CSUSM Researchers Could Be on Cusp of Banana Slug Breakthrough

CSUSM Researchers Could Be on Cusp of Banana Slug Breakthrough

By Jerry McCormick

An error occurred while preparing your download

One of the slowest creatures on Earth might be heading toward a scientific breakthrough at warp speed.

Cal State San Marcos researchers - along with colleagues from the San Diego Natural History Museum, the Natural History Museum of Los Angeles County and the Research Experiences for Southern California Undergraduate Ecologists network (RESCUE-Net) - recently discovered a population of banana slugs at Palomar Mountain.

Now they are trying to determine if they stumbled on a new species of the slug, or if it is an existing species far from its usual home.

The team was so excited about its discovery that it blasted the news on social media after a population was found in photos on a nature app.

Fun fact: The mascot for UC Santa Cruz is the banana slug, which was selected in the 1980s. And in 2024, Gov. Gavin Newsom signed legislation making the banana slug the state's official slug.

"This tells you about the importance of this slug … in the California culture," said Sergio Nigenda Morales, a CSUSM assistant professor of biological sciences. "If it's a different species and a really isolated population, that's what I'm interested in studying. And it's in our backyard, so we can just go and try to study them."

"Great discoveries often begin with curiosity," said Jackie Trischman, dean of the College of Science, Technology, Engineering and Mathematics. "Dr. Nigenda Morales recognized a fascinating scientific mystery right here in our area and engaged students in the process of exploring it. That's exactly the kind of hands-on learning and discovery we value at CSUSM."

The banana slug is a decomposer that eats dead organic matter such as fallen leaves, plants and moss, then recycles it into waste that fertilizes the soil. It got its name because it resembles a ripe banana, and it's usually found on moist forest floors in the Pacific Northwest.

Nigenda Morales said there are two ways that the population of slugs could exist here in San Diego County. One, they could have been introduced from a population farther north. Two, the slugs could have existed on Palomar Mountain for a while but are now trapped there because of the hot, dry conditions in the area surrounding the mountain.

An error occurred while preparing your download

California is home to all six currently recognized banana slug species, which explains the cultural importance of these slugs for Californians, according to Nigenda Morales.

CSUSM got involved in the project after Nigenda Morales, who joined the faculty in 2023, spoke about his genetic research on mammals, particularly whales, at UC Santa Cruz in the spring of 2025. That's when he was approached by Janet Leonard, a UCSC researcher who's a leading expert on banana slug reproduction.

Leonard asked if he'd be willing to partner on research on the genetic makeup of banana slugs. UCSC provided the samples, and Nigenda Morales decided that the research would make a great class and lab for students at CSUSM in a course called "Population Genetics."

The group started the process of extracting the DNA of the samples, and a few days later, Leonard connected Nigenda Morales with some researchers and scientists from Los Angeles.

The research and initial work began last August. A short time later, a discussion was held about possible banana slug sightings at Palomar Mountain, dating back to the 1980s or even '90s, that were never proven. However, on the iNaturalist app/website, which relies on citizen science for its data, photos of the slugs on the mountain were recorded for about 10 years before a decision was made to pursue the research.

The group decided to organize an expedition to the site and tried seven times to find the slug. Luckily, eight was the magic number.

As far as next steps, the group will send the samples it collected off to be studied against the other six known populations of slugs. Once that is done, an analysis will be conducted at CSUSM with students in Nigenda Morales' lab.

"Once we get the samples back, we can work on figuring out how much genetic diversity there is and how big the population is," Nigenda Morales said.

The researchers also are also studying to see if the slugs can survive on the mountain.

"With small populations, you can have genetic problems such as inbreeding; if we know about that, then we can take better care of them," he said. "We cannot intervene, but because Palomar Mountain is a California state park, we can give some recommendations to the rangers."

Media Contact

Brian Hiro, Communications Specialist

[email protected] | Office: 760-750-7306