Field Reports

After the Algal Bloom Cleared Out of Lake Merritt, the Nudibranchs Came to Party in Droves

But what will come next in this sea slug succession drama?

May 30, 2023

If you lay an old towel right down in the gull poop on Lake Merritt’s dock, sweep away the food wrappers and other detritus, hang your head over the side of the dock, and peer into the murk, a wondrous underwater forest awaits you.

This forest is thick with slippery green sea lettuce, and pocked with “trees” that are not plants at all, but colonies of tiny animals that, in their youth, stuck themselves to the sides of the dock and claimed it as their home. They waft in the current, grabbing or sucking prey from the water as it floats by. Hardly anyone has much of a backbone here; it’s mostly invertebrates, with a few visiting fish. Some of their names are familiar (mussels, barnacles, sea anemones, sponges) and some might not be (bryozoans, hydrozoans, tunicates), but each has its place in this aquatic landscape.

I’ve been coming here for years to photograph and learn about this forest, avoiding the poop and assuaging the fears of the concerned strangers who think I’ve either died or lost my cell phone. And last year, Lake Merritt’s dock turned out to be a courtside seat for me to observe the succession of species following an ecological apocalypse—the harmful algal bloom of August 2022.

It was a lot like a wildfire on land. The “dropped match” was the heat wave, both above and below the water’s surface, that led to a harmful bloom of algae (Heterosigma akashiwo, a “red tide” species associated with fish kills elsewhere). For the first time in 25 years of monitoring, dissolved oxygen in the lake dropped all the way to zero, according to Katie Noonan, the co-chair of Rotary Nature Center Friends, who worked with high school students to conduct the tests. “For five days, the oxygen level remained at zero [parts per million].” 

In late August last year, an algal bloom caused the oxygen in Lake Merritt to drop to zero, as this data from the Lake Merritt Boating Center dock shows. Dissolved oxygen typically drops over the summer, but this was the first time it bottomed out to zero in 25 years of recordkeeping. Data was collected by Rotary Nature Center Friends staff, student interns, and others, with assistance after the bloom began from Janai Southworth and Kristina Yoshida. (Chart by Katie Noonan)

It was heartbreaking to trudge along the concrete path encircling what is arguably Oakland’s most-beloved outdoor space, documenting the rotting carcasses of bat rays, striped bass, gobies and many other smaller lake life forms, such as shrimp and crabs, that washed ashore in staggering numbers. During those five days, thousands of trapped fish and marine invertebrates struggled to breathe at the surface. Most ultimately died. 

On the floating boat dock, empty mussel shells and the dead stumps of green algae seemed to be all that was left of the formerly vibrant invertebrate forest.

But the system began to correct itself surprisingly quickly. After those five dark days, Noonan’s tests showed that oxygen levels rose back to what’s considered normal for Lake Merritt in late summer: about 8 parts per million at the top of the water column, and 6 ppm at the bottom.

And once the anoxic “wildfire” had abated, that’s when the nudibranch magic began to happen.

Nudibranchs are my favorite members of Lake Merritt’s underwater forest environment—they are the apex predators of this tiny world. I’ve been photographing and learning about them for at least a decade, making up for time lost in a responsible corporate job that had nothing to do with marine biology. Sea slugs, which include nudibranchs and their mollusk kin, are small marine gastropods found worldwide in marine and estuarine environments, and they come in an astonishing variety of colors and shapes.

An Enosima Aeolid
An enosima aeolid (Sakuraeolis enosimensis) color-coordinates with its background. (Photo by Robin Gwen Agarwal)
A Flat Okenia
This flat okenia (Okenia plana) blends in as it glides forth. (Photo by Robin Gwen Agarwal)

If a wildfire burns down a forest, it can take decades for its ecosystem to be restored to a semblance of what was there before the fire. Forest diversity evolves over timescales longer than human lives. In Lake Merritt, however, many organisms live for just a few months—most nudibranchs live for a year or less—which means succession can happen much faster.  Nudibranchs have particularly picky eating habits, too, associating with only a few types of prey. That makes them ideal animals for studying ecological change. I was curious to see whether Lake Merritt’s invertebrate forest would be recolonized by fresh populations of species—and if so, which species. 

Naturally, I was hoping for nudibranchs. Maybe even nudibranchs that were new to Lake Merritt. But there was no guarantee.

