Shocking new footage captured during a deep-sea expedition shows a pair of bloodsucking parasites attached to the head of a rattail fish.
The video, shared by the Schmidt Ocean Institute in a Facebook post, reveals two copepods — small crustaceans — positioned symmetrically on either side of the fish's head. Each parasite sports a long pair of egg sacs at the rear, giving the unusual appearance of pig tails on the fish.
"They feed on blood and fluids from their host using their scraping mouth parts that are embedded in the muscle of the fish," James Bernot, an evolutionary biologist at the Smithsonian National Museum of Natural History who was not involved in the expedition, told Live Science in an email.
Scientists recorded the footage at a depth of 1,604 feet during a mission to explore the seafloor and biodiversity surrounding the South Sandwich Islands, a chain of 11 subantarctic volcanic islets in the South Atlantic.
According to the Schmidt Ocean Institute, the parasites belong to the species Lophoura szidati and are latched onto the head of a rattail fish from the genus Macrourus. Known commonly as grenadiers or rattails, these deep-sea fish are characterized by large heads and long, tapering tails. They inhabit frigid waters in the North and South Atlantic, as well as the Southern Ocean, and are typically found at depths ranging from 1,312 to 10,450 feet (400 to 3,185 meters).
While little is known about parasites in Antarctic waters, L. szidati is among the most common copepods found on Macrourus species in the region. Members of the Sphyriidae family, females of this species have been observed boring into fish to feed on their muscle tissue.
"These copepods are mesoparasites, meaning they are partly inside and partly outside of their host," Bernot explained. "In the video the middle and back end of the copepods stick out of the fish, while the anterior, or head-end of their body is embedded in the fish."
Copepods go through multiple developmental stages in their life cycle. The parasites typically attach to a host during their larval stage, burrowing into the skin to begin feeding. As they grow, they develop holdfasts at the front of their bodies to stay anchored to their host.
In the footage, each parasite is seen carrying a pair of sacs filled with hundreds of eggs. "Copepods are surprisingly good mothers for invertebrates," Bernot said. "They carry their eggs in sacs attached to their body until the eggs hatch into swimming nauplius larvae that will molt through several larval stages and eventually go on to find their own host."
Although their full life cycle and lifespan remain unclear, L. szidati is believed to remain attached to its host for several months, growing from microscopic larvae into mature parasites.
"Even after the parasite dies, remnants of the embedded head can still be found in their host for many years," Bernot noted.
