Phreatobius: a tiny naked pre-knifefish, pre-electric eel from Brazilian cisterns

Updated August 26, 2022
with the realization that I was identifying facial muscles as bones. This is not a living placoderm, but a transitional taxon between blind cave fish and knife fish + electric eels.

Putting taxa together that have never been tested together
This time the LRT takes on the tiny, blind, “puzzling” catfish, Phreatobious (Figs 1–3).

Figure 1. Phreatobious, a blind cistern ‘catfish’ nests with pre-knifefish in the LRT.

Figure 2. Anterior of Phreatobius with bones estimated.

Phreatobius sanguijuela
(Goeldi 1905; Muriel-Cunha and Pinna 2005; Fernández, Saucedo, Carvajal-Vallejo and Schaefer 2007; Ohara, Costa and Foseca 201; 4.2cm long) is the extant cistern ‘catfish’ from the Amazon. According to Wikipedia, “Phreatobius has been classified with a number of different families: Clariidae, Plotosidae, Trichomycteridae, Cetopsidae, and Pimelodidae. Most recently, it has been classified in Heptapteridae. Its phylogenetic position remains uncertain.”

Shibatta et al. 2007 report,
“The species displays typical adaptations seen in hypogean [subterranean] fishes, such as reduction of eyes and of dark integumentary pigmentation. In addition to those, many other morphological peculiarities also make the species distinctive at a glance from other Neotropical catfishes, such as a paddle-shaped caudal region formed by a caudal-fin extended dorsally and ventrally by numerous large procurrent rays, ventrally confluent with the anal-fin, a strongly prognathous lower jaw, massively hypertrophied jaw muscles and a bright red coloration in life. The internal anatomy of P. cisternarum is equally unusual, which has resulted in uncertainties about its familial allocation.”

Here
in the large reptile tree (LRT, 2106 taxa [then] 2135 taxa [August 26, 2022], Phreatobius nests between the blind cave fish, [Amblyopsis + Typhichthys] and [Kryptoglanis (Fig 3) + Electrophorus + Gymnotus]

Figure 3. The cave fish Kryptoglanis in 3 views. The LRT nests this taxon with knifefish and electric eels. Note the catfish-like barbels by convergence.

There is another small cave fish sporting catfish-like barbels.
Kryptoglanis (Fig 2b) is not related to catfish, but to electric eels (Electrophorus), itself derived from another clade of blind cave fish.

This novel topology is what happens when you add taxa
and let your software tell you how taxa are interrelated. This is how your own LRT can help you resolve similar enigmas. This is why you should never ‘cherry-pick’ your taxon list. Prior studies omitted pertinent taxa.

According to Muriel-Cunha and de Pinna,
“Individual fish hide in orifices of canga rocks at the bottom of wells. P. cisternarum is an opportunistic predator that feeds vigorously on earthworms and other live invertebrates. The subterranean catfish Phreatobius cisternarum is one of the most puzzling catfishes. Its unusual subterranean environment, bright red coloration, aberrant morphology, rarity in collections and uncertain phylogenetic relationships, all make the species an intriguing member of the Siluriformes.“

This appears to be a novel hypothesis of interrelationships.
If not please provide a citation so I can promote it here. This hypothesis needs to be confirmed, refuted or corrected by others.

References
Fernández LA, Saucedo LJ, Carvajal-Vallejos FM and Schaefer SA 2007. A new phreatic catfish of the genus Phreatobius Goeldi 1905 from groundwaters of the Iténez River, Bolivia (Siluriformes: Heptapteridae). Zootaxa. 1626: 51–58.
Goeldi E 1905. Nova zoologica aus der Amazonas-Region: Neue Wirbeltiere. In: Congrès International de Zoologie, 6. Compte-Rendu. W. Kundig & Fils, Genève, p.542-549.
Muriel-Cunha J and de Pinna M 2005. New data on cistern catfish, Phreatobius cisternarum, from subterranean waters at the mouth of the Amazon River (Siluriformes, Incertae Sedis). Papéis Avulsos de Zoologica 45(26):327–339.
Ohara WM, Costa I and Foseca ML 2016. Behaviour, feeding habits and ecology of the blind catfish Phreatobius sanguijuela (Ostariophysi: Siluriformes). Journal of Fish Biology 89(2): DOI: 10.1111/jfb.13037
Shibatta OA, Muriel-Cunha J and de Pinna MCC 2007. A new subterranean species of Phreatobius Goeldi, 1905 (Siluriformes, Incertae sedis) from the Southwestern Amazon basin. Papéis Avulsos de Zoologia 47(1):191–201.

cavefishes.org.uk
wiki/Phreatobius_sanguijuela
wiki/Phreatobius