Taxonomic Overview

Jellyfishes of the GBR fall into two phyla, the Cnidaria, or medusae, and the Ctenophora, or comb jellies. Most zoology textbooks give a good account of the classification and morphological differences between the two phyla. Other gelatinous zooplankton include salps, or pelagic tunicates in the Phylum Urochordata; these are not dealt with in this chapter (see Chapter 14).

Cnidaria is the more familiar of the two phyla, comprising numerous quite different forms, including:

• Hydroid jellyfishes (Class Hydrozoa, Figs 18.1, 18.2). Hydroids are characterised by having small, clear bodies with a shelf-like velum, gonads on the radial canals or manubrium; and they lack true oral arms (e.g. Obelia, Aequorea, Craspedacusta and Physalia).

• True jellyfishes (Class Scyphozoa, Figs 18.3, 18.4). These are characterised by having usually large, fleshy, colourful bodies, a greatly reduced velum, and gonads in internal pockets; the Rhizo-stomeae (blubbers) have eight oral arms and they lack marginal tentacles, whereas the Semeostomeae (sea nettles, moon jellies and their kin) have four oral arms and well developed marginal tentacles (e.g. Chrysaora and Aurelia). Cassiopea spp. are ben-thic rhizostomes that spend most of their time upside down, that is, lying with the umbrella down and oral arms up, gently pulsating water over their algal symbionts.

• Box jellyfishes (Class Cubozoa, Figs 18.5, 18.7). These are characterised by a box-shaped body, with eyes in cavities within the lower edge of the bell wall, and a gelatinous pedalium that emerges from each corner;

Figure 18.1 Some hydrozoans from the GBR. Clockwise from upper left: A, Turritopsis lata; B, Physalia utriculus; C, Zygocanna sp.; D, Olindias sp.; E, Porpita porpita. (Photos: L. Gershwin.)

the Carybdeida (Irukandjis and jumbles) have a single tentacle on each unbranched pedalium, whereas the Chirodropida (box jellies) have branched pedalia, with each branch leading to a tentacle (e.g. Carybdea, Chironex and Chiropsalmus).

• Benthic jellyfishes (Class Staurozoa). These less familiar jellyfishes are characterised by a stalked body, flared out into an 8-rayed flower-like form (e.g. Haliclystus, Manania and Lipkea)

The other jellyfish Phylum, Ctenophora (Figs 18.8, 18.9), includes sea gooseberries, sea walnuts, and Venus's girdles, none of which are harmful to humans. Ctenophores are divided into two classes: Nuda, which comprises only the genera Beroe and Neis, both found in Australia, which lack tentacles, and Tentaculata, which comprises most of the pelagic and all of the ben-thic forms, characterised by having two tentacles,

Ocellus (eye spot)

Ocellus (eye spot)

Figure 18.2 Features of hydrozoan jellyfish. (Photo: L. Gershwin.)

Stomach & gonad

Figure 18.2 Features of hydrozoan jellyfish. (Photo: L. Gershwin.)

Figure 18.3 Some scyphozoans from the GBR. Clockwise from upper left: A, Catostylus sp.; B, Pelagia sp.; C, Cyanea sp.; D, Netrostoma nuda. (Photos: L. Gershwin.)

which may be secondarily lost. Some of the more familiar forms on the GBR include:

• Order Cydippida (Pleurobrachia, Euplokamis), the members of which look like small grapes with two tentacles (Fig. 18.8A);

• Order Lobata (Bolinopsis, Ocyropsis), which are about the size and shape of an egg with two large lobes (Fig. 18.8C, D);

• Order Cestida (Cestum, Velamen), which look like ribbons, and

• Order Platyctena (Coeloplana, Ctenoplana), which look like a creeping, gliding, or swimming flatworm with two tentacles.

Jellyfishes are an ancient group, found in the fossil record before the Cambrian explosion; credible examples of fossil jellyfishes from the famous Ediacaran fauna date back about 585 million years, and appear relatively unchanged through the eons. Many interpretable fossils exist from the Cambrian and scattered more recent periods. Even the box jellyfishes, which are considered 'the pinnacle of development' among



Figure 18.4 Features of scyphozoan jellyfish. (Photo: L. Gershwin.)
Figure 18.5 Some cubozoans from the GBR. Clockwise from upper left: A, Carukia barnesi; B, Malo kingi; C, Chironex fleckeri; D, Chiropsella bronzie. (Photos: L. Gershwin.)

jellyfishes, were fully developed by the Pennsylvanian (about 300 Mya).

Currently, well over a hundred jellyfish species are recorded from the GBR region. Many of these species, however, are still awaiting formal classification. Most are tiny and inconspicuous hydromedusae, presenting no serious medical threat, giving a 'sea lice' sting at the very most. However, they may at times occur in such dense aggregations that visibility may become severely diminished and even minor stings become annoying if in large number. Furthermore, Irukandji jellyfishes are often found aggregating with dense swarms of salps (pelagic tunicates, see Chapter 14) and hydromedusae, so it is advisable to take extra safety precautions or not enter the water on days when there are dense gelatinous zooplankton blooms.

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