Saturday, November 13, 2010

Cone of Silence

Genus Conus LINNAEUS, 1758. Not really a taxa that many people give much though to, but cone shells (or cone snails) are ubber cool. There are about 500 extant species of Conus, that's the largest genus of marine invertebrates. These mollusks are found between latitude 40° North and the 40° South parallel. That means you can find them in tropical and subtropical oceans including the Indo-Pacific, Panamic, Caribbean, West African, South African, Peruvian, Patagonic, and Mediterranean Seas. You can find a few other species outside of this region but they tend to be localized in South Africa, Southern Australia, and Southern Japan. Cone snails live in the intertidal muds and sandflats, areas where the high and low tides alternate, but you can also find some offshore or in deep waters.

When picturing the structure of a cone shell, think of something like an underwater snail. They have a strong, muscular foot with a flat sole that is truncated or widely rounded at the front and pointed at the back. The foot can be striped or pimpled, but the coloring is really variable, not just due to genetics but environmental factors as well. On each side of the head they have an eye on a stalk, stalks that are wide at the bottom and narrow at the end. The mouth of this animal is very elastic and includes sharp and often hooked teeth, allowing the cone shell to swallow large prey. Being a cone shell, they are covered by a shell. This shell is spiral shaped and can have interesting patterns, and they are very desirable to shell collectors.

Most people, including me, find the cone snails' venom to be its most interesting feature. We're talking venom that is often fatal, or at the very least causes temporary paralysis, respiratory trouble, or swelling and inflammation (depending on the species). The composition of this venom varies depending on the species, the individual, or even between injections by the same individual. The active components are small, disulfide-rich peptides called conotoxins or conopeptides, and they cause paralysis in the victim. The specific paralytic components include alpha-, omega- and mu-conotoxins which all prevent neuronal communication, each targeting a different aspect of the process. Alpha-conotoxins target the nicotinic ligand gated channels, omega-conotoxins target the voltage-gated calcium channels, and the mu-conotoxins target the voltage-gated sodium channels. These toxins are particularly interesting to scientists, especially neurobiologists and medical researchers, because they can be used to identify specific ion channels.

To be effective the venom must be delivered from the cone shell to the prey. The cone shell itself is relatively slow and unable to swim, and yet it hunts other, faster marine organisms such as fish. The venom is synthesized in the epithelial cells of a long, convoluted venom gland and stored in the gland's lumen. When the cone snail zeros in on its prey it extends it's proboscis which is loaded with venom and tipped with a specialized radula tooth that functions as both a harpoon and hypodermic needle. The snail then shoots it (by a ballistic mechanism, we're talking around 400 miles per hour) into the prey to deliver the venom. It is known that the distal end of the venom gland dilates into an oval structure called the venom bulb and it has been suggested the this bulb functions in venom transport, perhaps like a peristaltic pump. If you look at other animals that use jet propulsion, like scallops and squid, you see that the closing of their valves requires a burst contraction of the adductor muscle. This muscle shows high levels of glycolytic enzymes as well as arginine kinase (a type of phosphagen kinase).

Figure 1 showing the venom apparatus of cone snails.
Also, Figure 1A is probably the best figure I've ever seen in a peer reviewed paper.
 A study in the Journal of Proteome Research takes a closer look at the Australian cone species Conus novaehollandiae and Conus victoriae in order to shed some light on the role of the venom bulb, or pump. Specifically they look at the levels of enzymes and kinases integral to pump function. In terms of methods they did a protein extraction and 2-dimensional gel electrophoresis, a one dimensional gel electrophoresis of the venom gland and bulb proteins, a cDNA (complementary DNA) isolation and identification of arginine kinase and BIP (immunoglobulin binding protein), and an in situ hybridization of the venom bulb using an arginine kinase specific probe.

After lots of tables and graphs, some colorful and pretty and some not-so-much, they found that the venom bulbs contain high concentrations of arginine kinase. The presence of this kinase enables the venom bulb to contract very rapidly and repeatedly. That means that the cone snail can quickly force the venom through the venom duct and out through the proboscis and into the harpooned prey. In addition to the kinase, morphological examination of the bulb showed the organ to be highly muscularized. Three distinct muscle layers are separated by a tunic-like collagen sheet and the outer muscle layer, in particular, contains radially, spirally organized collagen fibers. Ok, cool. Layered muscle. What does that matter? Well, if we go back to the squid comparison you see that squids have inner and outer surfaces of muscle lined with collagen tunics. These tunics are stronger than the muscles and prevent the muscle from stretching longitudinally during contraction. This restriction and contraction allows the squid to propel water through it's jet at very high speeds. Now, the cone's venom bulb is less complex but it is likely that the function is similar. So rather than just holding the venom, these researchers found that the venom bulb is an active participant in the injection event itself. Previous studies have shown that the venom is pressurized before injection. This study shows that repeated burst contractions of the venom bulb in combination with the relaxation of the proboscis leads to a sudden ballistic discharge of the radula tooth, where it is shot into the prey and the pressurized venom pumped in by ongoing, repeated burst contractions of the venom bulb (you got an image of that in your head right? Wow!).

Read more in the article:
Safavi-Hemami, Helena , Neil D. Young, Nicholas A. Williamson, Anthony W. Purcell (2010) Proteomic Interrogation of Venom Delivery in Marine Cone Snails: Novel Insights into the Role of the Venom Bulb. Journal of Proteome Research: 9(11), 5610–5619. (DOI: 10.1021/pr100431x)

Learn more about cone shells at these links:
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