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Sea Cucumbers, or, How to Really Lose Weight Fast

Via: www.starfish.ch

Common Name: Sea Cucumbers, Holothurians

A.K.A.: Class Holothuroidea

Vital Stats:

  • Approximately 1250 species
  • Size: 2-200cm (¾” to 6.5’)
  • Lifespan: 5-10 years in the wild

Found: Throughout the oceans, in both shallow and very deep regions

It Does What?!

Where to begin? This is an odd one… To start, despite the name sea cucumber, this isn’t a plant but an animal; a relative of starfish and sea urchins. One could be forgiven for mistaking the holothurians for plants, however. Most spend their lives lying on the ocean floor, looking like a sunken vegetable, and covering a distance of a couple metres or less per day in their search for food. The creatures feed on small particles, like algae and plankton. There is a tiny mouth at one end of their body, surrounded by between eight and thirty tentacle-like feet with which they grab their food and which can actually be retracted into their mouth. But that’s not really the interesting end of a sea cucumber, as we’ll see.

Via: www.answers.com

Lacking both eyes and any rapid means of locomotion, holothurians are tempting prey for crabs, fish, and other large sea creatures. When threatened, they have the single most bizarre and seemingly impractical defence mechanism ever evolved: self-evisceration. As a predator approaches, the sea cucumber violently contracts the muscles around its body wall and actually expels its own internal organs via its anus (demurely labelled as the ‘aboral pole’ in the diagram). Yes, really. In some species, these organs include most of the creature’s respiratory system, which takes the form of sticky threads that blanket and ensnare the predator. And just to add genuine injury to the insult, this discharge is accompanied by a toxic chemical known as holothurin, which kills whatever’s nearby. Disgusting, but effective. Once expelled, the missing organs can be regenerated in 1-5 weeks, depending on the species. Some researchers speculate that this ability may even be used as a means of ridding the organism of accumulated waste or parasites. The ultimate detox regime, if you will.

Are those your lungs, or are you just happy to see me?
Via: Wikimedia Commons

One such parasite is the pearl fish. You see, holothurians actually breathe through their rear end as well, so when one of them, umm… opens up… to take in some fresh, oxygenated water, in goes the fish, which then feeds on the sea cucumber’s internal organs. You can see why they might want to rid themselves of this visitor.

Strange as it all seems, the sea cucumber’s strategy is quite a successful one. At depths below five and a half miles (8.8km), they make up fully 90% of the mass of all macrofauna (i.e. any animal that’s not microscopic). Among the species that live at shallower depths, populations can reach a density of 1000 cucumbers per square metre. And it’s a good thing, because they’ve got one predator with whom spewing out their guts won’t work: humans. Sea cucumbers are a popular ingredient in Chinese and other Southeast Asian cuisines, although only about ten species are used for this purpose. These species are farmed commercially in artificial ponds, and are also used in traditional Chinese medicine. Perhaps not surprisingly, they are considered to improve male sexual health.

Does a Body Good.
Via: www.theworlds50best.com

[Fun fact: Sea cucumbers have a body wall made up of collagen fibres which they can ‘unhook’ at will, essentially liquefying their interiors and allowing them to squeeze into very small cavities as a means of hiding from predators. Once inside the cavity, they re-solidify themselves, making the creature very difficult to extract from its hideout.]

Says Who?

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If the Eyes are the Window to the Soul, this Fish has a Sunroof

Things are lookin’ up

Common Name: Barreleye Fish

A.K.A.: Macropinna microstoma  (and related species)

Vital Stats:

  • Size: 15cm (6″) long
  • Depth: 600-800m (2000′-2600′) below sea level
  • Discovered: 1939
  • First Photographed: 2008

Found: Subarctic and Temperate regions of the North Pacific

It Does What?!

As you have likely already noticed, fish don’t have necks. At least not in the sense that they are able to look upward. So for a bottom-dweller lurking about in the cold depths of the ocean, being able to see that tasty bit of food floating by above is something of a problem. Some species get around this issue by floating vertically in the water so their whole bodies are pointing upwards. Simple enough. But in the spirit of meeting every challenge with an impossibly bizarre solution, nature has also produced a fish with eyes directly on the top of its head. After all, why re-orient the entire fish when you can just shift a couple of parts?

Those things on the front that look like eye sockets?
That would be its nose.

But the strangeness of the Barreleye Fish goes a little further than that. These aren’t just normal fish eyes in an unusual location. This species’ main prey are jellyfish and their relatives, which frequently come equipped with stingers that could damage the eyes of most predators. So rather than a normal spherical eye perched on top of its head, Macropinna has a tubular structure with the lens buried deep within its head (the dark green areas in the images). Overlying the tubular eyes is a tough, fluid-filled, transparent shield which the fish can look through. That’s right, it looks through the top of its own head. This way, stings from jellyfish will never damage the delicate ocular tissue.

