Movin’ On Up: Hermit Crabs & the World’s Only Beachfront Social Housing

(Via: )
(Via: onestopcountrypet.com)

Common Name: The Hermit Crab

A.K.A.: Superfamily Paguroidea

Vital Stats:

  • There are around 1100 species of hermit crabs in 120 genera
  • Range in size from only a few millimetres to half a foot in length
  • Some larger species can live for up to 70 years
  • Most species are aquatic, although there are some tropical terrestrial species

Found: Generally throughout the temperate and tropical oceans, in both shallow and deep areas (I was unable to find more specific data on this.)

Trop. & Temp. Oceans

It Does What?!

If there’s one thing nature loves, it’s symmetry. Sometimes radial symmetry, as we see in starfish or sea anemones; sometimes bilateral symmetry, as in mammals and insects, which have a right half and a left half. External asymmetry is extremely rare in living organisms, and when it does occur, it is generally in a minor form, such as a bird species with beaks bent to the side, or a type of flower with oddly distributed stamens. One of the very few groups with entire bodies that lack symmetry are the gastropods; specifically, the snails. They develop helical shells with asymmetrical bodies to match.

caption (Via: Wikimedia Commons)
The shells also hide how ridiculous they look naked.
(Via: Wikimedia Commons)

But this post isn’t about snails. It’s one thing to evolve an unusual asymmetrical bodyplan to go with your asymmetrical home. It’s another to evolve an asymmetrical bodyplan to go with somebody else’s home. Which brings us to the hermit crab. When snails die in ways that leave behind perfectly good shells on the beach, these guys literally queue up for the chance to move in. Hermit crabs are part of the decapod order of crustaceans, as crabs are, but are not in fact true crabs, and unlike most other crustaceans, they lack any kind of hard, calcified plating on their abdomens (think shrimp shells). Instead, they have a soft, spirally curved lower body that fits perfectly into a snail shell, with muscles that allow them to clasp onto the interior of the shell. Paleobiologists have found that hermit crabs have been living in found shells for over 150 million years, and that they made the move to snail shells when their original shell-producer, the ammonite, went extinct. Living in shells has strongly restricted their morphological evolution, meaning the crabs of aeons ago look pretty similar to the crabs of today, because their housing situation doesn’t allow a lot of change.

caption (Via: Telegraph.co.uk)
Housing shortages hurt everyone.
(Via: Telegraph.co.uk)

Back to those line-ups I mentioned. Unoccupied snail shells are a limited resource, and an unarmoured crustacean is an easy lunch, so of course a lot of fighting goes on over them; crabs will actually gang up on an individual with a higher quality shell and just yank the poor bugger out. But it actually gets much more complex than that… these little pseudo-crabs aren’t as dim and thuggish as you might think. You see, as a hermit crab grows over the course of its life, it needs a series of progressively larger shells in which to live. A crab stuck in an undersized shell is stunted in its growth and is much more vulnerable to predation, since it can’t fully withdraw into its armour. The easiest way to find your next home? Locate a slightly larger hermit crab about to trade up and grab its shell afterward. This is how the crabs form what are called “vacancy chains.” A series of individuals will line themselves up in order of size (I’ve seen groups of schoolchildren unable to perform this task), waiting for hours sometimes, and as the largest crab moves to its new shell, each successive crab will enter the newly vacated one. Brilliant… new homes for everybody, and no one gets hurt. In fact, if a given crab chances upon a new shell that it judges to be too large for its current size, it will actually wait next to the shell for a larger crab to come along and a vacancy chain to form. That’s pretty impressive reasoning for a brain smaller than a pea.

[Fun Fact: Larger aquatic hermit crabs sometimes form symbiotic relationships with sea anemones; the anemone lives on the crab’s shell, protecting its host from predators with its deadly sting, while the crab shares its food with the gelatinous bodyguard.]

Today in Words You Didn’t Think Existed:
carcinisation / car·si·nə·ˈzā·shən / n.
a process by which an organism evolves from a non-crablike form into a crablike form.

That’s right, glossophiles, thanks to a British zoologist, we actually have a specific word for turning into a crab. English rules.

Says Who?

