By Paul on November 4, 2020

Scyphozoa

Scyphozoa is a large group of Jellyfish, with approximately 200 species spread across 4 orders, 20 families and 66 genera. These simple, soft-bodied, gelatinous creatures are found around the world, from tropical waters to the polar regions. They are generally coastal, although deep-sea species are also evident. Some species are familiar to humans, such as the Moon Jellyfish (Aurelia aurita), although others are scarcely seen. Scyphozoa are are closely related to other Cnidarians, all of which are ancient in their evolution.

These Jellyfish can be beautiful and graceful to witness in the water, although some species are known to have a potentially fatal sting.^1,2,3

Description

Scyphozoa range in size from just a few of millimetres, to over 2m across. The largest is Cyanea arctica (Lion’s Mane Jellyfish), with some reports suggesting that it may have tentacles over 40m in length! They exhibit the same basic structure as all Cnidarians, with radial symmetry and a double-layered body wall. The outer epidermis and inner gastrodermis are separated by the gelatinous mesoglia. Most exhibit two stages in their life cycle, growing from a sessile polyp, into a free-swimming medusae.

They are very simple creatures with no specialised organs for respiration, or excretion, however the presence of stinging nematocysts is characteristic of this phylum. The nematocysts are organelles, with millions of them being packed within the cells of the oral arms. They are used for defence, as well as predation. The majority of species have a relatively mild sting, which has little, to no effect on humans, although others are potentially fatal. With only one opening present, both food and waste products pass through the same orifice.

Whilst some species are sessile organisms, most are able to swim, through rhythmic contractions of the bell. This contraction creates a jet of water that propels the jellyfish forward. They show relatively simple behaviour, as they have no brain and are limited by a very basic net of nerves. Swimming is coordinated by nerve centres around the bell, which also hosts primitive sensory clubs, known as rhopalia. The rhopalia consist of basic light sensors (ocelli) and gravity sensors (statocysts). This enables them to distinguish light and dark, as well as up and down.

Scyphozoa can be solitary, however most form shoals that have been known to include thousands of individuals and extend across a number of kilometres.^1,2,3,4

Habitat

Scyphozoans are generally pelagic creatures, found in all oceans around the world, from the tropics, to the polar regions and at all depths. As juveniles, the polyps attach themselves to hard surfaces, such as rocks and shells, before developing into mobile adults.^1,3

Reproduction

Scyphozoans are extremely fecund and may reproduce sexually, or asexually. Even as juvenile polyps they are capable of reproducing asexually, through budding, which creates new polyps. In adults, the budding process is known as strobilation, which is usually triggered by environmental factors, such as changes in temperature or light levels. This can take as little as a few days, up to a few weeks, depending on conditions. Strobilation produces free-swimming medusae known as ephyrae, which mature over the course of a month, or longer in some cases, into sexually mature adults.

Most species have separate sexes, distinguishable only via inspection of the gonads, however some species are sequential hermaphrodites. In sexual reproduction, sperm strands are released into the water column, which are then digested by females during feeding. In most species, the eggs develop into small larvae, known as planulae, which swim to down toward a substrate and attach themselves. Once attached, they develop into polyps. Other species are retained by the female before settling and some lack a polyp stage altogether.^3,4

Lifecycle

The lifecycle of Scyphozoa is varied, although most begin life as a planula that swims down to the substrate and metamorphoses into a polyp, or scyphistoma. These attach to the substrate with a “foot” and have a central mouth, surrounded by a single ring of stinging tentacles. These polyps commonly clone themselves and reproduce asexually into more polyps, or free-swimming medusae. The distal end of the polyp eventually develops into an immature medusa, known as an ephyrae. Eventually, the ephyra develops into a sexually mature adult medusae.^1,2,4

Diet & Predators

Scyphozoans exhibit a variety of feeding methods, from carnivorous hunters, to filter-feeders, with some species even using symbiotic dinoflagellates to photosynthesise on their behalf. Characteristic of this group, is the presence of stinging nematocysts in the tentacles. Some use these nematocysts to paralyse, or kill their prey, before using their tentacles to pull the food toward their mouths.

Most carnivorous species feed of small crustaceans, zooplankton, Ctenophores and small fish. They also feed on the eggs and larvae of fish, which may pose a threat to fish populations.

In return, many organisms feed on Scyphozoa, including fish, other jellyfish, turtles and humans. They are considered a delicacy in parts of the world and there is a multi-million dollar fishing industry based on their exploitation.^1,3,4

Conservation

No species of Scyphozoa are listed by the IUCN and predictions suggest that these organisms will become more numerous with increasing ocean temperatures.

Jellyfish can have a number of adverse effects on humans, both directly and indirectly. Whilst most are relatively harmless, some can produce a nasty sting, which in rare cases can be fatal. Furthermore, they often clog up fishing nets, and engine intakes.

On the other hand, Scyphozoans are not only a delicacy, but they are also increasingly being held in aquariums due to their graceful swimming techniques.^1,2,3,4