Marui, on his own side-quest, was busy scooping as many not-crabs as possible into specimen containers. A crab in a box was one less not-crab that could crawl on him, and since he wasn't allowed to kill them indiscriminately anymore (and he had tried, ohhh he had tried,) it was the best option left to him.

"Crabs. They're crabs. Not crawlers. Just crabs." This thought kept him going as he attempted to de-crab a still sleeping Zaizen, only taking a break to swat off all the ones that the jerking shuttle had showered him with.