Fish in the Bay – Bad Mollusks & other critters, July – Aug 2026 (Part 1).

August 15, 2026 Jim Ervin
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Fieldwork, Fish in Lower South Bay, News

July/August report – Part1: Corbula Clam disaster in Lower South Bay (LSB).

Corbula Clams are found at all locations we survey in San Francisco Bay, but for some unknown reason, they decided to explode in LSB in 2025 and 2026. – We netted over 150,000 of the invasive clams in 2025.  That was vastly higher than the 3,000 to 8,000 per year we counted over the previous decade. 

  • These clams were first documented in Suisun Bay in 1986.  Since then, Corbula numbers surged then declined, and now they are surging again in LSB. They are ultra-efficient filter feeders.  When they surge, most other critters in the estuary starve!  
  • For 2026, we are seeing little relief.  The year-to-date count as of August is 87,806 and climbing!  It is likely this year’s total will come close to matching the previous year. 
  • Corbula overpopulation has already impacted other species by consuming phytoplankton and other microscopic foods. That’s what Corbula Clams do: they starve out most other creatures in the ecosystem. 

Note: Corbula are not the only problematic mollusk we find.  These bad clams are just one of many non-native mollusks we deal with.  Fairly or unfairly, I am developing a very bad attitude toward mollusks in general.  For that reason, this post is an expose about “Bad Mollusks.” 

1.  Bad Mollusk #1: Corbula Clam.  Corbula Hot Spots in Lower South Bay.

Corbula counts: July = 35,147, August = 21,686.  Year-to-date as of August: 87,803.  

  • Station Coy1 was Corbula Hot Spot #1 in July: 24,810 out of over 35,000 clams were collected at station Coy1 alone!

In August, Hot Spot #1 had shifted a little upstream to station UCoy2: 11,850 out of 21,686 clams were caught here.  Thousands more clams were again caught at stations Coy1 and UCoy 1.5.

Why is this particular stretch of Lower Coyote Creek a sweet spot for Corbula clams?  The most clams, and presumably densest accumulation, are usually found in the main stem of Coyote Creek – stations Coy1, UCoy2, and UCoy1.5.  Some thoughts: 

  1. Corbula are very sensitive to average salinity.  These clams tolerate a wide-range of short-term swings in salinity, but over longer periods of time, low salinity (below, say 10 ppt?) seems to kill them outright. 
  2. Higher salinities support predatory mollusks, like Oyster Drills and Philine Slugs, and fish predators, like English Sole and perhaps Sculpin that graze on very young Corbula,
  3. Salinity over about 15 ppt(?) also encourages attachment and growth of sessile organisms, like barnacles, tunicates, and hydrozoans.
  4. We have not directly measured clam ‘strangulation’ by sessile organisms nor predation by Oyster Drills, Philine or fish.  All we can say is, we never see large Corbula downstream from Corbula ‘hot spot’ areas.  Some combination of pressures prevent Corbula from reaching grand old age at higher salinities.  

Corbula create a food desert in their “Sweet Spot:”  We counted only 25 gobies, 71 shrimp, and 179 isopods at Coy1 in July.  August numbers were at least as bad: 8 gobies, 3 Anchovies, 136 shrimp, and 45 Isopods.  In most recent years, summertime shrimp counts at this station typically numbered in the thousands. Fish numbers were higher as well.  

A couple more images to emphasize the point: we caught a lot of Corbula at stations UCoy2 and UCoy1.5 in July and August.  Compared to station Art3, the clams were smaller, but many times more numerous in both months.   

  • More clams at UCoy2 and UCoy1.5 in August compared to July:  Increases in temperature and salinity in late summer season could be influencing factors.  Simple variability in trawl catches could also account for much of the difference.
    • It is worth remembering that clams are caught at UCoy2 under the preexisting LSB monitoring protocol of 10-minute trawls.  Clams caught at UCoy1.5 fall under the newer WRMP guidance of 5-minute trawls.  Ergo, clam numbers from WRMP trawls are doubled for direct comparison. 

The largest clams are always found at stations Alv1 and Art3. These clams are bigger, but usually not so numerous.

