Jellyfish And Their Relatives

The jellyfish, or medusae, are treated separately here as they are increasingly common, and of great interest to humans because of their sting and as a fishery. Jellyfish belong to Phylum Cnidaria, which is divided into three classes Hydrozoa, Scyphozoa, and Cubozoa (a fourth class of cnidarians contains all the benthic anemones and corals). They are distinguished from all other gelatinous zooplankton, and often from each other, by their stinging cells (cnidocytes, nematocysts, Ostman 2000). More useful characteristics for field identification include the presence, absence, shape and size of features such as the bell, oral arms, tentacles, stomach, and circulatory canals (Figure 8.9). Variation in these structures leads to organisms as diverse as a lion's mane (Cyanea) and a cannonball (Stomolophus). Jellyfish range in size from a few millimetres diameter (Solmundella or Obelia) to over 30 metres long (Praya).

tentacles

Figure 8.9 Some of the major anatomical features of rhizostome and semaeo-stome medusae. a) Sub-umbrellar view emphasising features of the oral arms (lower portions) and the bell (cut-away, upper portions). b) Side view of external and, with cut-away, internal features. The shape and size of these and other anatomical features can vary considerably and are used to distinguish among taxa from class level down to species. Rhizostome medusae have eight oral arms partially covered with numerous minute mouths, an oral disk, but no marginal tentacles. In contrast, semaeostome medusae have four oral arms, a single mouth, no oral disk, and generally many tentacles at or near the bell margin.

tentacles

Figure 8.9 Some of the major anatomical features of rhizostome and semaeo-stome medusae. a) Sub-umbrellar view emphasising features of the oral arms (lower portions) and the bell (cut-away, upper portions). b) Side view of external and, with cut-away, internal features. The shape and size of these and other anatomical features can vary considerably and are used to distinguish among taxa from class level down to species. Rhizostome medusae have eight oral arms partially covered with numerous minute mouths, an oral disk, but no marginal tentacles. In contrast, semaeostome medusae have four oral arms, a single mouth, no oral disk, and generally many tentacles at or near the bell margin.

Worldwide, there are approximately 200 described species of familiar, large jellyfish (scyphozoans) in three orders, although only two orders are relevant here: Rhizostomeae and Semaeostomeae. The bell (usually approximately 10 cm to 1 m diameter) is probably the most obvious structure in most scyphozoans, but it can also be adorned with numerous long tentacles, large oral arms, and other appendages, most notably in rhizostomes and semaeostomes (Figure 8.10).

The rhizostomes are the most taxonomically diverse order of scyphozo-ans, with approximately eight families, 25 genera, and 90 species described worldwide, mostly in the Indo-West Pacific. The rhizostomes are economically important as fisheries (Catostylus; Box 8.5), as introduced species (Rhopilema), and as problematic blooms (Box 8.6). The rhizostomes are also the youngest order of jellyfish, raising the question why this group

Figure 8.10 Some of the common genera of jellyfishes mentioned in the text that occur in Australian waters. Sizes range from 10 to 30 cm bell diameter. Class Scyphozoa: Order Rhizostomeae - (A) Cassiopea, (B) Catostylus, (C) Phyllorhiza; Order Semaeostomeae - (D) Cyanea (photo G. Edgar, reproduced with permission from Edgar 2000), (E.) Aurelia. Class Cubozoa: Order Cubomedusae - (F) Chiropsalmus sp., (G) Chironex fleckeri (photo J. Seymour), (H) Carukia barnesi (photo J. Seymour), Class Hydrozoa: Order Leptomedusae - (I) Aequorea (photo D. Miller); Order Cystonectae - (J) Physalia (photo J. Seymour).

Figure 8.10 Some of the common genera of jellyfishes mentioned in the text that occur in Australian waters. Sizes range from 10 to 30 cm bell diameter. Class Scyphozoa: Order Rhizostomeae - (A) Cassiopea, (B) Catostylus, (C) Phyllorhiza; Order Semaeostomeae - (D) Cyanea (photo G. Edgar, reproduced with permission from Edgar 2000), (E.) Aurelia. Class Cubozoa: Order Cubomedusae - (F) Chiropsalmus sp., (G) Chironex fleckeri (photo J. Seymour), (H) Carukia barnesi (photo J. Seymour), Class Hydrozoa: Order Leptomedusae - (I) Aequorea (photo D. Miller); Order Cystonectae - (J) Physalia (photo J. Seymour).

