Microstomum, natural selection, and flim flam.

[CreepingJesus]

Tom has been referring to Microstomum, a free living flatworm of the order Turbellaria which exibits the ability to retain the stinging cells of its hydroid prey for its own defence......

The point, I suppose is that such a thing could not be the work of natural forces.

Creationists often leap upon certain examples in nature whose development may not appear to follow the rules of natural selection as perceived by the creationists.

They grab such things with glee and wave them triumphantly. "Explain this!" they cry, "Where's your oh so clever Mr Darwin now?"

I had a look for microstomum around the web and found a couple of creationist sites using it as an example of the failure of Darwinism.


Here's a direct quote from a creationist site, which is more detailed, (and presumably more convincing to non-biologists) than most.


from http://www.nccg.org/Sc01-Evolution.html

To illustrate this problem in evolutionary theory, we shall consider a simple example provided by a type of flatworm called the Microstomum8. This flatworm is equipped with a kind of defensive cell called a nematocyst, which can fire a poisonous barbed thread. When the flatworm is attacked by a predator, the nematocysts, situated just beneath the surface of the worm's back, are discharged, thereby stinging the assailant and driving it away.

The most interesting aspect of this arrangement is that the nematocysts are not produced from the tissue of the flatworm itself. Rather, they are stolen from the hydra, an aquatic organism on which the flatworm preys. The hydra has tentacles armed with several kinds of nematocysts, which it uses to subdue and capture the small animals on which it feeds. Some of these cells fire poisonous barbs, and others discharge various types of coiled and sticky threads that enable the hydra to hold on to its prey.

The flatworms generally avoid hydras. But biologists have observed that when the flatworms need more nematocysts, they will eat hydras and digest all of their tissues except these particular cells. The nematocysts are neither damaged nor discharged, but are enclosed within certain cells, which carry them toward the flatworm's back. The nematocysts that fire coiled or sticky threads are then digested, but those that fire poisonous barbs are transported to sites beneath the outer layer of the worm's back.

There the nematocysts are oriented so that their stings will fire upward. The epithelial cells, which form the worm's outer layer, become very thin just above the newly positioned nematocysts, thus providing portholes for the firing of the stings. Finally, the cells that have encapsulated the nematocysts undergo extensive changes that enable these cells to act as trigger mechanisms. (The hydra's original trigger mechanism is contained in a type of cell called a cnidoblast, which the flatworm digests).

Let us consider whether or not these defensive arrangements of the flatworm could have evolved step by step. An evolutionary scenario would have to begin with an ancestral flatworm that ate hydras but did not make use of the nematocysts. In such a worm, what would be the fist evolutionary step leading to the eventual exploitation of the nematocysts as defensive weapons? Unless the nematocysts were actually used as weapons, for the worm to manipulate them internally would be useless. Indeed, it would be dangerous, since the flatworm can easily be killed by the discharge of the hydra's stings.

Yet each step in the internal processing of the nematocysts is essential for their eventual use as weapons. If they were not transported to the flatworm's back. they could not be usefully developed. If transported to the back but oriented incorrectly. they would be useless or even dangerous. If they were oriented beneath the epithelial of normal thickness, the discharged sting would lose its momentum whilst passing through the epithelium, and the worm would sting itself.

There is also a further problem. Evidently the nematocysts are not triggered simply by pressure applied to the worm's back. Rather, their firing is governed by a complex control mechanism within the worm. Without this trigger mechanism the whole arrangement would be useless, even if the nematocysts were properly oriented beneath epithelial "portholes".

When examined closely, each step in the internal manipulation of the nematocysts resolves into a complex of substeps. For example, for a nematocyst to be transported to the back of the flatworm, one of the worm's cells must first recognize it, and then the cell must initiate a process of motion that specifically carries the nematocyst to the worm's dorsal region. These are both complex procedures. Yet for the flatworm to take advantage of the hydra's nematocysts, it would seem that many complex arrangements of this kind must be present simultaneously.