  • These spots seem to provide ideal salinity and food availability that enables maximum Corbula growth.
  • Perhaps these locations are just a little better sheltered from salinity extremes?  Or perhaps these places afford optimal microscopic food availability?  Either way, clams grow biggest here.
  • We presume these biggest clams are also the most fecund. Imagine thousands of large males and females emitting clouds of sperm and eggs into the water column. External fertilization, in turn, produces clouds of free-swimming planktonic “veligers” that settle out into new baby clams after about 18 days – just a short distance upstream or downstream from these two Corbula havens.

Densest beds of Corbula clams often correlate with colonies of Mossy Bryozoans (“Bryo-Balls”).  This is not surprising considering that both Corbula and Mossy Bryozoans are primarily filter-feeders. 

  • In a similar, but opposite direction, Corbula numbers are anti-correlated with fishes and shrimp.  For example, Shimofuri Gobies numbered in the dozens to hundreds per month in Alviso Slough for most of 2021 through 2025.  In August 2026, we counted ZERO Shimos in all of LSB for the first time in years! 
  • Interestingly, we catch fairly large numbers of Synidotea Isopods at the densest beds of Corbula. Synidotea are ultimate survivor scavengers. These armored “marine sowbugs/wood lice” likely sustain themselves on a mixed diet of detritus, clam siphons, and dead/dying clams.

Alex Lama at left.  Right image adapted from Cloern & Jassby (2012)

The next question is “Why did Corbula populations explode in 2025 & 2026?” 
—  Two papers by researcher Jim Cloern and co-authors suggest a daisy-chain process that is  influenced by decadal-scale ocean oscillations. 

TLDR:  The Cloern papers propose that coastal Sea Surface Temperatures (SST) result in a trophic cascade that reverberates into SF Bay. 

  • Lower SSTs facilitate nutrient upwelling that feed organisms / Warm SSTs shut down coastal upwelling currents and thereby inhibit recruitment of fishes, crabs, and shrimp that predate Corbula clam larvae in the Bay.
  • Smaller populations of Corbula clams consume less phytoplankton and other microscopic food and thereby increase productivity in the Bay.   

Messy details are summarized below and explained in far greater detail in the papers themselves:

1) Cloern, Jassby, Thompson, and Hieb (2007) A cold phase of the East Pacific trigger new phytoplankton blooms in San Francisco Bay  https://www.pnas.org/doi/10.1073/pnas.0706151104  

“We report a large shift in the biological communities of San Francisco Bay, first detected as increasing phytoplankton biomass and occurrences of new seasonal blooms that began in 1999. …

Coincidental changes included sharp declines in the abundance of bivalve mollusks, the key phytoplankton consumers in this estuary, and record high abundances of several bivalve predators: Bay shrimp, English sole, and Dungeness crab.  The phytoplankton increase is consistent with a trophic cascade resulting from heightened predation on bivalves and suppression of their filtration control on phytoplankton growth.

These community changes in San Francisco Bay across three trophic levels followed a state change in the California Current System characterized by increased upwelling intensity, amplified primary production, and strengthened southerly flows.”

2) Cloern and Jassby (2012) Drivers of change in estuarine-coastal ecosystems: Discoveries from four decades of study in San Francisco Bay https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012RG000397

“[29] …  The remarkably fast colonization and dominance of the Suisun Bay benthos by C. amurensis is attributed to its capacity to utilize a broad range of food resources [Parchaso and Thompson, 2002] and its adaptability to a wide range of salinities, including tolerance of salinity <1 [Nichols et al., 1990]. The annual freshening of Suisun Bay during the wet season precludes colonization by marine bivalves, so C. amurensis rapidly occupied and filled a vacant niche. Abundance of this clam has fluctuated markedly since its establishment (Figure 11a), and the single most prominent reason is salinity variability in response to inflow changes [Peterson and Vayssieres, 2010]. In particular, clam abundance in Suisun Bay tends to increase as X2 shifts upstream (section 3.2) [Nichols, 1985; Nichols et al., 1990; Winder et al., 2011].

