So, Manta Rays Aren’t a Thing Anymore…

Neil_HexatrygonNeil Aschliman is an Assistant Professor of Biology at St. Ambrose University by day, a freelance artist by night. He earned his PhD in Biology from FSU by recovering the “Tree of Life” for rays, skates and their relatives, and is broadly trained in vertebrate biology and evolution. His personal website can be found at www.iceandshadows.com.

So mantas aren’t a thing anymore… – paraphrase, David Shiffman’s liveblog of my talk at the recent meeting of the American Elasmobranch Society.

Wow, is that going to require some explanation! Did I steal these guys away under cover of night? Did they pull a “so long, and thanks for all the fish” on us? No, this is a story about the power of naming, and one that may have serious implications for the conservation of these amazing animals and their close relatives.

Human beings love to name things. We do it vigorously and redundantly: a single fish species may be christened with a dozen or more common names by people in different geographic areas, times, or even marketing departments! It helps to have an international-standard system of naming animals to give this enterprise some consistency. It doesn’t matter if you know it as Chilean Sea Bass or the Patagonian Toothfish, people worldwide will be happy to confirm for you that Dissostichus eleginoides is indeed one ugly customer.

This system of precise identification in which scientists apply a formal code to name organisms is called binominal nomenclature (“two-part name”). This is often incorrectly called “binomial” nomenclature (“two number,” a mathematical expression), even by professionals! The first part of the name is the genus, which can apply to between one and many species that are closely related and resemble each other. The second part of the name is the specific epithet, which applies to one and only one species. These names are usually derived from Greek and Latin. For example, Batrachognathus volans translates as “flying frog-jawed” one, an apt appellation for an odd pterosaur from the Late Jurassic.

A genus (plural genera) should describe some small set of similar organisms. This group should be defined by a common ancestor, all of its descendants, and nothing else (Figure 1A). We call these natural groups, or clades.

This is the ideal case. What happens if you end up with something like Figure 1B? Here, you can see that one genus is “nested” inside the other one: Mobula is not a natural group because it inappropriately excludes Manta. It’s easy to analogize this to how, in some cultures, family names are passed down the paternal line. If you, your father, his father, and all male relatives in between are named “Lannister” but one of your brothers is named “Baratheon,” something’s gone wrong in the process of naming!

Under the current classification scheme, there are two genera of “devil rays”: two species in the genus Manta and nine species in the genus Mobula. Besides some slight differences in body proportions, there are a few characteristics that distinguish Manta from Mobula. Most notably, Manta lack teeth on the upper jaw and have a mouth at the end of the head, while the mouth of Mobula is underslung as in most sharks. In recent years it has usually been held that the classification of devil rays is a valid arrangement, following Figure 1A.

However…

Evidence from both DNA sequences and comparative anatomy suggests that current devil ray classification actually fits Figure 1B, making it invalid.

DNA sequence analyses from my doctoral dissertation work [1], followed by a large-scale study performed by my graduate advisor and colleagues [2], suggested that Manta is nested within Mobula. While not all species were sequenced, Manta birostris was indicated to be more closely related to Mobula japanica than it is to other sequenced Mobula. This is based mostly on evidence from one or two genes, with a couple of others rather uninformative. It was compelling enough to get me to delve back through the scientific literature on the anatomy of the group, and what I found was surprising.

It turns out that biologists who have taken a close look at devil ray anatomy have been quietly suggesting for nearly 15 years that Mobula is not a natural group! [3,4] For example:

• Most devil rays have comb-like teeth, while Manta, Mobula japanica, and Mobula mobular have distinctive peg-like teeth. [3,4]

Mobula japanica (>3m wide) and Mobula mobular (>5m?) are among the largest devil rays, comparable in size to the recently described Manta alfredi. [5,6]

• Among devil rays, only Manta, Mobula japanica, and Mobula mobular have caudal spines (the “sting” in “stingrays”). [5,6]

So is this enough to overturn the current system of classification for devil rays? Not yet. If any changes are to be proposed, someone first needs to perform a formal “taxonomic revision” of both Manta and Mobula. This means that they need to present a sound, peer-reviewed case for the reclassification of these species, which will require physically re-examining them and ideally presenting corroborating DNA evidence. This must include the “type species” for each genus: Manta birostris and Mobula mobular. A type species is the one to which the name of the genus is permanently attached; in this case, it defines “what a Mobula should be.” There are to date no available DNA sequence data for Mobula mobular, although we can predict that they will group most closely with Manta and Mobula japanica.