What’s more, the fish’s unique tubular eyes are supremely adapted for the dark depths of the ocean. They allow unusually accurate depth perception (due to a large overlap of the two visual fields) and enhanced light gathering compared the spheroid eyes. In an environment up to 2600 feet (800m) down, where little daylight penetrates and everything appears in monochrome, these adaptations enable the barreleye to distinguish even faint shadows and silhouettes moving above it, and to precisely gauge how far up they are.

The Barreleye Fish, failing to look at the camera.

Researchers had long been puzzled as to how the barreleye eats, since, with its eyes on top of its head, its visual field didn’t include the area around its mouth. The species has been known since 1939, but only as small mangled bodies caught up in deep-sea fishing nets (adults are only about six inches long). In each case, the transparent casing of the fish’s head had been destroyed by the nets and the rapid changes in pressure as the nets were pulled up, making its anatomy difficult to study. In 2008, however, scientists from the Monterey Bay Aquarium Research Institute sent remote operated vehicles with cameras down to try, for the very first time, to snap some photos of these oddballs in action. What they learned was that, when it spots prey, the barreleye can actually rotate its entire tubular eye downward, like moving the telescope in an observatory. This way, it can turn and look at its target straight on as it pursues. Most of the time, though, the fish was seen to use its large, flat fins to hold itself nearly motionless, looking up through its personal sunroof, just waiting for some unlucky jellyfish to float on by.

Says Who?

  • Robison & Reisenbichler (2008) Copeia 4: 780-784.
  • Monterey Bay Aquarium Research Institute

All images taken by the Monterey Bay Aquarium Research Institute (MBARI)

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EVOLUTION TAG TEAM, Part 1: Acacia Domatia

The first in an ongoing series of biology’s greatest duos. (Here’s Part 2 and Part 3)

Home, Sweet Home.
(via: Flickr)

Common Name (Plants): Bullhorn Acacias, Whistling Thorns

  • A.K.A.: Acacia cornigera, Acacia drepanolobium, and several other Acacia species

Common Name (Ants): Acacia Ants

  • A.K.A.: Pseudomyrmex and Crematogaster species

Found: Central America (Bullhorn Acacias) and East Africa (Whistling Thorns)

It Does What?!

Life as a tree is tough, particularly when you live in a part of the world that’s home to the biggest herbivores on Earth and happen to have delicate, delicious leaves. Such is the case for the African acacias. Without sufficient defences, they’d be gobbled up in no time by elephants, rhinos, and giraffes. The trees are known for having huge, sharp thorns, but even that’s sometimes not enough; the lips and tongues of giraffes are so tough and dexterous, they can often strip the leaves right out from between the thorns. So what’s a stressed acacia to do? Recruit a freaking army, that’s what.

Pseudomyrmex ferruginea: the giraffe’s worst enemy.
(Photo by April Nobile)

A few species of acacia in both Africa and Central America (where the herbivores are smaller, but no less voracious) have developed a symbiosis wherein they enjoy the services of ant colonies numbering up to 30,000 individuals, tirelessly patrolling their branches 24 hours a day. Should a hungry elephant or goat wander up and take a bite, nearby patrol ants will call in reinforcements and soon the interloper will be utterly overrun with angry, biting ants. What’s more, the protection extends beyond just animal threats. The ants will go so far as to kill other insects, remove fungal pathogens from the surface of the tree and even uproot nearby seedlings because, you know, they might eventually steal some sunlight from the beloved acacia.

“Trespassers Will Be Drawn and Quartered”
(via Wikimedia Commons)

So what do the troops get out of this? Quite a bit, actually. In ant-protected acacias (‘myrmecophytes’, they’re called), the thorns that normally grow at the base of a leaf swell up. In the Central American species, they grow into something that looks like a bull’s horn (hence their common name), while the African ones become more bulbous. These specialized structures, called domatia, are hollow inside and serve as very convenient housing for the ants. What’s more, the trees produce not one, but two different kinds of nourishment for the colony- regular, and baby food. The adult ants will feed from a sweet liquid exuded by nectaries on the branches. Meanwhile, on the tips of the tree’s leaflets, small white structures called Beltian bodies are formed which are high in the protein every growing child ant-larva needs. These are collected by workers and inserted right into the larval pouches, to be eaten before the ants are even fully formed.

