I've just written the question Are there “Space Jellyfish” in Florida? Would they be possible?

The phenomenon is described in Wikipedia as follows:

A space jellyfish (or jellyfish UFO; also rocket jellyfish) is a rocket launch related phenomenon that frequently is misidentified as an alien UFO. The phenomenon is caused by sunlight reflecting off the high altitude rocket plume gases emitted by the launching rocket, at pre-dawn and post-dusk, the twilight plumes. The observer is shrouded in darkness, while at high altitudes, sunlight is able to reflect off the exhaust that is being lit before dawn reaches lower altitudes or after dusk has already left lower altitudes, due the curvature of the Earth and its rotation causing the day-night cycle. This luminous apparition is reminiscent of a jellyfish.

In the title I used the plural form space jellyfish because it sounds humorous, as if there could be a school of such fish. But in the body of the question, I use space jellyfishes because I feel that this is more likely correct.

But I don't know why I feel this way nor how I made this choice.

Is there a grammatical rule that I'm using intuitively without realizing it? Is there some a priori reason that would tell us that the fishes form would be the correct choice, presuming that it is?

  • 1
    '[jelly]fish' is a collective term, best used when there are more than one at the same time. To indicate that they only occurred one-at-a-time on multiple occasions, '[jelly]fishes' is better.
    – AmI
    Commented Apr 2, 2019 at 10:41

1 Answer 1


The plural of "fish" is also "fish" . But under some rare circumstances, fishes can be used too.
For example if the specie is different, or if it is to be specifically shown that the amount was large etc.
(Source: https://www.grammarly.com/blog/fish-fishes/)

  • Thanks, please note though that these are not real fish.
    – uhoh
    Commented Apr 2, 2019 at 5:20
  • 3
    @uhoh Yes, but the rule is for the word "fish".
    – Bella Swan
    Commented Apr 2, 2019 at 5:22
  • okay thanks, I didn't realize it worked that way!
    – uhoh
    Commented Apr 2, 2019 at 5:31
  • 2
    I'm not sure the rule for the word itself. If for example you want to refer to Mr and Mrs Fish, you have to say the Fishes. You can't use Fish as the plural form, even though it's the same word, because they are not fish. Space jellyfish are not really fish either, but the metaphor treats them as a kind of fish, and for me that's the reason why we apply the same rule, and use jellyfish as the plural.
    – user339660
    Commented Apr 2, 2019 at 5:45
  • 2
    @Minty I agree, the same thing would apply if you knew people called "Deer" or "Sheep" where there is no alternative plural. It also applies if you know a family called "Stones". In those cases you would have to say "the Deers", "the Sheeps" and "the Stoneses" to indicate multiple people even though the normal, rather than proper nouns are already plural.
    – BoldBen
    Commented Apr 2, 2019 at 8:32

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