Tag Archives: poisonous

North America’s Big Five Centipedes

When Halloween comes around, snakes and spiders tend to steal the show. Yet centipedes, in my experience, tend to evoke even stronger reactions from people — I have met many entomologists who would happily handle a tarantula but recoil in horror when faced with a giant centipede.

In the United States there are five species of giant centipedes in the family Scolopendridae. Today, in the spirit of Halloween, I give you the Big Five: where they are found, what they do, and why I love them.

Blue Tree Centipede (Hemiscolopendra marginata)

The blue tree centipede. Photo by Sharon Moorman.

The blue tree centipede. Photo by Sharon Moorman.

This is the smallest of the five, seldom exceeding 3 inches, but still the largest centipede throughout most of its range. It is found through much of the East, from Ohio and Pennsylvania south to Florida and west to eastern Texas. Tree centipedes are also found in Mexico south to the Yucatan Peninsula. As the name suggests, the tree centipede is often an attractive blue-green, with yellow legs and orange fangs. The brightness of the color depends on the location, however, and some are paler than others.

The blue tree centipede is a habitat specialist, living under the bark of rotting trees, often before they have toppled to the ground. I have had the best luck finding them under the bark of pine logs. Because they are such good climbers, they occasionally wind up in buildings where they can cause quite a scare.

Bites from tree centipedes are painful but not much worse than a bee sting. They use their venom, as all centipedes do, to kill prey. Because they prefer to live in rotten pine logs, they may specialize in hunting beetle grubs that eat rotting wood. Like most centipedes, however, data on their feeding habits is severely lacking.

Green-striped Centipede (Scolopendra viridis)

The green-striped centipede is larger, reaching 6 inches or so, and usually pale yellow with a thick green or black stripe running down the back. Other patterns exist, however, and in parts of their range this species can appear more like a tree centipede or a tiger centipede (#4). These are adaptable centipedes, found from Florida west to Arizona, but don’t seem to venture further north than South Carolina.

The green-striped centipede. Photo by Jeff Hollenbeck, licensed under CC BY-ND-NC 1.0.

The green-striped centipede. Photo by Jeff Hollenbeck, licensed under CC BY-ND-NC 1.0.

Green-striped centipedes can live in a variety of habitats but they seem to prefer sandy forests. In Florida they can be found in scrub habitat, but like all centipedes they are not well-adapted to drought, and must stay moist by hiding underground or in rotting logs during the day.

Caribbean Giant Centipede (Scolopendra alternans)

The Caribbean giant is the only one of the Five with the russet-brown, mono-chromatic appearance of a “typical” centipede. It is probably our largest species, with a length easily exceeding 8 inches. However, the Caribbean giant is, as you might have guessed, a tropical centipede, and in the U.S. it lives only in southern Florida. It requires humid habitats, and the best place to find them is in and around the Everglades, in Dade and Monroe Counties.

A certain foreign species, the Vietnamese giant (Scolopendra subspinipes), is easily confused with the Caribbean giant at first glance. That wouldn’t be a concern, except that the Vietnamese giant has already become invasive in Hawaii and — this is just my speculating — is likely to become established in the Everglades at some point in the future. Because it is so large, often exceeding 10 inches, the Vietnamese giant is sometimes sold in the pet trade. Bites from either species are not deadly, but extremely painful.

Tiger Centipede (Scolopendra polymorpha)

A tiger centipede from Arizona. Photo by Sue Carnahan, licensed under CC BY-ND-NC 1.0.

A tiger centipede from Arizona. Photo by Sue Carnahan, licensed under CC BY-ND-NC 1.0.

Like the green-striped centipede, the tiger is a 6-inch-long animal found in a variety of habitats. Unlike the green-striped, this is a strictly western species, found from Idaho south through California into Mexico, and east all the way to Missouri. Its name comes from its color pattern: each segment is orange or yellow with a narrow, dark band.

Giant centipedes often move faster by undulating in a snake-like fashion, taking advantage of their long and muscular bodies. When a tiger centipede does this, the bands appear to “flicker,” rather like the brightly-banded milk snake and coral snake. This can make the centipede more difficult to track visually, and hence more difficult for a bird or mouse to grab.

Tiger centipedes, like their namesake, are voracious predators. They have been seen taking down prey much larger than themselves, including geckos and praying mantises. In turn, tiger centipedes are prey for scorpions, spiders, snakes, and many other predators.

A tiger centipede, fallen prey to a scorpion. Photo by Jasper Nance, licensed under CC BY-NC-ND 2.0.

A tiger centipede, fallen prey to a scorpion. Photo by Jasper Nance, licensed under CC BY-NC-ND 2.0.

Centipedes are adapted to moving fast, and their exoskeletons are thin and flexible. The drawback is that they dehydrate very easily. Although tiger centipedes are found in deserts, they still have to remain underground most of the time to conserve moisture.

Giant Desert Centipede (Scolopendra heros)

If you’ve ever seen centipedes used in a horror movie, they were probably heros*. They are big, reaching 8 inches or more. They are also brightly colored in black and orange — perfect for Halloween!

The Arizona form of the giant desert centipede. Photo by Aaron Goodwin, licensed under CC BY-ND-NC 1.0.

The Arizona form of the giant desert centipede. Photo by Aaron Goodwin, licensed under CC BY-ND-NC 1.0.

Heros are found in the desert Southwest, and color patterns vary by location. In eastern Texas and Oklahoma, they are typically jet-black with a bright orange head and yellow legs. In Arizona (above) they are usually red, with the first and last segments black. In New Mexico and western Texas the pattern is orange with black bands, much like a tiger centipede.

A giant desert centipede. Photo from NMNH Insect Zoo, licensed under CC BY-NC 2.0.

A giant desert centipede. Photo from NMNH Insect Zoo, licensed under CC BY-NC 2.0.

Why have black on just the head and the last segment? This an example of automimicry, in which one part of an animal’s body mimics the other. In this case, the tail-end of the giant desert centipede mimics its head-end. When faced with a giant centipede, predators usually attack the head, hoping to avoid a painful bite. If a predator gets confused, however, and attacks the tail instead, an unpleasant surprise awaits when the true head whips around to greet its attacker.