As it turns out, they did—and how! On the first day of exploring the boat docks at Lake Merritt, peering into the murk, I saw a record number of adult nudibranchs. They were everywhere, feeding and mating and laying down egg ribbons. Even more fun, this unprecedented slug party included multiple individuals of three species never before seen in Lake Merritt: the Morro Bay aeolid (Emarcusia morroensis) and white-crusted aeolid (Trinchesia albocrusta), both native species found on California’s Central Coast, and the Misaki aeolid (Leostyletus misakiensis), which was introduced into the San Francisco Bay from Japan since at least the early 1970s.

A Morro Bay aeolid
A Morro Bay aeolid (Emarcusia morroensis) lays its thin, spiral strands of white eggs. (Photo by Robin Gwen Agarwal)
A white-crusted aeolid
A white-crusted aeolid (Trinchesia albocrusta) ambles through the lake muck, its crusty name-sake patches on full display. (Photo by Robin Gwen Agarwal)
Two Misaki aeolids are mating.
Two Misaki aeolids (Leostyletus misakiensis) mate, forming a translucent tangle of tubes and tendrils. (Photo by Robin Gwen Agarwal)

As the weeks progressed, I saw, along with other citizen scientists, a significant increase in both the numbers and the diversity of sea slugs in Lake Merritt. We documented on iNaturalist 13 species of sea slugs (10 nudibranchs, two sap-sucking slugs, and a bubble snail) between September 28 and November 15.

To this not-really-a-lake, tides are everything

Normally, we sea slug enthusiasts tend to head to the outer coast of California, which has over 130 described species of nudibranchs. Their relatively large size, vivid colors and improbable forms lure us across the slippery rocks and into the tidepools that are revealed during the lowest tides. Even if I never find a single nudibranch, simply being in that magnificent coastal landscape makes the trip worthwhile. 

Yet sometimes, a nudibranch enthusiast like myself may occasionally give up the guaranteed sluggy and scenic beauties to seek out elusive species in murky Lake Merritt.

To understand its charms, you must understand that Lake Merritt is actually not a lake at all, but a large tidal lagoon, connected to San Francisco Bay by a channel. Since 1968, the lake has been managed by a flood control structure with tidal gates that can be opened and shut. Every six hours or so, the tides refresh some percentage of the seawater in Lake Merritt. “The tides are everything in understanding Lake Merritt ecology,” says James T. Carlton, professor of marine science emeritus at Williams College, who studied the lake extensively from 1962 to 1972. “If there’s seawater flowing in and out of the gates, things are relatively stable, at least by Lake Merritt standards.” Especially in the summer, it can be as salty as the Bay. But once the winter rains begin, this estuarine environment gets uncomfortably fresh for many fully marine species. Only the most adaptable aquatic creatures can survive the frequent and constant changes in temperature and water quality, which means that Lake Merritt’s “guest list” of species is constantly in flux. Since most nudibranch species can’t withstand the fresher, warmer part of this cycle, only a few of the tougher species are present in their adult forms, and possibly only for a limited part of the year.

Lake Merritt was rather nudibranch-poor when scientists started looking. UC Berkeley professor Sol Felty Light explored for decades from the mid-1920s and never turned up a single sea slug. And although Carlton listed 50 invertebrates in his 1966 Species List of Lake Merritt: Invertebrates and Fish, he didn’t include any nudibranchs until he discovered two introduced species in the 1970s. (Both Cuthona perca, the lost nudibranch, and Tenellia adspersa, the miniature aeolis, are the color of wet cardboard, tiny, and tough to find; the glory of finally spotting one among the detritus of Lake Merritt is a joy only a dedicated slug-lover could appreciate.)

Since then, Lake Merritt has become an international hub for non-native sea slug arrivals hailing from Europe, the US Atlantic coast, New Zealand, Chile and Japan. They’ve arrived as stowaways on ships and via the commercial oyster industry, according to Carlton. One such globetrotter, for example, is the lost nudibranch (Cuthona perca). It has been seen in the Chesapeake Bay, Hawaii, Florida—and, in California, only in Lake Merritt. Over the years, as the lake has gotten saltier, a few native nudibranch species have joined their non-native cousins. 