BOX 8.5 JELLYFISH FISHERIES

Dried jellyfish is eaten in many Asian nations and jellyfish have been harvested in China for over 1700 years. Approximately 500 000 tonnes of jellyfish are harvested annually, predominantly in Asia. Increased demand for jellyfish has, however, seen new jellyfish fisheries established in places such as the UK, USA, Namibia and Australia (Kingsford et al. 2000). Only rhizostome species, such as Catostylus mosaicus, are harvested because they have firm bodies and produce a product that has the desired, slightly crunchy texture. Jellyfish are semi-dried using a combination of alum and salt - the process can take 20-40 days. The dried product is initially prepared by soaking it in cold water to remove the salt. The jellyfish is then shredded into strips, blanched in boiling water and then mixed with sauces and other ingredients, such as chicken, and served cold as a salad. In many countries jellyfish are harvested using large nets or even trawlers, but in Australia fishers may only collect jellyfish using a hand-net. This method is more labour-intensive, but has the benefit of reducing by-catch of undersized jellyfish or other species.

BOX 8.6 JELLYFISH BLOOMS

The profiles of jellyfish blooms - rapid increases to high jellyfish abundance -and their causes have increased worldwide in recent decades (Mills 2001; Purcell et al. 2007). For example, blooms of Mediterranean Pelagia noctiluca in the early 1980s stimulated international meetings on environmental degradation. A 10-fold increase in the combined biomass of Chrysaora, Cyanea and Aequorea in the Bering Sea from the late-1980s into the 1990s raised concerns about over-fishing, climate change and trophic cascades (Mills 2001). In 2002, large swarms of medusae, which were tentatively identified as Crambionella orsini, bloomed in the Gulf of Oman blocking seawater intakes at the Oman Liquefied Natural Gas plant and clogging commercial fishing nets. In these cases, the blooms seem to be attributable to population fluctuations of endemic species. Elsewhere, it is likely that some blooms are due to introduced species, such as Rhopilema nomadica in the eastern Mediterranean (Mills 2001). However, all too often, the underlying causes for blooms remain unclear. Integrating data on weather patterns, biological, chemical, and physical oceanography, and jellyfish population dynamics (of both polyps and medusae) should increase understanding of the causes of jellyfish blooms and help mitigate future impacts. In the case of C. orsini, there may even be a silver lining because it is an edible jellyfish (Omori and Nakano 2001).

diversified so much so rapidly. Was it their environment or their biology, such as photosymbioses (see Box 8.7), or both that allowed rhizostomes to occupy so many niches?

In contrast to rhizostomes, semaeostomes are the most familiar jellyfish outside the Indo-West Pacific. Worldwide, three families, 18 genera, and 70 species are recognised, including Pelagia - one of the first jellyfish to cause international concern over jellyfish blooms (Box 8.6) - and Aurelia, the best studied of all jellyfishes. Long thought to be a single cosmopolitan species, A. aurita is now known to be a complex of at least 10 cryptic species, which has implications for identifying invasive species, jellyfish blooms, and interpreting decades of research (Dawson 2003, 2004; Dawson et al. 2005).

There are relatively few box jellyfishes (Cubozoa): five families, 13 genera and about 30 species described worldwide. They are generally easy to identify as the bell is square in cross-section with tentacles emerging only from the four

BOX 8.7 JELLYFISH SYMBIOSES

Jellyfish have symbiotic relationships with many other organisms. Some species of jellyfish contain photosynthetic dinoflagellates (zooxanthellae) within their tissues, as reef corals do (Figure 8.11A). The degree to which jellyfish derive their nutrition from their photosymbionts varies, with some species being only partially autotrophic (such as Cassiopea; Hofmann and Kremer 1981), while others are almost fully autotrophic (such as Mastigias in Palau; McCloskey et al. 1994). In some cases, the presence of symbiotic zooxanthellae is thought to be responsible for some remarkable behaviours displayed by jellyfish. For example, in the jellyfish lakes of Palau, Mastigias migrate along the length of the lake during the day to avoid shadows and maximise exposure to sunlight (Hamner and Hauri 1981). Another species, Cassiopea is known commonly as the 'upside-down' jellyfish because, unlike most medusae, it rests upside-down on the bottom to expose the zooxanthellae in its oral arms, to sunlight. A different type of symbiosis involves the association of fish and sometimes invertebrates (such as amphi-pods, barnacles and crabs) with medusae. Often large numbers of juvenile fish are seen swimming close to jellyfish and small crabs, copepods and other crustaceans can sometimes be found riding on the bell of jellyfish (Pages 2000; Figure 8.11B). Jellyfish probably provide effective protection against predation for these animals. How these animals avoid being stung by the jellyfish is not known. They may simply avoid contacting the tentacles or, as hypothesised for clownfishes that live in sea anemones, they may have some form of chemical or immune protection.

