An Unlikely Snail Plays an Important Role in Marine Ecosystems
October 29, 2025
October 29 is National Nudibranch Day or Sea Slug Day. A day to celebrate nudibranchs, it honors marine biologist Dr. Terry Gosliner, whose birthday is today. Dr. Gosliner, the Senior Curator of Invertebrate Zoology and Geology at the California Academy of Sciences (who literally wrote the book on Nudibranchs), has discovered over 1,000 new species of sea slugs. Nudibranchs are a shell-less Gastropod snails in the mollusk group, are often brightly colored, some with stinging tentacles. Recently, Dr. Gosliner and a team from the California Academy of Sciences described 17 new species of vibrantly patterned sea slugs from the Indo-Pacific, and constructed a new family tree for a group of nudibranchs.
An underwater photographer’s favorite, nudibranchs can play an important functional role and are indicators of ocean conditions. For example, in 1984 one of the most common sea slugs along the California coast species nearly disappeared from Southern California waters. The California Blue Dorid, (𝐹𝑒𝑙𝑖𝑚𝑎𝑟𝑒 𝑐𝑎𝑙𝑖𝑓𝑜𝑟𝑛𝑖𝑒𝑛𝑠𝑖𝑠) is a species of colorful nudibranch once commonly observed in tidepools and shallow subtidal zone between Monterey and Baja California. It was considered extinct in the region, likely caused by pollution and over-collecting, but possibly also associated with changes in sea surface temperatures. A note of hope, the once thought extinct California Blue Dorid was discovered living in marine protected areas in Catalina Island. These MPAs provide a number of benefits to fish, but also serve as healthier habitat and refuge for threatened species.
Many nudibranch populations may face threats from climate change, such as the range shifts of species, which may foretell larger ecological changes. In 2015, the population of the Hopkins’ Rose nudibranch exploded along the California coastline, during a large scale warm sea surface event in the northeast Pacific. These frilly purple sea slugs are the delight of tidepoolers, but are also harbingers of a shift in species related to climate change.
These innocuous snails may tell a big tale about the consequences of human impacts.
In the San Francisco Bay and coastal estuaries, the Taylor’s sea hare is a small, splendidly colored nudibranch that lives on eelgrass. Eelgrass is an important marine plant that supports a rich community of species from crabs, to larval salmon to sharks. It also plays an important role in bottom structure and carbon fixation. In 2017, Dr. Kathy Boyer, a professor at San Francisco State University’s Estuarine Ocean Science Center, documented complete disappearance of Taylor’s sea hares from sites monitored over many years. A dramatic drop in salinity in the San Francisco Bay that year demonstrate their sensitivity to water conditions.
The zebra striped slugs are gardeners on algae that can overgrow their host meadows, especially in areas of high nutrient input like the San Francisco Bay. Taylor’s sea hares are not listed as threatened or endangered, but like other nudibranchs, face significant risks from climate change, ocean acidification, and coastal development. Other research indicates they are vulnerable to changes in salinity and pH, which can cause population declines, and their habitat is shrinking due to the destruction of wetlands where eelgrass thrives.
Another fascinating study in the Elkhorn Slough revealed a trophic cascade affecting the Taylor’s Sea Hare, associated with the reintroduction of the endangered Southern Sea Otter. Like many species of shark, sea otters are apex predators and shape the ecology and biological make up of the community they live in. Connected to the Monterey Bay National Marine Sanctuary, the otters forage on crabs which are in turn predators of the sea slug. In this nutrient rich system, the sea hares are important in controlling algal overgrowth. When otters were absent and not eating crabs, the crab population grew and ate too many sea hares, so the eelgrass had more algae growth, thereby suffocating the seagrass.
In the San Francisco Bay, Shark Stewards is conducting a survey of disease and movements of small sharks and rays that are associated with eelgrass beds. As part of this study, we are collecting data on species, including eelgrass, and searching for the elusive Taylor’s sea hare on eelgrass present in the Aquatic Park lagoon. Recently, a volunteer described a strange nudibranch, the enteromorpha-eating Sapsucker Aplysiopsis enteromorphae, in the Aquatic Park lagoon.
The community can help scientists track nudibranchs with the iNaturalist app on your mobile phone, and celebrate the under- appreciated sea slug.
References
Goddard, Jeffrey H. R., Maria C. Schaefer, Craig Hoover, Ángel Valdés, 2013. Regional extinction of a conspicuous dorid nudibranch (Mollusca: Gastropoda) in California, Marine Biology, 160(6):1497-1510
Hannah E Epstein, Joshua M Hallas, Rebecca Fay Johnson, Alessandra Lopez, Terrence M Gosliner, Reading between the lines: revealing cryptic species diversity and colour patterns in Hypselodoris nudibranchs (Mollusca: Heterobranchia: Chromodorididae), Zoological Journal of the Linnean Society, Volume 186, Issue 1, May 2019, Pages 116–189, https://doi.org/10.1093/zoolinnean/zly048
Stewart T. Schultz, Jeffrey H. R. Goddard, Terrence M. Gosliner, Douglas E. Mason, William E. Pence, Gary R. McDonald, Vicki B. Pearse, John S. Pearse. Climate-index response profiling indicates larval transport is driving population fluctuations in nudibranch gastropods from the northeast Pacific Ocean. Limnology and Oceanography, 2011; 56 (2): 749 DOI: 10.4319/lo.2011.56.2.0749
Seely, Mike See Sea Slugs Scour Seagrass by the Seashore. KQED’s A Deeper Look, Nov 10, 2020