The marching scuttlecrab replaced its ancestors, the nomadic scuttlecrab, in all the habitats the nomadic scuttlecrab inhabited before being forced into extinction by it. They are the result of an unusual behavioural mutation in the nomadic scuttlecrab. Their aggressiveness by far exceeds the nomadic scuttlecrab's. They don't run, they swarm, and shred everything they can find, eating the remains. Even much larger creatures can be brought down by these ravenous little insectoids.
Marching scuttlecrab are organized into columns. Columns will literally leave barren landscapes in their wake, as they continuously march forward, consuming all in their path. At the forefront of the column is an alpha-female, leading the column towards its next food source. Columns themselves are made up of between a few dozen at the smallest, and nearing a thousand at the largest. When two columns meet, they will merge for a few meters, and then split off into two separate directions. It randomly choose a column, and this helps to mix up the gene pool, as they exchange members this way. However, this can reduce the 'strength' of a larger column, as the two resulting columns are roughly the sum of both cut in half, in most all cases. Those that die of age are consumed, while those that are apparently sick are thrown out, and the injured, if unable to defend themselves, are simply eaten as well.
The marching scuttlecrab themselves are virtually the same physically as their ancestors. The only difference is that their back legs are in reverse, letting them run significantly faster, similar in style to a horse. They use this increased speed to help them chase down food. However, their ability to survive long periods without food or water is much longer than the nomadic scuttlecrab, and they can go many months without food or water, as they store greater amounts of these, and use them more efficiently.
They will gather together at night to sleep, and prepare for the next day, in roughly a tight circle. 'Picket' marching scuttlecrab who are carried, sleeping, on the backs of their non-nocturnal brothers, then awaken, and form a defensive line that will wake the entire column to attack a predator. However, once the night is over, they once more return to sleeping.