Correction: European eels are neotonous swordfish

Before you say, “That’s crazy!” did you ever notice
that swordfish lack ribs and pelvic fins (Fig. 4)? So do European eels.

More importantly, did you ever notice
that baby swordfish look like eels (Fig. 4)? Okay. With that in mind, let’s start with a little backstory and cover all the bases.

Traditionally swordfish have been allied with 
barracuda, marlin and several extinct billfish, including Blochideae (i.e. Blochius, Fig. 4), based on overall appearance, open sea niche and apex predator status. According to Wikipedia, “They [swordfish] are the sole member of their family, Xiphiidae.” Gregory and Conrad  1937 compared the morphologies of the sailfish and swordfish. Earlier I followed the lead of these experts in nesting the sailfish, Istiophorus (Figs. 5, 7), and the swordfish, Xiphiias, with the barracuda, Sphyraena. That was a mistake.

Today we’ll compare
swordfish and sailfish morphology to two other more closely related taxa: the anchovy, Elops (Fig. 6) and the European eel, Anguilla, which turns out to be more closely related to swordfish despite their outward differences as adults. Turns out that swordfish go through a metamorphosis as they develop from eel-like hatchlings with teeth (Fig. 4).

The LRT scores skeletal traits 
rather than superficial morphologies, which are always prone to reversal and convergence. The large reptile tree (1793+ taxa) is designed to test taxa together that have not been tested together before. Some surprises were recovered earlier using this method here, here and here.

In their description of Bavarichthys
(Fig. 1) Arratia and Tischlinger 2010 did not mention or test the eel, Anguilla, or the swordfish, Xiphias. Turns out, they should have done so.

Recent revisions
of several fish taxa (now that I have 250 fish taxa and the experience that brings to bear) reveal a hitherto overlooked hypothesis of interrelationships between eels and swordfish. Sound crazy? Keep reading. This is one of those ‘moment of discovery’ moments I want to share with you.

Figure 1. Bavarichthys is a big head/ short body anchovy from the Late Jurassic. Now it is ancestral to European eels.

Figure 1. Bavarichthys is a big head/ short body anchovy from the Late Jurassic. Now it is ancestral to European eels.

Let the cheek plates evolve away in Bavarichthys
(Fig. 1) and you’l have the basic skull of both swordfish (Fig. 2) and European eels (Fig. 3). Note the triangular profile, the slender insertion of the nasal between the anterior frontals, the extreme brevity of the post-orbital portion of the skull, including a vertical quadrate. No other tested fish taxa have these traits.

Figure 2. Diagram of the swordfish (Xiphias) skull. Compare to figures 1 and 3.

Figure 2. Diagram of the swordfish (Xiphias) skull. Compare to figures 1 and 3.

Billfish came first. 
The European eel, Anguilla, is derived from swordfish, sailfish and Late Jurassic Bavarichthys. Hatchlings of swordfish are eel-like (Fig. 4) and Bavarichthys-like (Fig. 1). That means European eels are neotonous swordfish. They achieve adulthood while still in the hatchling swordfish stage. European eels also develop traits not found in swordfish, like additional vertebrae and a long, low operculum. European eels don’t develop pelvic fins of dorsal ribs. Neither do swordfish.

Figure 5. Skull of Anguilla, the European eel, compares well with that of Bavarichthys. Note the loss and reduction of preorbital bones.

Figure 3. Skull of Anguilla, the European eel, compares well with that of Bavarichthys. Note the loss and reduction of preorbital bones.

Earlier the LRT nested
Late Jurassic Bavarichthys with closely related anchovies (genus: Elops), then with more closely related European eels (genus: Anguilla). Now it nests basal to both swordfish and European eels.

Other eels,
like the moray eel (Gymnothorax) and electric eel (Electrophorus) nest elsewhere, both near the base of the bony fish. So eels were in the gene pool.

Figure 4. Swordfish ontogeny (growth series). Hatchings have teeth, a short bill and an eel-like body still lacing pelvic fins.

Figure 4. Swordfish ontogeny (growth series). Hatchings have teeth, a short bill and an eel-like body still lacing pelvic fins. Hatchlings go through an eel-like phase and a sailfish-like phase.

Xiphias gladius
(Linneaus 1758; Gregory and Conrad 1937; up to 4.5m in length) is the extant swordfish, nesting between Bavarichthys and Anguilla. 1cm long hatchlings more closely resembled little eels, then growing to little sailfish before reducing the long dorsal fin. The sword is not used to spear, but to slice and maim smaller fish traveling in schools. The pelvic fins and ribs are absent, as in eels. Larger females produce more eggs, up to 29 million.

Figure 5. Skull of the sailfish, Istiophorus. Compare to Elops in figure 6.

Figure 5. Skull of the sailfish, Istiophorus. Compare to Elops in figure 6.

Figure 2. Elops is the extant anchovy. Compare to Bavaricthys in figure 1 and Istiophorus in figure 5.

Figure 6. Elops is the extant anchovy. Compare to Bavaricthys in figure 1 and Istiophorus in figure 5.

Figure 1. Istiophorus, the sailfish, nests with the cobria (Fig. 2) in the LRT, not with the swordfish.

Figure 7. Istiophorus, the sailfish, nests with the anchovy, Elops, not with, but close to the sailfish, Xiphias.

Sailfish have long slender pelvic fins,
like those of anchovies, unlike swordfish and eels. Sailfish have a broad postorbital, like anchovies, unlike swordfish. Sailfish have a zig-zag frontal-nasal suture, like anchovies, unlike swordfish. The list of subtle, but scoreable differences continues. More importantly, no other tested taxa share more traits with swordfish and sailfish than eels and anchovies, respectively.

Figure 8. Subset of the LRT focusing on ray fin fish. Eel-like taxa are highlighted.

Figure 8. Subset of the LRT focusing on ray fin fish. Eel-like taxa are highlighted.

With the sailfish and swordfish gone, where does that leave the lonely barracuda?
In the LRT the barracuda nests with the similar long-bodied remora (Remora) and cobia (Rachycentron), derived from the mahi-mahi (Coryphaena) all with mandibular prognathism. This is non-tradional. Other workers prefer to nest billfish with barracuda.


References
Arratia G and Tischlinger H 2010. The first record of Late Jurassic crossognathiform fishes from Europe and their phylogenetic importance for teleostean phylogeny. Mitteilungen aus dem Museum für Naturkunde in Berlin. Fossil Record; Berlin 13(2): 317–341.
Gregory WK and Conrad GM 1937. The comparative anatomy of the swordfish (Xiphias) and the sailfish (Istiophorus). The American Museum Novitates, 952:1-25.
Linnaeus C von 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

https://pterosaurheresies.wordpress.com/2020/07/04/bavarichthys-a-late-jurassic-solnhofen-anchovy/

wiki/Istiophoriformes

wiki/Swordfish

 

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