The fish without a tail, part 2

Convergent with humans,
ocean sunfish (Figs. 1,2) also lack a tail. What they have instead was first called the ‘clavus‘ (= cornfield skin) by British ichthyologist, Alec Fraser-Brunner (1951),

Figure 1. Ranzania from Johnson and Britz 2004 in various degrees of dissection.

Figure 1. Ranzania from Johnson and Britz 2004 in various degrees of dissection. The clavus is where the tail used to be. Johnson and Briz determined that portions of the dorsal and anal fin migrated to produce the clavus. Compare to its relative Mola, figure 2.

From the Johnson and Britz 2004 abstract:
“Homology of the clavus has been a matter of debate since the first studies on molid anatomy in the early 1800s. Two hypotheses have been proposed:

  1. It is a highly modified caudal fin;
  2. It is formed by highly modified elements of the dorsal and anal fins.

We show that… the claval rays form from the posterior ends of the dorsal and anal fins toward the middle. We thus conclude that the molid clavus is unequivocally formed by modified elements of the dorsal and anal fin and that the caudal fin is lost in molids.”

Figure 2. The ocean sunfish (Mola) seems to be the opposite of the flying fish (Exocoetus), but these two nest with each other in the LRT based on skeletal traits.

Figure 2. The ocean sunfish (Mola) seems to be the opposite of the flying fish (Exocoetus), but these two nest with each other in the LRT based on skeletal traits.

Johnson and Britz 2009
stained tiny juveniles of the sunfish Ranzania (Fig. 1) to document their hypothesis.

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

Before finding Johnson and Britz 2009
decades older images of a hatchling and growing Mola mola (Figs. 2, 3) documented the disappearance of the tail prior to the appearance of the marginal clavus.

Figure 3. The queen triggerfish, Balistes, is related to Mola in the LRT.

Figure 4. The queen triggerfish, Balistes, is related to Mola in the LRT.

Mola mola (Linneaus 1758) is the extant ocean sunfish. As a hatchling it is similar to a pufferfish (Diodon) in shape, then undergoes metamorphosis to adulthood. It is the largest bony fish and the only one taller than long. In the large reptile tree (LRT, 1647+ taxa, subset Fig. 5) is derived from the the slow-moving queen trigger fish, Balistes (Fig. 4), and both are derived from the speedy high-fin amberjack, Seriola rivoliana apart from pufferfish. As we learned earlier, speedy flying fish (Exocoetus) and sunfish-like opah (Lampris) are also members of the amberjack + mola clade in the LRT.

Figure x. Subset of the LRT focusing on basal vertebrates (= fish).

Figure x. Subset of the LRT focusing on basal vertebrates (= fish).

That opens up a phylogenetic problem. 
The fish clade Tetraodontiformes traditionally includes trigger fish and puffer fish. In the LRT trigger fish are not related to pufferfish, except through a last common ancestor somewhere early in the genus Seriola. Both have different members of that genus in their ancestry and Seriola is not considered a member of the traditional Tetraodontiformes. This appears to be a novel hypothesis of interrelationships. If it has been published earlier, let me know so I can promote that citation.

Like Mola and Ranzania
Balistes swims by undulating those large dorsal and anal fins (see videos above) with the tail dragging without much movement behind, unlike most fish. That is why the question of clavus homology intrigued Johnson and Britz 2004.

On a side note,
sunfish have few to no pain receptors, an adaption to their diet of jellyfish. Predators sometimes take bites out of sunfish without causing a fight or flight reflex on the part of the victim.

A fossil genus traditionally aligned with Tetraodontiformes,
Plectocretacicus clarae, was earlier moved out of Tetraodontiformes to nest with the giant oarfish. Regalecus glesne. So, the clade Tetraodontiformes is likewise getting nibbled away, piece-by-piece by the LRT. Members are no longer monophyletic unless the definition or membership expands to include more taxa. 


References
Fraser-Brunner A 1951. The ocean sunfishes (Family Molidae). Bulletin of the British Museum (Natural History 1:89–121.
Johnson DG and Britz R 2004. 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). Journal of Morphology 266:11–21.
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.

wiki/Mola mola
wiki/Tetraodontiformes
wiki/Ranzania

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