Assuming the evidence continues to build that Manta is nested within Mobula, what happens next? The most likely case is that the genus Manta will cease to be valid, subsumed under Mobula as a “junior synonym.” Mobula has precedence under the rules since it was described in 1810, versus 1829 for Manta. There are mechanisms by which a name can be suppressed: the proposed renaming of the fruit fly species that’s the cornerstone of much of modern biology would be a nightmare beyond measure (see here), but Manta is unlikely to qualify. Their common names will remain “Manta Rays” and the world will go on turning. Given the scarcity of materials from some species and their often colossal size, I don’t envy the gal/guy to undertake the formal review of the devil rays… but it needs to be done. Five bucks says that Manta is going away in the next ten years.*

So why does this matter? Legal issues and conservation.

Savvy folks sitting in the conference room or reading David’s liveblog quickly seized upon this question. With Manta recently receiving protection under the Convention on International Trade in Endangered Species (CITES), would lumping these guys in with Mobula expose them (1) under a legal technicality and/or (2) as less unique and worthy of conservation than when they had their own genus? Fortunately, shark conservation experts including Sonja Fordham (President at Shark Advocates International / Deputy Chair at IUCN Shark Specialist Group / conservation rockstar) were on hand to address these concerns.

Fears of CITES loopholes appear to be unfounded. Current protection schemes are thought to be engineered to continue to protect the two named (plus a third suspected but undescribed) species of Manta. Prying at loopholes is sadly unnecessary: countries that would otherwise seek to do so can apparently just opt out of CITES protections (I am aiming an unimpressed glare squarely at you, Canada and Guyana).

In short, this is less a threat to mantas than it may be an opportunity to extend protections to the other devil rays. Most of these species are heavily affected by both targeted fisheries and as bycatch, but they exhibit human-like life histories that make them extremely vulnerable to such pressure. Despite this shared and imminent danger, only Manta – with its tremendous charisma – seems to have found many champions in conservation, evidenced by the fact that only Manta species were proposed for CITES listing. Changing its name won’t make these folks give up the cause, but folding all of the devil rays into a single genus can underscore the similarities in vulnerability across species in this group, strengthening the case for offering them equal safeguards in the future.

“Mantas aren’t a thing anymore”? Sounds good to me.


*Offer valid for first claimant as of ten years from the date of this article’s publication. If I win, I’ll pick one of you at random and demand my fiver.

References

1 Aschliman NC (2011) The batoid tree of life: recovering the patterns and timing of the evolution of skates, rays and allies (Chondrichthyes: Batoidea). Dissertation, Florida State University

2 Naylor GJP, Caira JN, Jensen K, Rosana KAM, Straube N, Lakner C (2012) Elasmobranch phylogeny: a mitochondrial estimate based on 595 species. In: Carrier JC, Musick JA, Heithaus MR (eds) Biology of sharks and their relatives, 2nd edn. CRC Press, Boca Raton, Florida, pp 31-56

3 Herman J, Hovestadt-Euler M, Hovestadt DC, Stehmann M (2000) Contributions to the study of the comparative morphology of teeth and other relevant ichthyodorulites in living supra-specific taxa of Chondrichthyan fishes. Part B: Batomorphii 4c: Order: Rajiformes – Suborder Myliobatoidei – Superfamily Dasyatoidea – Family Dasyatidae – Subfamily Dasyatinae – Genus: Urobatis, Subfamily Potamotrygoninae – Genus: Potamotrygon, Superfamily Plesiobatoidea – Family Plesiobatidae – Genus: Plesiobatis, Superfamily Myliobatoidea – Family Myliobatidae – Subfamily Myliobatinae – Genera: Aetobatus, Aetomylaeus, Myliobatis and Pteromylaeus, Subfamily Rhinopterinae – Genus: Rhinoptera and Subfamily Mobulinae – Genera: Manta and Mobula. Addendum 1 to 4a: erratum to Genus Pteroplatytrygon. Bull Inst R Sci Nat Belg Biol 70:5-67

