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 Zukan), Alvariño (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 length.

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 identification tutorial


Back to top of Page


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.