Centipedes, giant and otherwise, are pretty scary, and I never begrudge people who are afraid of them. Still, centipedes are amazing animals and if you see one, I encourage you to take a closer look. It will teach you, if nothing else, that just because an animal is frightening does not mean it can’t be beautiful.

*There is a centipede in one of the Human Centipede movies. People often tell me this after I tell them I study centipedes, so let me clarify a few things: I don’t know what kind of centipede the bad guy has for a pet. Not because I couldn’t identify it, but because I have never watched those movies and never will. I also don’t want to hear you describe your favorite scene with as many details as possible. Thank you.

Why Scorpion Venom is So Complex

by Joseph DeSisto

Scorpion venom, like many animal venoms, is incredible complex. It is made up of hundreds of different toxins and other proteins, each with a specific function, all mixed together in a lethal cocktail. Why do scorpions need so many different toxins? Last week, scientists at the Chinese Academy of Sciences published the results of their attempt to answer this question (Zhang et al. 2015).

They began by studying a particular class of proteins found in scorpion venom, which work by attacking the sodium ion-channel proteins in their victims.

The stripe-tailed scorpion, Vaejovis spinigerus, ready for action.. Photo by Joseph DeSisto.

The stripe-tailed scorpion, Vaejovis spinigerus, ready for action. Photo by Joseph DeSisto.

Sodium ion-channels help regulate the amount of sodium inside animal cells, which is vital for cells to function properly. In nerve cells, they are even more important: the change in sodium concentration inside and outside the cell is what transmits electric signals.

Toxins that inhibit sodium channels prevent the nervous system from working, which leads to death if the victim is small (like an insect). Scorpions have toxins called sodium-channel toxins to do exactly that. The puzzle is, scorpions have many different genes that produce sodium-channel toxins, each of which has a slightly different structure.

All proteins are essentially strings that are wound, twisted, and tied into a specific structure. The structure of a protein is critical to its function, since proteins need to have certain shapes in order to interact with each other like a lock and key. All sodium-channel toxins have a portion designated as the “interactive region” — the key — which attaches to a series of loops on the prey’s sodium ion-channel (the lock). If the key fits and the connection is successful, the prey’s ion-channel can no longer function.

A mother stripe-tailed scorpion, carrying young. Photo by Joseph DeSisto.

A mother stripe-tailed scorpion, carrying young. Photo by Joseph DeSisto.

Zhang and his colleagues studied the genome of their scorpion, a desert-dwelling East Asian species known as the Chinese golden scorpion (Mesobuthus martensii). They found no less than 29 different genes coding for sodium channel toxins.

There was a time, perhaps hundreds of millions of years ago, when scorpions only had one gene for sodium-channel toxins. Eventually that gene was duplicated, and thereafter the scorpion genome had multiple copies of the same toxin-producing gene. Since then, each copy of the gene has continued to mutate and evolve in its own direction. Now each toxin, despite having the same basic structure, is just a little bit different from the rest.

As it happens, the genes for sodium ion-channels in a scorpion’s prey also exist in multiple copies, each with minor variations. Zhang and colleagues hypothesized that scorpions need so many varieties of toxins because each toxin can only interact with a specific variety of ion-channel. In other words, scorpion venom needs lots of different keys because the prey have so many different locks.

To test this, the scientists examined the different toxin-gene copies to better understand how they had evolved. Sure enough, the “interactive region,” the key, of each toxin had mutated and evolved much more quickly than the “body” of the toxin. This provided strong evidence that natural selection has caused scorpion venom to evolve different types of toxins to keep up with the ever-evolving ion-channels in their prey.

Scorpions are incredible animals for so many reasons. They have been around for more than 400 million years — as long as there have been insects to hunt on land, scorpions have been there to hunt them. They are amazing and diverse in form, lifestyle, and hunting strategy. How fitting that they should be just as amazing on the molecular level.

Cited:

Zhang S., B. Gao, and S. Zhu. Target-driven evolution of scorpion toxins. Nature Scientific Reports 5:14973 doi: 10.1038/srep14973

Poisonous Frogs, Beetles, and Birds

by Joseph DeSisto

Meet the golden poison frog of Colombia’s coastal rain forests. This frog, one of nearly 200 species of poison frogs, is by far the most toxic. A single frog packs enough poison to kill 10,000 mice, or 10 or more humans (Myers et al. 1978).

The golden poison frog (Phyllobates terribilis). Photo by Brian Gratwicke, licensed under CC BY 2.0.

The golden poison frog (Phyllobates terribilis). Photo by Brian Gratwicke, licensed under CC BY 2.0.

For the golden poison frog and its close relatives in the genus Phyllobates, batrachotoxin is the weapon of choice. Batrachotoxin acts on the nervous system, opening up the membranes of nerve cells so they can no longer carry signals to and from the brain. Death comes from paralysis, which leads to heart failure.

The golden poison frog was only discovered in 1971 when scientists found them around an indigenous Colombian (Emberá Chocó) village (Myers et al. 1978). The Emberá use the frogs to lace poison darts, with which they hunt game in the surrounding forest. The frog-handlers were careful to cover their hands with leaves, with good reason. Scientists who touched the frog felt a strong burning sensation, and they stressed in their initial description that:

The new species is potentially dangerous to handle: One freshly caught frog may contain up to 1900 micrograms (µg) of toxins, only a fraction of which would be lethal to man if enough skin secretion came into contact with an open wound.”(Myers et al. 1978, pp. 311)

The black-legged poison frog (Phyllobates bicolor), closely related to terribilis but not quite as toxic. Photo by Drriss and Marrionn, licensed under CC BY-NC-SA 2.0.

The black-legged poison frog (Phyllobates bicolor), closely related to terribilis but not quite as toxic. Photo by Drriss and Marrionn, licensed under CC BY-NC-SA 2.0.

It is important, as the frog’s discoverers remind us, “to be cautionary, not alarmist” (Myers et al. 1978, pp. 340). Even though in theory these frogs are dangerous, there is no record of a person ever being killed by one. Although the poison can go through a person’s skin, it seldom does so in enough quantity to injure. The frogs do not bite. So if you see golden poison frogs while exploring in Colombia, do not panic, but be wary. However delicious and lemon-drop-colored they may seem, definitely don’t try eat them.