[31] Once established, Corbula amurensis quickly transformed Suisun Bay by reducing phytoplankton biomass and primary production fivefold [Alpine and Cloern, 1992], redirecting much of the remaining primary production from pelagic (zooplankton) to benthic (clam) consumers [Thompson, 2005], and creating a persistent state of low phytoplankton biomass and potential food limitation of herbivorous zooplankton. Other sources of organic matter are available to fuel production in food webs. The largest source to San Francisco Bay is river input of detritus [Jassby et al., 1993], but this is largely refractory and the labile components must be converted through the inefficient microbial loop into forms accessible to zooplankton [Jassby and Cloern, 2000; Müller-Solger et al., 2002; Sobczak et al., 2002]. This pathway has also been disrupted because C. amurensis consumes all components of the microbial loop, including bacteria [Werner and Hollibaugh, 1993], ciliates, and flagellates [Greene et al., 2011]. Introduction of a nonnative clam therefore reduced the microplankton food resource available to zooplankton and forced a shift toward their greater reliance on low-quality detritus [Jassby, 2008]. …

[34] Losses of rotifers, calanoid copepods, and mysid shrimp have contributed to the collapses of fish populations in low-salinity regions of San Francisco Bay because these are essential dietary components for resident fish. Rotifers are preferred prey of larval delta smelt [Nobriga, 2002], and many planktivorous fish, including adult delta smelt, longfin smelt, and early life stages of other species, selectively prey on calanoid copepods that are larger and have higher nutritional quality than cyclopoid copepods [Winder and Jassby, 2011]. Other species such as American shad, starry flounder, and juvenile striped bass feed primarily on mysids when available [Feyrer et al., 2003]. Losses of these zooplankton components provoked adaptations by their fish predators. Fish reliant on mysids shifted their diets to other prey, and those with the largest dietary shifts had the largest population declines in Suisun Bay marshes after 1987 [Feyrer et al., 2003]. Northern anchovy (Engraulis mordax) is the biomass-dominant pelagic fish in San Francisco Bay; summer abundance of this species fell 94% in the low-salinity region of the estuary as anchovies adapted to the decreased food supply in Suisun Bay by migrating seaward [Kimmerer, 2006]. In addition to food web transformations, the fivefold decrease in primary production implies a comparable decrease in the energetic carrying capacity for fish in Suisun Bay based on its primary production [Nixon, 1988].

The invasive overbite clam [Corbula amurensis] (studied extensively by researcher Francis Cloern) dramatically restructured the San Francisco Bay ecosystem after its 1986 introduction, reducing phytoplankton and occasionally serving as prey or experiencing population drops driven by predators like juvenile [Dungeness crab]. [1, 2, 3, 4, 5]”

Unfortunately, a state change in the California Current System equivalent to the one that caused a dramatic increase in upwelling intensity in 1999 is not as evident in the PDO, NPGO, CUTI, and BEUTI indices today. That is not to say that a similar but opposite (coastal warming) inflection didn’t happen sometime in early 2025, but so far, I have been unable to find a long-term index showing it.

Young shiny Corbula.  Corbula upstream and downstream of their preferred sweet spots in Coyote Creek and Alviso Slough always appear to be smaller and perpetually young. They simply don’t live as long and never look as brown and “crusty” much outside the preferred stretch of the main channels.

Additional smaller numbers of Corbula were caught in South San Francisco Bay in July & August at Bair Island (3 / 4) and Eden Landing (400 / 917).  (Those clams were caught in 5-minute trawls versus 10-minute trawls in LSB, so double the numbers for CPUE comparison.)  

2. Bad Mollusk #2:  Philine ‘Tortellini Snail’ (Philine auriformis).

Philine count in July / August = 281 / 37.  Year-to-date count in LSB = 1872.  Philine numbers exploded in June.  They have since dropped considerably.  Nonetheless, 2026 already stands as the biggest Philine year on record by far.

  • We find more Philine farther north at Bair Island where salinity is higher. Philine are not as numerous at Eden Landing.  The July / August 5-minute trawl counts were 297/467 at Bair and 33/11 at Eden.
  • After doubling the numbers for CPUE comparison, Bair Island is the clear Philine slug winner. The slimy slugs flourish along the fringes of Redwood Creek (stations Red1 and Red2.) We occasionally see big blooms of them in Middle Bair Pond (MBP1) and adjacent areas.

