Deep-sea crabs and shrimp have been appearing in droves at the oil-soaked site of the Deepwater Horizon rig. The likely reason, according to new research: Chemicals released from old, degraded oil mimic the hormones that put crustaceans in the mood for love.

But potential mates flocking to the polluted site 40 miles off the Louisiana coast aren’t exactly keepers.

“Their shells were black, and they had a lot of parasites on them,” said Craig McClain, director of the Louisiana Universities Marine Consortium, which led the research. “Many (crabs) were missing claws. They looked really unhealthy.”

It's been nine years since the Deepwater Horizon rig drilling BP's Macondo well blew up, causing an 87-day uncontrolled release of more than 160 million gallons of oil into the Gulf of Mexico. The site of the worst marine oil spill in U.S. history has recovered more slowly than expected, and it appears to be drawing in crustaceans and then making them sick, McClain said.

Crabs and shrimp were nearly eight times more abundant at the well site than in other survey areas, according to the LUMCON study, which was published in the journal Royal Society Open Science.

Several crustacean species are attracted to certain chemicals that are released as oil breaks down over time. These chemicals are often mistaken for sex hormones, and sometimes for food.

“Our hypothesis is that the degraded hydrocarbons mimic natural hormones, especially ones used in sexual attraction and mating,” McClain said.

More research is needed to determine how large an impact the well site’s oil is having on shrimp and crab populations. The deep-sea varieties of white and red shrimp and red crabs found at the site are not harvested commercially in the Gulf, so it’s unlikely people are eating the contaminated shellfish. But the shrimp and crabs are eaten by other marine life, potentially passing toxins up the food chain, McClain said.

The LUMCON team used cameras on remote-operated vehicles to survey the seafloor within 1,600 feet of the wellhead in 2017. The findings were compared with surveys of the same location just after the 2010 disaster as well as recent data collected from other parts of the Gulf at similar depths of just over a mile.

While the crab and shrimp numbers at the Deepwater Horizon site were unusually high, overall species diversity had hit rock bottom.

“We should be seeing glass sponges, giant isopods, fish, corals, sea cucumbers,” McClain said. “All of these are common deep-sea Gulf of Mexico animals. But when you get near the wellhead, you see none of that.”

Shrimp and crabs accounted for 92 percent of the animals found at the site, according to the study.

Oil rig bases and other deep-sea structures typically act as artificial reefs, drawing a variety of marine life. That's not the case with the Deepwater Horizon site.

“All the piping and other structures that are on the seafloor had nothing on them,” McClain said. “They were barren.”

Watching video footage of dying crabs wandering the sea bottom was “very depressing” for the longtime marine biologist.

“It was an emotionally draining dive — probably the most depressing in the course of my career,” McClain said.

The study offered what scientists say is an all-too-rare glimpse of the well site. Funding for researching the spill’s deep-sea impacts has dried up in recent years. The last published environmental impact assessment was in 2014.

The brief period of scientific monitoring at the site is a “definite mismatch” with the slow natural processes and recovery times in the deep sea, McClain said.

LUMCON’s surveys near the well were conducted during research cruises for unrelated projects. “If we were near enough to the Deepwater Horizon site and had some time, we’d do a little extra deep-sea work there,” McClain said.

LSU oceanographer Mark Benfield, who studied the area after the disaster in 2010, commended LUMCON for bringing new attention to the site.

“If (LUMCON) hadn't decided to take a look at the spill site … the conditions would still be out of sight and out of mind,” he said.