Scientists sequenced the genomes at the chromosome level of the Great Hammerhead.Sphyrna mokarran) and shortfin makoIsurus oxyrinchus) sharks. These results showed that these two sharks have experienced dramatic declines in their effective populations over the past 250,000 years.
Great Hammerhead Shark (Sphyrna mokarran). Credit: Christopher Vaughn Jones.
Class Chondrichthyes, also known as sharks, rays and chimeras has a rich history, which dates back to approximately 420,000,000 years. It has been through five major extinctions.
This ancient lineage is now in an epoch called the Anthropocene. It may be called the sixth mass extermination.
According to the IUCN Red List of Endangered Species report, 31% of shark species were listed as threatened in a new study. This is significantly higher than their 2014 estimates. These shark species are at risk of extinction in two categories: 6.5% and 10.5%.
A new report shows that the relative fishing pressure has caused a 18-fold increase of global oceanic shark and ray abundances.
Only 9 oceanic sharks or rays species were listed as endangered in 1980 by the IUCN Red List. However, this number has increased to 75% by 2021.
This is evident in the dramatic fall of two species: The shortfin mako and great hammerhead sharks.
Great hammerheads are the largest species in the Sphyrnidae family, reaching an average size of 6 meters.
It is found throughout the world in warm and temperate oceans.
This important predator is distinguished by its large hammer (cephalofoil), long sickle-shaped dorsal and dorsal fins.
The shark fin trade is highly dependent on large shark fins which make it a critically endangered species.
Shortfin mako, which is predominately a pelagic, is an apex predator and can reach a maximum size around 4m. It is also found in tropical and temperate seas worldwide.
This shark is part of the Lamnidae family, which also includes all known partially endothermic sharks.
This species’ endangered status is due to overfishing for commercial or sport purposes.
Shortfin sharks (also known as the Mako Shark)Isurus oxyrinchus). Image credit: Simon Hilbourne.
“With their entire genomes deciphered at high-resolution, we have a better window into how these endangered species evolved,” stated Professor Mahmood Shivai of Nova Southeastern University. He is also the director of Save Our Seas Foundation Shark Research Center as well as the Guy Harvey Research Institute at Nova Southeastern University.
Their DNA timeline shows that their population has declined significantly over the past 250,000 years.”
Professor Shivji, along with his colleagues, compiled complete genome sequences of great hammerhead sharks and shortfin mako sharks. They then compared these genomes to genome information for white sharks, whale sharks, brownbanded sharks, cloudy catsharks, and the whale shark.
These methods resemble complex puzzle-building techniques by scientists: they successively build from small fragments DNA various sequences, much like a large patchwork tapestry detailing the blueprint for life.
The latest high-quality whole genome sequencing research at the chromosome level is possible. This feat can be difficult for large species such as sharks, which have huge genomes.
Researchers discovered that great hammerhead sharks are more likely to inherit two copies (homozygosity) of the same DNA sequence from their parents.
Professor Shivji stated, “If you want to understand why it might not be desirable, think of it as a disease.”
To express recessive disorders, you need to have two copies of the same gene: one from your mother (and one from your dad).
If you’re homozygous, the gene sequence will have been inherited from your father and mother. The trait will then be expressed.
The great hammerhead sharks were found to have high levels of homozygosity in their genomes. This means they had large portions of their genome that were homozygous. It increases the likelihood of them expressing unwanted traits.
The dominant allele, however, can mask the effect of recessive genes if there are two distinct sequences (alleles), of a particular gene that one has inherited from both the mother or the father.
It will be suppressed if a trait that is undesirable in heterozygosis.
The homozygosity of shortfin sharks was lower than that of great hammerheads. This may be a benefit for their ability to adapt to changes.
The use of advanced techniques is amidst grim reports about sharks and other rays.
According to Dr. Michael Stanhope of Cornell University, “Technical advancements in genome study mean that DNA sequencing methods are now much more powerful”
We can use these technologies to get insights into the organism. This information can then be used to help protect sharks or rays.
While we do not know the exact effects of inbreeding on sharks, studies from wolves or cheetahs have shown that certain traits may become problematic over time. This often leads to lower survival rates for the species.
“The worrying picture of great hammerheads, overfished and sold for their fins, is alarming.”
We would not be able to change the way their fragile populations are managed without such critical genetic insight.
Published in the journal were the team’s findings iScience.
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Michael J. Stanhope The authors and others. 2023. The genomes of the endangered shortfin mako and great hammerhead sharks show historical population declines as well as high levels of interbreeding within great hammerhead. iScience 26 (1): 105815; doi: 10.1016/j.isci.2022.105815