Cubomedusae and Scyphomedusae
Introduction

The Cubomedusae and Scyphomedusae, classes of Cnidaria (formerly Coelenterata), include most of the larger solitary marine invertebrates. Their large swarms sometimes greatly affect the marine environment. Yet, paradoxically, for most regions of the world there is no reliable taxonomic account of them, and little is understood about their place in marine communities.

All Cubomedusae and Scyphomedusae have a relatively thick mesoglea or middle-layer which gives them bulk and provides some skeletal support. The popular name "jellyfish" reflects the prominence of the mesoglea. The name has no accurate zoological definition, and though these two groups might be considered the "true" jellyfish, the name is sometimes applied also to hydromedusae, siphonophores, and some other gelatinous plankton such as ctenophores and salps. Both the present groups are characterized by a radial symmetry. In fact, the scyphomedusae are the largest naturally-occurring perfectly symmetrical, non-bilateral, objects known from our planet. Their symmetry and the delicate nature of some of their structures give some of them great beauty. Many species are deeply or brightly colored, again mostly in a radial pattern which adds to their beauty. Color can be helpful in routine identification, but accurate systematics and taxonomy should rely on anatomical details.

The approximately 200 species of scyphozoans are exclusively marine. There are both pelagic (occurring at all depths) and benthic species. The Scyphozoa comprises Stauromedusae, mostly small, benthic polypoid medusae; Coronatae, mostly mid- to deep-water forms having a deep furrow (coronal grove) dividing the exumbrella into inner and outer zones (dome-shaped bell); Semaeostomeae (disk-shaped bell ;disk-shaped bell 2), typically large, conspicuous, inverted dish-shaped medusae, most abundant in shallow coastal waters (the genera Desmonema and Chrysaora can reach more than 1 m in diameter, and Cyanea was once reported at 2 m); and Rhizostomeae (hemispherical bell; hemispherical bell 2), typically roughly hemispherical, sometimes large, mostly warm-water coastal, filter-feeding forms, most species of which are distinguished by having innumerable mouthlets on the mouth-arms instead of a single central mouth. The Cubozoa (cuboid-shaped bell) or box jellyfish constitute a distinctive group of some 15 species, the Cubomedusae, several of which sting Man severely. The medusa stages of the great majority of both major groups have probably by now been described.

Reproduction is seasonal except in the high tropics. Although most species are dioecious, a few are hermaphrodite, typically being protandrous or protogynous. Some are slightly paedomorphic, becoming sexually mature before all asexual adult characters have developed (e.g. Chrysaora; see Agassiz and Mayer, 1898; Russell, 1970). Some species, especially the larger cubomedusae, and perhaps some deep-water scyphomedusae, evidently live for several years, but most live for just a few weeks or months. The life cycles typically include a small polypoid benthic stage, the scyphistoma. Small medusoid larvae, or ephyrae (Chrysaora lactea 3; Aurelia aurita 3), strobilate off from its upper end. Almost all scyphomedusae and cubomedusae have an alternation of stages between this asexual, sessile, benthic stage and a swimming, sexual planktonic medusa stage. The ephyrae each develop into a new medusa which is either male or female. A single scyphistoma will produce several to many medusae. Some scyphistomae are known to live for many years, perhaps a common pattern, and during their lives may liberate hundreds of ephyrae. It is a feature of some of the larger scyphomedusae, and of some cubomedusae, that those of large size may have been released hundreds, maybe thousands, of km from the area in which they eventually reach reproductive state. Much pan-oceanic genetic interchange must occur, and most of the species are extremely widespread in geographic range. Probably most scyphomedusae, and all cubomedusae of known life-cycle, have a benthic polyp stage. The life cycles of some coastal forms are known, but those of many are unstudied and the benthic stage of most is unknown. In a few coronate scyphomedusae the medusa stage is vestigial or entirely suppressed and the scyphistoma is dominant in the life cycle, paralleling an evolutionary trend common and well documented in the leptolid hydrozoans (hydroids and hydromedusae: see reviews in Boero and Bouillon, 1987; Cornelius, 1992). In contrast, in at least one species of Pelagia the scyphistoma stage of the life cycle is suppressed and bypassed, enabling them to range across the open ocean.

