Remora adhesion disc: origin, evolution and ontogenetic development

As reported at NationalGeographic.com
“The remora is so ridiculous that no one would try to make it up. The top of its head is a giant, flat suction cup. It uses the cup to lock onto the bodies of bigger animals, such as sharks, sea turtles, and whales. As the big animal swims for miles in search of a meal, the remora hangs on for the ride. When its host finds a victim, the remora detaches and feasts on the remains.”

“Remains”? According to Williams et al. 2003, 
remora diets are composed primarily of host feces.

As reported at NationalGeographic.com
“Their closest relatives include Mahi-Mahi and amberjacks, neither of which has anything on their head that even faintly resembles the remora’s sucker.”

According to the Friedman et al. 2013,
and the large reptile tree (LRT, 1556 taxa) the closest relatives of remoras include the barracuda (Sphyraena, Fig. 2), the cobia (Rachycentron, Fig. 3) and Opisthomyzon (Figs. 4, 5). Basal to these nests the open seas predator, mahi-mahi (Coryphaena) and its sister, the cave-dwelling wolffish (Anarhichas) and prior to these, the shorter-bodied carp and perch, derived from the more primitive piranha. Amberjacks, like Seriola revoliana, are more derived, basal another list of slower-moving taxa.

As reported at NationalGeographic.com
“Britz and Johnson’s research indicates that the remora suction disk started out, improbably enough, as a dorsal fin.” (Fig. 9)

That statement requires a bit of explanation
because outgroup taxa, like the barracuda (Fig. 2) and the cobia (Fig. 3) don’t have a traditional dorsal fin near the skull. The latter taxon does have a series of tiny hooks with spine bases (Fig. 9) and it is this structure that spreads laterally, develops a surrounding lip and moves forward over the flat skull during the ontogeny of remoras (Fig. 9), one step of which is tentatively shown in the early Oligocene pre-remora, Opisthomyzon (Fig. 4, 5). Those tiny little hooks could barely attach themselves to a larger host, a little like Velcro, not well and not often, but those fish that were better at it due to various morphological modifications (mutation), survived and reproduced better in their chosen niche, ultimately evolving to become full-fledged remora.

Rachycentron canadum (Kaup 1826; 2m; Fig. 3) is the extant cobia. Like the remora but without the adhesion disc, this fish also follows larger hosts (Fig. 8) seeking bits and pieces of the detritus and excrement. The first ‘dorsal fin’ has 6 to 9 short sharp spines. Females spawn 30 times a season, producing thousands of planktonic eggs 1.2mm in diameter.

Opisthomyzon glaronensis (Friedman et al. 2013; early Oligocene) is a small prehistoric remora with only a small posterior sucker. This specimen indicates that the adhesion disc originated in a postcranial position, and that other specializations (including the origin of pectination, subdivision of median fin spines into paired lamellae, increase in segment count and migration to a supracranial position) took place later in the evolutionary history of remoras.

This phylogenetic sequence of transformation
finds some parallels in the order of ontogenetic changes to the disc documented for living remoras (Britz and Johnson 2012).

Remora remora (Rafinesque 1810; 75cm) is the extant remora or shark-sucker. A flexible Venetian blind-like membrane rises due to blood flow atop the skull to produce suction (Flammang BE and Kenaley 2017). Hatchlings are less than a centimeter in length. At 3cm juvenile Remora has a fully formed 2mm sucking disc. Like its phylogenetic sister, the barracuda, the skull roof is otherwise flat and the lower jaw juts out beyond the upper one. Remoras eat the ectoparasites and feces of their host.

Notably
remoras lack a swim bladder. And they are more likely than not to attach themselves in an inverted or angled position on their host (Fig. 6). Most fish (Fig. 8) swim upright.

“We demonstrate that the “interneural rays” of echeneids are homologous with the proximal‐middle radials of Morone and other teleosts and that the “intercalary bones” of sharksuckers are homologous with the distal radials of Morone and other teleosts.”

Wish they had compared Remora to Rachycentron?

“The “intercalary bones” or distal radials develop a pair of large wing‐like lateral extensions in echeneids, not present in this form in any other teleost. Finally the “pectinated lamellae” are homologous with the fin spines of Morone and other acanthomorphs. The main part of each pectinated lamella is formed by bilateral extensions of the base of the fin spine just above its proximal tip, each of which develops a row of spinous projections, or spinules, along its posterior margin. The number of rows and the number of spinules increase with size, and they become autogenous from the body of the lamellae.”

And that’s the story, told both in ontogeny and phylogeny.

Finally let’s return to the Devonian – Carboniferous
and the chimaera-like clade Iniopterygidae (Fig. 10), which share with remoras the trait of large elevated pectoral fins. 

Distinct from remoras,
members of the Iniopterygidae (= Iniopterygiformes, 15-46cm in length) have pectoral fins with tiny hooks. Now we can wonder if these fins and hooks enabled iniopterygids to hitch a ride on larger hosts. Something about that hypothesis makes sense in light of what we’ve learned about the evolution of remoras. Let me know if anyone has promoted this idea before and I will publish the citation.

PS
Before reading Britz and Johnson 2012, and after nesting Remora with the barracuda, Sphyraena (Fig. 11), I wondered if the subdivided adhesion disc of Remora might be evolved from the oddly subdivided post parietal of Sphyraena.

Consider this: Most fish postparietals are flat to gently convex. By contrast the barracuda postparietal (Fig. 11) is depressed and subdivided with a tall neural spine-like spike or two… AND the similar adhesion disc of Opistomyzon (Fig. 5) essentially replaces the postparietal in shape, size and position. Like Remora, Opistomyzon has no postparietal. If I am wrong, please send the citation that indicates the presence of post parietal in either taxon.

References
Britz R and Johnson GD 2012. Ontogeny and homology of the skeletal elements that form the sucking disc of remoras (Teleostei, Echeneoidei, Echeneidae). Journal of Morphology https://doi.org/10.1002/jmor.20105 online here.
Flammang BE and Kenaley 2017. Remora cranial vein morphology and its functional implications for attachment. Scientific Reports 7(5914). https://www.nature.com/articles/s41598-017-06429-z
Friedman M, et al. 2013. An early fossil remora (Echeneoidea) reveals the evolutionary assembly of the adhesion disc. Proc. R. Soc. B 280.1766 (2013): 20131200.
Williams EH, et al. (6 co-authors) 2003. Echeneid-sirenian associations, with information on sharksucker diet. Journal of Fish Biology. 63 (5): 1176.

nationalgeographic.com/what-good-is-half-a-sucker/

scientificamerican.com/how-the-sharksucker-got-its-suction-disc/

https://en.wikipedia.org/wiki/Remora

https://en.wikipedia.org/wiki/Common_remora