Golden poison frogs are sometimes sold in the pet trade, since they lose their poison after being taken out of the wild. This is probably because frogs get batrachotoxin from their food: soft-winged beetles that make the toxin themselves (Dumbacher et al. 2004). In captivity, frog-keepers give their pets a blander diet of crickets and fruit flies, which don’t contain batrachotoxin.

An example of a soft-winged beetle, in the same family as those eaten by poison frogs. Photo by Udo Schmidt, licensed under CC BY-SA 2.0.

An example of a soft-winged beetle, not the same species, but in the same family as those eaten by poison frogs. Photo by Udo Schmidt, licensed under CC BY-SA 2.0.

Soft-winged beetles are found all over the world, especially in the tropics, but so far only a few other animals are known to eat them and use their batrachotoxins. Three of these are poison frogs, all found in a small rain forest region of Colombia. The others are birds. Yes, there are poisonous birds, and even though this blog is explicitly not about birds or mammals, I’m going to break that rule today.

The hooded pitohui (Pitohui dichrous) -- both males and females are brightly colored. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

The hooded pitohui (Pitohui dichrous) — both males and females are brightly colored. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

The toxic birds are all found in New Guinean rain forests, and most belong to a group of insect-eaters called pitohius (pronounced PI-to-hooies). Pitohui birds are related to the orioles and blackbirds found in more temperate climes. Shown above is the hooded pitohui, first found to be poisonous when a bird researcher handled one and left with a tingling, burning sensation in his hand.

Later study showed that the hooded pitohui, along with two other related species, has feathers laced with batrachotoxins (Dumbacher et al. 1992). More than a decade later, the same scientists demonstrated that toxin-wielding pitohui birds eat soft-winged beetles, and that these same beetles are loaded with batrachotoxin (Dumbacher et al. 2004). Despite being toxic, the birds are not nearly as dangerous as golden poison frogs, and there is little risk to a careful handler.

The variable pitohui (Pitohui kirhocephalus), not as showy as its cousin, but toxic all the same. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

The variable pitohui (Pitohui kirhocephalus), not as showy as its hooded cousin, but toxic all the same. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

Several more birds are now known to use batrachotoxins, and all are found in New Guinea (Weldon 2000). Many of them have similar red-and-black color patterns. By having similar colors, multiple bird species can work together to “educate” predators who might not be aware of the poisonous feathers (Dumbacher and Fleischer 2001). To make matters even more interesting, the toxins in bird feathers apparently serve as a repellent to parasitic lice (Dumbacher 1999).

We may continue to learn more about these amazing birds and their lives, or we may not. Most of these birds are becoming rarer and rarer as New Guinean rain forest is slashed and burnt, tilled and grazed into nothing.

I’ve written several articles about poisons and venoms: click here to learn about brown recluse venom and here to learn about tetrodotoxin, a poison used by many fish as well as newts, snails, and blue-ringed octopuses.

Darren Naish, writer of the superb science blog Tetrapod Zoology, writes often about birds. Click here for one of his articles on a poisonous New Guinean species. Note that the article is not on his most recent blog site, which is updated regularly at the first link to Scientific American.

To learn more about the relationship between lice and toxic pitohui birds, click here to read an excellent article by Bianca Boss-Bishop on the aptly-named blog Parasite of the Day.

Cited:

Dumbacher J.P. 1999. Evolution of toxicity in Pitohuis: I. effects of homobatrachotoxin on chewing lice (order: Phthiraptera). The Auk, 116: 957-963.

Dumbacher J.P., A. Wako, S.R. Derrickson, A. Samuelson, and T.F. Spande. 2004. Melyrid beetles (Choresine): a putative source for the Batrachotoxin alkaloids found in poison-dart frogs and toxic passerine birds. Proceedings of the National Academy of Sciences U.S.A. 101(45): 15857-15860.

Dumbacher J. P., B.M. Beehler, T. F. Spande, H. M. Garra¡o, and J.W. Daly. 1992. Homobatrachotoxin in the genus Pitohui: chemical defense in birds? Science 258: 799-801.

Dumbacher J.P. and R.C. Fleischer. 2001. Phylogenetic evidence for colour pattern convergence in toxic pitohuis: Müllerian mimicry in birds? Proceedings of the Royal Society of London B 268(1480): 1971-1976.

Myers C.W., J.W. Daly, and B. Malkin. 1978. A dangerously toxic new frog (Phyllobates) used by Emberá Indians of western Colombia, with a discussion of blowgun fabrication and dart poisoning. Bulletin of the American Museum of Natural History 161(2): 311-365.

Weldon P.J. 2000. Avian chemical defense: toxic birds not of a feather. Proceedings of the National Academy of Sciences U.S.A. 97(24): 12948-12949.

How Poisons Work: Tetrodotoxin

by Joseph DeSisto

This is the first of a series of short articles, each featuring a different type of poison or venom used by animals.

Poisons and venoms are some of the most complex substances in nature, often containing hundreds of different chemicals, each with a particular purpose. As technology advances, scientists have begun to look at some of these chemicals, usually proteins, and try to figure out what they do.

The deadly southern blue-ringed octopus (Hapalochlaena maculosa). Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

The deadly southern blue-ringed octopus (Hapalochlaena maculosa). Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

In some cases, the results give us a new perspective on an animal’s biology: rattlesnake venom, for example, contains a unique protein that allows the snake to track its prey after the initial strike. In other cases, we discover potentially useful surprises: giant centipede venom contains a single protein that inhibits pain in mice, using the same chemistry as morphine, but with greater efficiency. However, regardless of whether the results are useful to us, studying nature’s chemical weapons gives us a whole new appreciation and understanding of the creatures that wield them.

Tetrodotoxin is the poison of choice for a variety of animals, especially in the ocean. These include blue-ringed octopuses, cone snails, moon snails, certain angelfish, some ribbon worms, a handful of amphibians, and the puffer/triggerfish order Tetraodontiformes, for whom the toxin is named. Many of the animals that use tetrodotoxin are brightly colored, a warning to passers-by. Would-be predators, save the immune and the unlucky, heed this warning well.

An inflated pufferfish (Diodon holocanthus). Photo from Williams et al. (2010), licensed under CC BY 2.5.

An inflated pufferfish (Diodon holocanthus). Photo from Williams et al. (2010), licensed under CC BY 2.5.