The lost nudibranch
The lost nudibranch (Cuthona perca) has found a home in Oakland’s Lake Merritt—the only place in California it has been spotted so far. (Photo by Robin Gwen Agarwal)
The miniature aeolis
The miniature aeolis (Tenellia adspersa) is a tough one to spot, but it can also be found in Lake Merritt. (Photo by Robin Gwen Agarwal)

Goldilocks timing: Incoming tides bring new life

During the weeks after the algal bloom, the tide gates were open. So post-HAB, oxygen probably came back with the fresh seawater brought in by higher tides. With the new seawater came a zoo’s worth of plankton. Including microscopic veligers—the larval forms of several nudibranch species. 

Unlike adult sea slugs, which boldly roam with soft bodies, nudibranch veligers wear tiny, nautilus-like shells that protect them from predators. Like a human stuck in an inner tube, floating and kicking down a river, veligers power up and down the water column by beating the fine hairs on their outer membrane. They must spend at least a month feeding and growing in plankton form before they are ready to settle and metamorphose into juvenile slugs. 

Shortly after the new seawater entered Lake Merritt, Noonan and her students caught what they thought were live nudibranch veligers in plankton trawls, and photographed them through a microscope. I emailed their photos to nudibranch development expert Jeffrey Goddard of the UC Santa Barbara’s Marine Science Institute, who immediately replied. “Her images are of a recently hatched, planktotrophic nudibranch veliger (cool!).” Based on the coiled shape of the shell, Goddard was even able to rule out some species.

Nudibranch metamorphosis is as fascinating as that of butterflies. Juvenile nudibranchs absorb their outer membranes, leave their shells behind and start crawling as tiny versions of adult sea slugs. They have two chemosensory organs on their heads called rhinophores, which look like bunny ears, two eyespots for distinguishing light from darkness, and tiny sharp teeth, called radula, for munching on their prey.

A juvenile nudibranch under the microscope
A juvenile nudibranch, also known as a veliger, as seen through a microscope. (Photo by Katie Noonan)

These veligers are surprisingly capable of zeroing in on the tiny animals they will feed upon as adults: anemones, sponges, hydroids (these are delicate animals related to jellies that are often mistaken for plants, that are attached to a hard surface by a root-like ‘stolon’) and bryozoans (sedentary colonial animals whose exoskeletons can look like anything from lace to tiny underwater bushes). Veligers can even delay metamorphosis until they find the exact prey they like. “When their chemoreceptors tell them they’re in the vicinity of prey, fully developed veligers will settle and begin metamorphosis,” Goddard says.

No doubt these baby nudibranchs were delighted to find that some of their prey had survived the harmful algal bloom. Some invertebrates are incredibly hardy—anemones, for example, can be buried completely for months and survive once it clears, and Goddard told me hydroids can regenerate from a bit of stolon. “There are some species that seem to shrug off anything that isn’t hot water and bleach; they can be a weed,” says Allison J. Gong, who teaches marine biology at Cabrillo College, on Monterey Bay.

Whether they survived the anoxic conditions or settled out of the new seawater, these fast-maturing invertebrates seem to be among the quickest to recolonize rocks, shells and human-made surfaces, such as floating docks and the bottoms of boats. They found prime real estate left vacant by the demise of the normally dominant, slower-maturing invertebrates, like mussels, that did not survive the shock of low oxygen conditions.

“If you have nice clean seawater loaded with veligers, and they sense that their food is present, they’ll settle out into adults and have a ball,” said nudibranch researcher David W. Behrens.

I wonder, too, whether the post-apocalypse invertebrate forest might have had fewer nudibranch predators around, at least temporarily. There aren’t many animals that prey on adult nudibranchs, because they’re toxic, but nudibranch veligers are more vulnerable. “Bivalves were decimated during the fish kill and are really just beginning to recruit,” said Noonan, a month after the algal bloom. “Tunicates were initially wiped out, but barnacles were able to get back into business quickly.” With so many unknowns and relatively little data, however, we can’t know for sure.

New mystery nudibranch: new to science, or newly introduced?

To my delight, I found a few individuals of an undescribed nudibranch species that may be completely new to science. Most of California’s nudibranchs have already been described by researchers; spotting a new one is cause for celebration and speculation, much like when a vagrant bird appears unexpectedly in one’s spotting scope. 

