Crab egg predator found for the first time off US Pacific Coast: Tiny crustacean living on crab eggs has scientists worried about California fisheries

A routine research exercise at the University of California, Santa Barbara, led to a surprising discovery that could have implications for some of California’s commercially important crab species. A team of students studying crab egg broods discovered a tiny crustacean known as a nicothoid copepod living among the eggs of local rock crabs. The finding, described in the journal Ecology, marks the first observation of these copepods off the Pacific Coast of the United States. The discovery came after students returned to hands-on laboratory research following the disruptions caused by the COVID-19 pandemic.

In late 2021, UCSB undergraduate students Sophia Lecuona Manos, Gabrielle Plewe and Carson Gadler, along with doctoral student Zoe Zilz, began working in parasitologist Armand Kuris’s lab. During one of the early lab sessions, a student noticed what appeared to be an unusually large, moving crab egg.

Kuris immediately suspected it was something much more unusual. “Even before looking, I said to myself, ‘My G-d, that’s a nicothoid copepod,’” he said. The tiny creature was an adult female nicothoid copepod living on the egg brood of a yellow rock crab (Metacarcinus anthonyi).


For the students, what initially appeared to be a routine research exercise quickly became a two-year investigation into an organism that had rarely been observed and whose biology remained poorly understood.

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The tiny crustaceans target crab eggs

Nicothoid copepods are extremely small crustaceans that live on crab egg broods. Unlike conventional parasites that feed on or affect the host itself, these organisms target the host's reproductive output. “This matters ecologically because unlike a parasite that might weaken its host gradually, an egg predator strikes at the next generation directly,” said Jaden Orli, the paper’s lead author.

The researchers found the copepods on three commercially important crab species: yellow rock crab, Pacific rock crab (Romaleon antennarium) and red rock crab (Cancer productus). Some crabs were carrying large numbers of the tiny organisms.

“Some of these crabs were so highly infested,” said Plewe. “Their gills were just covered in juvenile nicothoids.” The researchers discovered that the copepod has an unusual life cycle that is closely synchronized with the crab's reproductive cycle. The organism begins life as an egg before hatching into a tiny larva called a nauplius. It then molts several times before becoming a copepodid, a faster-swimming juvenile stage.

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Eventually, female adult copepods settle into a crab's developing egg brood. Once there, they become almost completely immobile and lay their eggs in synchronization with the crab's own egg-laying cycle. “If she is separated from the crab from this point onward, the nicothoid will die within 24 hours,” the researchers noted. The timing is particularly important because crab eggs develop together and hatch within a very short period.

“The crab eggs all develop in unison,” Kuris said. “In a matter of six to eight weeks, depending on temperature, they’ll all hatch within a day.” After the crab eggs hatch, the crab cleans away the empty shells and other material from its brood, potentially removing the adult copepods as well.

Scientists found evidence of a continuous cycle

The students also discovered evidence suggesting that young copepods can remain elsewhere on the crab before eventually moving to the egg-bearing structures when the crab produces another brood. Plewe described witnessing this process during the team's second year of research.

“At some point in our second year of counting we had the opportunity to look into autoinfection,” she said. “And it was incredible to see the juvenile nicothoids come out during another oviposition.” The researchers believe this could allow the organisms to maintain a continuous cycle between generations of crab eggs.

Why scientists are worried about California’s crab fisheries

The discovery is particularly important because the copepods were found on three commercially important crab species. The researchers say the creatures do not feed on adult crabs, but their consumption of crab eggs could have consequences for crab reproduction. Nicothoids are described as partial egg feeders. Because the development of arthropod embryos is highly determined, damage to an individual embryo can have serious consequences.

The researchers also found that the copepods have a rapid life cycle of about 20 days, raising additional questions about how quickly their populations could develop under favorable conditions. “The team found it on three different rock crabs that are commercial species, but we don’t know about the Dungeness crab,” Kuris said. “Dungeness are officially regulated by the state, making them difficult to study. We are worried.”

Dungeness crab is an important commercial species along the U.S. West Coast, making the question of whether the copepod also affects it particularly significant. The discovery is also unusual because similar organisms have been documented in other parts of the world for more than a century, only to become difficult to find.

Researchers in France reported copepods living on crab broods as far back as the 1930s. Later observations were made along the German and English coasts, but those populations also became difficult to locate.

That history has left the UCSB researchers with another major question: why had this population gone unnoticed?

“Among the authors, we do not actually agree,” Kuris said. “There are only two possibilities. One is that they were always here in such low abundance that nobody ever saw them — maybe on another species of crab that we never looked at. Or that it’s an introduced species, but there’s nothing closely related described from Pacific waters.”

A discovery that changed the students’ careers

The project eventually became much more than a laboratory assignment for the students. The team spent two years collecting crabs from local waters with help from local fishers, keeping them in a seawater laboratory on campus and documenting the copepods through different stages of their life cycle.

Lecuona said the experience helped shape her future career in marine science. “It was surreal,” said Lecuona.

The research also earned unusually strong praise from the editors of Ecology, who highlighted the team's detailed documentation of the organism's life cycle and the potential for future research into its ecological effects.

For Kuris, who has published roughly 180 papers, the editorial response stood out. “In my roughly 180-publication experience no other editor has ever lavished such praise,” Kuris said.

Despite the discovery, researchers still don't know how widespread the copepod is or exactly how it arrived on the Pacific Coast. They are also searching for the male nicothoids, which have not been found during the team's observations.

Other questions include whether the copepod is genuinely a newly identified species, where it originated and how much crab egg mortality it causes.

Follow-up research is now underway, with another group of students continuing the work. “There are significantly more things to be done,” Kuris said.