Tetrodotoxin, sometimes abbreviated TTX, is a complex molecule that prevents its victims’ nerves from functioning properly. This eventually leads to paralysis, and death comes when the muscles that control breathing no longer receive signals from the nervous system. Basically, you suffocate. Humans can get TTX poisoning when they eat improperly-prepared pufferfish, but stings from blue-ringed octopuses and cone snails (both from the southwestern Pacific) are also a possibility if you are foolish enough to pick them up.

Although many animals use TTX, none of them actually manufacture the toxin themselves. Instead bacteria generate the toxin, to their host’s benefit. In exchange, the bacteria are allowed to live safely within their host’s body — the bacterium Pseudoalteromonas tetraodonis, for example, makes its home within the livers and skins of pufferfish.

A toxic moon snail (Naticarius orientalis) from East Timor. Photo by Nick Hobgood, licensed under CC BY-SA 3.0.

A toxic moon snail (Naticarius orientalis) from East Timor. Photo by Nick Hobgood, licensed under CC BY-SA 3.0.

The rough-skinned newt, from British Colombia and the western United States, has the bacteria needed to make TTX for itself — as a result, this newt has few natural predators. Two animals, however, have managed to work around and even benefit from the rough-skinned newt’s toxins, all without toxin-producing bacteria of their own.

The only animal capable of eating an adult rough-skinned newt is the garter snake. After ingesting the newt, any other predator would almost certainly die, but a few populations of garter snakes have evolved an immunity to TTX. What’s more, the snakes are able to sequester the toxins within their own bodies, so that the garter snakes themselves become poisonous (Williams et al. 2004).

The extremely toxic rough-skinned newt (Taricha granulosa). Photo by Rennett Stowe, licensed under CC BY 2.0.

The extremely toxic rough-skinned newt (Taricha granulosa). Photo by Rennett Stowe, licensed under CC BY 2.0.

Although many snakes inject toxins with their fangs, and so are venomous, garter snakes that eat rough-skinned newts are the only snakes that can truly be considered poisonous. The difference is that venomous animals have to inject their poisons into predators or prey, via fangs or stingers, while poisonous animals are laced with their chemical weapons and are dangerous to eat.

The other TTX-robber is, unexpectedly, a caddisfly. Caddisflies are insects whose larvae resemble caterpillars, but live underwater and surround themselves with cases made from pebbles, twigs, and other debris. Most are scavengers, eating decomposing plant matter and tiny invertebrates, while the flying adults are short-lived and do not feed.

A Limnephilus caddisfly larva, in its protective case made of twigs. Photo by Tom Murray, used with permission.

A Limnephilus caddisfly larva, in its protective case made of twigs. Photo by Tom Murray, used with permission.

A few predatory species in the genus Limnephilus, however, have developed an unusual appetite for rough-skinned newt eggs, which also develop in the water. These eggs are loaded with TTX, and the toxin-resistant Limnephilus larvae manage to eat so many that even the caddisfly adults are toxic (Gall et al. 2012).

Even though many marine invertebrates contain tetrodotoxin, we still don’t know how many contain the bacteria that make it, versus how many, like the caddisfly, “steal” the toxin from their TTX-laced prey. Certain sea slugs, for example, are often washed on shore where they are eaten by beach-combing scavengers such as dogs. In New Zealand, several dogs have died after eating sea slugs that contained TTX (McNabb et al. 2009), and in Argentina, a population of the same slugs appears to have become invasive (Farias et al. 2015). Yet we still do not know whether they have their own TTX-generating bacteria.

A toxic sea slug, Pleurobranchaea meckelii. Photo from Wägele and Klussmann-Kolb (2005), licensed under CC BY 2.0.

A toxic sea slug, Pleurobranchaea meckelii. Photo from Wägele and Klussmann-Kolb (2005), licensed under CC BY 2.0.

Experiments have shown that certain populations of sea slugs have TTX, while others do not (Khor et al. 2014) — just like the garter snakes of North America. Does this mean the sea slugs are obtaining TTX from some unknown, toxic prey item? The same researchers conducted a survey of all the marine invertebrates that these slugs might be eating, testing everything for TTX. The only positive result was a toxic species of sand dollar, but the sand dollars didn’t produce nearly enough TTX to explain the huge amounts found in sea slugs.

Ignorance has consequences, and there is still plenty of exploring left to do. The slugs may in fact make tetrodotoxin themselves (using bacteria). A more enticing possibility, and just as likely, is that there are still more toxic animals on the sea floor, waiting to be discovered.

One of the very first articles I wrote for this site featured the same sea slugs mentioned in this one, but that was when my only readers were a few sympathetic family members. If you’re curious about sea slugs, and/or you want to know the difference between a nudibranch and a pleurobranch, you can read that article here.

I also recently wrote about how rattlesnake’s use their venom to track once-bitten prey (mentioned in the second paragraph of this article). To learn more about that, click here. It’s amazing, I promise.

Cited:

Farias, N.E., S. Obenat, and A.B. Goya. 2015. Outbreaks of a neurotoxic side-gilled sea slug (Pleurobranchaea sp.) in Argentinian coasts. New Zealand Journal of Zoology, published online: DOI: 10.1080/ 03014223.2014.990045.

Gall, B.G., A.N. Stokes, S.S. French, E.D. Brodie III, and E.D. Brodie Jr. 2012. Predatory caddisfly larvae sequester tetrodotoxin from their prey, eggs of the rough-skinned newt (Taricha granulosa). Journal of Chemical Ecology 38(11): 1351-1357.

Khor, S., S.A. Wood, L. Salvitti, D.I. Taylor, J. Adamson, P. McNabb, and S.C. Cary. 2014. Investigating diet as the source of tetrodotoxin in Pleurobranchaea maculata. Marine Drugs 12(1): 1-16.

McNabb, P., L. Mackenzie, A. Selwood, L. Rhodes, D. Taylor, and C. Cornelison. 2009. Review of tetrodotoxins in the sea slug Pleurobranchaea maculata and coincidence of dog deaths along Auckland Beaches. Prepared by Cawthron Institute for the Auckland Regional Council Technical Report 2009/108.

Williams, B.L., E.D. Brodie Jr., and E.D. Brodie III. 2004. A resistant predator and its toxic prey: persistence of newt toxin leads to poisonous (not venomous) snakes. Journal of Chemical Ecology 30(10): 1901-1919.

