The structures are typically spherical, ovoid, lemon-shaped, or banana-shaped. The number and the forms of the types present help in species identification in many cnidarian groups. Often they are the only means for distinguishing one species from another, especially following stings, after which fragments of tentacles or nematocysts left in or on the skin may be the only objective evidence available.

Nematocysts can be retrieved from the skin following a sting by a very simple method: adhesive tape is laid sticky-side down on the dry sting area, then pressed to the skin and peeled up. The tape is then put on a microscope slide and examined. This provides a safe, effective, and noninvasive means of identifying the culprit jellyfish.

Treatment of jellyfish stings is largely species specific. For the life-threatening stings of species of box jellies and Irukandjis (see here, here, and here), dousing the wound with vinegar will instantly and permanently disable the stinging cells that have not yet discharged, preventing them from injecting any more venom. For other stings, it is just a matter of relieving the pain, which may be accomplished with ice or heat.


The majority of jellyfish are essentially passive drifters, at the mercy of the currents. Therefore, even though they may be large, they are nonetheless classified as plankton. Despite their planktonic nature, most possess structures that allow them to change direction, swim up or down, and in some cases, move against weak currents.

Medusa, Siphonophore, and Salp Locomotion

The most familiar type of jellyfish locomotion is the pulsation swimming of the medusae. The lesser-known siphonophores and salps employ this mode too. Medusae have well-developed muscles that allow them to contract the bell. Medusae use only half as much energy as any other organism would for the same propulsion, which makes their swimming action one of the most energetically efficient forms of locomotion known. During the contraction, which is the power stroke, the water cupped under the bell is rapidly forced out through an opening narrowed by a thin shelf of tissue called a velum or velarium, creating a thrust of jet propulsion. The counterstroke, when the bell expands back out again, is accomplished through elasticity and "memory" of the bell and therefore requires no energy expenditure. The cubozoans, or box jellyfish and their kin, have well-developed swimming ability. Some of the larger coastal species, such as Chironex fleckeri (see here), can swim against powerful currents and have been clocked swimming at up to four knots (about five miles per hour). The swimming speed of open-ocean species of the genus Alatina has not been measured, but these box jellies have been observed to be very fast and are likely to outpace Chironex.

Many types of siphonophores have special structures called swimming bells, or nectophores, used for locomotion. These are basically highly modified medusae that stay stuck to the colony rather than dispersing away and becoming free-floating. Nectophores contract and expand just as normal medusae do, except all the nectophores of the colony must pulsate in a coordinated manner in order to accomplish forward motion.

Salps and their kin also swim by means of a pulsing motion, in this case rhythmic contractions of their barrel-like bodies. Each species of salp has characteristic numbers and patterns of muscle bands encircling the body, like the metal hoops girding a wine barrel. The stimulation of these muscle bands produces the whole-body pulsations that power the animal through the water. Like siphonophores, salps in their aggregate stage need to coordinate their pulsations in order to accomplish forward motion.

Ctenophore Locomotion

Only one family of ctenophores is able to pulsate. When touched, species in the family Ocyropsidae (see here) are able to flap the body violently in a handclapping manner, or like a disturbed scallop, and are thus able to rapidly get away from a bothersome stimulus. This wild flapping motion probably also frightens away would-be predators in many cases.

Other species of ctenophores — at least the drifting species — move by means of a sort of tractor motion that involves rhythmic beating of ciliary plates. Recall that ctenophores have eight comb rows or tracks of cilia. These rows are actually composed of dozens to hundreds of clusters of cilia adhered together in small paddles or plates called ctenes (pronounced teens). These plates are located more or less along the length of each of the eight rows. A tiny primitive version of a centralized nervous system coordinates the ctenes so that waves of motion pass down the rows together, accomplishing forward movement with very little vibration. Ctenophores can even go backward, simply by reversing the beating of the ctenes.

Movement in Benthic Forms

Even the benthic forms of jellies have strategies and structures that help them solve the problem of getting around. Platyctenes lack the comb rows and paddles found in drifting ctenophores, but they are by no means stuck in one place, thanks to their ciliated undersurface. While they normally creep fairly slowly, they can glide at a pretty good clip if necessary. Similarly, although the stauromedusae spend most of their time affixed to rocks and algae, when the need arises, they are perfectly able to move. They simply bend over to one side, discharge some nematocysts to anchor an arm to the ground, release their sticky foot, and somersault away.


Excerpted from Jellyfish by Lisa-Ann Gershwin, David Price-Goodfellow, Amy K. Hughes. Copyright © 2016 Lisa-ann Gershwin. Excerpted by permission of The University of Chicago Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Table of Contents


Introducing the Jellyfish,
Introducing the Ocean,
About This Book,
Further Resources,
Picture Credits,

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