4 Adnet S, Cappetta H, Guinot G, Notarbartolo di Sciara G (2012) Evolutionary history of the devilrays (Chondrichthyes: Myliobatiformes) from fossil and morphological inference. Zool J Linn Soc 166:132-159

5 Notarbartolo di Sciara G (1987) A revisionary study of the genus Mobula Rafinesque, 1810 (Chondrichthyes: Mobulidae), with the description of a new species. Zool J Linn Soc 91:1-91

6 Marshall AD, Compagno LJV, Bennett MB (2009) Redescription of the genus Manta with resurrection of Manta alfredi (Krefft, 1868) (Chondrichthyes; Myliobatoidei; Mobulidae). Zootaxa 2301:1-28

Photos from Wikimedia Commons

Manta: LINK

Mobula: LINK

  1. I think its premature to say that Mantas are not a thing anymore. There’s another option: move the two mobulids that don’t fit the rest of the group – M. japanica and M. mobular – into Manta. Both Mobula and Manta would then be monophyletic taxa more accurately reflecting underlying biology, with peg-like teeth and presence of caudal spine as morphological synapomorphies. I don’t know where that leaves the subterminal mouth, though, which was always a convenient synapomorphy for mobulas; do M. japanica and M. mobular have that too?

  2. Al, that’s an interesting thought, but unfortunately it doesn’t work for two reasons:

    1) “Monophyletic” describes the actual branching pattern of relationships, not necessarily whether or not certain species share an anatomical character. No matter what they’re called, mantas are still nested within multiple distinct lineages of Mobula. I’ve alluded to that in Figure 1B, except that in reality “Mobula species 1 and 2” are actually clusters of species. There’s no way to cleanly keep the genus Manta without lumping in even MORE species that don’t share the manta-like characters (in this case, at least Mo. kuhlii and Mo. thurstoni would come along for the ride). One alternative would be to erect MULTIPLE NEW genera to describe these groups (Manta, Mobula, at least two more new ones), which I think we can all agree would be silly.

    [to answer your last question, only the two species of Manta have a terminal mouth]

    2) Mobula mobular is the type species of Mobula, which has priority over Manta. Even if we were to lump Mo. mobular with Manta, the rules still apply. Filing to suppress Mobula would work, but then /all/ Mobula would become Manta (and the nomenclature folks would be very unlikely to go along with that).

  3. Perhaps it is more important for the conservation of sharks and rays to identify the location of and monitor the of state their nursery grounds. Does anyone know of such monitoring? I have established a a cost effective methodology of mapping seagrass beds in known bay and inlet school shark (Galeorhinus galeus) nursery grounds – but have not yet, after 20 years of trying, been able to get either fisheries management or fisheries science to address the issues of monitoring or protective management.

    • What are you talking about? That’s been a focus of the research community for decades. There are hundreds of papers published on nursery grounds and critical habitat. There are dozens of labs around the world working on that topic.

      This post has absolutely nothing to do with that topic.

  4. But “stings” are plesiomorphic for myliobatids (and dasyatoids in general). Their presence in both mantas and the two “big” Mobula spp. could be evidence that this group is basal to the other Mobula spp., or that what’s depicted in Fig. 1A is correct. Fig. 1B would indicate that the sting has been lost independently in every other lineage of Mobula, which is unparsimonious. Of course, this is a single character, easily outweighed by the molecular data and other morphological features. But you shouldn’t cite it as evidence for a Manta + “big” Mobula clade: it’s a primitive, widespread ray feature.

    • Cay, you’re exactly right. The state distributions of the three characters listed are simply /concordant/ with the fairly strong evidence from molecular data. While the tooth character and possibly size are apomorphic, yes, the presence of a sting is plesiomorphic for myliobatiforms (crownward of Zanobatus). However – and interestingly! – according to the molecular trees, the most parsimonious explanation for the distribution of the sting character is that it is actually REGAINED by the Manta and friends group (1 step), versus two or more independent losses in other Mobula. As I mentioned, this was in no way a formal review of the group, but I’m highlighting a few characters that may, pending further study, be found to be apomorphic for the group. Yes, possibly even including the reacquisition of a sting. 🙂

    • Interesting! I imagine the ontogeny of big oceanic rays is pretty poorly-known, but how much work has been done on sting development? I know the stings are periodically lost and replaced in stingrays, are they lost early in life but never regrown in any of the ‘stingless’ pelagic taxa?
      Also, personally I don’t think it would be crazy to name two new genera to retain such an iconic and morphologically distinct genus as Manta, I think plenty of cichlid genera are separated on fewer characters than those Mobula species clusters. But at the end of the day it’s just a matter of taxonomic philosophy–names are just labels, the tree is the “science.”