Spiders Only Love Once

by Joseph DeSisto

In one of many vain attempts to show me that birds and mammals are far superior to invertebrates, a good friend once reminded me that swans only love once, in reference to some pop song or another. I reminded her that many spiders, too, only love once — for very different reasons.

Being a spider with parental care sounds like a good deal. Mother spiders of the Mediterranean lined spider (Stegodyphus lineatus) raise their young in silken nests and guard them as they grow, feeding them and reducing the likelihood that they will be found by predators. This is all well and good, but beneath it is a story, one of murder and suicide, sex and deceit, loyalty and sacrifice.

A mother spider, Stegodyphus lineatus. Photo by Joaquin Portela.

A mother spider, Stegodyphus lineatus. Photo by Joaquin Portela.

Romance

By the time a female lined spider has reached adulthood, her web is well-established (Maklakov et al. 2005). Rather than build a single, simple web like many other spiders do, lined spiders builds a shelter out of silk on a bush and then constructs many tangled webs, all radiating from the shelter. This way, she can capture prey from multiple locations, and when her young emerge, they have plenty of webs from which to harvest prey.

Males, on the other hand, spend most of their time wandering around looking for females. When a male finds one, he approaches with caution. She is much bigger than him, and may attack him if she is not in the mood for company. If, for example, she already has a clutch of eggs, or if she is not old enough to mate, or if she is just feeling hungry, he might be out of luck.

If, however, she is receptive, the male will mate with her and then stay for a few days, to prevent any other males from approaching. When he (or she) decides it is time to go, he rushes out to avoid being attacked, and then continues wandering about, in case he should find another female. This hardly ever happens. Almost invariably, male spiders only love once.

Murder

When she lays her eggs, the female spider wraps them in a silken ball called an “egg sac.” This she guards for up to two weeks until the young hatch. Although they are guarded by a ferocious mother spider, the eggs are very vulnerable. Ants are common predators and, in their armies of hundreds, can easily overwhelm a single spider.

A Stegodyphus web from South Africa (note: not S. lineatus). Photo by Harvey Barrison.

A Stegodyphus web from South Africa (note: not S. lineatus). Photo by Harvey Barrison.

Perhaps the greatest threat, though, is more familiar: male spiders. Just as male lions and polar bears kill the young of rivals, male lined spiders frequently steal and destroy the egg sacs of females (Schneider and Lubin 1997). Why do they do this? First, stealing a female’s egg sac removes competition — his offspring will not have to compete with the product of another male’s successful mating. Second, once a female loses her eggs she becomes able to mate again, allowing the infanticidal male to replace her former partner as the father of her progeny.

This seems cruel, but male spiders are dealt a hard lot in life. While a female may encounter many males in her lifetime, and indeed will mate with many of them (Maklakov et al. 2005). Her ability to do so, however, is because of her lifestyle as a sedentary, shelter-dwelling female. Males, on the other hand, suffer a high mortality due to their wandering habits, and many males will only encounter a single female in a lifetime. So, when he encounters her, if she already has an egg sac, he will do whatever it takes to make her receptive again, even if it means killing the young she already has.

Sacrifice

The female has ways of preventing this from happening. She can lay her eggs later in the season, when all the males have either mated or died (Schneider 1999). Or, she can simply chase away/beat up/kill any male that tries to bother her after she lays eggs.

Either way, if her eggs are fortunate enough to survive, she can look forward to caring for the young for a few weeks, catching food for them from the web and mashing it up so their adorable little mouthparts don’t get damaged. During this time, the young are completely dependent on their mother, but as they get older, they venture out onto the web themselves to harvest their own prey.

The Negev Desert in Israel, a location where some of the research on the life of Stegodyphus has taken place. These are arid and semi-arid specialist spiders, found in dry habitats surrounding the Mediterranean. Photo by Andrew Shiva.

The Negev Desert in Israel, an area where some of the research on the life of Stegodyphus has taken place. These are arid and semi-arid specialist spiders, found in dry habitats surrounding the Mediterranean. Photo by Andrew Shiva.

Meanwhile, the mother undergoes major changes. Her digestive organs begin to degrade, and ultimately disintegrate as she stops feeding herself (Salomon et al. 2015). By the time her young are ready to leave the nest, her internal organs have begun to liquefy. It is time for one last gift to her offspring: her body. The spiderlings, which she protected from infanticidal males, carefully incubated as eggs, and offered her own food as tiny spiders, now eat their mother as their last meal before leaving the web and heading out into the world (Salomon et al. 2005).

Why does she do this? Believe it or not, it is to her genetic advantage to give her life to ensure her progeny’s survival. The world is a harsh place for spiders, and not only are her chances of successfully raising a second clutch next to nothing, but each of her spiderlings has at best a meager chance of survival. Offering her own body gives them the best possible head start in life, and ensures that at least a few of them will be able to grow, mate, and raise young themselves.

Cited:

Maklakov, A.A., T. Bilde, and Y. Lubin. 2005. Sexual conflict in the wild: Elevated mating rate reduces female lifetime reproductive success. The American Naturalist 165(S5): S38-S45.

Salomon, M., E.D. Aflalo, M. Coll, and Y. Lubin. 2015. Dramatic histological changes preceding suicidal maternal care in the subsocial spider Stegodyphus lineatus (Araneae: Eresidae). Journal of Arachnology 43(1): 77-85.

Salomon, M., J. Schneider, and Y. Lubin. 2005. Maternal investment in a spider with suicidal maternal care, Stegodyphus lineatus (Araneae, Eresidae). Oikos 109(3): 614-622.

Schneider, J.M. 1999. Delayed oviposition: A female strategy to counter infanticide by males? Behavioral Ecology 10(5): 567-571.

Schneider, J.M. and Y. Lubin. 1997. Infanticide by males in a spider with suicidal maternal care, Stegodyphus lineatus (Eresidae). Animal Behavior 54: 305-312.

The Woodlouse Spiders

by Joseph DeSisto

In my last post, I talked about centipedes with unusually long, narrow fangs, and how in the woodlouse spiders of the genus Dysdera, such strange devices are an adaptation for hunting woodlice. These spiders, however, deserve more than a mention in an article about another animal, so let’s talk about woodlouse spiders, and why they are so cool.