The anatomy of jellyfish, and of all cnidarians, is mostly simple (refer to literature cited above; also Harrison and Westfall, 1991, for histology). This finds exception in their highly complex subcellular organelles, the nematocysts. Most function to inject toxin, or "envenomate", prey or enemies through long piercing tubular threads, but some threads entangle and some are sticky. The mechanisms of discharge and penetration, and the toxins, are among the most complex of subcellular structures of any phylum (review of their effects on Man in Williamson et al., 1996). The long survival of the Cnidaria may result largely from the evolution of nematocysts and their powerful toxins, and has given time for a wide range of nematocyst types to evolve. These are widely used in systematic diagnosis in some cnidarian groups, but not so much in the present ones.

Cubozoa and Scyphozoa are essentially carnivorous. They feed on swimming animals through a variety of fishing techniques. Many typically employ the pulsing movements of the bell simultaneously for movement and to provide presumably optimal vortices bringing prey into contact with the nematocyst-bearing surfaces (Costello and Colin, 1995). These may be the marginal tentacles, the mouth arms, and even the upper surface of the bell in one species or another. Some species hang essentially motionless in the water. Medusae of all kinds are typically capable of consuming a great variety of prey including copepods, small crustacea, fish eggs and larvae (e.g., Feigenbaum and Kelly, 1984; Alvariño, 1977, 1985; Fancett and Jenkins, 1988); and interspecific and intraguild predation are also recorded (Arai and Jacobs, 1980; Mianzan, 1986b; Purcell, 1991; review in Arai, 1997). Algal symbionts (zooxanthellae), present in a few species, secrete organic compounds typically used as metabolic substrates, and which are assimilated by the host medusa (review in Arai, 1997). A number of species occurs sometimes in remote island situations, indicating that some may be long enough lived to cross the great oceans even if feeding opportunities are reduced. Some species are known to metabolise existing tissue (Thill, 1937) and are suspected of uptaking dissolved organic matter (Ferguson, 1988), processes which if widespread may enable individuals to survive long periods between successful prey-encounters.

Gelatinous plankton, including Scyphomedusae, have been regarded as a "dead end" in pelagic marine trophic webs (Greve and Parsons, 1977). However, some research has demonstrated the importance of gelatinous prey for several fish groups and some higher vertebrates (Haedrich, 1967; Arai, 1988; Ates, 1988, 1991; Mianzan et al., 1996; Mianzan et al., 1997). Man himself can also be regarded a scyphomedusa consumer. More than 10,000 tons of Stomolophus nomurai auct. and 1,000 tons of Rhopilema rhopalophora and Lobonema smithii are fished for human consumption every year from the China Sea, Japan and several other southeast Asian regions (Omori, 1978). In 1981, 40 million $ US worth of scyphomedusae were imported by Japan alone (Omori, 1981, in Arai, 1997). In Taiwan Aurelia aurita (the moon jelly) is also eaten (Shulman, 1995), and various other species are taken in small quantities.

Local populations of some Scyphomedusae reach high densities in many regions of the world, occasionally causing problems for Man's interests. During certain periods they even dominate the zooplankton biomass (Möller, 1978, 1980a; Shushkina and Musayeva, 1983; Papathanassiou et al., 1987) in both volume and organic carbon content. Aggregation seems to be a widespread phenomenon for "jelly plankton" (Arai, 1992). It has even been suggested that blooms might be an essential part of the life history for some species (Boero, 1991); but populations of high density may quickly consume all the prey in a volume of water, leading to starvation of the medusae. Nevertheless, swarming for gamete release might still be hypothesised, and some cubomedusae certainly accumulate for the copulatory reproductive process that is unique to them in the cnidarian phylum.

