learn only 3 things about them ...
| Some may be highly toxic. Don't touch them!
are small and may retract at low tide. Don't step on them!
Their leathery skin is partly made up of chitin, the same
substance that insect exoskeleton is made up of.
seen? These tiny but tough flower-like animals often carpet
rocky and rubbly areas. Some are adapted be regularly exposed to the
air at low tide. These animals are often the first to settle on any
vacated space in a reef.
What are zoanthids? Zoanthids
belong to the same Class Anthozoa as sea anemones. But while true sea anemones belong to Order
Actiniaria, zoanthids belong to a different Order Zoanthidea.
Zoanthid taxonomy is undergoing some review so the number of known
zoanthids species range from 200 to 60 depending on how the species
Features: Zoanthids look like
tiny sea anemones. But while true sea anemones are solitary polyps, most
zoanthids live in colonies like corals do. But zoanthids don't produce
a hard skeleton like the hard coral colonies. Instead, their tissues
are leathery and composed partly of chitin (the same substance that
insect exoskeletons are made of).
The typical polyp has a cylindrical body column, topped by a smooth,
flat oral disk that is edged by short tentacles, usually in two rows
close to one another. The oral disk is often in a contrasting, bright
colour from the usually brown or drab tentacles. When exposed at low
tide, however, the animal retracts its tentacles into its body column
and then looks like a strange blob of jelly.
Two kinds of zoanthids
Pulau Hantu, Mar 05
Two kinds of zoanthids
Pulau Tekukor, May 07
Body column with oral disk edged with short tentacles.
Cyrene Reef, Jul 10
|Zoanthids may have three different living arrangements. Each zoanthid
polyp may be solitary but located near one another. These polyps are
large with thick, fleshy polyps on tall columns. Or the zoanthid polyps
are joined to one another by stolons (tube-like structures that spread
across the ground like a root or runner). Or the zoanthid polyps may
be embedded in a common mat of tissue. The tissue may be strengthened
by incorporating sand. The colony may form mats on the sand or encrust
The shape of the same zoanthid species may vary depending on where
they are found. Those inhabiting areas with strong waves tend to be
short and hug the surface. Others found in deeper, calmer waters are
taller, with longer tentacles.
Sometimes confused with sponges, ascidians and other
blob-like animals. Here's more on how
to tell apart blob-like animals. They are also sometimes confused
with sea anemones. Here's
more on how to tell apart animals
with a ring of smooth tentacles.
Sometimes packed so tightly that
they are mistaken for hard corals.
Chek Jawa, May 05
At low tide with their tentacles retracted
they look like a clump of sausages.
Chek Jawa, Aug 05
Sea mat zoanthids have polyps too,
embedded in a shared tissue.
Kusu Island, Oct 04
Toxic : Although
they look harmlessly pretty, some zoanthids contain powerful
toxins to protect themselves against predators. The most
toxic marine poison, palytoxin, was discovered in a zoanthid.
Minute quantities of palytoxin can paralyse and even kill.
So don't handle zoanthids with open wounds on your hand
or touch your mouth or eyes after handling them. It is
believed that the toxins are not produced by the animal
but by bacteria that live in symbiosis with the polyps.
How to stay safe: Wear covered shoes and long pants to cover all skin exposed
to water. Do not touch zoanthids.
Despite their toxins, some animals have adapted to eat zoanthids.
These include the Common
hairy crab, filefishes and nudibranchs.
What do they eat? Most zoanthids
feed on plankton, some also feed on finer particles. Many harbour
zooxanthellae (symbiotic algae) inside their bodies. These carry out
photosynthesis and may contribute nutrients to the host polyp.
Zoanthid babies: Zoanthids generally
reproduce asexually: new polyps bud to enlarge the colony. However,
they also reproduce sexually. The polyps may produce sperm or eggs,
but usually only either one at a time. Eggs and sperm are released
synchronously for external fertilization, in mass spawning similiar
to that practiced by hard corals.
Human uses: Palytoxin, the poison
extracted from a zoanthid, has been used to better understand how
our body works and may provide better treatment of hypertension, heart
disease and other disorders. Zoanthids are also a popular item in
the live aquarium trade, although their toxins make them dangerous
Status and threats: Zoanthids
are not listed 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. Trampling by careless
visitors, and overcollection also have an impact on local populations.
on Singapore shores
Polyps with broad oral disk
and long body column.
Polyps with small oral disk
and long body column.
Pink button zoanthids
Polyps with pink edges at the top of the body column, more obvious
in retracted polyp.
Zoanthidea recorded for Singapore
Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity
+from our observations
With grateful thanks to Dr James Reimer of JAMSTEC (Japan Agency for
Marine-Earth Science and Technology) for identification of the zoanthids.
Eric H. 2001. Aquarium
Corals: Selection, Husbandry and Natural History
T.F. H Publications. 464 pp
- 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.
- Wee Y.C.
and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.
National Council on the Environment. 163pp.
- Ng, P. K.
L. & Y. C. Wee, 1994. The
Singapore Red Data Book: Threatened Plants and Animals of Singapore.
The Nature Society (Singapore), Singapore. 343 pp.