Living Together

17 March 2014 Comments Off on Living Together


Allow me to get this out of the way: Textbook examples of symbiosis are boring. They are not boring because the interactions themselves are uninteresting—far from it, in fact, for even well-known mutualisms (such as that between sea anemones and clownfish) feature novel and curious aspects (such as the fact that clownfish may actively feed scraps of food to their anemone, that their feces supply the host with precious nitrogen, or that every anemone supports only a single breeding pair, accompanied by any number of non-dominant males—such that if the female clownfish dies, her mate becomes the new breeding female, and recruits a male from the existing pool). They instead seem lackluster because they do not fully portray the overlaps between different types of symbiosis—we are under the impression that mutualists are strictly beneficial toward each other, or that a parasite has no incentive to assist its host (this is, suffice it to say, not true). I also feel that these examples do not cover the sheer biological insanity that often comes with two unrelated species evolving in tandem with each other—there are aphids that can’t fly unless infected with a specific virus that gives them wings (and also kills them, but that’s a small price to pay), caterpillars that can only thrive in the webs of social spiders, and parrots that tear off pieces of fat from the backs of living sheep (yet another reason to stay far away from Australia and its environs). So, this week will be devoted to a couple of symbioses that I find to be particularly delightful (but then I find Loa loa and Medina worms to be delightful too, so your mileage may vary).

Ant-plant mutualisms are quite common in nature, as ants are notoriously territorial—and this territoriality means that you couldn’t ask for a better anti-herbivore defense than a resident colony. The quintessential ant-plant mutualism is that between the ant-acacias and acacia ants, and involves the housing of ant colonies within the hollow thorns of the plant: the ants feed on specialized feeding nodules, as well as nectaries found on the leaves, and protect the plant against anything that so much as looks at it funny—such that there is often a visible clearing around these acacias, all other plants having been destroyed by the colony (ants are notoriously territorial). A similar association is formed between ants and the fanged pitcher plant Nepenthes bicalcarata—the rim of each pitcher produces nectar for the ant colony, whose members feed the pitcher with their excretions in return. The ants, which can swim and even dive freely within the pitcher fluid, also clean the plant, protect it against herbivores, subdue particularly stubborn prey and devour the mosquito larvae that infest the pitcher before they can metamorphose and fly off. Other species of pitchers are also known to stray from their predatory habits and form mutualistic associations with shrews and birds—carnivorous plants mainly require nitrogen from their prey, and animal feces are a suitably nitrogen-rich substitute, making it a good idea indeed for the pitcher to exchange its nectar in return for animal droppings.

Such mutualisms are not immune to exploitation, however. The Peruvian tree Cordia nodosa also hosts structures specialized for maintaining ant colonies, but the ants it most frequently houses have grown too smart for the plant’s comfort: in addition to deterring herbivores and cleaning out rival plants, these ants castrate the tree by cutting off its flowers (ants are notoriously territorial), which induces it to divert more resources to its other activities—and these other activities include the production of more ant-feeding structures, which obviously benefits the colony. But just as a cooperative association may turn antagonistic, so too can a so-called parasite be revealed as helpful to its host: the spider genus Argyrodes, for example, is famous as the textbook example of kleptoparasitism. These audacious thieves sneak into the webs of other spiders and steal their prey, sometimes even killing the owner of the web along the way. But when Argyrodes fissifrons enters the web of Cyrtophora unicolor, it’s there to stay—for its silvery markings are strangely attractive to moths, which happen to be its host’s favorite quarry, while the tiny “kleptoparasite” is perfectly content to feed on small insects that the larger spider finds unworthy of attention.

As for my favorite symbiotic relationship…well, there’s some strong competition. You probably remember the infamous barnacle genus Sacculina, the larvae of which inject their own bodies into crabs through their hollow antennal syringes, and proceed to become a network of rootlike tissue that castrates the host and forces the males to look and behave like females (this change is necessary, as the brood pouch of Sacculina is where the egg mass of a female crab would be, and the infected crab spreads the barnacle’s larvae using the same set of motions that brooding females utilize). My single favorite example of symbiosis also features a barnacle and a crab, but in a rather different juxtaposition: the preference of the barnacle Octolamis unguisiformis is to grow on the sides of the crab Macrophthalmus milloti, and older females are more likely than similarly aged males to host these barnacles—possibly because the barnacles resemble the claws of male crabs, allowing the females to masquerade to any visually hunting predators as strongly armed males (another suggestion is that birds avoid eating these crabs because they **************** look like four-clawed mutant freaks, since the former do discriminate against sickly or aberrant-looking animals). And this in turn puts Octolamis, alongside the tongue-eating louse Cymothoa exigua and the leg-biting mite Marchocheles rettenmeyeri, into the rare category of “bionic animals” that replace or replicate existing structures on their hosts.

…But then there’s also the swashbuckling amphipod Hyperiella dilatata, which avoids predation by grabbing a sea angel (sea angels taste awful and are actively avoided by the local fish population) from the water column and using it to defend itself against unruly fish—and frankly, it’s impossible to dislike a real-life example of an animal wielding another as a weapon.