It’s generally thought that baleen whales are too large to be successfully attacked by most marine predators. Orcas are typically considered the only real predatory threat to large whales, and even they have to use teamwork to take down a young whale. Large sharks, which also sit near the top of the marine food web, are known to scavenge on whale carcasses as a nutritious and blubbery supplement to their usual diet of fishes and smaller marine mammals. However, evidence has been found that white sharks actually take a proactive approach to increasing the whale carcass supply by attacking live northern right whale calves. Now researchers in South Africa directly observed dusky sharks actively teaming up to bring down a humpback whale calf.
Though the fisheries news cycle has mostly been taken up by the 15-year anniversary of the Sea Around Us project (and some choice words between researchers), today also marked the official announcement of the 12-month finding on the petition to list dusky sharks on the U.S. Endangered Species Act. Long story short, the National Marine Fisheries Service has decided that the dusky shark population in the Northwest Atlantic doesn’t need ESA protection to avoid extinction. While it may be tempting to decry NMFS’ decision as falling short for a species that has long been a prime example of declining shark populations, what it actually means is that things are looking up for the dusky shark. Finding that out only takes a little reading into the decision documents.
When Dr. Gavin Naylor and his team started a genetic survey of existing shark and ray species, they didn’t expect the results of their project to make international news. Their recent paper (which, at over 250 pages and complete with more than 100 figures, is nothing short of epic), however, is too striking to ignore. The results indicate that there may be as many as 79 previously unrecognized cryptic species of sharks and rays.
A cryptic species is defined as a group that looks almost exactly like another, and may even live in the same region, but is genetically distinct. We’ve known that cryptic species of sharks and rays exist for some time, such as manta rays and scalloped hammerhead sharks, but 79 is a lot; as of the paper’s publication, only 1,221 species of sharks and rays were recognized.
According to Dr. Naylor,
“Organisms become genetically differentiated over time through the cumulative effects of mutation and recombination mediated via drift and selection. When they differentiate in isolation they eventually become so different from the parental stock from which they were derived that they can no longer produce fertile offspring when crossed with them. Some biologists use the point of reproductive inviability as the point at which new species should be recognized….. For practical purposes we recognize “new species” as being genetically or morphologically distinctive from previously recognized forms.”
The study’s methods, though enormous in scope, were relatively basic. According to Dr. Naylor, the study utilized a technique very familiar to geneticists: “standard DNA extraction, PCR, Sanger sequencing, alignment and analysis of a protein coding mitochondrial gene”. To achieve the goals of understanding both evolutionary relationships of sharks and rays and parasite host specificity ( where certain parasites associated only with one species), Dr. Naylor and his team obtained and analyzed samples from as many species as they could. The numbers are impressive- 56 of 57 known families of elasmobranchs were represented among the 4,283 samples from 305 species of sharks and 269 species of batoids. In other words, this study included approximately half of all known elasmobranch species, including many that had never been analyzed genetically before. Since 1986, when the project began, samples have been obtained in more than 50 countries, mostly through the team’s own field work!