‘Close relatives’ of the enigmatic Tetraodontiformes

Wikipedia reports,
“The Tetraodontiformes are sometimes classified as a suborder of the order Perciformes. They have no close relatives, and descend from a line of coral-dwelling species that emerged around 80 million years ago.”

“Most members of this order — except for the family Balistidae [triggerfish]— are ostraciiform swimmers, meaning the body is rigid and incapable of lateral flexure. Because of this, they are slow-moving and rely on their pectoral, dorsal, anal, and caudal fins for propulsion rather than body undulation. The tetraodontiform strategy seems to be defense at the expense of speed, with all species fortified with scales modified into strong plates or spines — or with tough, leathery skin (the filefishes and ocean sunfish).”

Figure 1. Diodon the pufferfish offers a problem. Are those facial spines circumorbital bones? Ore are they novel dermal ossifications?

Figure 1. Diodon the pufferfish offers a problem. Are those facial spines circumorbital bones? Ore are they novel dermal ossifications? Note pufferfish hatchlings have dorsal, caudal and anal fins. Compare to Mola hatchlings in figure 3.

In the large reptile tree
(LRT, 1533 taxa) three traditional members of the Tetraodontiformes are included, the pufferfish (Diodon, Fig. 1), the ocean sunfish (Mola, Fig. 2) and the triggerfish (Balistes, Fig. 3).

In the LRT all taxa have close relatives
and the Tetraodontiformes are no exception. In the LRT, the non-traditional mudskipper (Periophthalmus, Fig. 5) nests with the triggerfish, Balistes. All five are derived from the high-fin amberjack, Seriola rivoliana, Fig. 6) in the LRT (Fig. 2). This may be a novel hypothesis of interrelationships. If not, please provide the citation so I can give proper credit.

Figure 2. Subset of the LRT focusing on basal vertebrates (fish) with green applied to traditional ray fin fish.

Figure 2. Subset of the LRT focusing on basal vertebrates (fish) with green applied to traditional ray fin fish.

Diodon sp. (Linneaus 1758) is the extant porcupinefish. The long teeth are fused creating beak-like jaws. Dermal spines are distributed all over the body and skull. Pelvic fins are absent. The tail is reduced. The pectoral fins move the pufferfish slowly.

Figure 4. Mola mola is a relative of Diodon in the LRT. It has no circumorbital bones, but as a hatchling has pufferfish proportions and spines.

Figure 3. Mola mola is a relative of Diodon in the LRT. It has no circumorbital bones, but as a hatchling has pufferfish proportions and spines.

Mola mola (Linneaus 1758; Fig. 3) is the extant ocean sunfish. As a hatchling (Fig. 4) it is similar to a pufferfish (Diodon, Fig. 1) in shape, then undergoes metamorphosis to adulthood. It is the largest bony fish and the only one taller than long.

Figure 4. Mola larvae ontogeny. The caudal fin appears at first, then disappears as the dorsal and anal fin ossify and the body deepens.

Figure 4. Mola larvae ontogeny. The caudal fin appears at first, then disappears as the dorsal and anal fin ossify and the body deepens.

A third non-traditional taxon
nested heretically within the Tetraodontiformes, Periophthalmus sp (Bloch and Scheider 1801; Fig. 5) is the extant mudskipper, a goby often seen sunning itself above the surface of the water. The pelvic fins help grip the undersurface whether above or under water. The robust teeth of Periophthalmus are autapomorphies, unless one considers that the beak of tetraodontiformes evolved from similar long premaxillary teeth fusing together.

FIgure 2. The mudskipper (genus: Periophthalmus) is close to Diodon in the LRT.

Figure 5. The mudskipper (genus: Periophthalmus) is close to Diodon in the LRT.

At the base of the clade Tetraodontiformes
is an overlooked open seas fast-swimming predator that has never been associated with Tetraodontiformes before today. Seriola rivoliana (Valenciennes 1833) is the extant Almaco jack or high-fin amberjack. Note the tall dorsal and anal fins, further exagerated in Mola (Fig. 2). Pelvic fins are lost in derived taxa. Convergent with the unrelated tuna, Thunnus, the body is essentially stiff, powering the rapidly undulating tail. This is the precursor body type for rigid-body tetraodontiformes. Then add in all the matching skull traits and you have a ‘close relative’ of the Tetraodontiformes.

Figure 3. Seriola rivoliana is the high fin Amberjack is basal to gobies and tetraodontiformes.

Figure 6. Seriola rivoliana is the high fin Amberjack. This stiff-bodied morphology is basal to stiff-bodied tetraodontiformes in the LRT.

A sister to Seriola rivoiana,
Seriola zonata, nests at the base of the frogfish + cusk eels + European eels. We’ll discuss that surprising and heretical relationship in more detail in a future blogpost. It’s getting more and more interesting every day.


References
Bloch ME and Schneider JG 1801. M.E. Blochii, Systema Ichthyologiae iconibus cx illustratum. Post obitum auctoris opus inchoatum absolvit, correxit, interpolavit Jo. Gottlob Schneider, Saxo. Berolini. Sumtibus Austoris Impressum et Bibliopolio Sanderiano Commissum. Pp i-lx + 1-584, Pls. 1-110.
Cuvier G and Valenciennes A 1833. Histoire naturelle des poissons. Tome neuvième. Suite du livre neuvième. Des Scombéroïdes. 9: i-xxix + 3 pp. + 1-512. Pls. 246-279.
Johnson GD and Britz RJ  2005. Leis’ conundrum: homology of the clavus of the ocean sunfishes. 2. Ontogeny of the median fins and axial skeleton of Ranzania laevis (Teleostei, Tetraodontiformes, Molidae). Morphol 266(1):11-21.
Kuciel M. Zuwała K and Jakubowski M 2011. A new type of fish olfactory organ structure in Periophthalmus barbarus (Oxudercinae). Acta Zoologica 92(3):276-280.
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.
Valenciennes A 1833. In Cuvier and Valenciennes, 1833 (see above).

wiki/Almaco_jack
wiki/Diodon
wiki/Tetraodontidae
wiki/Mola mola
wiki/Periophthalmus
wiki/mudskipper

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