A Tale of Penis Fencers, Regeneration, and Other Invertebrate Adaptations. By Jonathon Perreira

Figure1. Two Turbellarians penis fencing. Image Credit: Leslie Newman (PBS “The Shape of Life” and National Geographic).

My Introduction to Turbellaria

Just a few weeks ago, I had the privilege of entering my first marine biology lab. Having never had the opportunity to observe so many of the invertebrate organisms I have learned about over the duration of many years studying biology, I was incredibly eager to make my way to the marine holding tanks in the back of the lab to see if I could catch a glimpse of one of the many marine organisms that have always struck me as alien in comparison to most terrestrial animals. I mean alien not merely in the sense that invertebrates are just weird and foreign, but in the evolutionary sense in which invertebrates have adapted, evolved, and persisted in ways that vertebrates simply have not. Immediately the largest organisms caught my eye in the intertidal tank, the Cnidarian sea anemones. Wonderfully interesting though they are, I wanted to really see something that I was entirely unfamiliar with. I continued looking and I saw what appeared to be a vibrantly colored pasta noodle moving melodically across the bottom of the tank. At the time I simply guessed at what the animal could be (initially I was wrong as usual), however in the following week I was fortunate enough to learn all about what I had seen…

As it turns out, what I was observing in the previous weeks was a Turbellarian, a class of the phylum Platyhelminthes, commonly known as flatworms. Initially, I was embarrassed in not having been able to identify what I had seen earlier, as I am very familiar with a variety of parasites and their associated pathologies (with Platyhelminth parasites being among the many). However, I soon learned that flatworms exhibit a great range of variety that far exceeds only the parasitic groups.

The common features of all flatworms

Fig 2. Free living Turbellaria planarian body plan
(Similar to in lab specimen).
Image Credit: J. Houseman.

A shared characteristic in all flatworms is that of bilateral symmetry,  resulting in their left and right sides being mirror images of one another. However, unlike most other animals that are bilaterally symmetrical, platyhelminths are acoelomate, meaning they have no internal body cavity other than the gut. also In addition to lacking respiratory and circulatory organs, many platyhelminths lack an anus entirely, requiring them to excrete digested material through their mouth. Other than simply appearing like a bilaterally symmetrical worm, some flatworms like Planarians (Figure 2) have these characteristic false eye spots which are observable dorsally, and a distinctly shaped arrow-head like anterior end.  On their face, flatworms seem unassuming, boring, and evolutionarily primitive in every way, however for what they lack anatomically, they make up for with some of the most spectacular adaptations, both behavioral and regenerative.

The highly derived, amply adapted, and far reaching implications

Figure 3. Penis fencing between two Turbellarians. Image Credit: Nico Michiels

If one day you awoke and found that all the knowledge you had gained about the organisms and diversity of life that surrounds you was lost, and the first organism you came across was a Turbellarian, what might you think? Mostly likely, even given the context that it’s your first encounter with a foreign organism, you might not be impressed (unless you’re an invertebrate zoologist at heart). Well, you should be. Among the many astonishing adaptive features Turbellarians possess, those concerning their behavior have to be the most interesting. Turbellarians are simultaneous hermaphrodites, fertilizing eggs internally after copulation. However, the larger species engage in a complicated courtship behavior known as penis fencing (Figure 3), in which two individuals duel, attempting to impregnate one another.  The one that “loses” adopts the female role of carrying the fertilized eggs (Michiels and Newman, 1998). Undoubtedly the most coveted (and worthy of species envy) adaptive traits are their interstitial or stem cells which provide the flatworm with regenerative abilities to aid them after suffering damage to their bodies (Rieger et al., 1991),  often a result of their dueling courtship behavior. Even when considering these few and most interesting adaptive qualities of the once seemingly unassuming Turbellarian, one could imagine the novel applications and societal implications (especially in medical research) a growing understanding of this phylum could have.

References:

Rieger RM, Tyler S, Smith III JP, Rieger GE, Harrison FW, & Bogitsh B.J (1991) Platyhelminthes: Turbellaria. Microscopic anatomy of invertebrates: Volume 3. Platyhelminthes and Nemertinea., 7-140.

Lange R, Reinhardt K, Michiels N K, & Anthes N (2013) Functions, diversity, and evolution of traumatic mating. Biological Reviews.

Michiels NK and Newman LJ (1998) Sex and violence in hermaphrodites. Nature 391: 647-647.