Electric eels and anglerfish? They don’t look similar…

Updated January 15, 2021
with the addition of more taxa the electric eel nests with the bowfin, Amia, while the goosefish, Lophius, nests with the tripod fish, Bathypterois.

…and yet
the large reptile tree (LRT, 1514 taxa; subset Fig. 4) continues to nest Electrophorus and Lophius together with every additional fish taxon ever since they entered the LRT, not quite on the same day, but close.

Either something is wrong…
or something is right. These two are such odd bedfellows. Why do they continue to attract one another. My curiosity was raised, so I dived into the candidate sister taxon list!

Traditionally
catfish, like Clarias, have been associated with the electric eel, Electrophorus, but the LRT separates them by a great morphological gap, as we learned earlier here.

There is an electric catfish
with small eyes, thick lips and a cylindrical body. Unfortunately, Malapterurus is indeed a catfish, not close or even transitional to Electrophorus.

Figure 1. Lophius in vivo. The pelvic fins are hidden from view beneath the large pectoral fins. So Lophius is all mouth and tail. Inset shows a larva/hatchling not to scale.

Figure 1. The goosefish Lophius in vivo. The pelvic fins are hidden from view beneath the large pectoral fins. This does not look much like an electric eel, but the two nest together in the LRT. Let’s search for a suitable, reasonable and valid transitional taxon. The inset hatchling goosefish does not provide a clue to the identity of the transitional taxon.

Figure 1. Bathypterois, the deep sea tripod fish, shown with diagram of jaws and palate from Sulak 2006, then colored and matched to the in vivo specimen.

Figure 1. Bathypterois, the deep sea tripod fish, shown with diagram of jaws and palate from Sulak 2006, then colored and matched to the in vivo specimen.

At times like this
my guess is to go looking for a long, narrow angler fish relative (if there is one) to bridge the morphological gap that currently separates the wide-mouth goosefish, Lophius (Fig. 1), from its elongate LRT sister, the electric eel Electrophorus (Fig. 2).

It turns out a good candidate 
is Forbesichthys (Fig. 2; Putnam, 1872; 9cm in length) the spring cave fish of Missouri, USA. Yes, that’s a long way from the Amazon, where Electrophorus is found, AND a long way from both sides of the North Atlantic, where Lophius is found.

de Santana, Vari and Wosiacki 2013 determined
“that Electrophorus possesses a true caudal fin formed of a terminal centrum, hypural plate and a low number of caudal-fin rays.”

Figure 1. Forbesichthys an apparent sister taxon to Electrophorus along with other sisters to the blind cave fish, Typhlichthys subterraneus. When I find skull material for Forbesichthys, I will enter it in the LRT.

Figure 2. Forbesichthys an apparent sister taxon to Electrophorus along with other sisters to the blind cave fish, Typhlichthys subterraneus. When I find skull material for Forbesichthys, I will enter it in the LRT. That’s the mandible of Typhlichthys superimposed on the skull of the electric eel, the only comparable data at present. Image from Armbruster, Niemiller and Hart 2016.

Figure 1. Amia juvenile with DGS colors added. Image from Digimorph.org and used with permission.

Figure 1. Amia juvenile with DGS colors added. Image from Digimorph.org and used with permission.

Unfortunately skull material for Forbesichthys agassizii
cannot be located at present, so its addition to the LRT will have wait until those data arrive. Wikipedia reports, “The head is sloped, and it has a protruding lower jaw” similar to the electric eel. “It has a well-developed sensory system. This system occurs in clusters on the head.” Like the electric eel, eyesight is poor in all cave fish, many of which are not related to Forbesichthys. Note the similar arrangement of all fins on cave fish (Fig. 2) to the goosefish (Fig. 1).

Figure 4. Electrophorus, the electric eel, in vivo.

Figure 3. Electrophorus, the electric eel, in vivo from de Santana et al. 2013.

Radiation and distribution
Given the present phylogenetic topology, the last common ancestor of Forbesichthys, Lophius and Electrophorus was in North American rivers before some descendants radiated along the coast to South America, while still others radiated along the perimeter of the then reduced North Atlantic. That primitive last common ancestor probably looked like Forbesichthys and sought dark and murky environments that it was already suited to. Both goosefish and electric eels are clearly derived.

I’m not sure how cave fish got into caves
in the first place, or how many thousands or tens of millions of years they have been there, but cave fish probably arrived from nearby rivers and lakes seeping into limestone fissures a long time ago, given that their non-cave sisters no longer inhabit North American lakes and rivers. (Let me know if this is incorrect!)

Figure 3. Revised subset of the LRT focusing on ray fin fish and kin.

Figure 3. Revised subset of the LRT focusing on ray fin fish and kin.

Interesting factoid:
Many cave fish are cannibals, not only to sustain themselves in low prey environments, but also to avoid overpopulating such environments.

Further updates will come
if and when skull data for Fobesichthys arrives.

If anyone knows
that this hypothesis of relationships was published earlier, please cite the reference and let me know so I can provide proper credit.


References
Armbruster JW, Niemiller ML and Hart PB 2016. Morphological Evolution of the Cave-, Spring-, and Swampfishes of the Amblyopsidae (Percopsiformes). Copeia 194(3):763–777,
deSantana CD, Vari RP and Wosiacki 2013. The Untold Story of the Caudal Skeleton in the Electric Eel (Ostariophysi: Gymnotiformes: Electrophorus). PLoS ONE 8(7): e68719. https://doi.org/10.1371/journal.pone.0068719
Putnam FW, 1872. The blind fishes of the Mammoth Cave and their allies. American Naturalist v. 6 (no. 1): 6-30. Also published in:
Packard, Jr. and Putnam 1872. Life in the Mammoth Cave, etc. chapter 3, pp. 29-54.

wiki/Spring_cavefish
wiki/Electrophorus
wiki/Lophius