Philine (aka Tortellini Snails, New Zealand Sea Slugs, or “Snotball Snails”) are slimy “Headshield Slugs.” There were first documented in San Francisco Bay in 1992. (https://en.wikipedia.org/wiki/Cephalaspidea) 

“Specimens of Philine auriformis have been observed in the southern portion of San Francisco Bay, California, since summer 1992. Specimens from Bodega Harbor were first discovered from intertidal mudflats in April 1994 and probably represent the spread of the San Francisco Bay population to a neighboring estuary. Their numbers have increased steadily and the species is now well established in both estuaries. Examination of the anatomy of specimens collected from California was compared with that described from New Zealand.” 

Nasty Philine slugs and unrelated mud tubes at Station Red1 near Bair Island on 11 Aug

Are Philine properly called “slugs” or “snails?” Mollusk phylogeny is very flexible in this area.  Philine are gastropods (from Greek “stomach-footed”), a broad class that includes both snails and slugs.  More specifically, Philine are members of the Philinidae family which are “headshield slugs. Ergo, Philine are more accurately described as “slugs” (even though they still retain snail-like shells inside their bodies.) 

Philine are aggressive and voracious predators of other mollusks.  They suck up small clams, snails, and worms and chew them up with internal gizzard plates.  Good news: Philine provide some control over Corbula clam populations where salinity is high enough to support Philine.  Bad news: Philine also predate all other native species.

Philine superpowers! 
1) Philine produce massive amounts of slime … enough slime to choke a horse; enough to gag a maggot!  We can attest that Philine are extremely slimy creatures. Just a few Philine in a standard counting tray easily suffocate gobies within minutes if we are not careful!  Philine are slimy beasts!

2) Foul taste and smell.  Philine smell is subtle.  Most of us feel slightly nauseous around these slimy slugs, but Sami has a particularly acute sense for Philine odor.  Sami has detected Philine in the net from smell alone on multiple occasions.  He likens the smell to a rancid fart.  Based on these observations, we concluded that Philine must taste bad. 

  • To test this hypothesis, we offered a Philine slug to a hungry Western Gull at Loch Lomond Wharf twice on 24 August 2026.  On both occasions, the gull immediately spat out the slug and gave us angry looks. This slug tastes bad!!! 

3. Bad Mollusk #3:  Japanese Bubble Snail (Haloa japonica).

Japanese Bubble Snail count in July / August = Bair Island 642/186.  Eden Landing 679/278. 

“Bubble Snails” are yet another non-native headshield slug in the mollusk family of “Philinidae.” They are cousins of Philine slugs discussed above.  Like Philine slugs, Bubble Snails (technically also slugs) exude a thick nauseating mucus. (They are also intermediate hosts for a number of bird parasites, including the microscopic flatworm that causes “swimmers itch;” just in case we needed another reason to dislike them.)   

  • Philinidae is a family of medium-sized sea slugs, marine opisthobranch gastropod mollusks. These are headshield slugs, in the order Cephalaspidea.
  • Nasty, puke-inducing snail slime is a defense strategy shared by both Philine and Bubble Snails.  To be nauseating is a heck of a defense strategy.
  • Thankfully, Bubble Snails have never been found farther south in LSB.  Lower and variable salinity must prevent their spread. 

4. Old School Bad Mollusks. 

These “old school” mollusk invaders have resided in SF Bay for so long now that we almost consider them naturalized citizens (almost).  Like Corbula clams, Philine slugs, and Bubble Snails, each of these non-native mollusks possesses different ‘special powers’ of great annoyance. 

Musculista Mussels (Musculista senhousia, now scientifically known as Arcuatula senhousia, also called Asian date mussel)were first identified and collected in the San Francisco Bay in 1946.

Eastern Mud Snails (Ilyanassa obsoleta, now scientifically known as Tritia obsoleta, also called the Mud Dogwhelk) were first collected and documented in San Francisco Bay in 1907.

  • Special Power:  Eastern Mud Snails are vectors for many fish and bird parasites, including occasional cases of swimmer’s itch in the Bay Area.  (Native California Horn Snails and the more recently introduced Japanese Bubble Snail also have this parasite special power.  Pro Tip:  Never eat raw snails!)  https://invasions.si.edu/nemesis/species_summary/74111

Atlantic Oyster Drills  (Urosalpinx cinerea) were first reported in San Francisco Bay in 1890.