Woodlouse spiders are weird. They look weird, they act weird, and naturally I think they’re great. Here in North America we only have one species of Dysdera, and that’s D. crocata. This species is actually introduced from Eurasia, where it lives with around 200 other members of the genus. These spiders are relatively large, mostly hairless, and do not spin webs. Dysdera are also quite handsome as far as spiders go, cloaked in red, orange, and pale brown.

Dysdera crocata, the only woodlouse spider found in North America. This one is just over a centimeter in length, not including the legs. Photo by Tom Murray.

Dysdera crocata, the only woodlouse spider found in North America. This one is just over a centimeter in length, not including the legs. Photo by Tom Murray.

The biggest reason these spiders look so strange has to do with their fangs. A spider’s fangs are called chelicerae, and are modified mouthparts, in contrast to a centipedes “fangs” which are modified legs. Fangs can come in all shapes and sizes, but the woodlouse spider’s are probably some of the biggest, relative to its body size, of any spider. The fangs are enormous, and indicate a specialized hunting strategy: woodlouse spiders, as their name suggests, are specialist predators of woodlice or pill bugs.

Or are they? The question, as it turns out, is more complex than you might expect. Pollard et al. (1995) experimented by offering D. crocata spiders different sorts of prey: two types of woodlouse, but also flies, beetle larvae, crickets, and other many-legged morsels. The result: the spiders did not prefer woodlice. So what’s going on here?

A woodlouse spider's fangs, up close. Photo by Tom Murray.

A woodlouse spider’s fangs, up close. These are adapted for killing heavily armored woodlice. Photo by Tom Murray.

More than a decade later, Řezác and Pekár (2007) did pretty much the same experiment, and again saw that the spiders didn’t prefer woodlice over the alternative, in this case fruit flies. In fact, their spiders actually ate more flies than woodlice! But they also conducted a second experiment in which they raised young Dysdera on diets of either woodlice, flies, or a mix of the two.

When the spiders weren’t allowed to choose their food, those that ate woodlice developed significantly faster than those that ate only flies. It seems that, although woodlouse spiders are adapted to be woodlouse specialists, certain circumstances, including captivity, can cause them to change their preferences. Perhaps in the captive setting, flies are just easier prey than woodlice.

Woodlice, the primary prey of Dysdera spiders ... most of the time. Probably. Photo by Tom Murray.

Woodlice, the primary prey of Dysdera spiders … most of the time. Probably. Photo by Tom Murray.

It is worth pointing out that Řezác and Pekár conducted their experiment with a different but closely related species of spider (Dysdera hungarica) than Pollard et al. This may seem trivial, but in fact there is a lot of variation in feeding strategies among the woodlouse spiders. Although all appear to be specialized to some degree, the way in which their enormous fangs help them dispatch their armored prey varies quite a lot between species.

For example, Řezác et al. (2008) found that Dysdera spiders with concave chelicerae use them to stab woodlice from beneath, whereas others have flattened fangs they can slide between the prey’s plates of armor. Not only do different species have different methods of killing woodlice, some species with relatively “normal-looking” fangs refuse to attack woodlice in captivity.

So, not to worry if you were hoping to study woodlouse spiders: there is plenty of work still to be done.

Cited

Pollard, S.D., R.R. Jackson, A. Van Olphen, and M.W. Robertson. 1995. Does Dysdera crocata (Araneae Dysderidae) prefer woodlice as prey? Ethology Ecology & Evolution 7(3): 271-275.

Řezác, M. and S. Pekár. 2007. Evidence for woodlice-specialization in Dysdera spiders: behavioral versus developmental approaches. Physiological Entomology 32: 367-371.

Řezác, M., S. Pekár, and Y. Lubin. 2008. How oniscophagous spiders overcome woodlouse armour. Journal of Zoology 275: 64-71.

Some Strange Male Centipedes

by Joseph DeSisto

Time for some centipedes! Recently I’ve been looking at a lot of centipedes in the family Lithobiidae, and in particular the striking modifications of some of the males, and I thought I’d share them here.

For those of you who don’t spend your free time studying centipedes, they are arthropods with 15 or more pairs of legs and venomous fangs. I say “fangs,” but they are technically a highly modified pair of legs, positioned beneath the head, that centipede biologists may refer to as poison claws, forcipules, maxillipeds, prehensors, prehensorial feet, forcipular telopodites, toxicognaths (Bonato et al. 2010) … you get the picture. Fangs. They look like this, when viewed from below:

multidentatus

Bothropolys multidentatus, a common lithobiid centipede from eastern North America, viewed from beneath. Photo by Joseph DeSisto.

The fangs are the things that look like fangs. Pretty cool, huh?

Lithobiid centipedes belong to the order Lithobiomorpha, which includes centipedes with flattened bodies, spiracles (breathing holes) on the sides of their body, and 15 pairs of legs as adults. They are also show anamorphic development, which means the young add legs as they grow until they reach the final 15. Those in the family Lithobiidae have spines or spurs on their legs, which are helpful in identification.

Without a microscope, all lithobiids look pretty much the same. Some are bigger than others, but here in the eastern U.S. none exceed an inch or so. But on a smaller scale, the diversity in body form is fantastic, and one of the reasons they are probably my favorite family of centipedes.

To tell the difference between males and females, we need to move our view to the rear end of the centipede, and look at it again from beneath. While males are pretty nondescript in this regard, the females have a set of gonopod claws, which they use to manipulate the eggs they lay. A female centipede can lay an egg and then carry it around with her until she finds a suitable place to leave it, then use her legs and claws to coat the egg in dirt for camouflage .

Gonopod Claws

The gonopod claws on a female lithobiid, Lithobius forficatus. Please excuse the red writing, I originally made this image for a poster. Photo by Joseph DeSisto.

But in a few North American lithobiids, the males make themselves known by other means. This is most obvious in the last two pairs of legs, which may be highly modified into strange, contorted forms. Here’s an example, viewed from the side:

A male centipede in the genus Pearsobius. Photo by Joseph DeSisto.

A male centipede in the genus Pearsobius. Photo by Joseph DeSisto.

Pearsobius is a poorly known genus from Virginia and North Carolina (Causey 1942). The specimen above is unidentified. There will be a later post devoted just to Pearsobius, but for now, let’s look at more pictures! Pictures are great.

Male Pearsobius again, this time viewed from above. Photo by Joseph DeSisto.

