> Subclass Alcyonaria/Octocorallia > Order Pennatulacea
learn only 3 things about them ...
Each sea pen is a colony of many animals living together.
Small crabs and other creatures often live among the fronds.
low tide, they retract into the ground. Don't step on
seen? These strange animals are commonly seen on our Northern
shores. In soft, silty sand near seagrasses. At low tide, they are
often retracted completely into the sand. Sometimes, an "uprooted"
sea pen might be seen washed up on the shore or stranded in a pool.
Please do not step on or pull sea pens out of the sand. You will hurt
a whole colony of animals and the small creatures that live on them.
What are sea pens? Sea pens belong
to Phylum Cnidaria which includes
the more familiar sea anemones, hard corals and jellyfishes. Sea pens
are members of the same Class Anthozoa
as sea anemones. Unlike sea anemones which are large solitary polyps,
each sea pen is a colony of polyps. Sea pens belong to the Subclass
Alcyonaria (Octocorallia) that includes the soft
corals. Members of this subclass have tentacles which are branched
and in multiples of eight. There are about 300 species of known sea
Features: Those seen on our shores
average 15-25cm long. Sometimes really small ones 2-4cm long are seen.
Each sea pen is a colony of different kinds of polyps connected to
one another, each playing a different role.
There is usually a central 'stem', called the axial or primary polyp,
that supports the whole colony. The bottom half of the primary polyp
forms a muscular 'foot' (called the peduncle) that anchors the colony
and retracts the whole colony into the ground at low tide. This portion
lacks other kinds of polyps. The central stalk is usually stiffened
by an internal 'bone' made of calcium. You might sometimes come across
this bone washed ashore.
The upper half called the rachis sticks out of the surface. The rachis
is the budding zone where other kinds of polyps emerge. Some may have
leaf-like structures (called oozoids). These leafy shapes make the
colony resemble a feather: sea pens are so named because they resemble
feather quill pens.
Tiny polyps with 8 branched tentacles, called autozooids, emerge from
these leaf-like structures when submerged. The stinging tentacles
filter feed at high tide. These can retract fully.
There is also another kind of highly modified polyp, called siphonozooids,
that pump water into the colony to keep it rigid, and circulates water
through the colony. Siphonozooids do not have tentacles and usually
look like bumps or holes in the colony. They are usually much smaller
and found among the autozooids.
Some sea pens don't have leaf-like structures and the autozooids and
siphonozooids are found directly on the 'stem'. At low tide, the autozooids
are usually retracted. As a result, some sea pens look just like a
stick stuck in the sand, or a floppy sausage on the sand.
The colony might be stiffened by sclerites (tiny bits of calcium).
Some have long sharp sclerites that support the feathery structures.
Sea pens are adapted for life on soft sea bottoms. Here, they can
dig into the ground for support. They retract completely into the
soft ground when alarmed or at low tide. It is said that they can
move along the bottom by looping their bodies.
What do they eat? A few sea pens
may harbour zooxanthellae (symbiotic algae) inside their bodies. These
carry out photosynthesis and may contribute nutrients to the host
polyp. But most gather edible bits from the water.
Snacking on sea pens: Sea pens are preyed upon by some
snails and nudibranchs. The striped Armina
nudibranch (Armina sp.) is among those seen near sea
pencils, and appear to feed on these sea pencils.
Pen pals: Sea pens are often homes
to other small creatures. The tiny Painted
porcelain crab (Porcellanella picta) is often found in
the Common sea pen (Pteroides sp.).
Sometimes, tiny transparent shrimps are seen on Flowery
sea pens (Family Vertillidae)
Human uses: These beautiful animals
are sometimes taken for the live aquarium trade. However, they usually
don't do well in captivity and eventually die of starvation.
Status and threats: Sea pens are
not listed as among the threatened animals of Singapore. However,
like other animals harvested for the live aquarium trade, most die
before they can reach the retailers. Without professional care, most
die soon after they are sold. Those that do survive are unlikely to
breed successfully. Like other creatures of the intertidal zone, they
are affected by human activities such as reclamation and pollution.
Trampling by careless visitors, and overharvesting by hobbyists also
have an impact on local populations.
sea pen sometimes seen
uprooted and washed ashore
Chek Jawa, Jul 05
The sea pencil looks more
like a pencil than a pen!
Changi, Apr 05
Spiky sea pen with polyps expanded
Changi, Jul 12
Closer look at the polyps.
Changi, Jul 12
Flowery sea pen with some polyps
Changi, Jun 05
A closer look at the polyps.
Changi, Jun 05
Half dead sea pencil washed ashore,
showing the stick-like skeleton in the centre.
Changi, May 06
Pennatulacea recorded for Singapore
Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity
Names from Erhardt, Harry and Daniel Knop. 2005. Corals: Indo-Pacific
+from our observation.
pens awaiting identification
are difficult to positively identify without close examination.
On this website, they are grouped
by external features for convenience of display.
Veretillidae (Flowery sea pens)
Harry and Daniel Knop. 2005. Corals:
Indo-Pacific Field Guide
IKAN-Unterwasserachiv, Frankfurt. 305 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.