  • Special power:  Oyster Drills are efficient predators of oysters. They are a major impediment for recolonization of native Olympia Oysters. https://serc.si.edu/book/export/html/22146

Good News?  The counts of all three “old school” mollusks have been very low this summer: at or near single digits at Bair, Eden and LSB.  For LSB, the current 2026 totals of all three are the lowest we have seen in close to a decade. 

  • Are low numbers a result from the current surge of Corbula Clams?  Or, is this yet another response from a shift in Sea Surface Temperatures and ocean upwelling? 
  • Investigation continues!

5. Kleptomaniac Sea Slugs.

Finally, we need to mention kleptomaniac sea slugs.  We never more than a few of these magical beasts.  The total nudibranch count from Bair Island and Eden Landing for both July and August was 13, with zero seen in LSB. (We occasionally see enosima nudibranchs in LSB, but there have been none since November 2023.) 

Sakuraelois enosimensis, Enosima Aeolid, White-tentacle Japanese aeolis, Enoshima Nudibranch.  https://www.reeflex.net/tiere/10534_Sakuraeolis_enosimensis.htm 

  • Also known as: Enosima Aeolid.  https://en.seaslug.world/species/sakuraeolis_enosimensis
  • From Google AI:  Branchial papillae in nudibranchs refer to the finger-like or spike-like dorsal outgrowths—more commonly known as cerata or branchial plumes—that function as external gills for breathing, digestion, and defense.
  • Key Functions of Branchial Papillae
    • Respiration (Breathing): They absorb oxygen directly from the surrounding water.
    • Digestion: In aeolid nudibranchs, extensions of the digestive gland branch into these structures.
    • Defense: They can store stolen stinging cells (nematocysts) from prey like anemones to fend off predators.

Aplysiopsis enteromorphae, commonly known as the enteromorpha-eating sapsucker or string algae sapsucker, is a unique species of sacoglossan sea slug.

  • Algae Specialist: Unlike carnivorous sea slugs (nudibranchs), it is an herbivore. It uses its needle-like teeth to pierce filamentous green algae (such as Chaetomorpha, Cladophora, and Urospora) and suck out the cellular fluids.
  • “Solar-Powered” Slugs: As a member of the Sacoglossa superorder, it can steal chloroplasts from its algal meals. It retains them within its body to capture energy via photosynthesis.
  • Sacoglossa, the sap-sucking-sea slugs.  https://en.wikipedia.org/wiki/Sacoglossa#:~:text=Sacoglossa%20is%20a%20superorder%20of,members%20are%20known%20as%20sacoglossans.
    “Some sacoglossans simply digest the fluid which they suck from the algae, but in some other species, the slugs sequester and use within their own tissues living chloroplasts from the algae they eat, a very unusual phenomenon known as kleptoplasty, for the “stolen” plastids. This earns them the title of the “solar-powered sea slugs”, and makes them unique among metazoan organisms, for otherwise kleptoplasty is known only among other euthyneruans and single-celled protists.”

The smaller Saccoglossan was a new discovery for us.  Robin Agarwal, a volunteer with California Academy of Sciences, quickly helped us identify this newest critter. It turns out that Saccoglossans are very common in the Bay.  They are just so tiny (around 8 mm in length) and well-camouflaged that few of us ever notice them.  

  • Special power: Kleptomania.  Both these sluggy creatures steal living cells from their prey and repurpose them for stinging defense or food production. 
  • My mind has been blown ever since the August catch.  I now nurture a love-hate relationship with all mollusks. I admire their special powers. But, for most of them – with exception of Nudibranchs and Saccoglossans, I loathe their specific impacts on the local area. 

6. Free Sci-fi Poster. 

I asked ChatGPT to encapsulate my newfound feelings about mollusk invaders into 1950s-style science fiction movie posters.

  • AI is evolving quickly.  AI will probably be making movies for us soon.
  • For better and for worse, we are all heroes of our own story!

Rock Lobster – The B-52s https://www.youtube.com/watch?v=vz65vonktMA

Fieldwork, Fish in Lower South Bay, News