Male Pearsobius again, this time viewed from above. Photo by Joseph DeSisto.

These centipedes are about half an inch in length, but the “spike” on the femur of the last pair of legs is visible even without a microscope. Although impressive, the purpose of these structures is unclear. My best guess is that the females use them to recognize males of the same species. While butterflies might use colors to achieve this effect, and birds might use songs, female centipedes live their lives in leaf litter and soil, where sight are of little use, and they can’t hear. So in an area where multiple centipede species might roam the same patch of leaf litter, a female needs something she can feel to avoid getting friendly with a male of a different species.

Not all leg modifications are so striking. Here is the 15th pair of legs on a male Paitobius zinus, also from Virginia.

The 15th pair of legs on a male Paitobius zinus from western Virginia. Photo by Joseph DeSisto.

The 15th pair of legs on a male Paitobius zinus from western Virginia. Photo by Joseph DeSisto.

Not as cool as a massive spike, but the modification here (the long indent on one of the segments) is still enough to make Paitobius distinguishable from other lithobiid genera. However, in Paitobius zinus, this is not the most striking male modification. Uniquely in this species, the male and female fangs/forcipules are different. The female’s forcipules are normal, and look pretty much like the ones from earlier (on Bothropolys). The male’s however … well, they look like this:

The forcipules on a male Paitobius zinus. Photo by Joseph DeSisto.

The forcipules on a male Paitobius zinus. Photo by Joseph DeSisto.

Yeah.

So far, P. zinus is the only species known to have modified male forcipules, and nobody knows why they have these. Long, narrow fangs could be an adaptation to extracting prey from narrow spaces (like in the woodlouse-eating spider, Dysdera crocata) … but why aren’t they found in females? Usually when we find a structure that is present in one sex but not the other, the function is related to reproduction. But as far as we know, the only thing forcipules are used for is killing prey.

Crabill (1960) was the first to write about this phenomenon in P. zinus, and since then, not a single person has bothered to study it. Why? Because despite being totally and undeniably awesome, centipedes are hard, and barely anyone studies them. I am currently planning a summer collecting trip to Virginia, though, and while I’m there I’ll see if I can learn anything. I have no idea why this species is so strange, but whatever reason there is, I bet it’s amazing.

A big thank you is owed to Dr. Bill Shear at Hampden-Sydney College in Virginia, who kindly sent me the specimens of Pearsobius and Paitobius, which he collected.

Cited:

Bonato, L., G. Edgecombe, J. Lewis, A. Minelli, L. Pereira, R. Shelley, and M. Zapparoli. 2010. A common terminology for the external anatomy of centipedes (Chilopoda). ZooKeys 69: 17-51.

Causey, N.B. 1942. New lithobiid centipedes from North Carolina. Journal of the Elisha Mitchell Scientific Society 58: 79-83.

Crabill Jr., R.E. 1960. A remarkable form of sexual dimorphism in a centipede (Chilopoda: Lithobiomorpha: Lithobiidae). Entomological News 71: 156-161.

A Sea Slug Story

by Joseph DeSisto

If you watch science news, or if you happen to have spent a lot of time researching slugs in the last few days like me, you might have noticed a few stories with a titles something like, “Toxic Sea Slugs Invade Argentinian Coastline.” A few days ago I wrote about terrestrial, predatory slugs and how they fit into our understanding of gastropod evolution. But although the land-dwellers are the most familiar, more than two-thirds of all gastropod (snail and slug) diversity is found in the ocean. And of course, there are marine slugs as well.

Nembrotha lineolata, an Indo-Pacific nudibranch. Photo by Nick Hobgood.

Nembrotha lineolata, an Indo-Pacific nudibranch. Note the gills (red) which are held outside the body, unlike the pleurobranchs, whose gills are internal and located on the sides of the body. Photo by Nick Hobgood.

As on land, shell-lessness evolved several times within the marine snails, but most of the “sea slugs” belong to a single lineage, Nudibranchia, which contains around 3,000 described species. Unlike most terrestrial slugs, nudibranchs begin their lives as larvae with shells, then quickly lose them during development. Many nudibranchs are predatory, highly toxic, and brightly colored, and the group includes some of the most visually stunning animals on earth.

Two individuals of the predatory nudibranch Nembrotha kubaryana, eating a tunicate colony. Photo by Nick Hobgood.

Two individuals of the predatory nudibranch Nembrotha kubaryana, eating a tunicate colony. Photo by Nick Hobgood.

The name Nudibranchia means “naked gills” and refers to the fact that in many species, the gills are held outside the body, exposed to the elements. A closely related group of sea slugs, the Pleurobranchomorpha, have internal gills on their sides. There are far fewer species of pleurobranchs than nudibranchs, and they aren’t typically quite as colorful, but many are predatory and powerfully toxic nonetheless.

Back to the story. Since 2009, a group of scientists in Argentina have been monitoring an outbreak of sea slugs off the coast of Mar del Plata, in the southwestern Atlantic (Farias et al. 2014). These sea slugs belong to Pleurobranchaea, a pleurobranch genus, and while they closely resemble P. maculata, their exact identity is unknown. Marine invertebrate diversity is poorly understood, even for relatively charismatic groups such as the sea slugs, so it wouldn’t be all that surprising if these turned out to be an undescribed species. What is known is that these mystery slugs contain a neurotoxin, also unidentified. Incidentally, P. maculata contains tetrodotoxin, 1-2 milligrams of which can kill a person, and while you might think the scientists who discovered that fact were the sea slug experts themselves, the real story contains far more intrigue, and a little tragedy, so brace yourself.

Pleurobranchia meckelii, a close relative of P. maculata. Photo by Heike Wägele & Annette Klussmann-Kolb.

Pleurobranchia meckelii, a close relative of P. maculata. Photo by Heike Wägele & Annette Klussmann-Kolb.

Pleurobranchaea maculata isn’t known from Argentina — yet. Its known range is halfway across the world, in the Pacific waters off the coast of New Zealand. In 2009 (McNabb et al.), after visiting beaches near Auckland, fourteen dogs died with similar symptoms of poisoning. Veterinarians contacted the National Centre for Disease Investigation, of the Ministry of Agriculture and Forestry, and a formal investigation began. As local media ran with the story, public interest grew and speculations were made as to the cause of these mysterious dog deaths.

