The cnidarian jellies in the class Scyphozoa (link from UC Berkeley Museum of Paleontology) include most of the species that people think of when hearing the word “jellyfish.” Several Pacific Coast species are among the most stunning of all marine creatures and may attain truly astounding body dimensions. Compared to the relatively inconspicuous hydromedusae, our local scyphomedusan fauna consists of very showy forms that are easily seen, even by casual observers.
In scyphozoan jellies, the medusa phase is the dominant part of the life cycle, at least as far as size and mass. This class can be distinguished from the hydromedusae by the presence of fingerlike nematocyst-bearing projections in the stomachs (gastric cirri, which are also found in the Cubozoa), multiple stomach pouches, cells in the mesoglea, and the absence of a flap of tissue known as the velum surrounding the inside margin of the bell. The gastric cirri nematocysts help to subdue prey that may still be kicking. The cirri also secrete digestive enzymes and have cells that serve to absorb products of digestion. A complex gastrovascular system lined with cilia transports water and food products from the stomach pouches, through unbranched radial canals to a circular canal surrounding the bell margin. Branched interradial canals then bring the return flow to exhalant grooves on the oral arms. This flow throughout the mesoglea transports oxygen and food products to all of the tissues. Incompletely digested food can be taken up for complete digestion by endodermal cells lining the gastrovascular system. Scyphomedusae are tetraradially symmetrical, which means that certain structures, such as the stomach pouches, are found in multiples of four. Moon jellies, for example, have four easily visible stomachs.
Although usually characterized as drifters on the ocean currents, scyphozoan jellies often possess decent swimming ability. Radial and circular muscles in the subumbrella of the bell contract to enable locomotion via jet propulsion. A network of subumbrella nerve cells with fairly rapid conduction of impulses helps to coordinate these muscular pulsations. Following a bell pulsation, the somewhat elastic gelatinous mesoglea forming the bulk of the bell recoils to its resting state, allowing the pulsing cycle to continue anew. Complex sensory structures known as rhopalia are situated around the bell margin. Each contains a statocyst for determining orientation (with many statoliths), pigmented light sensitive spots, and sensory pits that detect chemical scents. Associated with each rhopalium is a marginal nerve center that serves to set the frequency of bell pulsing. Sensory information from the rhopalia are integrated by the nerve centers, which can lead to changes in the rate of pulsing. In addition to the subumbrella net that coordinates swimming, a second nerve net weaves its way over the entire exumbrella and subumbrella, and into the tentacles, manubrium and oral arms. This system coordinates certain localized movements such as feeding, and passes impulses relatively slowly.
Nematocyst-laden marginal tentacles carry the potent stinging punch that scyphozoans use to capture and immobilize prey. Lacking teeth, claws and other hard body parts, they rely on toxins to acquire their food. These toxins can also be a painful deterrent for any creature that may entertain thoughts of eating or molesting a jelly. Although some of the scyphozoan jellies that visit California waters pack a relatively potent punch (sea nettles and purple-stripe jellies in particular), none has the capability of seriously harming people. Once captured by the tentacles, food items are usually transferred to frilly oral arms for transport to the mouth and then into the gastric cavity.
Scyphozoan jellies get the best of both worlds, often exploiting asexual and sexual reproduction to increase their numbers. Jellies such as moon jellies and purple-striped jellies have separate male and female sexes. The gonads that produce sperm and eggs in scyphozoan jellies are derived from gastrodermal tissue, hence the close association with the stomach pouches. When the mood is right, males release sperm that are taken up by the females to fertilize the eggs. These are cast into the water through the mouth (moon jellies, however, brood the eggs on the manubrium for a period of time). Soon the eggs hatch to form planktonic, ciliated planulae that eventually settle on appropriate substrates, with each forming a polyp (or scyphistoma). Scyphistomae can reproduce by a variety of asexual means (budding, development of stolons or production of tough podocysts) to increase their numbers. They may spend months or years fattening up, using long tentacles to capture passing zooplankton. Eventually each scyphistoma undergoes transverse divisions that produce offspring which are budded off sequentially, a process known as strobilation. Factors that induce strobilation are poorly known, but temperature, light and food supply are probably involved. Aurelia scyphistomae (and probably other scyphozoans) release an organic iodine compound during strobilation that induces nearby polyps to join the crowd. The small medusae that are released in great numbers (known as ephyrae) then spend several months growing into the familiar jellies we all know and love. Many scyphozoan populations have peak production periods during certain parts of the year when ephyrae release is highest – these seasonal cycles result in uniformly aged cohorts that may appear about the same time every year.
Scyphozoans are classified into 3 planktonic orders and one that is entirely benthic. Most of the scyphozoan medusae that you may encounter in Monterey Bay and other West Coast nearshore habitats are members of the Order Semaeostomeae. Some, like sea nettles, can be incredibly abundant at times, only to disappear for months before returning. This order is characterized by jellies with 4 or more long frilly oral arms and a scalloped bell margin. A scyphistoma stage with production of medusae by strobilation is also typical. The Order Coronatae is also well represented in Monterey Bay. Unless you do a midwater trawl or have a deep-water ROV however, you are unlikely to see any of them since they tend to be deep dwellers on the West Coast (some tropical species are nearshore surface inhabitants). A prominent feature of this group is the distinctive horizontal groove that surrounds the umbrella (hence the name for the order). Vertical groves often extend from the coronal groove to the middle of marginal lappets. Many coronate species possess relatively stiff tentacles that are held above the bell. Coronates also differ from semaeostomes and rhizostomes in the retention by medusae of the vertical septa found in polyps of all these groups.
Several species in the tropical Order Rhizostomeae occasionally visit southern California waters, but rarely if ever venture farther north to central California. In this group the central mouth is closed off and oral arms are fused to form a system of enclosed branching canals along which lie many small mouth openings. They also lack marginal bell tentacles and are restricted to feeding on small zooplankton. Many species also incorporate symbiotic algae which provide a source of nutrition. The scyphozoans in the Order Stauromedusae are not really planktonic jellyfish. They lack any free-swimming medusa stage, and instead live as stalked anemone-like benthic forms attached to various substrates.
The East Coast of the United States also has its share of scyphozoan jellies (link from the South Carolina Department of Natural Resources).