da " SCANORAMA " - Maggio 1985

NB. - Si tratta di un servizio foto-giornalistico redatto dall'amico Michael Sedge per la rivista in lingua inglese "Scanorama" e
corredato da una serie di foto di mia produzione. Ripubblico l'intero servizio, completo di un interessante fuoritesto, che bene illustra
le caratteristiche di quei particolari organismi che sono le Meduse.
 

JELLYFISH

THE SOFT STING

 

It glides eerily toward you through the silent water. You won't even
 feel the sting of this rather benign
Aequora. But avoid the embrace
of its relations, with tentacles that welcome you to pain. In social
get-togethers or slipping singly through the water,
jellyfish are feared as dangerous beauties of the deep.

by MICHAEL H. SEDGE (*), photos by GUIDO PICCHETTI


Just a passing fancy: Rhizostoma meets a diver.

m


Compass jellyfish,
Chrysaora hyoscella, finding their way. You'll find them in the Atlantic, the English Channel and the North Sea.

Vegetation sways with eerie abandon in the currents twelve meters below the surface. Through the crystal-clear waters appears a creature that hangs stationary, as if floating in space. The bluish white, almost transparent parachute-shape pulsates, enabling it to move. "Fringes" hang from it. Most swimmers would call it a jellyfish. Zoologists would refer to it as "moon jelly." Marine biologists might categorize it as an Aurelia aurita.

Distributed among all the world's oceans, the Aurelia, or moon jelly, is one of a vast number of jellyfish. Off the Bermudan coast the minute By-the-Wind-Sailor, Velella, is often seen riding the waves. In Greece, the colorful Pelagia noctiluca appears by the thousands every summer. In the Mediterranean, the Cotylorhiza tuberculata looks much like a fried egg, sunny-side-up. Off the shores of Greenland float the deep-living Atolla jellyfish. Near Rio de Janeiro, the Chrysaora hyoscella, better known  as compass jellfish,  is  frequently  found  in fishermen's

nets. In Cuba, the remains of once-beautiful sky-blue Rhizostoma jellyfish are found on the beaches, victims of strong winds. Chrysaora hyoscella and Rhizostoma are common in the North Atlantic and North Sea.

The monstrous appearance of the Cyanea (the most renowned of the stinging jellyfish found in Scandinavian waters) often fright-ens swimmers on the east coast of the United States, while the deadly tentacles of the Physalia, or Portugese-man-of-war, cause panic among vacationers along the coast of Mexico, in the North Atlantic and the Mediterranean. In the North Sea, tales are told of the giant sea "gooseberry", Pleurobrachia jellyfish. The "sea wasp", Chironex fleckeri, with its fine poisonous tentacles, causes several deaths each year off the coast of Australia.

Jellyfish are one of the simplest forms of marine life, consisting of 96% water, a large sac made up of only two cell layers, and a single opening that serves as both mouth and anus. Jellyfish are coelenterates, species that also include the flowerlike sea anemones, rock-like corals and the siphonophores. Diverse though they may seem, these animals all possess a large internal cavity, or coelenteron. (The phylum is further divided into three classes: Scyphozoa are predominantly free-swimming animals such as jellyfish. Hydrozoa takes in the siphonophores. Anthozoa consists largely of sedentary creatures such as anemones, corals and sea pens, but no jellyfish.)

m

Scyphozoa, true jellyfish, are what we find pulsating in the sea off many beaches. They often travel in schools, stinging every-thing they touch. Scientists have documented cases in which groups of pale white Stomolophus meleagris have driven off schools of sharks and porpoises with their stinging attack and defense systems. On each of the jellyfish's retractable tentacles are rows and rows of barbed, poisoned hypodermic syringes called nematocysts. A nematocyst is like a coiled whip ready to lash out.  When a potential source of food, normally small planktonic crustaceans or perhaps fish larvae or small fish, touches a tentacle's hair, the whip shoots out like a harpoon. Its force is so great that it punctures the victim's skin, injecting paralyzing poison. The tentacle then retracts, pulling the catch into the mouth, located in the center of the underside of the jelly-fish. As a defense system against large animals or unsuspecting swimmers, the poison barbs of the nematocyst break off as the victim escapes, leaving the wounded area sore for some time after. If the victim is weak and the wound serious, death may occur. Despite the stinging potency of many jellyfish, there are certain fish that live among their tentacles, unaffected by the poison. The delicate Nomeus fish live among the trailing tentacles of the Portuguese man-of-war, Physalia.

Pulsate to keep from sinking
carrying life-support materia! with it.

Jellyfish have an efficient system for moving food,   oxygen and


The sting of Rhizostoma is negligible, experts say. They are found in the Atlantic, the North Sea and the Mediterranean.

m

waste products around their bodies through a series of canals and pouches that link the central gatric cavity with a narrow circular channel around the edge of the bell-shaped body. Within these channels water circulates, Jellyfish swim by alternately opening and closing their bell-shaped form, creating a jet-propulsion system that thrusts them through the water. Because they are denser than water, jellyfish must constantly pulsate to keep from sinking. As their normal habitat is near the surface of the water, they must also be able to distinguish up from down. To do this, their seemingly simple body contains light-sensitive zones that help locate the surface. In addition, jellyfish have a series of balancing organs, statocysts, that permit the animal to remain in an upright position.

