With three classes of Cnidaria covered, let’s move on to the fourth, Cubozoa. The cubozoans,
or “box jellyfish” were once categorized as scyphozoans, or “true jelly-fish,” but their
numerous morphological distinctions prompted their recategorization as a separate class of Cnidaria.
Unlike the hydrozoans we learned about and anthozoans we will investigate in a moment,
but like the scyphozoans and staurozoans we just discussed, the medusa stage of the box
jellyfish is dominant. If you were to take a transverse cross-section of a box jellyfish
it would appear to be almost square, hench the name “box jellyfish.” They generally have four
stalks called pedalia, where each pedalium hangs at one of the lower corners of the box.
Each of these four stalks then further diverges into one or more long, slender, hollow tentacles
that house the stinging nematocysts. The rim of the bell is folded inwards to form a shelf known
as a velarium, which increases swimming efficiency by pulling the bell into the characteristic box
shape. This creates a powerful jet when the bell pulsates and allows box jellyfish to move more
rapidly than the true jellyfish and hydrozoans. Underneath the bell is the manubrium that
terminates in the mouth and opens into the gastrovascular cavity. This cavity
is divided by septa into a central stomach and four gastric pouches that also house the gonads.
Along with their general shape, another aspect of box jellyfish that separates them from
true jellyfish is their more developed nervous system. The box jellyfish, like all Cnidarians,
have no brains, but instead possess a “nerve ring” around the base of the bell that coordinates their
pulsing movements. In addition, box jellyfish have fully formed human-like eyes complete
with retinas, corneas, and lenses, making box jellyfish the only animals outside of bilateria
to have such sophisticated sensory structures. These eyes are set in pocketed clusters called
rhopalia, about halfway up the outer, flat surfaces of the bell. Each rhopalium contains
two lensed eyes, one pointing upwards while the other points downwards and inwards.
Along the sides of the lensed eyes are two pairs of simple pigment-cup ocelli: one pair of slit
eyes, and one pair of pit eyes next to the upper lensed eyes. This means that box jellyfish have 4
different eye types and a total of 24 individual eyes, since they have four rhopalia and each
rhopalium contains six eyes. In addition to eyes, cubozoan rhopalia also have statoliths, or balance
stones, attached to their complex eyes that help the animals orient themselves in the water column.
What is fascinating is that box jellyfish possess no brain to process the visual information they
obtain. Instead, their complex, visually guided movements, and well-documented decision-making
capabilities, are controlled by the four rhopalia. It is still debated whether the eyes form images
in ways not yet fully understood or whether the animals simply see blurry images and shadows.
However, the eyes of box jellyfish are just one aspect of these animals that is still the subject
of ongoing research. The life cycle of the box jellyfish is still unknown for most species.
In general, we know that a fertilized zygote becomes a planula-larva that metamorphosizes
into a creeping polyp. The creeping polyp eventually attaches to its substrate and becomes
sessile. In some species, this polyp has been documented to reproduce asexually through
budding to form new creeping polyps, somewhat like the freshwater hydra. These stationary
polyps can metamorphosize into individual medusa buds that go through a series of transitions
before becoming sexually mature adults. Though box jellyfish have a range of unique
features, they are perhaps best known for their painful sting, which in some species can be
deadly. Although they have garnered a reputation as some of the most venomous animals in the world,
most pose no serious threat to humans. The most infamous box jellyfish is probably
Chironex fleckeri, also known as the “sea wasp.” Though it has a reputation for being a killer,
most stings result in only mild envenomation. However, since a single tentacle may contain
up to 500,000 nematocysts, they do have the potential to cause excruciating pain.
Other notable box jellyfish include the 1 centimeter-sized Irukandji jellyfish,
that can cause cardiac arrest, as well as the highly venomous viper jellyfish.
Like the sea wasp, stings from these species and a few others can also be deadly, but if the wound
is dosed in vinegar the undischarged nematocysts are disabled and the victim can be further treated
in a hospital. For example, among 225 reported envenomations from 1991 to 2004 in Australia,
only 58 reported severe pain while 167 reported moderate to mild irritation.
Out of the 58, 18 required hospitalization, and only one resulted in a fatality.
Like many dangerous animals, the box jellyfish of our imagination are far deadlier than the
box jellyfish in reality. And with that we wrap up our investigation of the box jellies.
Next let’s examine the corals, anemones, and sea pens of class Anthozoa.