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Phylum Cnidaria > Class Scyphozoa
Jellyfish
Class Scyphozoa
updated Mar 13

if you learn only 3 things about them ...
They are not fish!
Many can sting. Don't touch them.
Turtles eat jellyfishes. They often mistake plastic bags for jellyfishes and die when they accidentally eat the bags.

Where seen? Almost everyone knows what a jellyfish looks like! These blobs of jelly are creatures of the open waters and are only sometimes encountered on the shores. In the water, these elegant creatures are a delight to watch. Sometimes, they are trapped in a pool. More unluckily, they are sometimes stranded on the shore and look like sorry heaps of jelly. But no matter where you see them, don't touch them!

What are jellyfish? Despite their name, they are not fish and are more closely related to sea anemones and corals. They are Cnidarians (Phylum Cnidaria).

Jellyfish may belong to various groups within the Phylum Cnidaria. Many cnidarians undergo a life cycle in which they take a jellyfish-like form (the medusa) in one stage, and a stationary sea anemone-like form (the polyp) in another stage.

Other cnidarians only take the form of a jellyfish in their life cycle and don't take on the stationary form. These include members of the Class Scyphozoa; Class Cubozoa (which includes the highly venomous box jellies); Class Hydrozoa (which includes the highly venomous Portuguese-Man-of-War in the Order Siphonophora which is a colony and not a solitary animal like other jellyfish). In Class Scyphozoa and Class Cubozoa, the jellyfish form is the dominant and most conspicuous form in the life cycle.

Features: Jellyfish are more notable for the features that they lack: no head, no organs, no bones. The body called the bell is jelly-like, transparent or semi-transparent. The body has eight-fold symmetry. Like other cnidarians, the jellyfish does not have an anus and wastes are eliminated through the same opening where food is taken in.

The animal moves by contracting its bell. Some may have long thin tentacles on the edge of the bell, others lack these. The mouth is under the bell in the centre. Often surrounding the mouth are long structures called oral arms (not tentacles). These can be short and stout, branched, or long and ribbon like.

Don't play with jellyfish! Like other cnidarians, jellyfish can sting and some, very painfully. Stingers are still active even after the jellyfish is dead or dying. So don't touch jellyfish, even if they are stranded on the beach.

How to stay safe: Wear covered shoes and long pants to cover all skin exposed to water. Do not touch jellyfishes.

What do they eat? Some jellyfish sting fishes and crustaceans. Others are suspension feeders, trapping plankton in mucus on the underside of their bell-shaped bodies.

Jellyfish babies: Most jellyfish belonging to Class Scyphozoa and Class Cubozoa have of separate genders and stay the same gender. The reproductive organs are in the stomach and eggs and sperm are released through the mouth. Some species brood fertilised eggs. In the first larval stage, the offspring looks like a sausage and goes through a brief free-swimming stage. It eventually attaches to a surface. It then changes form to become a polyp that looks like a tiny sea anemone. Here, it feeds and grows. In the next stage, the polyp changes form again, budding into several tiny bell-shaped bodies that are stacked on top of each other like stacked cups. Each tiny cup breaks away to form a tiny jellyfish. In some other species, the polyp may bud off to form more polyps, and each polyp transforms into only one jellyfish.

In some small jellyfish, these larvae eventually settle down and develop into polyps that feed and grow. These polyps may reproduce asexually by budding off more polyps. Eventually, the polyp may reproduce asexually by budding off medusa forms. These medusa swim off and develop into adults that may eventually reproduce sexually. The original polyp may remain alive to produce medusa forms again later on.

Role in the habitat: Among the creatures that eat jellyfish are sea turtles. Because plastic bags and balloons look like jellyfish, sea turtles may eat them and eventually become ill and/or die. This is why it is important to dispose of plastic bags and balloons properly. Our litter can kill!

Human uses: Jellyfish are a delicacy in Chinese cuisine. But so far, there has not been an outcry against harvesting them. Jellyfish themselves can become a threat. Introduced jellyfish (through ballast water) can upset the natural balance by out-competing native animals. Some jellyfish are seasonally abundant and those that sting can be a danger to swimmers when they are plentiful. A sudden increase in jellyfishes can also impair commercial fishing as they clog up nets. Explosions of jellyfish populations are considered to be an indicator of an imbalance in the ecosystem, or pollution.

Status and threats: Jellyfish are not listed as among the threatened animals of Singapore. However, like other creatures of the intertidal zone, they are affected by human activities such as reclamation and pollution.

With ribbon-like oral arms
Raffles Marina, Jul 05


With fat oral arms
Changi, Apr 05


Mangrove jellyfish
Pulau Ubin, Apr 09


With thread-like tentacles this is a hydroid!
Tuas, Apr 04


Huge jellyfish washed ashore
Changi, May 06


The Upside down jellyfish is usually seen
facing the 'wrong' way up.
Pulau Semakau, Mar 08

Upside down, it is often mistaken
for a sea anemone.

Class Scyphozoa on Singapore Shores
from Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.
*names from The Scyphozoan website by Dr Michael N Dawson
+from our observation

Order Rhizostomeae
  +Huge jellyfishes
  Family Cassiopeidae
  Cassiopea sp. (Upsidedown jellyfish)

  Family Catostylidae/previously Crambessidae
  +Acromitus sp. (Mangrove jellyfish)
Catostylus sp. (Fat-armed jellyfish)

  Family Linuchidae
  Linuche sp. (Thimble jelly)

Order Semaeostomeae
  Family Cyaneidae
  Cyanea lemarckii (Blue jellyfish)

  Family Pelagiidae
  +Chrysaora sp. (Ribbon jellyfish)

  Family Petasiidae
  Craspedacusta sowerbii/previously Trastadacusta sowerbryi (Freshwater jellyfish)

  Family Physaliidae
  Physalia utriculus (Blue Bottle or Indo-Pacific Portuguese Man-of-War)

Class Cubozoa on Singapore Shores
from Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.

  Family Chiropsalmidae
  Chiropsalmus quadrigatus (Box jellyfish)

Unidentified jellyfishes on Singapore shores
Photos for free download from wildsingapore flickr

Acknowlegement
With grateful thanks to Dr Michael N Dawson of the University of California, Merced for identification of the jellyfishes.

Links

References

  • Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore. National Council on the Environment. 163pp.
  • Edward E. Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate Zoology Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
  • Pechenik, Jan A., 2005. Biology of the Invertebrates. 5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
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