Erik V. Thuesen
This page is designed to guide the
reader through the process of chaetognath identification. This is the second
part of this identification guide, and I suggest reading the Chaetognatha:
Identification Hints page first. Links
to chaetognath taxonomy lists and keys can also be found at the bottom
of this page.
Here is the specimen. It was collected off Santa Barbara, California.
Click on image for larger picture
Is the sample benthic or planktonic?
This is a planktonic specimen, so it is most likely a
sagittid or eukrohniid chaetognath.
Is the specimen transparent, translucent,
opaque or pigmented?
This specimen is not transparent, because its body is
tightly packed with bouyancy sac mesentaries. This results in reinforcement
of the body giving it a solid looking appearance.
How long is the specimen?
This specimen is 25 mm long, longer than most surface
How many fins are there and what do
they look like?
This specimen has two pairs of lateral fins, that means
it is in the family Sagittidae.
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.