Jellyfish make up one of the oldest groups of creatures that roam the world’s oceans, fossil records showing certain specimens very similar to modern ones existing 500-700 million years ago. Jellyfish are extremely simple animals, consisting of an umbrella-shaped bell and trailing tentacles that capture prey. They swim through the water by pulsating the gelatinous bell and squeezing the surrounding water behind them. Because Jellyfish are invertebrates and very far removed from true fish, biologically, it is becoming increasingly common to call them sea jellies in order to prevent the use of the misnomer suffix, “-fish”
Many species of jellyfish do not have specialized systems for digestion, respiration or even central nervous functions. On the underside of their bell, they have a manubrium, which is a stalk-like structure that features a mouth at the end. Food is passed by contraction of the tentacles up into the mouth where it is digested. Jellyfish do not have gills and have no need for respiration – their skin is so thin that they can oxygenate their body through diffusion of the surrounding seawater. The jellyfish body actually consists of 95%-98% water, with only a small percentage of biological matter serving to support the life of the creature.
Jellyfish generally do not have a nervous system, although many species support a network of loosely connected nerves that serve to control the jelly’s movements and metabolisms. Even in the most complex of jellyfish species, there is no structure even close to that of a mammalian brain, and the jellyfish is limited to floating along and eating whatever it may happen to come across. Some jellies have light-detecting organs that they can use to orient themselves in the dark waters of the ocean.
The tentacles of many jellyfish are extremely potent and dangerous weapons. Certain species of box jellyfish in the class Cubozoa are among the most venomous creatures in the entire world, and the nematocyst stinging mechanism is the world’s fastest biological reaction, with the discharge of poison taking place within a timeframe of a few thousand nanoseconds at impossibly high speed. Because the nematocysts are triggered automatically by ionic pressure, even beached and dead jellyfish can still sting and represent a clear danger to human beings. Most nematocysts can only discharge once before requiring a longer rest time to “recharge”, although very few jellyfish have any control over when and whether the discharge happens.
Upside Down Jellyfish
The genus of true jellyfish known as Cassiopeia, or the upside down jellyfish, includes 8 different species that thrive mainly in warmer region and can be found anywhere from mangroves and coastal regions to canals and mudflats. The coast of Florida and the Caribbean are commonly teeming with these types of jellyfish. The name of the genus is given by the species’ tendency to live on the seabed upside down. They are also quite varied in shape and color, ranging from green and brown to various shades of white and blue.
Species of the genus Cassiopeia are often mistaken for sea anemone due to their similar appearance. They are mainly photosynthetic, so they have only a mild sting bean. Also, as their name would suggest, they are upside down, so their bell points to the seabed and acts similarly to a suction cup designed to stabilize the jellyfish on the bottom. The tentacles point upwards, and they feature many branches meant to help the jelly capture its prey more easily. Replacing the mouth are a number of small oral openings that guide the food from the tentacles to the stomach.
The upside down jellyfish uses a type of mucus and nematocysts that work together to paralyze its prey and get it ready for consumption. They feed mainly on small organisms, and they use the mucus – which is essentially invisible – as a net designed to catch small organisms and immobilize them. Some of the species, such as Cassiopeia andromeda, live in a symbiotic relationship with algae species like the photosynthetic dinoflagellate, Zooxanthellae and various species of shrimp. The Zooxanthellae and their photosynthetic algae are assisted by the jellyfish to gain access to more sunlight. In exchange, the upside-down jelly is provided with extra food. Shrimp benefit differently from their relationship with the jellyfish, living underneath it for protection, while clearing its tentacles from parasites.
Cassiopea xamachana and C. Andromeda are two of the most significant varieties of Cassiopeia species. The relatively small medusae can grow up to sizes between 10 and 30 cm, and they are both widespread – usually being found in the warmer areas of the Atlantic Ocean and the Caribbean. Other notable species of upside down jellyfish include C. depressa, medusa, ndrosia and frondosa.
