With the class Hydrozoa covered, let’s check out the next two classes in the phylum Cnidaria,
starting with Scyphozoa. The scyphozoans, or “true jellyfish,” are generally characterized
by their dominant medusa stage. Though there are only about 200 recognized species of scyphozoans,
a great deal less than the 3,700 or so species of hydra,
they are perhaps some of the most widely recognized Cnidarians, second only to the corals.
Though most scyphozoans are small, about 2 to 40 centimeters in diameter,
some species, such as those in the genus Cyanea, like the Lion’s Mane jellyfish, can attain
a bell diameter of over 2 meters with tentacles over 70 meters long. Like the Lion’s Mane,
most scyphozoans are found floating in the open sea. But some, like the rarely seen Deepstaria
jellies, have been found living at depths of over 1,750 meters, and others, like the
bioluminescent Atolla Jellyfish that is found all over the globe, inhabit the so-called “midnight
zone,” at depths of 1,000 to 4,000 meters. The “jelly” that is characteristic of the
scyphozoans is really the mesoglea layer that is sandwiched between the epidermis and gastrodermis.
Though the bell of many scyphozoans may feel firm to the touch, this is due entirely to
the pressure of their hydrostatic skeleton. The “jelly” or mesoglea is really about 95% water,
and unlike the hydrozoa, this layer in scyphozoans also contains amoeboid cells and fibers.
Superficially, the medusae of the hydrozoans resemble that of the scyphozoans, but the
“true jellyfish” are often larger, with many more tentacles, and oral arms equipped with stinging
nematocysts. True jellyfish also have four gastric pouches that extend out of the stomach
by way of gastric filaments that are lined with additional stinging nematocysts. True jellyfish
also have a complex system of radial canals that branch out from the pouches to the ring canals.
Some jellyfish, like the Aurelias, or moon jellies, a common visitor of the East and West
coasts of the United States and beaches around the world, have a shallow saucer shape bell
with comparatively short tentacles that line the ridge of the umbrella margin. They feed on small
planktonic animals, like larval crustaceans and rotifers, which they capture in the mucus of their
bells and store in their “food pockets” before passing them into the gastrovascular cavity.
Others, like the increasingly common ghost jellyfish, have long tentacles covered in
nematocysts that aid in capturing small vertebrates and even other jellyfish.
Still others, like the Cassiopea, or “upside down jellyfish” lack tentacles,
have highly modified oral arms, and no mouth. They instead have tiny pores on their frilly oral arms
that serve as multiple “mouths” for capturing planktonic organisms caught in their mucus.
Many “upside down jellyfish” also house tiny symbiotic dinoflagellates, known as zooxanthellae,
within their tissues and spend their lives upside down with their oral arms outstretched in sunny
areas to allow their symbionts to photosynthesize. True jellyfish are dioecious, meaning the male and
female sexes are separate. Both have gonads located within their gastric pouches. Males
release sperm in spawning events and internally fertilize the eggs within the body of the females.
In some species these fertilized eggs, or zygotes, are brooded within the folds of the oral arms. The
zygote then develops into a tiny ciliated larva called a planula. Once a young planula settles
into the substrate it develops into a scyphistoma, which feeds and further develops into a strobila.
Finally, through the process of strobilation, a form of asexual reproduction unique to the
Cnidarians and some species of parasitic worms, young ephyrae are produced and swim away. The
young ephyrae then develop into medusae and sexually mature. A single strobila can asexually
produce many ephyrae in its lifetime and a single medusa can sexually reproduce multiple times.
Jellyfish have a high rate of fecundity, or the ability to produce an abundance of offspring,
given that a single female sea nettle can produce up to 45,000 eggs per day. Jellyfish also exhibit
high tolerance of ocean acidification and global warming, so quite interestingly,
some species of jellyfish have greatly benefited from human-induced climate change and human-caused
depletion of fish species worldwide. Some species, like the blue jellyfish and moon jellies,
have become so common that they can clog the cooling systems of nuclear powerplants,
wipe out entire fisheries, sink fishing boats, interrupt military operations,
and even disable aircraft carriers. In addition, since most jellyfish aren’t eaten by humans,
they are essentially protected due to their inedibility, while “tastier” species of fish and
crustaceans are wiped out. However, some species, like flame and cannonball jellyfish are captured
by commercial fishing vessels for consumption. That’s just about it for the “true jellyfish,” but
before we finish, let’s briefly cover the “stalked jellyfish” that belong to the class Staurozoa.
These animals are unique in that they share many characteristics with the scyphozoans,
but they lack an alternating polyp and medusa life cycle. They are instead “attached medusas”
that resemble individual hydras, or scyphistoma, though they do not asexually produce medusas.
Instead, an individual zygote develops into a planula that produces a single sessile adult.
These adults attach themselves by a tubular stalk to seaweeds and other objects on the sea floor,
from which their tentacles project upwards. Most are small and flower-like in appearance with eight
or more arms arranged in pairs. The arms are covered in hollow nematocyst tipped tentacles
used to capture prey. Unlike most Scyphozoans, which prefer the open ocean and warmer waters,
stalked jellyfish are more common in cold shallow waters. Some, like Haliclystus antarcticus,
have even been found off the coast of Antarctica. Others, like Haliclystus sanjuanensis, are found
in the Pacific Ocean off the West Coast of North America, and have a wide range of color morphs.
And that wraps up the “true jellies” of class Scyphozoa as well as the “stalk jellies” of class
Staurozoa. Let’s move forward and cover the venomous Cubozoa next.