  5. Does the fact that M. birostris is more genetically similar to M. japanica really mean that the only explanation is that the Manta genus is nested within Mobula? Surely there are other viable explanations for this one piece of evidence. It sounds to me as though you’re making some huge assumptions based on a few inconclusive pieces of molecular evidence, backed up by flawed morphology: M. alfredi does not have a caudal spine, and M. birostris has a vestigial, nonfunctional spine which is not morphologically similar to the caudal spines of other mobula rays. Furthermore, size is not a useful metric in this case, especially as mobula species have been historically misidentified, even by ‘experts.’ M. japanica rarely approach the 3m mark that your suggest, with M. tarapacana most closely resembling the manta species in size.

    While you’re the first to admit that this is not a formal review of the group, you’re making quite a stir based on very, very little empirical evidence.

    • Josh, yes, there are alternative explanations – lineage sorting artifacts among the mitochondrial DNA is the most likely. The signal among available data is quite strong, however, you’ve apparently dismissed the shared-derived dentition for these taxa. See Notarbartolo di Sciara 1987 or Adnet et al 2012 for figures. I would not call concordance between molecular data and a unique morphological character “very, very little empirical evidence,” and discussions following my talk revealed that several other workers in chondrichthyan systematics have been zeroing in on this for some time (Ebert, Compagno, et al).

  6. If you’re in the mood to solve fish phylogenies, you should take a gander at the perciformes/scorpaeniformes – now that is a taxonomic mess!

    Though, I’m intrigued by this gain/loss of sting… Just how far back do the phylogenies suggest stinging goes? Was the basal ray (or even ray/skate) venomous?

    • Christie, under the current best phylogenies of batoids, the caudal spine arose after the split between Zanobatus and crown group stingrays. There’s a long branch between that split and the current candidate for basalmost stingray (Hexatrygon), so based on Aschliman et al. 2012 (MPE), likely somewhere between 150 and 80 MYA, ballpark. It looks like the earliest recovered fossils are from the Late Cretaceous (Marmi et al, JVP May 2010).

  7. Just to be clear, nobody has actually synonymised Manta and Mobula yet, right? So what you’re really saying is that one day in the not too distant future, you’re betting that Manta as a genus name, but not manta rays as a common name, might not be a thing anymore. That’s not QUITE what David’s comment/your title implies, but it certainly starts a lively conversation…

  8. Concordance between molecular data and dentition, perhaps, but what of the discordance between the molecular data and other morphological features such as the ventral/terminal mouth, which I imagine would represent a more significant evolutionary step than dentition? Furthermore, M. alfredi has been left out of the dentition analysis of Adnet et al., and both Ma. alfredi and Mo. mobular have been left out of your molecular analysis—unless I’m mistaken—and these two species are critical in your argument for mobula absorbing manta.

    • Exactly right Josh, and while trying to maintain brevity for the post, I’ve highlighted the need to include some of [but all are needed] these things in a formal taxonomic revision by some enterprising scholar in the future. I /suspect/ that the final story may suggest that Manta derived a terminal mouth and other unique features from a common ancestor with one or more of the cited Mobula with peg-like teeth, but if that’s wrong the alternative should be interesting as well! Thanks for your contributions!

  9. I don’t know anything about rays, but reading your cladograms, it doesn’t look to me that either genus name need be dropped. According to this research, Manta is a monophyletic genus within the paraphyletic genus Mobula, which is a perfectly satisfactory arrangement (it’s only if Mobula or Manta were polyphyletic that you’d have a problem).

    Genera can be paraphyletic – indeed, it appears to necessarily be the case if every organism is to be placed in genera. Otherwise, every lifeform is in the same genus, the original ‘Genus species’ that is at the base of life.

  10. Folks I would love opinions on a siting of “Manta style Rays” which we recently had here at Johns Pass Florida. Below is a link to the post with the highest quality pictures we have. (Not the best but…..) Appriciate any guesses on Manta vs Mobula as we would like to categorize them on our identification pages. Thanks
    Floyd Roberts