From: itannman@boris.ucdavis.edu (Ann Mansker)
Subject: Critter of the Week: Pseudoceros bifurcus
Date: 03 Aug 1998 00:00:00 GMT
Newsgroups: ucd.life

Pseudoceros bifurcus Prudhoe is one of many species of hermaphroditic
marine flatworm of the order Polycladida.  These large (for a flatworm)
and gaudy invertebrates are found around tropical reefs around the would.
The subject species is found along the Great Barrier Reef off the coast of
Queensland, Australia.

A number of polyclad worms use hypodermic insemination in mating; that is,
the worms have a "penis papilla" topped by a sharp stylet.  When two worms
meet, they attempt to stab each other with the stylet, sometimes causing
moderate to serious wounds.  A successful strike anywhere embeds the
stylet in the body of the other worm, and insemination takes place.

In P. bifurcus, this behavior has become highly competitive, to the point
where the worms engage in "penis fencing," rearing up and actively
striking and avoiding in an attempt to be the sperm donor in the
partnership.

Images of the worms, an excellent short description of the process and
its selective advantages can be found at
http://www.rzuser.uni-heidelberg.de/~bu6/flatintr.htm#reprodu .

The original source for all web articles on this topic is a letter to
Nature (Vol 391:647) submitted by N.K. Michiels and L.J. Newman.

My thanks to someone who wishes to remain anonymous for pointing me to
this extremely interesting critter.