  • Angel (2000) J. of Experimental Marine Biology and Ecology 243: 169-184
  • Cunningham et al. (1992) Nature 355: 539-542
  • Fotheringham (1976) J. of Experimental Marine Biology and Ecology 23(3): 299-305
  • Rotjan et al. (2010) Behavioral Ecology 21(3): 639-646
  • Tricarico & Gherardi (2006) Behav. Ecol. Sociobiol. 60: 492-500
Say hello to my little friend. (Via: dailykos.com)
“Say hello to my little friend.”
(Via: dailykos.com)

Death from Below! (The Purse-Web Spider)

(Via: Wikimedia Commons)

Common Name: Purse-Web Spiders

A.K.A.: Family Atypidae

Vital Stats:

  • The family contains three genera; Atypus, Calommata, and Sphodros
  • Females reach up to 30mm (1.2”) in length
  • Fangs can measure up to half the spider’s body length
  • Prey includes crickets, beetles, millipedes, ants, wasps, and other spiders
  • Web tubes measure up to half a metre (20”) from top to bottom

Found: Africa, temperate regions of North America, Europe, and Asia

It Does What?!

Imagine you’re a beetle, peacefully strolling along the forest floor, minding your own business, when suddenly, two enormous black spikes drive up out of the earth and impale you through the abdomen. As everything fades to black, your last beetle-ly thought is, “What the hell was that?!

You have just become a tasty lunch for the purse-web spider.

So how does this work? Well, unlike most of the spiders we’re familiar with – those with small, pincer-like mouths that sit in webs all day – purse-webs are a type of primitive spider called a mygalomorph. In this group, the fangs are like a pair of large (relative to the spider) tusks that only move up and down; they don’t pinch, and this feature lends itself to some rather creative hunting methods.

Rather than constructing a flat, aerial web designed to have something fall into it, the purse-web spider spins what is essentially a silken tube-sock. The ‘foot’ of this sock lies along a slight depression in the ground, while the upper part lies vertically against a tree or rock (or, in some species, angles downward into the earth). The spider will then place bits of bark and lichen onto both parts of the web as camouflage. Over time, moss will actually begin to grow on the web, completing the disguise. All the spider needs to do now is wait, suspended from the ceiling of her underground lair, for some unwitting creature to walk over it. When this happens, she rushes to the source of the disturbance and spears her prey from below with her fangs before they realise what hit them (like this).

Invisible by spider standards, anyway.
(Via: Wikimedia Commons)

The spider will be vulnerable to larger predators if she ventures out into the open, so she simply cuts a slit in the web, drags her impaled prey inside, and seals up the hole again. Having sucked out their delicious insides, she then drops the dead husks out of the top of her sock like so much household garbage. In fact, researchers determined the diet of the purse-web spider by noting the various exoskeletons hanging from the outside of the web, having gotten caught on their way down. Apparently, all the dead bodies seemingly stuck to the side of a nearby tree aren’t much of a deterrent to other passersby.

So, since these spiders never leave their burrows, and kill anything that approaches, mating must be tricky, right? Right. The male is attracted to the female’s web by means of pheromones, and ventures out to find it. Once he locates the web, he must be very careful, tapping at the outside of the tube in a way that indicates he isn’t prey. Ultimately, though, whether he’s prey or not will be up to her. If the female inside isn’t yet mature or is already pregnant, she won’t hesitate to eat him when he attempts to enter the burrow. Researchers experimenting with placing male spiders in or near the webs of unreceptive females noted, essentially, that they run like hell as soon as they figure out what’s what. Research is amusing sometimes.

A male purse-web spider on what will be either the best or worst day of his life.
(Via: Florida Backyard Spiders)

But in the happy instances where the female is willing to mate, the male enters safely, and in fact continues to live with her for several months of domestic bliss before he dies naturally. And then she eats him anyway. Spiders are not sentimental creatures. Her eggs will take almost a year to hatch, and the young will stay with her for nearly another year after that, before striking out in the world to spin their own tube-sock of death.

Says Who?

  • Beatty (1986) Journal of Arachnology 14(1): 130-132
  • Coyle & Shear (1981) Journal of Arachnology 9: 317-326
  • Piper (2007) Extraordinary Animals: an encyclopedia of curious and unusual animals. Greenwood Press, Westport CT.
  • Schwendinger (1990) Zoologica Scripta 19(3): 353-366