The Bullhorn Acacia, now with more Beltian bodies!
(via Flickr)

Sounds like the perfect partnership, right? Usually, yes, but in nature, a symbiosis is only a symbiosis until one side figures out how to take advantage of the other. From the ants’ side, for example, any energy spent by the tree on reproduction is energy not spent on new homes and sweet, sweet nectar for them. Therefore, the ants will sometimes systematically nip all the flowers off the tree as it attempts to bloom. They’ll also prune the acacia’s outward growth if those new shoots may come into contact with a neighbouring tree, allowing invasion by another ant colony. Conversely, if herbivores become scarce and the acacia no longer requires such a strong protection force, it will begin to produce fewer domatia and less nectar in a move to starve some of the ants out. This has been shown to actually be a bad strategy for the acacia, since the soldiers, not to be outsmarted by a tree, turn to farming and begin raising sap-sucking insects on the bark, thereby getting their sugar fix anyway. And so it goes, oscillating between advantageous partnership and opportunistic parasitism… like so many things in life.

The roomier, more spacious African domatium.
(Image by Martin Sharman)

[Side note: While I’ve never personally encountered ant-acacias, I have disturbed an ant-protected tree of another family in the rainforests of Guyana, and can attest to the fact that the retaliation was both swift and intense. I was in a small boat at the edge of a river collecting botanical specimens, and I nearly jumped in the river to escape the onslaught. Don’t mess with ants.]

Says Who?

  • Clement et al. (2008) Behav. Ecol. Sociobiol. 62: 953-962.
  • Frederickson (2009) American Naturalist 173(5): 675-681.
  • Huntzinger et al. (2004) Ecology 85(3): 609-614.
  • Janzen (1966) Evolution 20(3): 249-275.
  • Nicklen & Wagner (2006) Oecologia 148: 81-87.
  • Stapley (1998) Oecologia 115: 401-405.
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A Shellfish Goes to the Dark Side (Sacculina carcini)

The crab barnacle, hitchin’ a ride.
(Image by Hans Hillewaert)

Common Name: Crab Barnacle, or the charmingly descriptive Dutch term “krabbenzakje,” meaning “crab bag”

A.K.A.: Sacculina carcini (and other Sacculina species)

Found: In the coastal waters of Europe and North Africa

It Does What?!

Most barnacles, those almost quaint crusts seen decorating old piers and ships, live their lives by cementing themselves to a hard underwater surface and using their arm-like limbs to pull passing bits of food into their mouths all day. Not so for the crab barnacle, who decided that all that arm-waving was for chumps and set about evolving into the ultimate free-loader.

Normal, hardworking barnacles, for the sake of comparison…
(Image by Michael Maggs)

In its immature larval form, Sacculina has a similar body plan to other barnacles and is able to swim about freely; however, rather than finding a surface to settle down on, it finds itself a crab. Typically, this will be a green crab, species Carcinus maenas. The female barnacle (more on the males later) crawls along the surface of the crab’s shell until she comes to a joint – a chink in the armour – where she turns into a sort of hypodermic needle, injecting herself into the crab and leaving her limbs and shell behind. Now nothing more than a tiny slug-like mass, she makes her way to the crab’s abdomen and proceeds to grow rootlike tendrils throughout her host’s body, drawing nutrients directly from the bloodstream.

If that wasn’t disturbing enough, consider Sacculina’s mode of reproduction. In addition to its internal root system, the parasite forms an external sac (hence the nickname ‘crab bag’) where the female crab normally keeps her fertilized eggs. This is where the male barnacle comes into play. Upon finding a crab already infected by a female, the male will do the same needle trick, injecting himself into the external sac and living for the rest of his life as a parasite inside the female’s body. Fertilization takes place and the sac is soon full of microscopic Sacculina larvae.

In case you needed a closer look.

Since the barnacle infection has rendered the host sterile, and because crabs aren’t very bright, the crab will now care for this sac of larvae as if they were her own young. But what if the infected crab was male, you ask? No problem. The parasite is able to interfere with his hormones to such an extent that, in addition to changing his body shape to that of a female, he now actually behaves like, and even carries out the mating gestures of, a female crab. Horrified yet?

Now, this may not seem so bad from the point of view of the crab; I mean, it doesn’t know it’s carrying around evil changeling spawn, right? But it’s a bit worse than that. Wanting to keep all the available energy for its own use, the parasite prevents the crab from moulting its shell or re-growing lost claws, as crabs normally do. This leads to a variety of secondary infections which, coupled with malnutrition, leads to the premature death of the crab. But nature isn’t without a sense of fair play… research has now found that Sacculina sometimes succumbs to viruses and yeast naturally present in the crab’s body, via infection of its rootlets. Take that, bloodsucking barnacle!

Says Who?

