Hundreds or thousands of giant crabs cross the coastal road and marsh in a concentrated surprise advance toward the sea.
The migration blocks a poorly maintained road near the island's southern coast. A massive car pileup, wetlands on both sides, and the sheer density of crabs prevent Carl and Donut from going around.
The approaching army produces cracking, skittering, and the murmur of a thousand conversations while shaking treetops. The crabs crawl over one another, consume fallen body parts, and splash rapidly into the marsh without regard for casualties.
Their concentration exploits the monk seals' weakest coastal deployment. Once in the water, the crabs intend to circle the island and continue the reproductive assault. Carl and Donut use invisibility and conceal Mongo to avoid detection.
An endless river of enormous armored crabs crossing wrecked cars and flooded marsh toward a visible ocean.
The migration is both a strategic invasion and the immediate obstacle between the party and Samantha.