Ways to Die: Snake Venom

The vast majority of snakes that one encounters in the wild (unless you live in Australia or India) are either non-venomous to humans or want nothing to do with you.

However, should you stumble upon a rattlesnake nest or coral snake hole while texting in the middle of nowhere, there will probably be a combination of different enzymes and polypeptides pumped into your body, via the modified parotid salivary glands (right below the ear in humans) that snakes have evolved over the ages, to disable their prey. Of course, you’re not prey, but you stepped on a snake while texting. It has every reason to envenomate you.

While all snakes have multiple active enzymes in their venom, all snakes dangerous to humans have either neurotoxins or cytotoxins as a significant component in their venom. For the most part, elapids (such as the cobras and mambas) create neurotoxins, while the viperids (such as vipers, adders, and rattlesnakes) create cytotoxins.

Neurotoxins

• Dendrotoxins: Inhibit neurotransmission by blocking the exchange of positive and negative ions across the pre-synaptic neuronal membrane, causing paralysis. Found in some rattlesnakes (such as the Mojave) and mambas.
• Fasciculins: Destroys acetylcholinesterase (AChE) in synaptic clefts of nerves. Without AChE, acetylcholine (ACh) is not broken down, and remains bound to the postsynaptic vesicles of the nerve, leading to constant contraction of the related muscles. This is called tetany or tetanic paralysis. Found only in mambas.
• α-neurotoxins: Very large group of toxins that mimic ACh and bind to post-synaptic vesicles, leading to numbness and paralysis. Found in cobras, kraits, and sea snakes. 

Cytotoxins

• Cardiotoxins: Target muscle cells and cause depolarization. If enough of these components reach the heart, the depolarization can cause irregular heartbeat or spontaneous stopping of the heart. Can cause fasciculations in skeletal muscles. Found in the Naja genus, and in King Cobras. Minor but important component of mamba venom.
• Phospholipases: Proteins that target the phospholipid bilayer of cells, causing cellular rupture. Can cause extreme blistering at site of bite. Relatively uncommon, found in the Japanese Habu.
• Hemotoxins: Burst red blood cells (hemolysis), causing thin blood, internal bleeding, and blood clots due to the massive clotting response. Found to some degree in almost all vipers, as well as some cobras.

Images:
Top: Bungaris fasciatus - Banded Krait. An elapid, and the largest of the kraits. Has neurotoxic venom. [source]
Center Right: Hydrophis robusta [now Hydrophis spiralis] - Yellow Sea-Snake. The longest sea snake, at 3 m (9.8 ft). A member of the Hydrophiinae, separate from other elapids. Though they have some of the most toxic venom in the world, bites are extremely uncommon and often unnoticed. [source]
Center Left: Vipera russellii - Russell’s Viper. A particularly aggressive viperid. Necrosis and amputation following envenomation not uncommon, due to hemolysis and local cell damage. [source]
Bottom: Vipera caudisona [now Crotalus horridus] - Timber Rattlesnake. A venomous viperid endemic to the United States. Primarily hemotoxic venom, very low fatality rate, but very painful bites. [source]

Thornback ray (Raja clavata) and thornback ray skeleton

Like sharks, rays and skates have fully cartilaginous skeletons, which provide a stable structure but more flexibility than bone. You can see that, much like fish, rays have defined, er, rays, in their fins. The difference is that while fish tend to have a few unconnected rays and a taught taut tissue between them, the Rajiforms (skates and rays) have many, many rays, which are all connected perpendicularly by collagen. The body is then formed around these rays, which propel the Rajiforms forward in an undulating (wave-like) motion.

A history of the fishes of the British Islands. Jonathan Couch, 1863.

Note: Not “historical”, but a subject I’ve dealt with recently, by way of a friend who bought a skull online.

I’m partial to cow skulls, myself…but this is a good comparative anatomy overview for anyone interested in big animals!

shadyufo:

So often I see “horse skulls” for sale that are actually cow skulls. A lot of folks automatically assume that if it is a big skull with no horns then is must belong to a horse. Here is a side by side comparison to show how vastly different these two critters are.

Skull on the left in the first photo is a horse. The skull on the right is a cow. In the second photo the skull on the bottom is a horse and the skull on top is a cow. 

Cows have broad, thick heads, especially near the top of the skull while horses have longer, more tapered skulls. The most obvious difference between the two though is the teeth. Horses have both upper and lower incisors (twelve teeth in total) at the front of the mouth while cows only have lower incisors (six). Since horses don’t ruminate or have four stomachs like a cow they need to be able to break down their food a little more thoroughly before they swallow it. Having upper incisors helps them get the good out of every bite. 

Sometimes horses will also have canine or even wolf teeth that grow between the incisors and the molars. Wolf teeth are actually vestigial premolars which are often removed if the horse is a work animal because they can interfere with a bit. Canine teeth look like fangs and horses can have anywhere from zero to five of them. Most often they are found in stallions and geldings (my big draft gelding has some scary canine teeth) but mares can have them too. I think it’s something like less that 30% of mares have them though and they often times only have one or two while stallions usually have a full set of four (two uppers and two in the bottom jaws).

So to recap: Horses have long, thin skulls with incisors in both the skull and lower jaws  while cows have thick, broad skulls, sometimes with horns, with incisors only present in the bottom jaws. 

Nine-Banded Armadillo (Dasypus novemcinctus)

Did you know that the nine-banded armadillo (and a few of its Dasypus cousins) gives birth to identical quadruplets in almost every litter? Shortly after the zygote implants in the uterus, it splits into four (or occasionally three or five) separate embryos, each of which develop their own independent placenta. This means that, unlike in identical human fetuses, blood and nutrients are not shared, and the death of one fetus is unlikely to affect the survival of the others. After the pups are born, they remain in the burrow for approximately three months, and over the next year of their life, slowly wander farther and farther away from their place of birth.

As nine-banded armadillos have few natural predators in their Northern range, this highly effective reproduction strategy means that one female will often produce upwards of 50+ offspring in her relatively short lifetime. Those offspring have been expanding the armadillo’s known range for the past several decades. However, as armadillos are poor at thermoregulation, they’ve just about reached the limit of the area that they can survive in - any farther north, and they would not be able to survive the longer winters.

Images:

Top: Tatusia novem cincta [now Dasypus novemcinctus] - The Nine-Banded Armadillo. From Biologia Centrali-Americana. F. Ducane Godman and Osbert Salvin, 1918.

Bottom: Fetal Nine-banded Armadillo Pups. The American Journal of Anatomy. Vol. III, 1900-1901. “Enamel in the teeth of an edantate.” A. M. Spurgin.

Limbs of the Cephalopoda

Whether squids, octopuses, and nautilus have “arms” or “tentacles” is often simply a matter of semantics, but the most accepted definitions (from what I’ve found) tend to define the “arm” as a tapered limb, with two rows of suckers along its entire length. “Tentacle” is typically a length of tapered limb with no suckers, leading to a distal club-like appendage, covered in suckers.

One exception would be limbs in the nautilus - they have up to 90 un-suckered limbs, but their limbs are called “tentacles” by those who study them, even without the terminal club.

Images:
Top right: Octopus vulgaris and detail of beak and arms
Top left: Detail of tenticular clubs in squid, from the Expedition of the Valdivia
Bottom right: Arm of Illex illecebrosis (Northern Shortfin Squid)
Bottom left: Tentacle of Illex illecebrosis