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Hello, I am fishhead0103666 and I have some Tetranematichthys that I would like to get an id on if possible as I can’t tell if they are T wallacei or T quadrifilis. I have been told they are 5”-6” and were collected in Rio Napo Peru.
Hopefully someone can help me with this identification but I understand if no one can help me with this since I can’t see any distinguishing traits between the two species myself.

More pictures can be gotten if it will help with the identification.

Key to the species of , source:
https://www.scielo.br/scielo.php?script ... 0000100009

1. Lateral margin of head at anterior portion of orbit, parallel to the posterior limit of operculum, in ventral view (Fig. 5a); ventral region light or dark colored; presence or absence of digit-like projections on distal portion of mandibular barbel; length of mandibular barbel 5.2- 2.6% HL; length of main ray of upper caudal-fin lobe 14.5- 20.5% SL; body width at pectoral-fin insertion 82.1- 110.0% HO; head length 24.4- 31.0% SL ..................................................2

1a. Lateral margin of head at anterior portion of orbit, diverging laterally to posterior limit of operculum, in ventral view (Fig. 5b); uniform color pattern with dark patches along the body; presence of digit-like projections on distal portion of mandibular barbel; length of mandibular barbel 6.3-23.5% HL; length of main ray of upper caudal-fin lobe 18.1-19.9% SL; body width at pectoral-fin insertion 117.0125.0% HO; head length 28.5-32.3% SL ............................................. .............................................................. T. quadrifilis (rio Guaporé Basin)

2. Dorsal region without blurred light area; dark patches of pigmentation present along the body; ventral region dark colored; one light colored stripe present on the caudal-fin; pelvic-fin without distinct light colored stripe; anterior margin of mandibular barbel with digit-like projections; length of mandibular barbel 7.4-24.6% HL; snout length 41.2-49.3% HL; body width at pectoral-fin insertion 69.279.3% HL ................................................... T. wallacei (rio Tocantins, rio Orinoco and several stretches of the rio Amazon, except rio Guaporé)

2a. Dorsal region with blurred light area; dark patches of pigmentation absent along the body; ventral region light colored; two light colored stripes present on the caudalfin; pelvic-fin with distinct light colored stripe; anterior margin of mandibular barbel serrated on distal portion; length of mandibular barbel 4.5-6.0% HL; snout length 28.834.5% HL; body width at pectoral-fin insertion 82.1-88.7% HL..................T. barthemi (rio Saracá, rioAraticum and rio Urubu).

I can't really see from the pictures, but you'd go 90% at least for T. wallacei.

Jools

Thank you Jooles for your response, from what I understand my best chance at getting a proper id is to look at the snout since T wallacei is at 28.8% hl and T quadrifilis is at 41% - 49% hl.
Due to my limited scientific background I’m afraid that I do not know what HL means in this context, could you enlighten me to it?

HL = head length.

These two images may help with key.

Cheers, Eric

I also think is correct for the following reasons:

1. Tetranematichthys barthemi
   has a conspicuous pale underbelly, whereas the other two species are relatively evenly pigmented along the flank and underbelly. Your fish appears to be rather dark on the sides and underbelly, so, not T. barthemi.
2. From an older paper (Vari, R. P., & Ferraris Jr, C. J. (2006). The catfish genus Tetranematichthys (auchenipteridae). Copeia, 2006(2), 168-180.) which can be obtained here (https://bioone.org/journals/Copeia/volu ... 0.CO;2.pdf), their key for distinguishing T. wallacei from
   Tetranematichthys quadrifilis
   relies on (among other traits) a clear distinction in the distance from tip of snout to dorsal-fin origin relative to standard length (SL): 0.29–0.32 of SL for T. wallacei vs. 0.33–0.36 for T. quadrifilis (meaning the head of T. wallacei can be significantly shorter than, and is never longer than, that of T. quadrifilis).

I took one of your photos and did what all good scientists disdain - I attempted to perform loosey-goosey morphometrics from an uncontrolled photo of an unposed fish from an unorthodox angle!

I did so to make a coarse... and I mean COARSE... estimate of this ratio. It was relatively straighforward (and straight) to draw a line from tip of snout to origin of dorsal fin. What was more difficult was getting a reasonable estimate of SL, since your fish's body is bent and the camera angle is poor. But with that, I did my best and obtained a vague estimate of the ratio, which came out to be about 0.289, very close (considering the sloppy method) to the range for T. wallacei and well outside of the range for T. quadrifilis. If anything, I might have underestimated SL, which would drop your ratio even lower than the range accepted for T. wallacei, but that would be for scientists to then figure out, right?

For comparison, I asked the question, "how short would the body need to be to obtain the minimum 0.33-0.36 ratio for T. quadrifilis? I drew that on the photo too (green lines), in order to show just how different the body length would need to be in order to qualify this fish as T. quadrifilis. Not even close!

Therefore, I conclude you have .

As further confirmation, I checked my technique two ways:

1. I went back to the Vari & Ferraris paper and extracted the photos for both T. quadrifilis and T. wallacei, to measure the same ratios and to show visually the shorter head of T. wallacei. See comparison photo below. My ratios for both species fall below the ranges in their paper, but even so, the relative proportions are obvious. I might be using the wrong landmark for origin of dorsal fin - I'm trying to estimate the first measurement from the anterior edge of the dorsal spine insertion. I discovered that if I used the posterior edge of the insertion of the dorsal spine on each fish, that would add 10 pixels to the first measurement for each fish, and in each species that would be sufficient to bump the ratios up to 0.29 and 0.33, respectively).
2. I analyzed image 5 in the CLOG for T. wallacei to image 1 for T. quadrifilis and again obtained similar ratios.

Unrelate aside: I include here a photo from the Vari & Ferraris paper showing an scanning electron micrograph of the barbel of T. wallacei.