Manta ray: curious reversals + head fins

Manta rays (= devil rays)
(genus: Manta; Figs. 1, 6; sometimes genus: Mobula) have an odd mixture of derived and primitive traits. We’ll review those today.

Figure 1. The largest manta ray to scale with humans.

Figure 1. The largest manta ray to scale with humans. Note the unique cephalic fins hanging down at lower left.

The manta is different from other rays

  1. Mantas have a unique third set of ‘fins’, the cephalic ‘fins’. These are not connected to the pectoral fins, but originate ventral to the eyes (Fig. 1).
  2. In skates, rays and guitarfish the anterior pectoral fin attaches in front of the orbit, on the prefrontal. In mantas + cownose rays (genus: Rhinoptera (Figs. 4, 5) the pectoral fins attaches further back, near the jaw articulation. This is a reversal going back to Rhincodon, the whale shark (Fig. 3) and Thelodus (Fig. 3), the jawless outgroup taxon of the clade Gnathostomata (jawed vertebrates) in the LRT.
  3. Most sharks, guitarfish, skates and rays have a long nasal bone that gives them a rostrum (which gets really long and toothy in saw fish). A similar elongate nasal is missing in cow nose rays and mantas. This is a reversal, too.
  4. Typical guitarfish, skates and rays have a ventral mouth adapted for bottom feeding (Fig. 2). By contrast, the manta ray mouth opens anteriorly, as in the whale shark, for open water feeding. This is a reversal.
  5. Typical guitarfish, skates and rays have electric prey sensing organs. Manta rays do not. This is also a reversal.
  6. Typical guitarfish, skates and rays cruise sandy sea floors seeking buried hard-shelled prey. Mantas do not. They cruise open waters seeking tiny planktonic and mid-sized oceanic prey they trap using internal gill bars prior to digestion. This is also a reversal going back to whale sharks.
  7. Because their mouths are buried in sediment, typical guitarfish, skates and rays ‘inhale’ through an alternate dorsal opening, the spiracle. The manta ray does not. It ‘inhales’ through its cavernous anterior mouth, like most fish, including the whale shark. This is also a reversal.

    Figure 4. Rhinobatus jaw mechanism animation. This is how skates and rays eat, distinct from the Thelodus/ whale shark/ manta ray method of ram feeding.

    Figure 2. Rhinobatus jaw mechanism animation. This is how skates and rays eat, distinct from the Thelodus/ whale shark/ manta ray method of ram feeding.

  8. Typical guitarfish, skates and rays have eyes essentially on the top of their flattened heads. The eyes of manta and cownose rays are placed laterally, as in the whale shark. This is also a reversal.

With so many reversals
I wondered if Manta might be the taxon that broke the ability of the LRT to lump and separate 1558 taxa based on a small (238) character list originally created to lump and separate reptiles. Fortunately, everything came out okay. We’ve seen reversals several times previously here and elsewhere [use keyword: “reversal” in the search box.]

The cephalic ‘fins’ in Manta 
are not used for propulsion. Instead, these lobes help funnel and gather food. They can curl into a spiral to cut drag while swimming rapidly. A series of flexible cartilage rods, like fins rays, fits inside each one (Fig. 6). The origin of these appendages baffled workers for decades.

Figure 1. Rhincodon typus, the extant whale shark, shares traits with jawless Thelodus, armored Entelognathus, and the walking catfish, Clarias.

Figure 3. Rhincodon typus, the extant whale shark, shares traits with jawless Thelodus, armored Entelognathus, a basal placoderm with bone that informs the identity of the cartilage in Manta. Even Thelodus is not far from Manta.

Swenson et al. 2018 shed new light on the origin of manta cephalic ‘fins’. 
They studied a series of embryo cownose rays (genus: Rhinoptera bonasus) because cownose rays are the closest relatives of the manta. Cownose rays also have movable cephalic lobes (Figs. 4, 5), similar to the cephalic fins of mantas, but completely attached to the head along the long axis, like a tailgate.

Figure 2. Cownose ray lobes open and close like flaps.

Figure 4. Cownose ray lobes open and close like flaps. These are homologous to the cephalic fins seen in Manta. See these in operation in figure 5.

Swenson et al. report, “In cownose rays, cephalic lobes do not develop from independent fin buds emanating from the head; rather, they develop as modifications to the anterior pectoral fins, closely resembling the hooks at the anterior of developing skate and ray pectoral fins (Babel, 1966; Luer et al., 2007; Maxwell et al., 2008). However, unlike the anterior pectoral fins of other batoids, cephalic lobes are distinguished from the rest of the pectoral fin by a small region of reduced tissue outgrowth we call the ‘notch.'” 

