Snails Go On the Offensive

by Dr. Elizabeth Mitchell on May 21, 2011

Sciencemag: “ScienceShot: How the Snail Picked Its Poison The venom gland in a poisonous mollusk sheds light on the origin of defense/attack structures.

Predatory marine cone snails paralyze their prey with neurotoxic peptides delivered through a harpoon-like tooth. The venom gland originates late in metamorphosis by “rapidly pinching off” a ventral channel from the esophagus. In other mollusks, a similar structure develops into mucous glands or digestive enzyme glands.

This esophageal “out-pocketing has no function in larvae,” but it differentiates into the venom gland while the larva continues to feed through the other esophageal channel. Developmental biologist Louise Page dissected many snail larvae to find the elusive origin of the gland. She notes that this non-disruptive modular development of the adult venom gland addresses “a core issue for evolutionary biology: how can any component of a complex system change during evolution without disrupting the functional integrity of the whole?”2

The evolutionary scenario is limited by the irreducible complexity of the system: all parts—harpoon tooth, venom sac, and neurotoxic peptides—must already be present before the feeding system would be selected for preservation.

While the harmlessness of the out-pocketing meets one criterion for a successfully evolving structure, the fact that it offers no survival advantage until the poisonous enzymes develop argues against the evolutionary paradigm. The evolutionary scenario is limited by the irreducible complexity of the system: all parts—harpoon tooth, venom sac, and neurotoxic peptides—must already be present before the feeding system would be selected for preservation.

This development represents not evolution, but speciation, as well as the apparent expression and preservation of a defense/attack structure. Just as thorns and thistles are modifications of other plant parts, so the cone snail’s venom sac is a modification of a larval structure which develops in other snails to become more benign structures.

An organism’s genome typically contains more information that it expresses. The genes producing the venomous destiny for the esophageal out-pocketing may have well been present in the original created genome. Their noxious expression awaited God’s curse on His creation. And by equipping cone snails to, at some point in time, expand their diet to include their neighbors, God enabled them to compete successfully in a hostile world and pass this trait on to their offspring, producing the 500 extant cone snail species we see today.

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Footnotes

  1. Page, Louise R. 2011. Developmental modularity and phenotypic novelty within a biphasic life cycle: morphogenesis of a cone snail venom gland. (Published online before print May 18, 2011, doi: 10.1098/rspb.2011.0501 Proc. R. Soc. B).

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