Stars of the Sea

All hail the echinoderms! Among them, a strange and wondrous creature that motors through salt-soaked currents and intertidal pools with self-generated hydraulic power—and tube feet.

Call them by their common name, sea star or starfish. But know they are uncommonly marvelous and important scouts of the ocean’s health. Though frequently five-armed, some sea star species can have up to 50.

Brainless, and heartless, they still have the ability to regenerate lost limbs, reproduce by splitting themselves in half, and see in the dark with a tiny eye on the tip of each arm.

So, really, how could anyone resist complete infatuation with these mysterious stars of the sea?

Sarah Cohen ’82, a professor of biology, evolutionary ecology, and genetics at San Francisco State University, has devoted a lot of recent attention to understanding their evolutionary significance—and, more recently, new threats to their survival.

“I am pretty possessed by sea stars lately,” says Cohen, who researches the Leptasterias, a genus of six-rayed sea stars. “I spend a lot of time thinking about sea squirts, or tunicates, too.”

Both sea stars and sea squirts are intriguing, says Cohen, because they have amazing powers of regeneration and really different body organization. “Sea stars, in particular, have body plans that operate in completely different ways than we are used to,” she says. “Their body surfaces, under magnification, show a teeming forest of spines, waving tube feet, and small pincers on stalks that act to clean off debris and protect against predators.”

Their small size makes them convenient subjects for lab study. Cohen’s husband, Peter Barschall ’80, has even used his engineering degree to help with experiments, working with a collaborating physics professor to construct tiny grippers used to measure attachment strength in the lab and field. And they have used a miniature shower system to replicate the tidal flow in coastal habitats. Cohen’s lab at San Francisco State’s Estuary & Ocean Science Center can mimic storm surges, now more common because of climate change. This is one way to study their attachment strength—think of those tubular feet—and their tolerance to temperature shifts.

Cohen looks at sea stars through the lens of ecological, behavioral, and environmental features, questioning how those characteristics shape their evolution and genetic systems.

“They are an eccentric organism that’s globally important,” says Cohen. “They’re also cute.”

In Greek, echinoderm means “spiny skin”; the upper side of their body is a nubbly surface of calcium carbonate plates with tiny spikes, just one of their many protective adaptations.

The variety and adaptability of echinoderms is so expansive, in fact, that defining that bumpy skin literally scratches the surface of operating mechanisms of these carnivorous predators.

“We look at the ways many of the species perceive their environment and if they use fine-tuned adaptation versus acclimation,” she says. “I have always been fascinated by organisms that keep their offspring near them, rather than letting them disperse great distances.” Heartless—yet somehow connected.

She wonders, though, how this strategy works successfully in a population sense. Don’t they become inbred and lack new genetic diversity to adapt? And most important, what happens when the environment changes a lot? As coastal storms are more intense and frequent, she says, the organisms clearly face more stress.

“These species may be adapted to local conditions and be good at changing as local habitats change, up to a limit,” she believes, “and then once that limit is exceeded and they can no longer adapt, they will disappear, at least temporarily, from those areas.”

For an animal with an ancient fossil record dating to the Ordovician period around 450 million years ago, disappearing—even temporarily—is a red flag. Another new worry for the communities Cohen studies is the threat of sea star wasting disease, or SSWD. The virus manifests in the spread of white patches of eroded tissue and devolves into destroying its host so entirely that only a gooey mass of the sea star remains.  The cause of the epidemic that has affected dozens of species is mysterious, says Cohen, and likely related to viral pathogens and added environmental stress. She and her students and collaborators are exploring multiple possibilities. The community of sea star researchers remains vigilant for explanations.

“I really enjoy carrying out research in a collaborative way, with colleagues, students, postdoctoral associates,” says Cohen, who even plans family vacations around the Pacific Coast’s best intertidal pools. “A great thing about combining ecology and genetics is that the puzzles involve varied research, from fieldwork to wet lab experiments to genetic analyses and computational bioinformatic comparative work.”

In general, Cohen says that staying local, and not dispersing far, gives sea stars the best opportunity for adaptation. “Another way of saying it is that the scale of dispersal matches the scale of environmental variation,” she says.

In her most recent study, Cohen was surprised to find a unique genetic group of sea stars clustered around the outflow of the San Francisco Bay into the Pacific Ocean. The discovery of the population began as a practical quest: She wanted to find “wave-bashed rocky intertidal field sites” near campus to bring her students to during class periods.

“We want to see how evolution happens in the wild,” she says.

Hands-on expeditions help her classes see science in new ways. “For some students, getting out at low tide to look for sea stars and seeing this unexpected home of strange and colorful diversity is a life-changing experience,” says Cohen, who still remembers the stimulating labs and inspiring field trips that marked her time as a Swarthmore biology major. “We went out and did biology. It made bio real and electric, and the labs taught us to think and write.”

Researching with Cohen was a gateway into a science career for Alejandro Bellon ’15. He was part of Swarthmore’s extern program and spent a week in her lab, followed by an experiential summer internship funded by Swarthmore.

“Dr. Cohen was a great guide to have helping me through it all,” says Bellon, who just started a Ph.D. program in Biology and Society at Arizona State University incorporating ecology with social sciences. Time in Cohen’s lab involved going out to the marina, at first to collect tunicates, and later to check on field experiments. With a collection of juvenile tunicates, Bellon would test the water chemistry and feed the invertebrates while taking pictures to keep track of their development. There were long hours analyzing all those photos, too.

“I still love the fact that tunicate larvae start off with a brain and tail for mobility and then get rid of both as they settle down for a life as filter feeders,” he says. “Goes to show that in some circumstances, a brain costs more than it’s worth.”

Ultimately, Bellon gave his first scientific presentation on his tunicate work at the national meeting of Society of Integrative and Comparative Biology, in Austin, Texas. Cohen works to impart that same enthusiasm in all her students and interns—hoping that together they will find answers about Leptasterias that will ensure the population continues.

Serendipitously, because San Francisco Bay sea stars don’t move around too much at any life-history stage, she says, they are great narrators for changes in local conditions.

Even when the news isn’t good.

“Recently, all our populations associated with the local bay outflow disappeared over the space of about a year, first declining and then disappearing completely,” says Cohen. It’s not what they wanted to discover, but they have been able to genetically compare contemporary samples with historic ones from the California Academy of Sciences and other collections.

“We found that the genetic composition of these sea stars along our Pacific coast has changed dramatically over the past 100 years,” she says. Her guess is that a feature of species that do keep their brood nearby is that there may be frequent local extinctions, and that recolonization may take a long time.

Often at the end of a day of faculty meetings and writing, Cohen strolls down to the bay to check on experiments in the water.

“I get to laugh a lot with students in our open-ended discussions,” she says. “It’s a highlight of being a professor and researcher who teaches the practice of research to new students.”

The tide laps, the papers are graded, the students’ projects are moving forward. And somewhere, a sea star devours its evening meal by pushing its stomach out of its body and into the bivalve. Evolution advances.