SCIENCE
Genetic research helps uncover the Atlantic sixgill shark. (National Geographic)
Teachers, scroll down for a quick list of key resources in our Teachers Toolkit, including a link to today’s MapMaker Interactive map.

Photograph by Emory Kristof, National Geographic
Discussion Ideas
- So, tell us about the species of shark described in the new research.
- The Atlantic sixgill shark (Hexanchus vitulus) lives in the deep, cold water of the tropical Atlantic Ocean. Individuals were studied around Belize, the Gulf of Mexico, and Bahamas.
- Atlantic sixgill sharks are closely related to the two other sixgill species: bigeye sixgills and bluntnose sixgills. All sixgill species are most closely related to sevengill sharks and dogfish sharks. Sixgill sharks are sometimes called cow sharks or mud sharks.
- As their name implies, sixgill sharks have six pairs of gill slits on either side of their head. (Most sharks have five; sevengills have … guess how many.)
- Sixgills are among the most “primitive” of shark species, meaning their anatomy resembles fossils of shark ancestors.
- Sixgills have unusual teeth. Teeth in their upper jaw are sharp, familiar points. (Like these.) Teeth in their lower jaw, however, are weird comb-like or saw-like plates. (Like these.)
- Sixgills are ovoviviparous, meaning they do not lay eggs, like most fishes. Instead, mother sharks give birth to live pups—sometimes 100 at a time!
- How was the new species of sixgill discovered?
- genetics. Researchers “analyzed 1,310 base pairs of two mitochondrial genes to see if there were enough molecular differences between samples.”
- The discovery was not an entire surprise. Researchers had long suspected the Atlantic population of the bigeye sixgill was a separate species. The scientific name (Hexanchus vitulus) was first proposed in 1969.
- genetics. Researchers “analyzed 1,310 base pairs of two mitochondrial genes to see if there were enough molecular differences between samples.”
- How do Atlantic sixgill sharks differ from their bigeye and bluntnose cousins?
- genetics. The fish look and behave almost identically. The Atlantic sixgill was “hiding in plain sight” until researchers analyzed its DNA.
- size. Atlantic sixgills are about 1.8 meters (6 feet) long, while their cousins can be nearly three times that length.
- species range. Atlantic sixgills live in the tropical Atlantic, while their bigeye cousins mostly inhabit the tropical Pacific and Indian oceans.
TEACHERS TOOLKIT
Nat Geo: New Species of Shark Discovered in Deep Sea
Nat Geo: Shark Sanctuaries
(extra credit!) Marine Biodiversity: Resurrection of the sixgill shark Hexanchus vitulus Springer & Waller, 1969 (Hexanchiformes, Hexanchidae), with comments on its distribution in the northwest Atlantic Ocean