  • Powell & Rowley (2008) Diseases of Aquatic Organisms 80: 75-79.
  • Zimmer (2000) “Do parasites rule the world?” Discover Magazine (August issue).
  • Russell et al. (2000) Journal of the Marine Biological Association of the U.K. 80: 373-374.
  • Mouritsen & Jensen (2006) Marine Biology Research 2: 270-275.
  • Goddard et al. (2005) Biological Invasions 7: 895-912.
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Every Day is a Crappy Day for the Bird-Dropping Spider (Celaenia excavata)

Celaenia excavata
(via: http://commons.wikimedia.org/wiki)

Common Name: The Bird-dropping Spider

A.K.A.: Celaenia excavata

Found: Eastern and Southern Coastal Australia

It Does What?!

Quick, what’s the first thing that comes to your mind when someone says “disgustingly inedible” ?

If you said “Why, poop, of course!”… congratulations, you think just like Celaenia excavata. And if the thing you’re trying to look inedible to is a bird, naturally, you go with bird poop. Such is the evolutionary reasoning behind the politely-named Bird-Dropping Spider. And while remaining motionless is a must, looking the way it does allows the spider to sit comfortably atop a leaf all day, secure in the knowledge that spiders’ main predators, birds and wasps (who apparently aren’t into eating bird poop either), won’t take an interest.

“Nobody here but us droppings.”
(Thanks to Ron Atkinson at www.findaspider.org.au)

But the mimicry doesn’t end there for this sneaky little guy- by day it sits inactive and gross-looking, but by night, it hangs upside down from a leaf and releases the mating pheromones of a female moth. When some unlucky male moth comes looking for a good time, the spider snatches it right out of the air with its powerful front legs and wraps it up for dinner. The moth may be eaten right away or, if its capturer isn’t feeling hungry quite yet, be hung under a leaf next to the spider’s egg sacs, which, oddly enough, look like nuts (see top photo).

Believe it or not, Celaenia excavata isn’t the only spider out there masquerading as merde. Another such trickster is Mastophora cornigera, a North American species which is part of a group known as the Bolas Spiders, or Fishing Spiders. Not content to hope their prey wanders into arm’s reach, bolas spiders release pheromones to attract male moths, then dangle a line of silk with a sticky blob on the end. Once a moth gets close enough, the spider swings its line and –yoink– rips the poor thing right out of mid-air. Whoever thought up Spiderman’s web-slinger clearly had a bolas spider in mind.

So there you have it, the leisurely lifestyle of a successful spider: pile of poo by day, upside-down fisherman by night.

Says Who?

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The Curious Case of Turritopsis nutricula

Turritopsis nutricula

Common Name: The Immortal Jellyfish

A.K.A.: Turritopsis nutricula

Found: Tropical and temperate oceans around the world

It Does What?!

Ever been under a lot of stress and found yourself longing for the simplicity of childhood? What if, by force of will, you could actually turn back into your childhood self? And once you’d re-grown up, you could do it again. And again, and again… Welcome to the unusual lifestyle of Turritopsis nutricula, the so-called immortal jellyfish.

Jellyfish, also known as medusae (singular: medusa), are the mature life stage of Phylum Cnidaria, Subphylum Medusozoa. They start off as a bottom-dwelling structure that looks a lot like a series of plants connected by stolons (like strawberry plants… translucent, underwater strawberry plants). These “pseudo-plants” are called polyps, and when they mature, they bud and release many tiny medusae into the ocean, like a plant releasing pollen.

The polyp stage of Turritopsis nutricula

In most species, these medusae go off and live the jellyfish version of the good life- swimming, eating plankton, releasing sperm or eggs to be fertilized and form polyps for the next generation, and finally dying at the ripe old age of anywhere from a few hours to six months, depending on the species. Not so for the Immortal Jellyfish.

Reaching a size of only 4.5mm across, when Turritopsis nutricula becomes stressed, whether due to aging or a change in its environment, it can begin a process called transdifferentiation. First, its tentacles (80 to 90 of them in adults!) shorten and are re-absorbed into the body. The medusa becomes unable to swim and settles onto the bottom. It there transforms into a blob-like mass of cells and, within two or three days, forms a new polyp. In about a month, new jellyfish are ready to be released.

In theory, T. nutricula can pull this trick any number of times, which would effectively make it immortal. However, as scientists point out, these little guys frequently die from disease or predation before they can regenerate (Whovians, insert your own Doctor Who joke here), keeping the population under control. Not entirely under control, though, apparently- one researcher describes the spread of T. nutricula through the world’s oceans as a large-scale, “silent invasion.”  Beware the Immortal Jellyfish.

Says Who?

  • Miglietta & Lessios (2009) Biological Invasions 11: 825-834
  • Piraino et al. (2004) Canadian Journal of Zoology 82: 1748-1754

[Thanks to The Marine Biology Image Database for the use of these images: Migotto AE, Vellutini BC (eds). 2011. Cifonauta: marine biology image database. Available at http://cifonauta.cebimar.usp.br/ ]