Among Scyphozoa jellyfish, the sex organs are found only in the free-swimming medusoid (bell-shaped) stage, in which the animal passes most of its life. Adult medusoid forms have separate sexes and frequently travel in like-sex groups. When a group meets the opposite sex, the reproduction cycle begins; sperm emerge from the mouth of the male and enter the cavity of the female. Fertilized eggs then grow into ciliated larvae that settle on the sea bed or rocks. These larvae develop into polyps. As seasons pass and the water reaches the required temperature, the polyps develop eight arms. Eventually the polyps mature and break free like a pile of plates from the top, to become free-swimming adult medusae.

Hydrozoa jellyfish are distinguished by their colony form. A mature, plant-like colony bears polyps that, in turn, form free-swimming medusae. These carry male or female reproductive organs and, when separated from the parent colony, shed their sperm or eggs into the water. Fertilized eggs subdivide, forming

ciliated larvae that swim until they locate a suitable surface on which to settle and grow. As they develop, the larvae will bud free-swimming medusae asexually until becoming a mature colony.

A well known example of specialized Hydrozoa is the beautiful Physalia or Portuguese man-of-war, an inhabitant of tropical water the world over and sometimes found in temperate seas. The tentacles of this colony of Hydrozoa, consisting of many organisms that appear to be one, may grow to 18 meters. Physalia rides on the surface, using its gas-filled body as a sail.

Another Hydrozoa common to mid-Atlantic waters is the Velella or By-the-Wind-Sailor. This two-to-five-centimeter diameter surface drifter looks like a tiny sailing craft as it is carried along by the wind. Millions of Velella frequently are seen floating in the open Atlantic or washed up on Bermudan beaches after a gale. A marine creature that is not even a coelenterate but often is considered a jellyfish is Pleurobrachia, more commonly known as the sea gooseberry. These ball-like animals swim by coordinated beating of rows of cilia (hair-like out-growths) that stretch along their body from top to bottom. They are Ctenophora, or comb jellies. Not all Ctenophora have tentacles. And  those  that do are not equipped with stingers  but adhesive

 


Sunny-side up for Cotylorhiza tuberculata, common in midsummer in the Mediterranean.

cells that capture tiny animals as the tentacles sweep through the water.No matter what biological classification appplies, jellyfish are passive creatures of beauty. Their unique defenses have won them a high-ranking position on man's list of dangerous marine animals, yet it is man's lack of common sense and caution that creates the danger.
 

 

Some Jellyfish Don't Like Salt
 

Jellyfish in freshwater are rare; called Craspedacusta, they look like salt-water medusae, only smaller. The umbrella or parachute-like part of Craspedacusta may range in color from blue-gray to straw-yellow. Its diameter is 6 to 18 millimeters.  The  lower  section   of   the jelly-like  body  contains 150 to 520 minu-

 
Cotylorhiza cautiously meets
Homo sapiens.

m

tee tentacles, arranged in unequal lengths around the umbrella in three or more sets. These dangle downward to sting and entrap microscopic life that sustains the Craspedacusta. The sex organs appear as large sacks hanging within the umbrella.

Like its saltwater cousins, the freshwater jellyfish is a coelenterate. Unlike many of thŔ species lýving in saltwater, however, Craspedacusta is classed as Hydrozoa rather than Scyphozoa. This lake creature has two life forms, a jellyfish-like state and a polyp stage. Its life-cycle begins as a polyp, the body a cylindrical stalk without tentacles. At this stage, the stinging cells used for capturing food are located around its mouth. The polyp stage of a Craspedacusta is commonly called microhydra. This microhydra has three methods of reproduction: budding, breaking off or dropping a group of cells. In budding, the polyp forms a bulge on its side that continues to grow until it breaks free as a new individual. In breaking  off,  the microhydra break  off tiny   pieces  that  form    free-swim-ming   larvae   that  intime  settle  to the

m

bottom to become microhydrae. In the third reproduction method, dropping cells, the mature polyp breaks off a group of cells that have formed a "cup" at the top of the parent. This cell group then floats through the water as a jellyfish.

All reproduction by the Craspedacusta polyp is asexual, as the animai has no sex. The medusa stage, however, not only maintains separate sexes but also travels in like-sex groups. When oppo-site sex groups meet, they give birth to free-swimming larvae that settle to the bottom to become microhydrae.

There have been 143 recorded sight-ings of the tiny Craspedacusta in various parts of the world. In Europe there are 55 lakes containing the marine creatures. In 1880, they even appeared in the fountains of the Regents Park Gardens in London. Minute Craspedacusta have been found in freshwater aquariums.

(*) Michael Sedge is a free-lance jour-    nalist living in southern Italy.


   torna alla "Homepage"

all'altro articolo della serie  

torna a " fotogiornalismo "

copyright Guido Picchetti - 17/4/2009