Chaetognatha: Identification Tutorial

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 living chaetognaths.

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 identification tutorial


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Links to chaetognath taxonomy lists and keys

Go to a species list of the phylum Chaetognatha

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 page

Comments? Send me an email at thuesene@evergreen.edu.