The rootmouth jellyfish is unique in its biological makeup as being the only member of its genus. Belonging to the family Rhizotomatidae, it is a large cnidarian that can grow to sizes comparable with that of a human. Its smooth surfaced bell and large mouth are some of its most distinguished features. Despite its size, Eupilema inexpectata – as it is known by its scientific name – is equipped more for consuming large quantities of microscopic food. This jellyfish is more commonly found in the southern hemisphere.
Like other jellyfish, E. inexpectata features a rounded, dome-shaped bell, and somewhat resembles a large mushroom. Its large tentacles together with the bell assist the jellyfish in propelling itself through the water, and the smooth-surfaced bell features no tentacles. Unlike most species of jellyfish, the root mouth jellyfish is much larger, reaching diameters that can exceed 1 meter. Also one of the features it is most known for it the large manubrium or mouth, meant to facilitate an easier acquisition of food. The mouth has numerous microscopic holes leading straight to its stomach that are responsible for making the jellyfish’s feeding cycle easier.
The place where the rootmouth jellyfish is most prevalent is South Africa. Unlike most species it is not as widespread, however, its localized distribution is associated with the entire South African coast. From one side to the other, you can find root mouth jellyfish of varying sizes at depths that are not as large as the ones where other species can be found. These jellyfish can swim closer to the surface, up to about 35 meters below sea level.
The genus Eupilema only features this single species, and it belongs to a larger order of jellyfish known as Rhizostomae. These jellyfish have no additional structures around the bell – such as the tentacles some species possess – but, instead, they feature eight oral arms that are branched out in order to facilitate the gathering of food. This order is quite large, with no less than 10 different families, each featuring several genera, however, the root mouth jelly is unique among them in its behavior, appearance, size and shape. An interesting fact about the rootmouth jellyfish is that it can often be found swimming along with large numbers of fish, which use its bulk to avoid and attempt to hide from their predators.
The moon jellyfish is one of the most well-known species of jellyfish in the world. Belonging to the genus Aurelia, it is closely related to many other species of the genus. A few of its most easily distinguishable features include a translucent, circular body measuring between 25 and 40 cm in diameter, gonads shaped in the form of a horseshoe and the lack of any type of respiratory system. Members of the species don’t have any gills or lungs, but are instead able to diffuse oxygen from water through the use of their thin membrane. The species is resilient and extremely widespread, being present in all major oceans of the world and in waters with temperatures ranging from 6 to 31 °C.
The moon jellyfish is also known among researchers as Aurelia Aurita. Aside from its ability to efficiently extract water from oxygen, the body structure of this species of jellyfish confers many more advantages and surprises. The animal not only lacks a respiratory system, but also has no use for an excretory or circulatory system. The food it consumes is broken down by digestive enzymes in the gastro-vascular cavity, and the same cells that produce the digestive enzymes also trap nutrients and carry them throughout the body to wherever they are needed. While these creatures can reproduce sexually, their bodies are equipped for much more. According to a recent study, A. Aurita is capable of a process known as lifecycle reversal, where individuals grow younger rather than older.
Like most types of jellyfish, A. Aurita has a diverse diet consisting of crustaceans, tunicate larvae, protozoans, fish eggs, mollusks and other small creatures. Their nematocysts allow the jellyfish to protect themselves and hunt for food even from their youngest age. That being said, the species has a large number of predators that also include the leatherback sea turtle, a large medusa known as Aequorea victoria and the ocean sunfish.
The lack of any complex organs and its remarkable regenerative capabilities allow this jellyfish to thrive under the harshest conditions. While one might find the moon jelly in many tropical regions including the Pacific Ocean, the species is also prevalent in areas like New England and Eastern Canada on the North American coast, being able to withstand temperature drops even at latitude 70°N. The moon jellyfish generally thrives in temperate waters, and can exist in water that features a severe drop in salinity to 6 parts per thousand.