Marine flatworms devouring an ascidian colony
Three flatworms are in this photograph of a small area of a compound ascidian (or sea squirt) Botrylloides sp..
The native species of this ascidian is Botrylloides leachi but the invasive ascidian Botrylloides violaceus is spreading in the English Channel and it is not yet known whether this ascidian colony is the invasive species or the native species.
Dr. John Bishop and Dr. Gretchen Lambert tell me that the easiest way to tell the ascidian colonies apart is by looking at the ascidian larvae, which are not visible in this colony.
World authority on ascidians, Dr. Gretchen Lambert wrote "I also wondered if the orange Botrylloides is Botrylloides violaceus. I looked very carefully at the photo but there are no visible brooded embryos, which are the best way to tell this species (apart from the native species Botrylloides leachi)." Dr. Lambert recommends that I collect some colonies, tear them apart carefully, and look for huge brooded larvae in the tunic. "They will usually be pink, no matter what the colour of the colony is."
She writes "Botrylloides violaceus colonies are always a solid color--all orange, or all purple, etc. The larvae are more than 1 mm in diameter, spherical, and when mature have 24-32 lateral ampullae arranged in parallel around the anterior end like a little mop. In Botrylloides leachi the larvae are small, and brooded inside the zooids and not in the tunic."
There appear to be two species of flatworm in this image of a close-up of the ascidian colony. One of the flatworms is gliding over another and heading to the lower right of the image. This image was taken south of the Lihou Island causeway on Guernsey's west coast on the 10 May 2009.
The cryptic coloration of the flatworms makes them difficult to see. After 14 years of rock pooling on Guernsey's coast this is the first time I have noticed these flatworms. Their identity has not yet been determined. But they appear to be common.
This link provides a possible identification, which remains to be verified: http://www.rzuser.uni-heidelberg.de/~bu6/flat0482.html
File No. 100509 3973
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