Chaetognatha: Hints for Identification.
Erik V. Thuesen
This page is designed to instruct
the reader on what to look for when identifying chaetognaths. By jotting
down your observations as you answer the questions below, you should be
able to identify successfully most chaetognaths using good illustrations
and descriptions. For the Pacific Ocean, I suggest using the figures of
Tokioka (in Yamaji's Nihon Kaiyo Purankuton
(Naga Report series) and the recent papers by J.-P.
Casanova. For the Atlantic Ocean, I suggest using the chapter on chaetognaths
by Jean-Paul Casanova in the South
Atlantic Zooplankton guide. There are many rare species of chaetognatha,
and these hints will help, but they are mostly designed to get the reader
started with identification of the more common chaetognaths.
There is a link to a short tutorial
at the bottom of this
page. Links to chaetognath taxonomy lists and keys can also be found
at the bottom of this page.
Is the sample benthic or planktonic?
The most common benthic chaetognaths are in the family
Spadellidae. They are some times caught in plankton nets that hit the bottom
or trawled close to the bottom. Common planktonic chaetognaths are usually
in the family Sagittidae or Eukrohnidae. Deep-living chaetognaths also
include the Heterokrohnidae.
Is the specimen transparent, translucent,
opaque or pigmented?
Most planktonic sagittid chaetognaths are transparent
when alive. They become translucent as they die. The transverse musculature
of spadellid, euchaetiid and heterokrohniid chaetognaths gives them an
opaque appearance. Several spadellid and heterokrohniid chaetognaths have
distinctive pigmentation patterns on their bodies consisting of circles,
patterns, or dots. The color is usually a shade of orange. Several deep-sea
species of chaetognaths have orange guts.
How long is the specimen?
The smallest chaetognath is Spadella boucheri.
It's maximum length is 1.3 mm. It lives between the grains of sand around
the tropical island of Miyakojima in southern Japan. The largest chaetognath
is Pseudosagitta gazellae. It's maximum length is 105 mm. It lives
in the Southern Ocean from the shores of Antarctica to about 40°S.
Most chaetognaths caught near-shore in plankton nets are around a centimeter
in length. Deep-sea chaetognaths are often in the range of 20-50 mm in
How many fins are there and what do
they look like?
Sagittid chaetognaths have two pairs of lateral fins.
The other common chaetognaths have one pair of lateral fins. Pay attention
to the starting and ending points of the lateral fins in relation to other
body parts, such as ventral nerve ganglion, ovaries, trunk/tail septum,
and seminal vesicles. The fins are easily damaged, so this requires gentle
positioning of the specimen and adjusting the lighting under the microscope.
If the fins are not damaged, look at their shape. Pay attention to the
fin rays, noting if the are completely rayed or partially rayed.
How big is the tail section in relation
to the trunk section and total body length?
This is an important characteristic that can often give
you a first hint on which species you are dealing with once you have determined
the general group to which your specimen belongs. Make a measurement of
the tail section then divide by the body total length. For example, compare
Pseudosagitta maxima (tail section ~23%) with Pseudosagitta lyra
(tail section ~12%).
Is the specimen mature or a juvenile?
If you can clearly see the ovaries and developed seminal
vesicles, the specimen can probably be identified. If the seminal vesicles
are not yet developed and the ovaries only incipient, the specimen will
take more work to identify. The seminal vesicles can change shape rapidly
(24 hours) as the specimen gets ready to mate.
What is the shape of the seminal vesicles?
Are the seminal vesicles ornate or simple? Pay attention
to any protuberences, knobs, etc. Where are the seminal vesicles located
relative to the tail fin and trunk/tail septum?
What do the ovaries look like?
Eggs can be arranged in one, two or three neat rows in
each ovary, or they may be packed randomly. Pay attention to the length
the ovaries extend into the trunk section and to the size of the individual
eggs. Notice the shape of the seminal recepticle. A thin opaque white or
cream coloured line along the ovaries may be sperm that has worked its
way up to fertilize the eggs.
Do you want
to try your hand at a chaetognath identification tutorial?
Open this link for the chaetognath
Back to top of Page
Links to chaetognath
taxonomy lists and keys
Go to a species list of the phylum
Go to list of the Chaetognatha
found in the eastern North Pacific Ocean
Go to higher chaetognath nomenclature
Key to deep-sea chaetognaths
of the eastern North Pacific
Key to common nearshore chaetognaths
of the eastern North Pacific
Return to Chaetognath main
me an email at email@example.com.