Whose line is it, anyways? The search for Nannostomus marilynae..or whatever it is!

As fish geeks, we get into some pretty esoteric stuff, huh? 

We all have our geek-out about different stuff...one of the things I geek out about is making sure that I have the fish that I actually think I have. And one of my favorite groups of fishes, characins, specifically, Pencilfishes of he genus Nannostomus- just happens to be chock full of fishes that bear a rather annoying resemblance to each other, making positive identification a real challenge. What this typically means is that a shockingly large percentage of the time, the fish you think you are purchasing is not what it is supposed to be!

Case in point: 

I recently acquired a small group of what were offered as "Marginatus Pencilfish" from my LFS. Of course, being the fish geek that I am, I immediately was charmed by the fish, but also realized that...they...were not...N. marginatus! Of course, that made them even MORE attractive to me (that's a fish-geek truism- mis-identified fishes are irresistible! Why is that? Hmm...).

(The "real" N. marginatus- note the red in the fins...)

Of course, I did what any self-respecting fish geek would do: I bought the whole group! I mean, it was only six fish, but they were perfect sized for my new tank, and they looked awesome!

Once I got them home, acclimated, quarantined, and introduced into the display aquarium- the whole deal- I spent some time looking at them, once again attempting to make a "best guess" at their identification. If you're a Pencilifish lover, you'll notice that a lot of different species seem to trade under just a few names, N. mariginatus and N. trifasciata being two of the more commonly "over-applied" names in the trade. 

The first thing I noticed was the distinct absence of red almost anywhere on my fish's bodies. Now, that in and of itself wouldn't be 100% conclusive as a reason why my fish are NOT Marginatus, but it's a pretty good clue. Through the power of the internet, I was able to locate a paper by the two guys who really know the genus better than anyone, Stanley Weizmann and J. Stanley Cobb, and it was pretty cool. Of course, not being a scientist, these papers generally make my head spin...However, there was some good information for a non-scientist who was persistent!

After perusing this paper, I realized that the real "X Factor"with Pencilfishes, at least to the layman,  who isn't about to do scale counts, fin ray counts, or DNA testing, is the stripes. Ubiquitous in the genus, they do serve as a starting reference point for at least a rough species identification. Indeed, Eitzmann and Cobb did a thorough study on the genus for the Smithsonian in 1975, and made the following, very interesting statement in their abstract:

"Evolutionary trends toward specialization within the genus appear to be development of longitudinal stripes, development of oblique bands, loss of ossification of the sensory canal in the second infraorbital bone, an elongation and thickening of the anal-fin rays of males as an aid to fertilizing eggs, possibly the development of an ocellus in the dorsal lobe of the caudal fin, and development of nuptial tubercles on the ventral surface of the head."

So, in other words, stripes and bands on the body are pretty important in establishing the identification of species, and each species' stripes were some form of evolutionary adaptation.

Wow, I was hooked even more on these fish! But I still wasn't sure what I had. I narrowed it down to three species: N. beckfordi, N. minimus, and N. eques. Notice N. marginatus wasn't even in the running after I read the Smithsonian paper?

After reading this paper, looking at a lot of other pictures, and hobby-related websites and material, I've sort of zeroed in on N. marilynae as my primary candidate. And I think I'm as correct as any other amateur!

Of course, there are still a few other nagging candidates that share similar charachteristics. For example,  at times, Nannostomus marilynae may have a moderately developed secondary stripe, and weak secondary stripes may occur in N. beckfordi and especially in N. digrammus.

The official description of N, marilynae is at least sort of helpful, but of course, the amateur can see the same things in a bunch of Pencilfish species: "Primary horizontal stripe black, continuous onto ventral lobe of caudal fin, cover- ing proximal one-third to more than one-half of 5 or 6 dorsalmost rays of that fin lobe. Secondary stripe black but usually weakly developed. Silvery stripe between primary and secondary horizontal stripes with a greenish and pale golden reflectant color, producing what essentially looks like a burnished greenish silvery stripe."

Okay, pretty helpful. If I can just get a good pic and if the fish could just cooperate a bit more!

(From the Smithsonian paper...helpful, sort of?)

N. marilynae was named after Stanley Weitzman's wife, Marilyn Weitzman.  Kind of romantic, actually!  Described in 1975 by Weitzman and Cobb, but I understand that it was occasionally imported as N. bifasciatus before that. That name was attached to any two stripe Pencil for a long time. That's how confusing the ID of Pencilfishes is!

 (My video frame grab...even less helpful)

And, these fishes are supposedly rare in the trade, typically arriving in the hobby only as a "by-catch" of more commercially viable fishes like Cardinal Tetras and such, and are simply called "Pencilfish" in most importer's stock lists, often misidentified as the "catch-all" N. marginatus- a usually safe bet, because everyone know that most fish don't show their "comfort colors" while in transit or at the LFS, right? Yeah.

I suspect that they are brought in rather frequently..and rampantly misidentified (perhaps likeAI'm doing here right now?)...

And here I am, perhaps perpetuating the same misidentification that has plagued the group for decades! Just reading a couple of papers and detailed articles doesn't make me remotely qualified to draw many conclusions. However, being a hobbyist, I am not going to go crazy, either. I just want to know which one I have, to a reasonable degree of certainty.

(From the fine hobby site, Seriously Fish- N, marilynae)

It reminds me very much of the confusion we had with coral ID in the reef aquarium hobby/trade. Misidentification and use of "common names" and "morphs", none of which have any scientific meaning- is rampant, and it's really hard to know exactly what species you're dealing with. Most of the "rare" varieties of Acropora, for instance, are just color variations of common species, like Acropora tenuis.

Ahh, just when I thought life could get more simple, right? 

The conclusion? It's fun to know what you have, to do some digging. But it's also fraught with problems, and publicly concluding something without 100% positive identification is perhaps even more detrimental than the misidentification that as already going on, so I'm not going to state with any degree of certainty that I know exactly what I have! 

Regardless of what my little brown Pencilfish are, I love them dearly. 

That's good enough for me.

For now, anyways.

Todays tale of stripes, fin ray counts, and excessive hobbyist curiosity.

Until next time- stay persistent. Stay educated. Stay focused.

And stay wet.


Scott Fellman

Tannin Aquatics


Scott Fellman
Scott Fellman


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