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Marine
flatworms
Class Turbellaria |
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Pseudobiceros
sp
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| Marine
flatworms are common but are more active when it is dark and
cool. |
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'Platyhelminthes'
means 'flat worm'. There are about 18,500 species of
flatworms, but only about 16% of these are free-living
flatworms. The rest are parasites.
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Flatworms range
from tiny ones found under rocks to larger ones that roam out in the
open. Some are brightly coloured and patterned, others blend with
their surroundings.
Flatworms belong to the Phylum Platyhelminthes. Most members of this
Phylum are internal parasites. These infest fish and other animals
including humans; such as tapeworms and liver flukes.
The flatworms we see on Chek Jawa, however, are not parasites. They
are free-living flatworms, most about 1cm long or less, although some
'monster' specimens of 8-10cm are sometimes seen. There are also countless
minute free-living flatworms that live among sand grains. These flatworms
stick onto the sand grains with paired glands on their underside.
One gland secretes a glue, and the other gland another substance to
release the glue.
How flat are flatworms? Unlike
bristleworms and earthworms which are segmented
and belong to Phylum Annelida, flatworms are unsegmented and really
very very flat. Usually less than 1mm thick! Being flat has its advantages.
For example, flatworms breathe using their entire body. Being flat,
oxygen diffuses quickly across the skin and to all parts of the body,
so they don't need a blood circulatory or respiratory system. Nutrients
are also quickly diffused from the central gut to the rest of the
body, although larger flatworms may have a highly branched digestive
system to bring food to the furthest reaches of the body.
Flatworm features: The skin of
a flatworm is covered with cilia (tiny beating hairs). The swirling
of their constantly beating cilia gives their class name 'Turbellaria'
which means 'whirpool'. In bigger flatworms, the cilia are often only
found on the underside. They also produce a mucus that protects them
from drying out or perhaps as protection from predators. Flatworms
have a central nervous system and a simple brain to co-ordinate their
well developed muscular system.
Flatworm food: Many flatworms
are carnivores that prey on tiny animals (protozoa, copepods, worms)
or feed on immobile animals such as bryozoans
and ascidians. Some are scavengers, feeding
on dead animals. The mouth of a flatworm is on the underside of the
body, in some, towards the centre of the body. Some flatworms can
push out their pharynx (a part of the gut) through the mouth to engulf
the meal. Others use their extendible pharynx to inject digestive
juices into their meal then suck in the resulting liquified meal.
Most flatworms don't have an anus and they spit out indigestible bits
through the mouth.
Flatworms on the hunt: Flatworms
are quite adept hunters. Flatworms may capture prey by with their
bodies or entangle it with slime. Some produce paralysing mucus and
a few use their penis to stab their prey! To help them in the hunt,
some have a pair of tentacles made out of folds of their body margins
to sense their surroundings. Others have sensory cells to detect water
currents and chemicals released by potential food. A few also have
balance sensors that tell them which way is up. Some have simple eye
spots on their head or along their body margins. These don't form
an image and only help flatworms avoid the light.
Flatworms on the move: To move
about, small flatworms secrete a mat of mucus and crawl on this mat
with a dense layer of cilia on their underside. Bigger ones may swim
by undulating the sides of their bodies. Some large flatworms even
have a sucker on their undersides to get a grip on the surface.
Flatworm babies: Marine flatworms
are hermaphrodites, that is, each flatworm has both male and female
reproductive organs. When two flatworms meet, they exchange sperm.
Some species simply insert their needle-like penis anywhere in the
body of the partner. This is not surprisingly called 'hypodermic impregnation'!
In yet other species, each flatworm tries to impregnate the other
without itself being impregnated, as it involves more energy to produce
eggs. This results in a sort of 'penis-fencing' when the two flatworms
meet! Eggs are laid in gelatinous strings or masses. Some flatworms
only lay a few eggs. Most flatworms do not have a free-swimming larval
stage. Instead, miniature flatworms develop within the protective
egg capsule. Here is a photo
of a flatworm larva on Image
Quest 3-D Marine Library
Many flatworms can also reproduce asexually by budding or splitting
apart. Each segment then regenerates into a new flatworm.
Fragile worms: Flatworms are very delicate and tear easily
when handled. So please avoid touching them.
Nudibranch or flatworm? Flatworms
are commonly mistaken for nudibranchs. Unlike most nudibranchs,
flatworms don't have flower-like external gills on their backs. Flatworms
are also much flatter and tend to move faster.
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Pseudobiceros
sp.
Pseudobiceros
gratus
Unidentified
flatworm
Pseudobiceros
fulgor
Unidentified
flatworm
Unidentified
flatworm
Unidentified
flatworm
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quick
facts
2-5cm, common under rocks and in the coral rubble area.
Classification:
Class Turbellaria
Phylum Platyhelminthes |
