How the devil ray uses its horns
Manta rays are large rays belonging to the genus Manta (sometimes called Mobula) comprising 11 different species. The two largest species are the giant oceanic manta (M. birostris reaching 7 m (23 ft) in width and the smaller reef Manta, (M. alfredi more than 5m (16 ft) in width. Both species have triangular pectoral fins and two symmetrical horn-shaped cephalic (head) fins flanking the flat forward-facing mouths. The cephalic fins at the front of the body are extremely malleable, and can even be rolled up and unrolled (see picture), depending on if the animal is traveling or feeding. While it is underway, it can roll up its fins to help them move quickly through the water. The fins then corkscrew into neat and thin forward-projecting appendages. In contrast with earlier views, the manta rays prefer to stay in patches of the ocean as small as 140 miles (220 kilometers) across and rarely if ever journey outside of them.
Left: the cephalic fins rolled up (upper picture) and unrolled (middle picture). Lower picture: the 'aileron' effect causing a sharp roll to the right: pectoral and cephalic fins bent upward at the right side, and downward at the left side of the body. Pictures were taken at Raja Ampat.
The mantas have horizontally flattened bodies with eyes on the sides of their heads behind the cephalic fins, and gill slits on their ventral (belly) surface. The belly contains distinctive markings, allowing individuals to be identified by the unique belly spot pattern, like a human fingerprint. Dorsally (backside), they are typically black or dark in color with pale markings on their shoulders. All-black color morphs are also known to exist. Mantas are sometimes observed to make spectacular breaches, leaping partially or entirely out of the water. The reason for breaching is not known; possible explanations are mating rituals, the removal of parasites and commensal remoras, or perhaps just ‘fun’.
The function of the Mantas horns is still a matter of speculation. When we observe the manta swimming it seems that its movements are driven primarily by the flapping of its massive pectoral fins. Mimicking the movements of a large bird's wings. The major function of the fleshy face fins is believed to funnel the plankton (its major food source) during filter feeding. Additional functions that may have developed during evolution are communication, steering, and sensations. A reef manta may sometimes flip open one cephalic fin while swimming past, potentially serving a sensory or communication function. Assisting movement could be a third function. Watching the swimming manta you get the impression that movements of the big pectoral fins and cephalic fins are nicely coordinated to guide locomotion. When making sharp turns, the horns seem to behave like ‘ailerons’ in a small aircraft (ailerons moving in different vertical directions produce the aircraft to roll: to move around the aircraft's longitudinal axis; see lower picture above).
The cephalic fins seem to have developed analogous to forelimbs of other vertebrates. Recent research suggests that the genes that guide the development of the rays’ cephalic lobes play the same role in the fins of a closely related ray species, the little skate, which doesn’t have cephalic lobes. The results suggested that the ray’s horns aren’t a third set of appendages at all – they’re simply the foremost bit of fin, modified for a new purpose. Suggesting that cephalic lobes are not independent appendages but rather modified pectoral fins.
John D. Swenson et al. 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