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.

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.

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.