A ray by any other name

 

Recent rumours of the manta ray's taxonomic demise have been greatly exaggerated.

  The two species of manta ray now have a new genus.   Shannon O'Toole/Flickr  (CC BY-NC-ND 2.0)

The two species of manta ray now have a new genus. Shannon O'Toole/Flickr (CC BY-NC-ND 2.0)

 

This piece is published in partnership with the Australian Society for Fish Biology, a scientific organisation that promotes research, education and management of fish and fisheries in the region.

Manta rays are undoubtedly one of the ocean’s most charismatic fish. Growing up to seven metres wide, they look and feel a bit like stealth bombers when they glide over you during a dive. A recent publication about the family tree of manta rays and their smaller relatives, the mobula rays, has received quite a lot of attention in marine biology circles, but has also led to some confusion. Has the manta ray really ceased to exist?

Before we begin in earnest, a quick introduction to scientific names, or binomial nomenclature. Scientific names consist of two parts: The first part is the genus of the species (a bit like your surname) and the second part is the actual species name (like your first name). A few examples: Antennarius pictusHomo sapiensWunderpus photogenicus. Names are usually in Latin or Greek, or anything that vaguely sounds like either one of those. Unlike common names, a species' scientific name is the same wherever you go in the world, which is helpful when talking to scientists who speak a different language than you do.

So what is the manta vs. mobula article all about? Until recently, two species of manta rays were recognised: oceanic manta rays (Manta birostris) and reef manta rays (Manta alfredi). Mobula rays look very similar to manta rays, but are smaller and differ from mantas in a few other ways. The most obvious one is that the mouths of mantas are at the very tip of their body, facing forward (‘terminal’), whereas the mouths of mobulas are underslung, just below the tip of the body (‘subterminal’).

 
  A reef manta ray, formerly classified as  Manta alfredi .   Shiyam ElkCloner/Wikimedia Commons  (CC BY-SA 3.0)

A reef manta ray, formerly classified as Manta alfredi. Shiyam ElkCloner/Wikimedia Commons (CC BY-SA 3.0)

 

This newly published paper, by fish taxonomist William White and colleagues, compared a combination of mitochondrial and nuclear DNA sequences to see just how closely mantas and mobulas are related, and they turn out to be closer than we previously thought. To put this into human perspective, the manta ray species were thought to be something like cousins to mobulas, but they're actually more like siblings. In biology-slang: Manta rays are now seen as belonging to the same genus as mobula rays. Which, in turn, means that their scientific names change from Manta to Mobula, so Mobula birostris and Mobula alfredi. A bit like an adopted child getting a new last name.

What the new article does not claim is that manta rays are now suddenly a different species to what they were previously. It just means they are classified differently by taxonomists (and that they might get more invitations to Mobula social events). The common names will remain the same; manta ray species do not suddenly disappear or behave differently. It will take a while before ID guides will pick up on the name change and a lot longer, if ever, before most ocean enthusiasts will notice.

In terms of conservation, a different scientific name means that certain official documents concerning the trade in protected species might have to be adjusted. Luckily, the status of manta rays as a species is not questioned, so existing conservation laws should not need to be changed. What is more, manta rays and mobula rays are both listed on CITES Appendix II, a crucial international agreement that regulates trade in endangered species. So even if the entire “manta”-section were dropped, they would still be protected as “mobula” under CITES.

 
  Both manta and mobula rays are already protected worldwide under CITES Appendix II.    Klaus Stiefel/Flickr  (BY-NC 2.0)

Both manta and mobula rays are already protected worldwide under CITES Appendix II.  Klaus Stiefel/Flickr (BY-NC 2.0)

 

But how does this happen? Why do scientists decide that a species has a different family tree than we’ve always thought? This is actually not an uncommon event: In recent years, many species — from nudibranchs to red pandas — have received different names or classifications. One reason is that science is constantly evolving, and, as we learn more, we update our knowledge and correct mistakes from the past (or make new mistakes which might in turn be corrected later). In the manta/mobula case, by using modern methods we found out that the family relations were different than we had assumed from only looking at the species' anatomy.

Another, more surprising reason is that we still don’t have a good definition of what a species is. Human nature impels us to order the world around us into categories with different names, initially very broad (animal / plant / rock), then more detailed (fish / mammal / bird), more detailed still (ray / shark / frogfish), until you reach the scientific naming system (Mobula birostris / Mobula alfredi / Mobula mobular). But sometimes it is difficult to decide where one species stops and another one begins.

At school, you were probably taught that two species are different when they can’t produce fertile offspring; for example, horse + donkey = mule, but mules are infertile, so horses and donkeys are different species. This definition works to an extent, but it breaks down when you start looking closer, especially among ocean species. The question of how to define a species is a surprisingly hot topic in biology that isn’t likely to be resolved any time soon.

In the meantime, you can call manta rays ‘mantas’, ‘devil rays’, ‘big-ass mobulas’, or anything else that floats your boat. As long as you have a great time watching them, and try to protect the environment they live in, I’m happy.

A version of this article was originally published on the blog Critter Research. Edited by Andrew Katsis.