The Cantercrab

The Cantercrab (Megacursoriarthrus bracchiatus) is the largest terrestrial isopod yet to evolve, reaching over 400 grams and about 20 centimeters long. It lives in hot savanna environments and possesses several features that allow it to survive the long dry season. For example, the exoskeleton of its face forms a "brow ridge" to block harsh sunlight, and the antennules can act like wipers to scrub off any sand or dust that gets caked onto the eyes. It is also adept at finding any source of moisture to wet its gills. In some cases, it may dig up to 50cm straight down to find groundwater, but more often it simply tears open a succulent plant and applies the water-rich innards to its underbelly, letting moisture soak in slowly enough that a single large leaf can sustain it for many hours.

As a member of the upright-walking clade within Scansoriarthriformes, the cantercrab can run about as quickly as a ground-dwelling bird or mammal of its size. It is a member of the stagmice, which ancestrally walked on their first three pairs of legs. This species is also functionally hexapodal, but it walks on pairs 2, 3, and 4, with the front legs modified into opposable claws. These are structurally very different from the quick, grabbing claws of the cantercrab's snatcher relatives, whose last foreleg segment is a spiny, serrated, backward-pointing cutting apparatus that pins and cuts its victims against the proximal segments. By contrast, M. bracchiatus's claws are made from the three segments, with the first one growing a large projection past the point of articulation with the second. The final two segments - propodus and dactyl - are smaller, very flexible, and capable of fine movement to oppose and hold objects against the outgrowth of the larger carpus. 

This is not to say the cantercrab's pincers are weaker than those of snatchers; they are about equal in strength, adjusting for body size, but specialized for very different uses. While snatchers are devoted tree-defenders with claws designed to tear attackers apart, cantercrabs are, for the most part, solitary seed-eaters who use their claws to pluck, carry, and crush large grains of prairie grasses. The smaller, jointed "digit" can also puncture the shells of seeds and arthropod foes alike using a forward-and-back motion not dissimilar from the maxillae of harpoonjaw mosquitoes. Megacursoriarthrus's mouthparts, like those of most members of its order, are relatively weak, for its ancestors ate primarily liquid food before they abandoned their tree-mutualist niche. The main purpose of its jaws is now to catch fragments of seeds broken by the claws, then funnel them into its gut. This diet and lifestyle suits the cantercrab well for most of the year in its native range of Loxodia's ten-month savanna. However, during the short rainy season, the population of other seed-eaters like Muridiungulates swells, with millions of mothers migrating into the formerly parched landscape to find a rich, seemingly endless meal to support their calves. For the cantercrab, though, the onslaught of stomping hooves and careless cropping bites poses an extreme danger. Any individuals with enough fat reserves to survive a two-month period of estivation do so, while the unlucky ones are forced to remain active and face the very real possibilities of starvation, trampling, or being opportunistically eaten by the ravenous grazing rats.

It is only when the rodents leave that the isopods can begin their own reproductive cycle. In the rats' absence (and supported by hundreds of thousands of tons of dung), grasses regrow quickly and produce another round of flowers. Before the last of the rainy-season moisture has evaporated from the savanna's soil, tens of millions of cantercrabs search for each other, forming breeding colonies of up to a thousand. If males outnumber females, they may form a large lek, snapping their claws at each other to show off their fitness. Females can carry many thousands of eggs at a time, which they cradle under their bodies by flexing their tail and last three body segments under their midsection, using their last three pairs of legs to latch onto the base of the running legs. This forces them to adopt a semi-sprawling posture while the eggs are developing, greatly reducing their speed and ability to evade predators. The males are thus forced to stick around and defend their mates if they want their progeny to have a chance at hatching successfully.