Eventually, tetrodotoxin (TTX) was found in the vomit of one of the dogs, which led to suspicion that the dogs had eaten something on the beach containing the deadly toxin. TTX is used, often as a defense mechanism, by a variety of marine animals, including puffer fish (family Tetraodontidae, for which the compound gets its name), cone snails, and several species of blue-ringed octopuses (Kohr et al. 2014). McNabb et al. (2009) tested animals found on the beaches near Auckland and found that only one, the sea slug P. maculata, contained TTX in significant concentrations. The scientists then filed a report to the Auckland Regional Council, concluding that the dogs had eaten the poisonous sea slugs, and recommending that efforts be made to increase public awareness of the danger P. maculata presents to pets. Because TTX had never before been recorded in a sea slug, the investigators also implored the government to support continuing research on the biology, ecology and distribution of the slug, which at the time was very poorly known.

The gray side-gilled sea slug (Pleurobranchaea maculata) from New Zealand. Photo from McNabb et al. 2009.

The gray side-gilled sea slug (Pleurobranchaea maculata) from New Zealand. The one on the right is a female laying eggs. Figure 3 from McNabb et al. 2009.

We now know quite a lot more about P. maculata, the gray side-gilled sea slug, but to this day it is unclear whether the slugs synthesize the toxins themselves, or sequester it from TTX-containing prey. Across the world in North America, some garter snakes have evolved an immunity to the toxin and are capable of sequestering TTX from the rough-skinned newts they eat (Williams et al. 2004), and at least one species of caddisfly does the same thing by eating the newt’s eggs (Gall et al. 2012). So how do the slugs get their poison?

It turns out that only some populations of the species have TTX, and concentrations of TTX vary widely between individuals in each population (Wood et al. 2012). Khor et al. (2014) conducted an experiment in which they captured and maintained in aquariums 18 specimens of non-toxic P. maculata from South Island. Twelve of these slugs were fed a diet laced with TTX, and these specimens rapidly sequestered TTX both in the stomach and in the mantle (the fleshy “body” of a mollusk). In an interesting twist, they also found the females that were fed TTX laid toxin-laced egg masses.

The common garter snake (Thamnophis sirtalis). Some populations of garter snake can sequester tetrodotoxin by eating toxic rough-skinned newts (Taricha granulosa). Photo by Mark A. Wilson.

The common garter snake (Thamnophis sirtalis). Some populations of garter snake can sequester tetrodotoxin by eating toxic rough-skinned newts (Taricha granulosa). Photo by Mark A. Wilson.

Khor et al. then conducted a comprehensive survey of the benthic (ocean floor) invertebrates in an area where TTX-containing populations of P. maculata are known: the Auckland beaches. Despite the first experiment showing that P. maculata can in fact sequester TTX from food, the only other natural source of TTX they found was in a species of sand dollar, at concentrations far too low to explain the high toxicity of the slugs. So, while P. maculata is clearly capable of sequestering TTX from food, it still seems likely that they also produce it themselves.

The issue of whether wild populations of P. maculata manufacture their own TTX or sequester it is still very much unresolved. That said, I started this article as a report on toxic sea slugs in Argentina, and I’ve barely discussed them! So, here goes.

We still don’t really know that much about the Pleurobranchaea species in Argentina, mostly because we don’t know what species it is. The slug is readily distinguished from the only other known Argentinian Pleurobranchaea, P. inconspicua. The fact that these slugs possess neurotoxins may indicate they are an introduced population of P. maculata, or they may simply be an undescribed species that we only know about now because its population is growing. Farias et al. (2014) tested the slugs for neurotoxins, but further work is needed to determine if TTX is the toxin in question.

Pleurobranchus inconspicua (A) and the unknown Pleurobranchus (B), the two members of the genus known from Argentina. Figure 1 from Farias et al. 2014.

Pleurobranchus inconspicua (A) and the unknown Pleurobranchus (B), the two members of the genus known from Argentina. Figure 1 from Farias et al. 2014.

What we do know is that the population is indeed growing. Farias et al. claim, and rightly so, that this outbreak demands both additional research and public education, mostly because of the ecological consequences of a probably invasive sea-floor predator. In other parts of the world, invasive benthic predators such as sea stars have had disastrous consequences for local shellfish industries. However, they also emphasize that this slug is highly toxic, and the potential public health implications for the region have yet to be determined.

Sea slugs are amazing animals, and important components of the ocean floor ecosystem. Some are beautiful, some are deadly, and many are both. All deserve, and many demand, more scientific and public attention than is currently awarded them.

For a short news article that inspired me to write this one, click here.

Cited:

Farias, N.E., S. Obenat, and A.B. Goya. 2015. Outbreaks of a neurotoxic side-gilled sea slug (Pleurobranchaea sp.) in Argentinian coasts. New Zealand Journal of Zoology, published online: DOI: 10.1080/ 03014223.2014.990045.

Gall, B.G., A.N. Stokes, S.S. French, E.D. Brodie III, and E.D. Brodie Jr. 2012. Predatory caddisfly larvae sequester tetrodotoxin from their prey, eggs of the rough-skinned newt (Taricha granulosa). Journal of Chemical Ecology 38(11): 1351-1357.

Khor, S., S.A. Wood, L. Salvitti, D.I. Taylor, J. Adamson, P. McNabb, and S.C. Cary. 2014. Investigating diet as the source of tetrodotoxin in Pleurobranchaea maculata. Marine Drugs 12(1): 1-16.

McNabb, P., L. Mackenzie, A. Selwood, L. Rhodes, D. Taylor, and C. Cornelison. 2009. Review of tetrodotoxins in the sea slug Pleurobranchaea maculata and coincidence of dog deaths along Auckland Beaches. Prepared by Cawthron Institute for the Auckland Regional Council Technical Report 2009/108.

Williams, B.L., E.D. Brodie Jr., and E.D. Brodie III. 2004. A resistant predator and its toxic prey: persistence of newt toxin leads to poisonous (not venomous) snakes. Journal of Chemical Ecology 30(10): 1901-1919.

Woods, S.A., D.I. Taylor, P. McNabb, J. Walker, J. Adamson, and S.C. Cary. 2012. Tetrodotoxin concentrations in Pleurobranchaea maculata: temporal, spatial and individual variability from New Zealand populations. Marine Drugs 10(1): 163-176.