25 Jan 2013

Spider Art and Bioluminescent “Bombs”: Extreme Animal Mimicry

Exciting news for the world’s arachnophiles (spider lovers) broke late last year – a spider was discovered in the Peruvian Amazon that constructs a much larger fake spider in the center of its web. The as-yet-unnamed arachnid, likely belonging to the genus Cyclosa, creates its faux-spider masterpieces (some of which have eight legs!) out of leafy debris and dead insect parts. To further enhance the big-spider illusion, the real spider hides at the head of its creation and plucks strands of the web to make it vibrate.
The decoy spider's elaborate fake spider (Phil Torres)
So, does this spider make art? Nope, sorry – it’s not art. Creating a larger decoy of itself to frighten off potential predators is a form of self-protective mimicry that’s hardwired into the spider’s genes.
The most common form of mimicry is camouflage. Such visual mimicry can be as basic as a lion’s tawny coat blending in with the African savannah, or a gray-patterned moth disappearing when it lands on a similarly colored tree. More complex examples include caterpillars that pose as twigs, or leafy seadragons that are almost indistinguishable from seaweed, or spiders that, along with their webs, can be mistaken for bird droppings in both color and shape.
In Batesian mimicry, harmless species evolve to visually impersonate poisonous or distasteful species, which makes it much less likely that they, themselves, will be eaten. A master of this particular adaptation is the Indonesian mimic octopus.
The mimic octopus impersonates a flatfish, a scorpionfish, and a sea snake.
Octopuses are masters of disguise and can change color and pattern, and even texture, to match their surroundings. The mimic octopus’s self-defense strategy goes one step further – it’s a shape-shifter. This diminutive cephalopod has evolved an elaborate collection of behaviors that allow it to ape other marine animals. It stretches out its arms to imitate a scorpionfish flaring its poisonous spines, or hunkers down with only two legs extended in either direction to impersonate a venomous sea snake, or swims flattened out to mimic the shape of a flatfish. To date, this little octopus has been observed posing as fifteen other marine creatures! A fun twist to this amazing story is that scientists have recently witnessed a timid little black-marble jawfish, whose markings are strikingly similar to those of the mimic octopus, swimming perfectly disguised among the arms of one of these octopuses. A mimic mimics a mimic!
Australian chameleon gecko (Stewart MacDonald)
Decoy mimicry is yet another variation, in which animals have developed a means to either divert the attention of a predator or, conversely, to attract the attention of a potential meal.
One of the most radical practitioners of decoy mimicry is the gecko who, when threatened, is capable of discarding its own tail. The cast-off tails of some geckos flop around and writhe, offering a tantalizing alternative meal to predators while the geckos themselves escape. In Australia, the writhing, discarded tail of the chameleon gecko is made even more irresistible – it actually squeaks! Though self-amputation may sound like an extreme adaptation for survival, geckos are capable of growing replacement tails within a few months.

The adaptation of decoy mimicry isn’t confined to land animals. While studying life in the ocean’s depths aided by remote-controlled submarines, a group of scientists led by research zoologist Karen Osborn discovered seven new marine worms (annelids), some of which have developed a novel form of decoy mimicry – we just don’t know exactly what they’re mimicking.
Deep-sea marine worms release bioluminescent "bombs" (Karen Osborn)
These small bristly worms, the biggest no longer than a finger, come equipped with tiny balloons attached to their bodies near their heads. When one of these worms, which have been dubbed “green bombers,” is touched, it releases one or more of these bioluminescent “bombs” that give off intense green light for several seconds. It’s likely that a would-be predator, searching for a tasty morsel in the darkness, would be distracted by these mini floating lights while the worm is able to avoid being eaten by quickly swimming backwards out of harm’s way.

Here’s a link that has a video showing forward and backward movement of one of the newly discovered marine worms (genus Swima): http://news.sciencemag.org/sciencenow/2009/08/20-02.html
Here’s another that shows the mimic octopus in action: http://www.youtube.com/watch?v=H8oQBYw6xxc
And a delightful recording of the moment the decoy spider was discovered!: http://www.youtube.com/watch?v=RrWnZ7VySac


Paula said...

Is it just me, or do those Swima marine worms look like the Hallucigenia fossils in the Burgess Shale?

Jan Thornhill said...

Hallucigenia were, I think, walkers - would love to see one in action (or, for that matter, any of those wacky Burgess Shale characters).