learn only 3 things about them ...
Often mistaken for plants, they are animals!
Some can sting powerfully. Don't touch them.
'bush' is a colony of many little polyps.
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.
Features: 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.
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
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.
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.
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
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.
Tiny amphipods often found in large
numbers on some hydroids.
Tuas, Apr 05
hydroids on Singapore shores
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
Jan A., 2005. Biology
of the Invertebrates.
5th edition. McGraw-Hill Book Co., Singapore. 578 pp.