蟹は甲羅に似せて穴を掘る
(Kani wa koura ni nisete ana wo horu;
“A crab digs its hole to match its shell”)

Definition:

People think and act according to their personal circumstances and abilities. Just as a small crab will dig a small burrow and a large crab will dig a large burrow to live in, matching their size, so will people think and act based on their understanding of a given situation and the tools and skills they possess and can bring to bear. A more general version of “to a man with a hammer, every problem looks like a nail.”

Note that this saying can be used descriptively, as above, and also prescriptively: “Everyone should tailor their actions to their strengths, limitations, and needs.” “Cut your coat according to your cloth.” In this sense, the saying becomes a slightly cynical admonition against trying to reach beyond your station in life or predetermined function or role.

Breakdown:

We begin with the noun 蟹 (kani), “crab,” marked as the topic of discussion by the particle は (wa). Skipping forward, we find that the crab is also the implied subject of the comment on this topic. The object, marked by particle を (wo), is noun 穴 (ana), “hole,” and the verb is 掘る (horu), “to dig,” in conclusive form. Finally, in the middle, we have a verb phrase that gives us extra details about the digging: the noun 甲羅 (koura), “shell,” is marked by the particle に (ni) as the indirect object of the verb 似る (niru), “to resemble,” in causative form.

Notes:

This saying comes to us from the writings of Zen Buddhist priest Takuan Sōhō (沢庵宗彭), via the poetic haikai treatise Kefukigusa =『毛吹草』.

There are a number of variations on the theme of this saying, from a variant that simply contracts 甲羅 to 甲 (kou) without any change in meaning, to an alternate metaphor that replaces crab, shell, and hole with bird, wingspan, and nest: 鳥は翼に従って巣を作る (tori wa tsubasa ni shitagatte su wo tsukuru, “the bird makes its nest in accordance with its wings”).

Example sentence:

「蟹は甲羅に似せて穴を掘るというが、一日中テレビを見てばかりいる人には不適切な職もあるかもしれない」

(“Kani wa koura ni nisete ana wo horu to iu ga, ichinichijuu terebi wo mite bakari iru hito ni wa futekisetsu na shoku mo aru kamoshirenai.”)

[“They say that the crab matches its hole to its shell… perhaps if you watch TV all day long there are some jobs you’re not suited for.”]