What I’ve temporarily and rather unimaginatively nicknamed the “Undescribed Cladobranchia” is very small, about 5 millimeters long, and very hard to see. Imagine peering through silty saltwater in bad light while you’re bouncing up and down on a floating dock, trying to locate a single grain of rice that’s hiding deep in a rice-colored environment. The nudibranch’s white-and-brown speckled body and orange-banded rhinophores blended in well with the small hydroids (possibly in genus Hydractina) I saw it perched among. At first glance, it looked a bit like a small Trinchesia albocrusta, but lacks the white markings.

The undescribed nudibranch species
The Undescribed Cladobranchia. (Photo by Robin Gwen Agarwal)
A white-crusted aeolid.
A white-crusted aeolid (Trinchesia albocrusta) looks rather similar to the Undescribed Cladobranchia. (Photo by Robin Gwen Agarwal)

I’d seen one other slug of this Undescribed Cladobranchia species once before, when the first one in the United States was found by Luan Roberts, a fellow community scientist, in 2021. She found it on a boat bumper while we were dock-fouling together in the Berkeley Marina, a few miles north of Lake Merritt, and called me over. Side by side, we lay on the dock scratching our heads over its identification, until we decided to simply put up photos on iNaturalist and wait for the nudibranch experts to weigh in. 

Some scientists who saw my photos and videos wrote that this Undescribed Cladobranchia might be a new species, while others suspect that it might be a known, introduced species. “I wonder if it is related to Trinchesia hiranorum, found in the Sea of Japan and the Kuril Islands between Japan and Kamchatka, Russia,” said Washington-based nudibranch researcher Karin Fletcher. “To me, the photo of T. hiranorum looks similar to your aeolid. As far as I know, no specimens of T. pupillae or T. hiranorum have been sequenced, but I bet the one you’ve been seeing will prove to be related to those two species, if not prove to be one of them.” Another researcher in the Netherlands wrote that it might be related to a Trinchesia species seen there and in France. Hopefully its DNA will be sequenced and the animal studied more, so it can join the pantheon of scientifically described California nudibranchs.

What’s next for Lake Merritt’s nudibranchs?

Every party has to end sometime. 

About two months after the massive resurgence of nudibranchs observed by myself and other citizen scientists in Lake Merritt, the nudibranchs started laying egg masses—and disappearing. “Some of the soft-bodied nudibranchs like these species have very rapid life cycles,” says Goddard. “Egg-to-egg in no more than a few months, even six weeks. They’re fast growing and opportunistic.” It is unclear whether subsequent generations of nudibranchs will follow the same path as their pioneering ancestors. Observing these population shifts over time is one of the best things about being interested in nudibranchs; it’s like watching a soap opera or following a sports team. Who’s on top today? Who’s in decline? Who has disappeared, and will they make it for another season? 

Nudibranch eggs on a feather.
A ribbon of nudibranch eggs rests on a feather. (Photo by Robin Gwen Agarwal)

Heavy winter rains likely shook up the balance even more. Noonan said that in 2017, a prolonged freshwater period due to an atmospheric river event caused havoc in the lake’s invertebrate communities, but—based on citizen science data—the sequence of repopulation that time was different than what we saw after the 2022 algae bloom. 

So I’m curious what species—nudibranchs or otherwise—will follow this infusion of fresh water into Lake Merritt. When the rains began in December, Noonan noticed that surface salinity was already down to 7 parts per thousand (ppt), down from 29 ppt before the rains. By early January 2023, it was down to 2 ppt. That’s essentially fresh water. This dramatic change completely knocked out all the nudibranchs in the lake, and we are now back to square one again. When the rains stop and the salinity returns to the unique “normal” of Lake Merritt’s marvelous urban underwater environment, these resilient species will likely reestablish in yet another iteration of the invertebrate forest.

When that happens, you’ll surely find a few of us nudibranch paparazzi hanging our heads over the docks, waiting with cameras poised for their sluggy grand entrance.

About the Author

Robin Gwen Agarwal was a bug-and-tidepool kid who went astray and graduated with a liberal arts degree. In the last decade, she has returned to her childhood passions, photographing and writing about nudibranchs and other natural marvels. She is co-editor of the California Sea Slugs Guide on iNaturalist, where she is a curator with more than 54,000 geotagged observations. Find out more at iNaturalist