In a rare discovery, a team of scientists has identified a type of crab that can be transmitted between fish, a phenomenon previously recorded only in a very few animal species
Scientists hope this discovery will help them understand the mechanism of cancer metastasis within the human body, where in this case the cancer cells appeared to be behaving more like parasites than ordinary tumors.
The study, published in the journal Nature, focuses on brown catfish living in Lake Mumfremagog, a 42-square-mile lake stretching from the U.S. state of Vermont to the Canadian province of Quebec.
Since 2012, scientists have noticed strange dark lesions appearing on these fish, described by Peter Emerson, a biologist with the Vermont Department of Fish and Wildlife as resembling "black tar," appearing in the mouth, on the fins, eyes, lips, and gills, negatively affecting the fish's health.
Between 2014 and 2017, scientists detected these malignant melanomas in between 23 and 37 percent of the fish samples examined, a surprising number, especially since these fish live at the bottom of the lake, explained Dr. Julie Dragon, associate professor at the University of Vermont and co-author of the study.
Using whole-gene sequencing technology, the team compared tumor tissues with healthy fish samples, and the surprise was that the cancer cells in each infected fish were genetically identical to each other and more closely related to each other than to the host fish, confirming that it was a single metastatic cancer and not independent tumors.
Dr. Dragon believes that understanding how this cancer spreads among fish could give scientists a deeper insight into cancer biology in general, saying: "By studying how cancers survive and spread outside their original host, we can learn a lot about the factors that confine cancer, and what happens when those boundaries break down. These fish give us a rare opportunity to observe cancer development."
Scientists believe the infection spreads during breeding season when fish come into close contact, and the sharp spines on their pectoral fins scratch other fish, creating wounds that allow cancerous cells to spread from one fish to another. This theory is supported by the fact that young fish show no signs of infection, suggesting that the infection is transmitted through direct contact rather than inheritance.
The analyses also revealed high levels of arsenic in the cancer cells, with a relationship between the concentration of arsenic in the soil surrounding the lake and the geographical distribution of cancer among the fish, raising questions about the role of environmental factors in the emergence of this tumor.
Researchers confirm that this type of cancer is not transmissible to other species; it is confined to brown catfish in this lake. However, this discovery marks the fourth documented case of a contagious cancer in the animal kingdom, following cancers found in dogs, Tasmanian devils, and certain mollusks.
Dr. Vincent Lynch, an evolutionary biologist at the University at Buffalo, believes this discovery could be a game-changer in our understanding of how cancer metastases develop—secondary tumors that arise when cells from the primary tumor break away and migrate to other parts of the body—opening the door to new research that could help in developing cancer treatments for humans
