hydroid text index | photo index
Phylum Cnidaria > Class Hydrozoa
Class Hydrozoa
updated May 2020
if you learn only 3 things about them ...
Often mistaken for plants, they are animals!
Some can sting powerfully. Don't touch them.
Each 'bush' is a colony of many little polyps.

Where seen? A wider variety of these animals are commonly encountered on our Nothern shores. Some fiery stinging ones are also commonly seen on our Southern Islands. But they are often overlooked as they resemble plants.

What are hydroids? They belong to Phylum Cnidaria. 'Hydrozoa' means 'water animals' in Greek. Hydrozoans may look like jellyfish or appear to be branching plants. There are about 3,000 known species of the Class Hydrozoa.

Hydroids are colonial animals. The polyps are tiny (1mm tall with a smaller diameter). In branching forms, the polyps are encased in a 'skin' made of chitin (the same substance that insect exoskeleton is made of). In some, each polyp lives in a bell-shaped 'container' with a lid. The colony often takes on feathery, branching plant-like forms. The branches arise from a central stalk that is attached to a hard surface.

Colourful Candy hydroids.
Tuas, Apr 04

Grass-like pale stinging hydroids.
Beting Bronok, Aug 05

This jellyfish belongs to Class Hydrozoa.
Tuas, Apr 04
The colony may be made up of two different kinds of polyps. Feeding polyps look like sea anemones with tentacles armed with stingers like other Cnidrians. These stingers are used to gather food particles from the water.

Other polyps function as reproductive organs and often don't have tentacles. Some hydroids have defensive polyps, usually club-shaped and well armed with stingers that can inject toxins or are sticky. These not only protect the colony but may also help to capture tiny prey.

Fire corals (Millepora sp.) are hydroids that produce a massive skeleton so they appear to be hard corals. As their name suggests, these animals have powerful stingers.

Pale stinging hydroids inflict powerful stings that can leave painful and hideous scars on the bare skin of careless visitors or divers.

Tiny polyps of the fern hydroid.
Beting Bronok, Jun 10

Polyp of fluffy hydroid.
Changi, Apr 12

Capsules of candy hydroids.
Pulau Sekudu, May 12
What do they eat? The feeding polyps gather food with tiny tentacles armed with stinging cells. The digested nutrients are shared with the rest of the colony. Some hydroids such as Fire corals (Millepora sp.) harbour symbiotic algae (zooxanthellae) in their bodies. The algae undergo photosynthesis to produce food from sunlight. The food produced is shared with the host, which in return provides the algae with shelter and minerals.

Living on a hydroid Despite their powerful stingers, some tiny animals such as amphipods actually live on them. Their stingers don't deter some nudibranchs from eating them.

Tiny tentacles of stinging hydroids.
Changi, Aug 12

Tiny amphipods often found in large
numbers on some hydroids.
Tuas, Apr 05

Cuthona nudibranch on Fern hydroid
Beting Bronok, Aug 06
Hydroid babies: To increase in size, the plant-like form buds new polyps and branches. Some species may produce 'runners' at the base along the surface that sprout new plant-like forms.

Hydroids may also reproduce in a more complicated way. In some hydroid species, the plant-like form produces tiny jellyfish-like forms (medusa) that break off from the branches and swim away. These forms are reproductively mature adults. Each hydroid usually produces either all male or all female medusa. The medusa produces eggs or sperm that are released into the water. For most hydroid species, however, the plant-like form does not release medusae. Instead, these egg- or sperm-producing forms remain attached to the branches. In yet other hydroids, the free-swimming reproductively mature medusa may bud off more genetically identical medusa before engaging in actual reproduction. Fertilised eggs eventually hatch into free-swimming planula larvae that eventually settle down onto a surface and growing into a new branching colony.

Some hydroids on Singapore shores

Seagrass hydroids
grows on seagrasses.


Hydroids on Singapore Shores
text index and photo index of hydroids seen on Singapore shores



  • Ria Tan. 6 June 2014. A blue button at Sisters Islands, Porpita pacifica. Singapore Biodiversity Records 2014: 151
  • 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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