Parasite That Dissolves Crabs Discovered

Researchers at Scripps Institution of Oceanography at UC San Diego have described a new parasitic single-celled organism known as a ciliate that kills one of the West Coast’s most common crabs by dissolving and eating its muscle and connective tissue.

Crabs infected by the parasite were five times more likely to die than uninfected crabs. Evidence suggests it increases overall mortality of local populations of the species, the striped shore crab, or Pachygrapsus crassipes, by 22 percent.

The study, published today in the Journal of Eukaryotic Microbiology, was initially supported by a SEEDS Fellowship from the Ecological Society of America.

“This parasite really eats these crabs from the inside out. It has five distinct stages living inside the crab. There’s a huge feeding stage that forms a gigantic mouth, something never seen before. Imagine these furry basking sharks swimming around in a crab’s blood,” said Dan Metz, a PhD candidate at Scripps Oceanography and first author of the study.

“And this is also probably just the tip of the iceberg,” said Ryan Hechinger, an ecologist at Scripps Oceanography and senior author of the paper. “It’s clear that there’s a massive, unseen diversity of these sorts of parasites influencing our marine ecosystems. We just haven’t been looking enough.”

The new parasite has so far only been seen in striped shore crabs, which are ubiquitous residents of West Coast shores and are important parts of coastal food chains — for instance, serving as food for birds such as green herons. The shore crabs are not used as seafood, but Metz and Hechinger said there is a possibility for the parasite to spread to other crabs that are important for commercial or recreational fisheries. The Pacific Rock crab (Cancer antennarius) is a prime candidate, the authors said, having substantial habitat overlap with the striped shore crab.

“It is also very possible that populations of the crabs that we like to eat are actually already getting hammered by this or a similar parasite without us even knowing it,” Hechinger said. "We really need to survey these other crabs.”

Metz and Hechinger used several methods to understand how the parasite attacks crabs, beginning when both were at UC Santa Barbara in 2013. They drew blood from crabs without killing them to check for infection. Metz also came up with a new way to use silver staining — a method to visualize cellular structures — to better reveal some of the fine details of the parasite’s anatomy, including capturing their special method of sexual reproduction.

The parasite is not only a new species, but is also in a new genus. Metz and Hechinger dubbed the species name, grapsolytica, which means “crab dissolving,” after its host and its pathology. They conferred the genus name Lynnia after the late biologist Denis Lynn of the University of Guelph in Canada, who specialized in ciliates like the parasite they discovered.

"Lynn studied ciliates, which are tiny single-celled organisms covered in little hairs called cilia," said Metz. "He contributed so much to the biology of ciliates, including important work on species closely related to this new genus we found. We were very happy to honor him with this genus name."