Metazoa: important evolutionary changes associated with the transition to multicellularity. Phylum Cnidaria (Coelenterata). Genus Hydra. Genus Obelia

At a glance

1. Being many-celled lets an organism grow beyond the limits of diffusion and share out work among specialised cells. Animals took this step once, from ancestors they share with the choanoflagellates, and have 100 to 150 kinds of cell.
2. Every animal has cells held in a matrix of collagen instead of cell walls, and grows from a hollow ball of cells, the blastula. In gastrulation it folds in to make the germ layers: ectoderm and endoderm, and in most animals a mesoderm between them.
3. Cnidarians are diploblastic: an outer epidermis and an inner gastrodermis, with the jelly-like mesoglea between them. They are radially symmetrical, with a gut cavity that has one opening, a nerve net, and stinging cells, the cnidocytes.
4. They live as attached polyps or swimming medusae. Hydra is a freshwater polyp that buds, regenerates, and reproduces sexually when conditions turn harsh. Obelia is a marine colony whose polyps bud off medusae, which reproduce sexually: metagenesis.
5. Jellyfish stings are a common summer injury on Mediterranean beaches. First aid is lifting off the tentacles, rinsing, usually with seawater and never with fresh water, and heat for the pain.

01From one cell to many

The Metazoa are the animals: organisms of many cells. Multicellularity has evolved at least 25 times among eukaryotes, in plants, algae and fungi among others, but only once in animals. Their closest living relatives are the choanoflagellates, single-celled or colonial flagellates, each cell with a funnel-shaped collar of microvilli round its flagellum. The simplest animals, the sponges (Porifera), have specialised cells but no true tissues, and their collar cells, the choanocytes, look much like choanoflagellates. The animals with true tissues, from the cnidarians on, are the Eumetazoa.

How the step was taken is not known. The colonial theory, Haeckel's, holds that a colony of flagellated cells, like those of choanoflagellates, came to share out its work, its cells specialising; the syncytial theory, that a cell with many nuclei divided into separate cells.

What the step to many cells brought:

• Size. A single cell that grows has less surface for its volume and cannot take in enough food or oxygen by diffusion. A body of many cells can grow far larger, though it then needs ways to bring food and oxygen to cells far from the surface.
• A division of labour. Cells specialise for one task each: animals have 100 to 150 kinds of cell, against 10 to 20 in plants and fungi.
• Cells that stick together and speak to each other. Adhesion molecules, junctions and signals hold the cells in place and tell each one what to become and when to divide.
• A germ line. When a many-celled organism reproduces sexually, its body must be rebuilt from a single cell, the zygote, made by two germ cells, the sperm and the egg. In some animals, among them vertebrates and arthropods, the germ cells are set apart early from the body's somatic cells, which die with it; in Hydra the same stem cells make both gametes and body cells.
• The risk of cancer, when a cell escapes the body's control of its growth. Many of the genes that arose with the first animals, genes for cell differentiation, adhesion and communication between cells, are deregulated in cancer cells.

02What makes an animal

Animals eat other living things or their remains, breathe oxygen and, for at least part of their lives, move. Their cells have no cell walls; they sit in an extracellular matrix of collagen and elastic proteins, which lets cells move about in the embryo and be rearranged into tissues and organs.

An animal made by sexual reproduction begins as a zygote that divides into a hollow ball of cells, the blastula. In most animals the blastula then folds in on itself, in gastrulation, to form a gastrula with a gut chamber and two germ layers: the ectoderm outside and the endoderm inside. Most animals then add a third, the mesoderm, between them. Animals with two germ layers are diploblastic, those with three triploblastic.

Two more features divide the animals:

• Symmetry. A radially symmetrical animal, such as Hydra or a jellyfish, has its parts arranged round one axis, from the mouth to the opposite end, and meets the world equally from every side. Most animals are bilaterally symmetrical, with a left and a right side, a front and a back end, and nerve cells and sense organs gathered at the front: cephalisation.
• The body cavity. Between the gut and the body wall a triploblastic animal may have solid tissue, a cavity lined with mesoderm on one side only, or a true coelom lined with mesoderm all round.

Arthropods and molluscs are coelomates too, but in them the coelom is small and the main body cavity is a haemocoel, filled with blood. These are grades of body plan, not a family tree: the coelom was lost again in several lines of animals. In a person the coelom of the embryo becomes the pericardial, pleural and peritoneal cavities.

03Phylum Cnidaria

The Cnidaria are over 11,000 species of jellyfish, hydroids, sea anemones and corals, almost all of them in the sea; Hydra is one of the few that live in fresh water. Their name comes from the Greek for nettle. They were once grouped with the comb jellies as the Coelenterata, "hollow gut", a name still met in older books, until the two groups were found to differ and became separate phyla.