Figure 8.11 a) Mastigias from Palau, left, and a close up of zooxanthellae clusters on the underside of the bell, right, showing concentrations of zooxanthel-lae on muscle bands along the lower edge of the picture and around the gut in the upper right-hand corner. b) Small juvenile yellowtail horse mackerel (Tra-churus novaezelandiae) swimming among the tentacles of the semaeostome jellyfish Desmonema in New Zealand, the two large leatherjackets (Parika scaber) are preying on the medusa (photo M. Kingsford). c) The parasitic gooseneck barnacle Alepas on the bell of Cyanea caught in the Huon estuary, Tasmania (Tubb 1946).

Figure 8.11 a) Mastigias from Palau, left, and a close up of zooxanthellae clusters on the underside of the bell, right, showing concentrations of zooxanthel-lae on muscle bands along the lower edge of the picture and around the gut in the upper right-hand corner. b) Small juvenile yellowtail horse mackerel (Tra-churus novaezelandiae) swimming among the tentacles of the semaeostome jellyfish Desmonema in New Zealand, the two large leatherjackets (Parika scaber) are preying on the medusa (photo M. Kingsford). c) The parasitic gooseneck barnacle Alepas on the bell of Cyanea caught in the Huon estuary, Tasmania (Tubb 1946).

corners of the bell, and a rhopalium (eye) set in the lower-middle of each of the four sides. Box jellyfish include Chironex fleckeri - the most venomous marine animal - and Chiropsalmus (Halstead 1988; Nagai et al. 2002).

Most of the planktonic cnidarians are hydrozoans. Worldwide there are approximately 45 families, 200 genera, and 700 species of hydromedusae, plus 15 families, 45 genera, and 150 species of siphonophores. Compared with the scyphozoans, many are relatively inconspicuous because of their small size or habitat (under rock ledges or wharves). There are a few obvious exceptions, such as Aequorea (order Leptomedusae), which grows to about 15 cm bell diameter and is common in near-shore waters and Physalia, the colonial blue-bottle (or Man-o'-war; order Cystonec-tae), with its distinctive float, or pneumatophore, which is often blown onto the shore. Physalia is of particular interest because it causes tens of thousands of stings each year in Australia, South Africa, and the eastern United States (Box 8.8, Box 8.9). Other colonial hydrozoans washed up on beaches are distinctively blue, coin-sized discs with polyps beneath (Velella, 'sail-by-the-wind' and Porpita). Sometimes clear firm gelatinous cubes or shapes can be found, which are the reproductive stages of siphonophores (Figure 8.8I3).

Most jellyfishes are meroplanktonic (only in the plankton for part of their lifecycle) and have a second life-history stage - the bottom dwelling polyp. One major group (Stauromedusae) occurs only as polyps. Polyps are similar in form to anemones and corals (which comprise the cnidarian class Anthozoa), but are rarely larger than a few millimetres, and are generally benthic. They reproduce asexually to generate other polyps or new medusae

BOX 8.8 THE BLUEBOTTLE, PHYSALIA, AND ITS RELATIVES

The bluebottle is often seen in the summer surf and washed up on beaches associated with an on-shore wind. It can inflict a painful sting (see Box 8.9 on treatment). Its habitat is the surface water of the open ocean, where it stings and consumes small fish and zooplankton. It is a holoplanktonic, colonial hydro-zoan (one of the classes of cnidaria). The colony is dominated by a highly modified polyp, which forms the float, and other polyps (or zooids) which are specialised as long, stinging polyps, short feeding polyps and thick reproductive polyps. The long stinging polyps have many stinging cells, which can discharge when touched by something the jellyfish does not recognise as itself. A related blue hydrozoan also washed up is the 'sail-by-the-wind' (Velella) - a blue disc about the 2-3 cm across with a small triangular sail. Velella is harmless to humans, although it also stings and feeds on plankton.

BOX 8.9 HANDLING JELLYFISH: A NOTE ON SAFETY

Most jellyfish stings are not lethal, but a few are. Many more cause rashes, swelling and other symptoms such as nausea, sweating, muscle and joint pain and difficulty breathing (Fenner 1997). Wear rubber gloves when handling jellies and avoid water into which cnidocytes might have been released. Wear a wet-suit (with gloves, booties and hood) if swimming with them. As jellyfish are generally fragile, avoid taking them out of water. Instead capture and move them in bags and buckets.

An effective and practical treatment for pain from bluebottle stings is immersion in warm to hot water (45°C for 20 minutes), which is more effective than the traditional icepack method. Many marine venoms are heat labile and are quickly denatured by moderate heat (Loten et al. 2006).

(which usually have separate sexes producing eggs or sperm) depending on environmental conditions. This is one reason why massive blooms of medusae can seem to appear out of nowhere - in reality, they're coming from minute asexually reproducing polyps - which causes problems for coastal management (Box 8.6).

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