Occasional swarms of great density have a notorious effect on many human economic activities such as tourism, fisheries and various coastal industries. Cnidarians are the most frequent marine envenomators (stingers). More than one hundred species of cnidarians are now regarded as venomous to Man, including some 25 of the larger jellyfish, and the list is continuously expanding (Kokelj et al., 1993; Williamson et al., 1996). Swarms of Pelagia noctiluca in the Mediterranean Sea affected tourism for several years in the 1970s and 1980s (Rottini-Sandrini and Stravisi, 1981; Aubert, 1984; Bernard, 1984; Maretic, 1984; F.E. Russell, 1984; Rottini-Sandrini and Avian, 1991), but the prevalence of that species seems to wax and wane over the decades. The sea nettle Chrysaora quinquecirrha is considered a pest in Chesapeake Bay (Cargo and Schultz, 1966, 1967; Schultz and Cargo, 1969). The serious, even lethal, sting of the box-jellyfish Chironex fleckeri of Australia and some regions to the north, and the slightly less dangerous Chiropsalmus quadrumanus which occurs pan-tropically, are notorious (Williamson et al., 1996; see also Williamson, 1985; Burnett, 1991; Fenner, 1991) but some other species also sting Man severely.

The fishing industry is sometimes affected by jellyfish swarms (Rottini-Sandrini et al., 1984; Möller, 1984a, b; Mianzan, 1986b; Hay et al., 1990). Jellyfish have probably been preying on fish and their food resources for some 200 million years. They still remain competitors for the same plankton resources that are consumed by many commercial fish (Parsons and Seki, 1995). Overfishing seems to have resulted in the increase of some scyphomedusa species. In the Mediterranean Sea, Avian and Rottini-Sandrini (1988) and Legovic (1991) concluded that increase of the Pelagia noctiluca population followed increased pressure on fish stocks. In the Black Sea, Aurelia aurita increased from an estimated stock of 30 million during the 1960s to 400 million during the 1980s, putting pressure on pelagic fisheries by competition with fish stocks for food and by predation on fish larvae and their food (Shushkina and Musayeva, 1983; Bingel, 1991; Niermann et al., 1994; Mutlu et al., 1994). In the Benguela Current off Namibia, increase of Chrysaora hysoscella populations from the 1960s onward, from 5 million tons to 30 million tons, has been concurrent with a decrease of the pilchard (Sardinops ocellata) (Fearon et al., 1992).

Cooling-water intake ducts of power stations in all parts of the world are occasionally clogged by swarms of large, coastal species (e.g. Möller, 1984a; Werner, 1984, Baltic Sea; Rajagopal et al., 1989, India). Clogging of the refrigeration-plant intake ducts of ships has required SCUBA divers to clean the pipes (Mianzan, 1986b, 1989a, Puerto Madryn, Patagonia, Argentina).

Some medusa swarming and its consequences, as outlined above, may result from Man's activities. The destruction of a major geographic barrier by the building of the Suez Canal, and the transport of scyphozoans in ship ballast waters, have been invoked to explain the Mediterranean invasion by Rhopilema rhopalophora (see Cornelius, 1996; Galil et al., 1990; Lotan et al., 1992, as R. nomadica). Cassiopea andromeda entered the Mediterranean from the Red Sea much sooner after the Canal was opened (review in Spanier and Galil, 1991); and populations of Phyllorhiza punctata have become briefly established locally at isolated localities far from its original Australasian and southeast Asian distribution (Brazil, California, Mediterranean, Hawaii; review in Cornelius, Silveira and Mianzan, in prep.). Eutrophication of semi-enclosed areas has also been considered the cause of some scyphozoan blooms (Shushkina and Musayeva, 1983; Papathanassiou et al., 1987; Legovic, 1991).

No cubomedusa or scyphomedusa is known to be parasitic, but a few parasitic species of several other phyla are known to infest them (Laval, 1980). Amphipods of the genus Hyperia can debilitate a large medusa by gradually consuming it from within. Mostly, however, medusae of these groups are notably free of parasites.

For the present review, the classification adopted broadly follows that of Kramp (1961) with minor revisions but is considered provisional and several systematic questions await evaluation. Two interesting and well-founded genera are still best referred to Incertae sedis at the end (Tessera, Tetraplatia).