What cnidarians share:

• Radial symmetry, round the axis from the mouth to the opposite end.
• Two cell layers: the epidermis, from the ectoderm, and the gastrodermis, from the endoderm, each mostly one cell thick. Between them lies the mesoglea, a jelly with few cells or none, thin in a polyp and thick in a jellyfish's bell. Cnidarians have tissues, such as their nerve net and their muscle, but few organs.
• A gut cavity, the gastrovascular cavity (coelenteron), with a single opening that is both mouth and anus. Enzymes from the gastrodermis begin digestion in the cavity, and its cells take up the fragments and finish digestion inside them.
• No organs for breathing, circulation or excretion. Oxygen comes in, and ammonia goes out, across the body surface.
• A nerve net spread through the body, without a brain, and simple sense organs: statocysts for balance and, in some, simple eyes.
• Stinging cells, the cnidocytes, found in no other animals.
• Two body forms, the polyp and the medusa, and a ciliated larva, the planula. Many reproduce both without sex, by budding, and with it, and many regenerate lost parts.

The stinging cells

Each cnidocyte holds a capsule, the cnida or nematocyst, with a long thread coiled inside, often barbed and armed with venom. On its outer side the cell has a hair-like trigger, the cnidocil. When prey touches it, in the usual explanation, calcium leaves the capsule, water floods in, and the thread shoots out, turning inside out, in a few microseconds, one of the fastest movements in nature. It pierces or wraps the prey and injects the venom. Each cnidocyte fires once and is then replaced. On the tentacles of Hydra they sit in clusters, the batteries.

Polyp and medusa

In many cnidarians the two forms alternate in one life cycle, but some groups have lost one of them, and Hydra has only the polyp.

The classes

• Hydrozoa: hydroids, Hydra and Obelia, and colonies that float, such as the Portuguese man o' war, Physalia. Their polyps are mostly small and colonial and their medusae small, with a shelf of tissue under the bell, the velum. Most free-living freshwater cnidarians are hydrozoans, such as Hydra and the small freshwater jellyfish Craspedacusta.
• Scyphozoa: the true jellyfish, whose large medusa is the form usually seen, such as Pelagia and Aurelia.
• Cubozoa: the box jellyfish, among them Chironex fleckeri of northern Australia, one of the most venomous animals known.
• Staurozoa: the stalked jellyfish.
• Anthozoa: sea anemones and corals, which have no medusa; their polyps reproduce sexually.
• Myxozoa: microscopic parasites, long taken for protozoa, now known to be highly reduced cnidarians.

04Genus Hydra

Hydra is a small polyp of ponds and slow rivers, usually about a centimetre long when stretched out, which lives alone and never forms a medusa. The figure shows one cut open along its length.

• Its basal disc holds it to a plant or a stone with a sticky secretion. It can let go and move, looping or turning somersaults, or float away.
• At the top, the mouth opens on a dome, the hypostome, ringed by four to eight hollow tentacles armed with cnidocytes, which catch small animals such as water fleas and push them into the mouth.
• The body wall is the cnidarian plan at its simplest: epidermis, mesoglea and gastrodermis round the gut cavity, which runs on into the tentacles. Its cells include epitheliomuscular cells, which contract to shorten and bend the body, gland cells, sensory and nerve cells of the net, cnidocytes, and small interstitial cells, stem cells that make the cnidocytes and the gametes, among others.
• A Hydra has 50,000 to 100,000 cells, made from three populations of stem cells that keep renewing the body. In the laboratory it does not appear to age, though this is still debated.

It reproduces in two ways:

• Budding, when food is plentiful. A fold of the body wall, with a pocket of the gut cavity inside, grows out, forms a mouth and tentacles, and breaks away as a small new Hydra; a well-fed animal can start a bud every two days.
• Sexually, when conditions turn harsh, often before winter. Testes (one to eight, conical) swell from the body wall high on the column, below the tentacles, and ovaries (one or two) lower down. In many species the sexes are separate, though some, such as Hydra viridissima, carry both at once, as the figure draws them. Sperm swim to the egg in the ovary of another animal; the fertilised egg grows a tough coat and rests on the bottom through the bad season after the parent dies, then hatches into a small Hydra.

It also regenerates: cut in two, each half grows back what it has lost, the head end a new foot and the foot end a new head. This is how it got its name, after the many-headed monster of Greek myth.

05Genus Obelia

Obelia is a marine hydrozoan that grows as a small, branched colony on seaweed, rocks and piers, and shows the full cnidarian life cycle.

