Mating and Reproduction
Since the visual sense of polyclads is quite primitive, chemical compounds
are thought to play a key role for finding food and mating partners. The
elaboration of the anterior margin into large pseudotentacles is one evidence
that these worms rely substantially on a chemosensory instrumentarium to
perceive the reef environment and to determine their behavior.
In general, polyclads are hermaphroditic having male and female reproductory
organs. They copulate to engage in mutual cross-fertilization. Once, two
adult worms of the same species meet a quite crude copulation act, which
can be described as hypodermic insemination, is performed (upper images,
bifurcus). During copulation worms were observed to move towards
each other, touch, then roll around together (left image below, Pseudoceros
gravieri), simultaneously everting their penis papillae and stylets
outward (both images on the right below, Pseudobiceros
bedfordi). They then try to stab each other anywhere, sometimes
causing considerable damage to their partner. Worms with wounds were able
to heal within 24 hours. When one animal is successful in penetrating the
other, it holds on with its stylet embedded in the epidermis of its partner
for several minutes. During that time, spermatozoa are injected into the
partner (upper image, right) .
Recently, observations on the mating behaviour of Pseudoceros
bifurcus (Michiels & Newman, Nature, Vol. 391: 647), show that
polyclad individuals seem to increase sperm donation over sperm receipt.
For males, injecting sperm offers direct access to eggs, whereas females
bear the costs of wound healing and lose control over fertilization. Therefore,
a strong selection on females to avoid these costs is observed. This is
achieved by "rearing up", a mating behaviour that is effective for both
"striking and parrying". Individuals try to stab one another, but show
strong avoidance behaviour when struck by their partner. In terms of evolution,
this attempt to increase the benefit of sperm donation over the cost of
sperm receipt, means that avoidance of insemination might not only cut
costs but also offer the benefit of being inseminated by better "stabbers",
resulting in more successful offspring.
After hypodermic insemination sperm actively move into the parenchyma towards the oviducts. They are probably attracted by a gradient of molecules released by oocytes or other cells of the female reproductory tract. Fertilized eggs are then deposited as an irregular shaped eggmass containing a few hundred eggs which are tightly packed in a single layer. Often, they can be found underneath boulders nearby colonial ascidians, one of their preferred prey. After about ten days of development free swimming, transparent larvae hatch (=Müller's larvae).
As depicted on the scheme where three consecutive stages of development are shown, Müller's larvae are characterized by eight lobes. Lobes carry beating cilia which allow a ciliate-like swimming (very left photo: larval stage under dark field microscope). Larvae enter the planktonic phase and swim around for a few days prior to settling and metamorphosing. During development, larval lobes continue to be absorbed whereas the digestive glands grow. Metamorphosis is completed when miniature adult worms, only a few millimeters in length, enter the benthic phase of life. As known from developmental studies on nudibranchs metamorphosis of larvae can be triggered by chemical compounds produced by the favorite food of the species. This mechanism makes sure that the place of settling provides enough food for the juvenile organism to grow and, therefore, it has a much better chance of survival. Since polyclad larvae under lab conditions only survived for two weeks after hatching without settling in this time, it is not known if an external trigger is necessary for polyclads to enter the benthic life phase.
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