I am aware that animal names are generally written in katakana (although sometimes hiragana), but kanji is sometimes used. I stumbled upon the Japanese word クラゲ, meaning jellyfish. The dictionary said that there were two different kanji combinations for the word: 海月 and 水母, both pronounced as クラゲ. The first means 'sea moon', and the other, 'water mother' (possibly 'aquatic mother'). What is the difference between the two and are they interchangeable? If not, which is more appropriate if either at all. Any dive into nuance (Why strange words like moon and mother were used) could be useful.

  • Don't know the full story, but seems that "jellyfish" in Chinese is 水母, so that might be a lead.
    – dainichi
    Commented Feb 25, 2013 at 5:17
  • If 海月 is of Japanese origin can we just assume that they called it that because it looks like a moon in the sea?
    – ssb
    Commented Feb 25, 2013 at 6:02
  • There's a quite rambly analysis of it here
    – ssb
    Commented Feb 25, 2013 at 6:08
  • 4
    Don't start trying to make sense of multiple 義訓 writing and (often non-existing) nuances between them. There lies madness.
    – Dave
    Commented Feb 25, 2013 at 6:53

1 Answer 1


According to 大辞林, 水母 and 海月 are 熟字訓{じゅくじくん} ways of writing the same word. (See the angle brackets? Those indicate 熟字訓.)

熟字訓{じゅくじくん} readings are a type of 当{あ}て[字]{じ}. They're "word readings", where an entire kanji compound is given a reading. (Usually, readings are given to single kanji.) As Wikipedia explains, these are established word readings assigned to a group of kanji; established means, more or less, that people commonly know how to read them. This is the opposite of 義訓(ぎくん) readings, which are not established and usually require furigana to be read.

How is this relevant to your question? You asked about the difference between "the two words for 'jellyfish'". But they're the same word! It's just written more than one way. (In fact, it's also been written 水月, 久羅下, and 鏡虫.)

Look at 大辞林's entry again. If they were different words, they'd have separate dictionary entries. If they were the same word but had important differences, the dictionary would probably make a note of that (as in the entry for いとこ), or divide it into senses and show a particular kanji where the difference matters (as in the entry for つく). In fact, I checked three other dictionaries, and not one mentioned a difference between them.

So I don't believe there are any important differences. Even in the idiom listed by 大辞林, 「水母{くらげ}の[骨]{ほね}」, it's easy to find it written online with the other kanji. Personally, I'm going to keep writing it クラゲ.

  • Oh, gotta love how many combinations there are for いとこ!
    – istrasci
    Commented Feb 25, 2013 at 15:15

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