With their an ontogenetic series of cownose ray embryos
Swenson et al. documented the disconnection of the anteriormost portion of the pectoral fin from the face in younger embryos and the later fusion to the face prior to hatching. Mantas are similar, but skip this last stage and so retain more flexible anteriormost pectoral fins arising from the lower lateral face.

Figure 2. Cownose ray feeding by dropping their cephalic lobes to increase the suction on their oyster prey.

Figure 5. Cownose ray feeding by dropping their cephalic lobes to increase the suction on their oyster prey. See figure 4 to see these lobes closed like landing gear doors, streamlined prior to swimming.

The Swenson et al. results
showed that the devil ray’s horns are not a third set of fins after all – they’re simply the foremost bit of pectoral fin, modified for a new purpose and separated from the main portion of the pectoral fin. The cownose ray documents a modified transitional phase.

Figure 6. Three views of the skeleton of Manta, colors added. Green represents the maxilla. Note the terminal mouth, distinct from other rays, skates and guitarfish. The pectoral fins do not reach the orbit. The cephalic fins are highly modified maxillae, still gathering food. Note the attachment to the quadrate. The premaxilla extends across the mouth.

Figure 6. Three views of the skeleton of Manta, colors added. Note the near terminal mouth, distinct from other rays, skates and guitarfish. The pectoral fins do not reach the orbit. The cephalic fins are independently mobile extensions of the anterior pelvic fins.

Also added to
the large reptile tree (LRT, 1558 taxa) is one of the basal rays, the guitarfish, Rhinobatos, (Figs. 7, 8). Both taxa enter alongside Isurus, the mako shark. Manta (Figs. 1, 6) is, perhaps, the most derived ray (clade Batoidea). Rhinobatos is, perhaps, one of the most basal batoids, closer to sharks, in morphology. Very likely all the rest of the batoids will nest in the LRT between these two taxa. We’ll test that rather orthodox guess as time goes by.

Figure 2. Rhinobatos, the guitarfish, and Rhina the bowhead guitarfish, are transitional to skates and rays, but not mantas. Note the ventral mouth and pectoral fins extending anterior to the orbits.

Figure 7. Rhinobatos, the guitarfish, and Rhina the bowhead guitarfish, are transitional to skates and rays, but not mantas. Note the ventral mouth and pectoral fins extending anterior to the orbits.

Fun fact: brain size
According to Wikipedia, “The oceanic manta has one of the largest brains (ten times larger than a whale shark) and the largest brain-to-mass ratio of any cold blooded fish.”

Figure 5. Guitarfish (Rhinobatos) skull. Colors, eyeballs and spiracles added.

Figure 8. Guitarfish (Rhinobatos) skull. Colors, eyeballs and spiracles added.

Fun fact #2:
That anterior portion of the pectoral fin lateral to the eye of the guitarfish (Figs. 7, 8) is what evolves to become the flap-like cephalic lobes in cownose rays (Figs. 4, 5) and the anteriorly detached, ever-curling cephalic ‘fins’ in manta rays (Figs. 1, 6).


References
Bancroft EN 1829. On the Fish known in Jamaica as the Sea-Devil. The Zoological Journal. 4: 444–457.
Bloch MC and Schneider JG 1891. Systema anclystoma.
Fisher RA 2010, revised 2012. Life history, trophic ecology, & prey handling by cow nose ray, Rhinoptera bonasus, from Chesapeake Bay.
Garman S 1884. An Extraordinary Shark. Bulletin of the Essex Institute: 47–55.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Smith A 1829. Descriptions of new, or imperfectly known objects of the animal kingdom, found in the south of Africa. South African Commercial Advertiser 3: 2.
Rafinesque CS 1810. Caratteri di alcuni nuovi generi e nuove specie di animali e piante della sicilia, con varie osservazioni sopra i medisimi. Per le stampe di Sanfilippo: Palermo, Italy. pp. 105, 20 fold. Pl., online
Swenson JD et al. 2018. How the Devil Ray Got Its Horns: The Evolution and Development of Cephalic Lobes in Myliobatid Stingrays (Batoidea: Myliobatidae). Front. Ecol. Evol, published online November 13, 2018; doi: 10.3389/fevo.2018.00181
White WT et al. 2018. Phylogeny of the manta and devilrays (Chondrichthyes: mobulidae), with an updated taxonomic arrangement for the family. Zoological Journal of the Linnean Society, 2018, 182, 50–75.

wiki/Manta
wiki/Giant_oceanic_manta_ray
wiki/Whale_shark
sci-news.com/biology/manta-rays-cephalic-lobes.html

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