I'm into the marine fish hobby and quite a few are aggressive to those that are too similar; usually fish that are congeneric to themselves; however most people just refer to the group as conspecifics. The point being, much of the vernacular in the trade can't be taken literally.

I was looking up a specific fish and in the description was, "It is best not to keep [it] with fish of the same genera". It makes sense to me that it shouldn't be kept with members of its genus, but how can it be in the same genera? Does that mean within the same family? Was the word just used incorrectly or am I not understanding the usage?

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    genera being the plural of genus, yes, it was used incorrectly, but media and data suggest you shouldn't lose too much sleep over this. – stevesliva Oct 24 '15 at 4:22
  • I figured as much, but I don't like to assume. – Gary Oct 24 '15 at 4:23
  • @stevesliva: media and data mean different things from medium and datum, and cannot be replaced by them. But in this case, genera can easily be replaced by genus. – herisson Oct 24 '15 at 4:23
  • @sumelic: right. I'm just joking that English words can lose enough of their attachment to the Latin, that the plural and singular forms can evolve to mean both, or entirely separate things. But yes, seriously, no one says genera when speaking of one genus. – stevesliva Oct 24 '15 at 4:26
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    @Mitch: the word “datum” (with the plural “datums”) has a specialized meaning in the context of geodetic surveys: oceanservice.noaa.gov/facts/datum.html – herisson Mar 22 '18 at 14:07

Genera is a plural noun; it's used as the plural of the singular genus. Since any single species only belongs to one genus, the sentence you quote ("It is best not to keep [it] with fish of the same genera") doesn't seem to use proper grammar. "It is best not to keep it with fish of the same genus" would be correct.

If some plural noun referring to a family of fish, or some larger category of fish, were used in place of "it", I think the sentence might be acceptable. Something like "It is best not to keep acanthurids with fish of the same genera" or a general statement like "It is best not to keep certain species of fish with fish of the same genera". Even though this structure doesn't seem incorrect, it still seems more awkward to me than the structure with a singular noun phrase after "keep" and a singular "genus" after "same". I guess the way I would try to evaluate it is by replacing the word "genera" with some word that takes a regular plural like "families": if the situation was that the fish were aggressive to other fish that belonged to the same families, what kinds of sentences would sound natural?

  • "When dealing with members of the order Perciformes, keep them away from others of the same families."
  • "It is best not to keep perciforms with fish of the same families."
  • "It is best not to keep certain species of fish with fish of the same families."

I think these sentences are technically correct because they have consistent pluralization but "of the same families" does sound a bit odd to my ears even so. Probably, sentences of this structure with plural noun phrases are not as common as sentences of this structure with singular noun phrases.

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    They were definitely talking about an individual fish and I couldn't think get past it to think about what they probably meant. The second paragraph is almost exactly what I was commenting when your edit went through. They were talking about the family 'Acanthuridae', which almost every member is notoriously belligerent towards members of their own genus but not necessarily the rest of the family. Therefor, the general rule of thumb when referring to a member of the family as a whole, keep them away from another of the same genera. – Gary Mar 22 '18 at 15:46
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    Ah, thanks, I'll edit to replace my random example with "acanthurids" then. I think it's a bit tricky: in the last sentence of your comment, you used the phrase "a member of the family as a whole", and "a member" is singular and refers to some single species, which makes the use of "genera" seem not quite right to me in the exact sentence as written. But if you had said something like "when dealing with members of the family Acanthuridae, keep them away from others of the same genera", it does seem to me that it would be appropriate to use the word "genera". – herisson Mar 22 '18 at 15:52
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    Searching Google Scholar for "a genera" yields nothing remotely readable, but searching for "same genera" yields thousands of hits. Discarding typos, line hyphens, and legit plural usage, it looks like well over half are referring to a single genus. I have a sneaking suspicion that species is rubbing off on genus. – Phil Sweet Mar 22 '18 at 20:49

the usual major subdivision of a family or sub-family in the classification of organisms usually, consisting or more than one species.

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    You haven't answered the question genera or genus. – Chenmunka Mar 22 '18 at 10:09
  • Your answers should be in the form of complete sentences; and please include links to corroborating sources, and provide examples of correct usage, if possible. – Bread Mar 22 '18 at 12:54

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