We can conclude that the standard Darwinian or neo-Darwinian theory of evolution cannot readily explain the origin of complicated interlocking arrangements such as the flatworm's defensive system. Yet such systems are by no means rare in nature. Indeed, it might be argued that nearly all complex organs and systems of organs in living beings involve many essential interdependent elements and that they are therefore not amenable to explanation by traditional evolutionary concepts.

So apart from some serious inaccuracies and a deep misunderstanding about the structure of platyhelminthes, the gist is that the use of second hand nematocysts by these animals is too complex a development to have evolved by natural selection and (with the help of our old friend the post hoc fallacy) must have been designed by god.


Several other organisms of different types display this "recycling" of nematocysts. Microstomum is not unique. Animals which exhibit this behaviour are known as "kleptocnidae"


Right.

I'm going to respond to some points made by the above writer.


The nematocysts are neither damaged nor discharged, but are enclosed within certain cells, which carry them toward the flatworm's back, The nematocysts that fire coiled or sticky threads are then digested, but those that fire poisonous barbs are transported to sites beneath the outer layer of the worm's back.

No mystery here. The stinging nematocysts are made of keratin on the outside, the same as your fingernails and the claws of predatory animals, and the barbs are made of chitin, the stuff that makes up insect carapaces. Not a very digestible mix.

The ancestors of modern kleptocnidae were the ones who survived ingesting nematocyst-armed prey. That's how evolution works

All descriptions of this process I've seen describe a "migration" by the nematocyst which would imply that they work their way through the tissue propelled by the normal body movements of the worm itself, a common enough process for foreign objects in animal tissue.

This person's proposition of specialised cells which "carry" the nematocysts appears to be his own hypothesis, not those of current zoological thought, unless I've missed whole screeds of articles on this.

Of course it's these specialised cells that carry his whole argument.

If the nematocysts work their way through the tissues of the worm themselves, and by virtue of their own geometry point the right way - they're pointed at one end and bulbous at the other- you can see this working in your head, this statement:

When examined closely, each step in the internal manipulation of the nematocysts resolves into a complex of substeps. For example, for a nematocyst to be transported to the back of the flatworm, one of the worm's cells must first recognize it, and then the cell must initiate a process of motion that specifically carries the nematocyst to the worm's dorsal region. These are both complex procedures. Yet for the flatworm to take advantage of the hydra's nematocysts, it would seem that many complex arrangements of this kind must be present simultaneously.

is simply bollocks.

There is also a further problem. Evidently the nematocysts are not triggered simply by pressure applied to the worm's back. Rather, their firing is governed by a complex control mechanism within the worm. Without this trigger mechanism the whole arrangement would be useless

This is just wrong. Nematocysts will fire in response to chemical as well as tactile stimuli.

They don't need help from surrounding cells

"Nematocysts are capable of discharging with an initial acceleration of 40,000g; two factors contribute to the force of this firing. The first of these factors is the tension contained within the collagenous capsule walls, and the second factor is the osmotic pressure within the capsule itself (Tardent 357). A resting nematocyst possesses an osmotic pressure of 150 barr based on capsule wall tension and elevated concentrations of K+ and poly ?-L-glutamic acid (Tardent 356)." (Lifted from a zoology discussion website.)


So here's his problem with the evolution of this adaptation:

Let us consider whether or not these defensive arrangements of the flatworm could have evolved step by step. An evolutionary scenario would have to begin with an ancestral flatworm that ate hydras but did not make use of the nematocysts. In such a worm, what would be the fist (sic) evolutionary step leading to the eventual exploitation of the nematocysts as defensive weapons? Unless the nematocysts were actually used as weapons, for the worm to manipulate them internally would be useless. Indeed, it would be dangerous, since the flatworm can easily be killed by the discharge of the hydra's stings.

Ancestral worms eat hydra - some survive - some of their offspring inherit the gene that lets them survive nematocyst venom - more survive - evolving into a new species as the generations undergo more adaptation, including the retention of nematocysts in their bodies. Those with nematocysts are better equipped to survive predation and the adaptation contibutes to the success of the organism.

no mystery.

just natural selection.