The warm waters of El Niño are helping hot pink sea slugs return to the Northern California coastline.
by Connor Ito
Dec. 11, 2015—A salt-laden breeze buffets a scientist and her volunteers as they clamber on the rocky reef near Half Moon Bay. One observer pauses when she sees an eye-catching object clinging to a rock in a tide pool. From a distance, it looks like a piece of trash—a Day-Glo pink tentacle toy from a 50-cent vending machine. When she looks more closely, the volunteer sees a small but stunning specimen of Okenia rosacea, more commonly known as the Hopkins’ Rose nudibranch.
“Over here!” she calls out. “We’ve been seeing them all morning.” I hop from rock to rock over to the waving volunteer in a doomed attempt to stay dry. She points into the pool near her feet, and I zero in on the slug right away. Its coloring is magnetic. From its squishy body to the waving tentacles covering its back, the slug is uniformly pink. It edges its way under an encrusted rock while I watch, entranced.
Though the Hopkins’ Rose has immigrated to Monterey Bay and the rest of the central coast, it isn't an invasive species. It’s just normally very rare. Traditionally, the nudibranchs populate the tide pools of southern California. But in spring and summer 2015, these beautiful pioneers were carried en masse along the coast by warmer waters. Their movements are just part of the many unusual conditions near our shores—and they hint at longer-term trends that may take hold as Earth’s climate settles into new patterns.
Small Travelers on a Long Journey
The Hopkins’ Rose is a tiny type of sea slug. Unlike its terrestrial relations, it doesn’t mind all the salt. Only about one inch long, the slugs make up for their petite size with the sheer vibrancy of their coloring, clashing brilliantly with the dull browns, greens and grays of the intertidal. They take their coloring from their sole food source, a pink bryozoan that encrusts rocks from Baja California to Canada. The slug couldn’t move further north until it had the right conditions: warmer currents that have heralded its arrival in past blooms.
A study published in 2011 examined prevous nudibranch migrations and forecast that similar travels would happen again under certain conditions. A team led by Stewart Shultz, now at the University of Zadar in Croatia, predicted many species of nudibranchs would respond to El Niño-like conditions—masses of warm water in the central Pacific Ocean that disrupt standard conditions on the west coast. During such events, cold coastal upwelling is cut off and warmer water circulates along our coastline.
The study found that the factor most relevant to blooms of the slugs in the north was the transport of tiny newborns riding the currents. Young nudibranchs float up the coast until they find abundant food or adults of their species. The team’s analysis of records from 1969 to 1995 appeared in Limnology and Oceanography.
In El Niño years, the California current, which usually flows from north to south, reverses direction. This allows hatchlings from southern waters to drift up north and settle into new tide pools. The nudibranchs hatch, relocate, reproduce, and die within one year. Their fast life cycles help them take advantage of the ocean’s changing conditions. When it’s just right for the slugs, their populations bloom.
Leading the morning’s tide-pool survey was Rebecca Johnson, a UC Santa Cruz banana slug alumna and a nudibranch researcher at the California Academy of Sciences. “I thought that when we saw range shifts, it would be a slower progression,” she says. “To see this rapid and sustained bloom is surprising.” Instead of being restricted to southern California, the slugs have traveled as far north as southern Oregon. That’s hundreds of miles in a few months—a long way for something smaller than your thumb. Johnson’s team is using genetic analysis to explore whether the nudibranchs have become so successful that they’re now reproducing and thriving here.
However, the longer-term range shifts of oceanic organisms may not stem solely from El Niño. Shultz and his colleagues pointed to the Pacific Decadal Oscillation (PDO), an oceanographic pattern like El Niño that can last for decades, not just months. The PDO can shift from warm to cold, much like the change between El Niño and La Niña. Over this past century, scientists have observed two full PDO cycles, but they lack the information needed to predict this complex climate-driven phenomenon.
A Changing Ocean
In spring 2015, El Niño conditions began taking shape. Oceanographers observed borderline El Niño conditions in March; they have now strengthened dramatically. But the Hopkins’ Rose bloom began in late 2014, when neither warm nor cool water dominated. A bloom of Hopkins’ Rose in the 1990s was linked to El Niño, but this year's bloom is much larger. Something is drawing up bigger swarms of slugs—and scientists think the broader PDO may explain why.
“These last fifteen years, the temperatures off central California have been cooling,” says John Pearse, an emeritus marine biologist at UCSC and coauthor of the Shultz study. “So this last year, we’re getting all these reports of different things appearing in the north and the [ocean] temperature was going up. When we saw Hopkins’ Rose, we went ‘My goodness, just what we said would happen.’”
Scientists could use the population surges of the slugs to monitor changes in the intertidal community with and without them. The nudibranchs reproduce quickly due to their short lifespans, making them ideal to track seasonal and annual patterns. “In the past, they stayed for one or two generations, and then they disappeared,” Pearse states. “So they are good for [measuring] short-term changes. We don’t have very many animals like that.”
The profusion of pink slugs in new coastal settings will have ripple effects. They could act as a novel prey for some creatures, while their bryozoan dining habits could alter tide-pool communities. “One of the things people have noticed is that Okenia is in large numbers in some places, but the bryozoan they eat is not there,” Pearse says. “Maybe they eat something else.” A new picture of the Hopkins’ Rose life history may emerge.
Hopkins’ Rose isn’t the only species of nudibranch moving up the coastline. The ornate Spanish Shawl, with its dazzling orange and purple coloring, is traveling north. A more pugnacious slug charging up the coast is Phidiana hiltoni. It’s a dark red-and-white slug that has no qualms about feeding on a coral-like hydrozoan—or munching on its fellow slugs. Scientists are worried that Phidiana may eat too many local slugs and cause a decline. Though not as colorful as its compatriots, the California Sea Hare also is trudging northward.
Johnson encourages citizen scientists and tide-pool fans to post observations and photos of these marine wanderers at iNaturalist.org—a site that helps researchers track species as they spread and shift.
As these and other migrants arrive, the central coast may undergo some visible changes within the next few years. If warmer waters persist, local tide pools may start resembling ones from the south. It’s a picture both lovely and potentially terrible in its implications, Johnson says: “You’re seeing these nudibranchs that are beautiful and incredible. But combined with sea star wasting and other changes, it’s hard not to think: We’ve been talking about global change, and this is what it looks like.”
Connor Ito studied marine biology at UC Santa Cruz. He wrote this story for SCIC 160: Introduction to Science Writing.