• A creeping stolon anchors the colony, and upright stems branch from it. Stems, stolon and polyps are enclosed in the perisarc, a clear, chitinous case secreted by the epidermis. The living tissue inside, the coenosarc, with the gut cavity, runs on through the whole colony, so food caught by one polyp feeds them all.
• The polyps (zooids) are of two kinds, and with the medusa make three forms in one species, which is called polymorphism. The feeding polyps, or gastrozooids, each a hydranth sitting in a cup of perisarc, the hydrotheca, have a ring of solid tentacles round a mouth on a raised hypostome, and feed the whole colony. The reproductive polyps, or gonozooids, have no mouth and no tentacles: inside a case of perisarc, the gonotheca, a central stalk, the blastostyle, buds off tiny medusae, which swim out of its open top.
• The medusa is a transparent saucer a few millimetres across. Its parts come in fours: a manubrium hangs from the middle with the mouth at its tip, four radial canals run from the stomach to a ring canal round the margin, and a gonad sits on each radial canal. Twenty to thirty tentacles fringe the margin, and eight statocysts are spaced evenly round it, each at the base of a tentacle.

Each medusa is male or female, and releases sperm or eggs into the sea. The zygote becomes a blastula, then a ciliated planula larva, which swims for a while, settles and grows into a new colony. A generation of polyps that reproduce without sex alternating with a generation of medusae that reproduce sexually is called metagenesis, an alternation of generations. Unlike the alternation of generations in plants, both generations are diploid; only the eggs and sperm are haploid.

06Jellyfish stings

Swimmers, surfers and divers meet cnidarians, and not all of them can pierce human skin, but many jellyfish can, small ones among them. How bad a sting is depends on the species and the dose of venom: on how much skin the tentacles touched and for how long, and on the person's age and health. The most dangerous are the box jellyfish of the Indo-Pacific, Chironex fleckeri and Carukia barnesi, and the Portuguese man o' war, Physalia. In the Mediterranean the most venomous native species is the mauve stinger, Pelagia noctiluca, a small pinkish-purple jellyfish whose swarms are carried inshore by wind and current in summer. Unusually, its bell stings as well as its tentacles, and even dead ones stranded on the beach can sting. In the eastern Mediterranean the nomad jellyfish, Rhopilema nomadica, which came in through the Suez Canal, also forms large swarms.

A sting causes burning pain, redness and raised, often lined weals where the tentacles lay, then itching. The pain and redness usually settle within hours to days, though after a mauve stinger they can last a week or two, and the rash can come back weeks later or appear away from the sting, and can leave dark marks or scars. Muscle pain, difficulty breathing, low blood pressure and, rarely, an allergic reaction can follow a heavy sting.

First aid:

• Leave the water. Lift off any tentacles with tweezers or a gloved hand, since detached tentacles can still sting, including the rescuer.
• Rinse with seawater, the rinse most guidelines advise. Fresh or distilled water and urine are avoided, since they may make the remaining cnidocytes fire.
• For the pain, hot water as hot as can be comfortably borne, about 45 °C, for 20 minutes, by immersion or shower, or a hot or cold pack; lidocaine helps too.
• Vinegar is the standard rinse for box jellyfish. For other species the studies disagree: for the mauve stinger some found that vinegar, and others that seawater, makes the stinging cells fire, so local advice is followed.
• Difficulty breathing, collapse or a widespread reaction needs emergency help.

The evidence behind these steps comes from few and small trials, and seawater and heat are recommended mostly because they are safe and close at hand.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1What lies between the epidermis and the gastrodermis in the body wall of Hydra?

Mesoderm, a third germ layerMesoglea, a jelly with few or no cellsA coelom lined with peritoneumThe perisarc

2What makes a cnidocyte fire its thread?

An order sent from the brainTouch or chemicals on its trigger, the cnidocil, after which water rushes into the capsule and the thread turns inside outContraction of the muscles of the tentacle squeezing the capsuleFresh water only

3Which form of Obelia reproduces sexually?

The feeding polyp (gastrozooid)The reproductive polyp (gonozooid)The medusaThe planula larva

4When does Hydra usually reproduce sexually?

When food is plentifulWhen conditions turn harsh, often before winterOnly after it has been cut in twoNever, it only buds

5A swimmer off Sardinia is stung on the arm by a small mauve jellyfish, and pieces of tentacle are still stuck to the skin. What should be done first?

Rinse the arm under fresh tap waterLift off the tentacles with tweezers or a gloved hand and rinse with seawaterPour urine on the stingRub the sting with alcohol