When most people hear about a fishing expedition in Mexico’s Yucatán Peninsula, they might picture researchers snorkeling through coral reefs in the Caribbean Sea or diving into crystal-clear cenotes, surrounded by postcard-perfect scenery.
But nothing I had imagined—or Googled—before my first field trip came close to the reality we faced in our search for pupfishes. We were after something far more elusive: the Laguna Chichancanab adaptive radiation, a group of pupfish species named for the way their tails wag like those of playful puppies. These fish exhibit remarkably different head shapes, a clue to their fascinating evolutionary story.
Tucked deep in the Yucatán and known to the ancient Maya as the “Little Sea,” Laguna Chichancanab is a 12-square-mile, landlocked lake. It’s home to one of only two known examples of pupfish adaptive radiations occurring in sympatry—a term biologists use when multiple species evolve and live together in the same habitat. Once described as five distinct species just four decades ago, the Chichancanab pupfishes rapidly evolved over less than 8,000 years, developing specialized feeding behaviors, and striking craniofacial differences to avoid competing for the same food.
The team
The Fish Speciation lab at the Department of Integrative Biology and the Museum of Vertebrate Zoology at UC Berkeley directed by Dr. Christopher H. Martin studies the ecology, evolution, development, and genomics of fish adaptive radiations. To do this, the team uses microendemic radiations of trophic (or dietary) specialist pupfish found only on San Salvador Island, Bahamas, and in Laguna Chichancanab, Mexico.
As a postdoctoral fellow in his lab, I study the genetic changes underlying the development of craniofacial divergence during embryonic and larval development in Bahamian pupfishes. This work is an effort to identify novel genes and gene variants involved in adaptive craniofacial morphologies and their potential implications for understanding clinical craniofacial variation in humans.
In 2022, Martin and his collaborators from Universidad Nacional Autónoma de México (UNAM) and UC Riverside traveled to Laguna Chichancanab to establish local collaborations and estimate the diversity and number of pupfishes in the lake.
They explored several sites along the southern edge of the lagoon and successfully found three of the five originally described pupfish species: C. beltrani, the generalist of the group; C. labiosus, a carnivorous pupfish with thick, fleshy lips; and C. simus, known as the “boxer pupfish” for its distinctive jaw, which juts out at a sharp angle—similar to the scale-eating pupfish C. desquamator found in San Salvador Island.
But their discoveries came with a sobering reality. The numbers of these rare fish were lower than ever, continuing a troubling trend of decline and extinction likely driven by pollution and the introduction of invasive species like the Mayan cichlid (Mayaheros) and Mozambique tilapia (Oreochromis mossambicus).
While the 2022 field expedition was relatively successful, I can’t say the same about our fieldwork in 2024. Described less than a century ago, the Chichancanab pupfish radiation is becoming a poignant symbol of the ongoing loss of animal diversity across the globe in the Anthropocene.
Searching for the Chichancanab pupfishes
Cancún welcomed us with the almost breathable inherent sticky humidity of the Caribbean, skies heavy with clouds, daily temperatures soaring above the nineties, and a strolling pace inconsistent with our hectic schedules as scientists in the Bay Area. With our truck loaded with the essentials—seine nets, cast nets, field clothing, snorkels, and protein bars—we left behind the idyllic Riviera Maya’s beaches, the tourists, the inviting streets of Tulum and Cancún heading into the heart of Quintana Roo, delving into Mayan communal lands. Slowly were we transitioning—without knowing it—into a vida sin afán, a life without the haste, without the urgency that we are accustomed to in populated cities.
Illuminated by a beautiful August full moon, the Mayan jungle greeted us at night with the mesmerizing sound of chirping crickets, croaking frogs, blowing-kiss geckos (or besucones in Spanish), and of course, the hum buzzing mosquitoes.
We gathered to eat with our local collaborators from Universidad Autónoma de México, El Colegio de la Frontera Sur, Instituto Tecnológico Nacional, and the UNAM’s Laboratorio de Conservación Biológica.
As our ties grew closer by sharing guacamole and freshly made totopos, we started planning the next day’s fieldwork in the search of the Chichancanab pupfishes. We were optimistic about finding some of the more extreme pupfish species; hopeful to observe again C. beltrani and C. labiosus sampled in 2022 though we were certainly less confident about encountering C. simus, which possess a ninety degree vertical lower jaw.
As a native speaker, I was heartened by watching my lab mates trying their best to speak Spanish, to learn and use new words to better communicate with our collaborators and the local community. It takes a lot of courage to speak—or even trying to speak—in another language. And yet, there we were, a bunch of Hispanic and Anglo-speakers ichthyologists planning an exciting search for these highly endangered pupfishes while eating ‘tacos de aRRachera’ in the town’s sin afán restaurant.
Into muddy waters
The next day we headed out to a sampling site that hadn’t been visited before, ‘El Rancho’, located along the southeastern shore of the Laguna Chichancanab. It took Dr. Martin three minutes to don his snorkel and dive into the water, eager to survey the abundance and diversity of the new sampling site. To study fish abundance and their diet stomach content (as their diet proxy), we split into teams—some snorkeling along the extended shores of ‘El Rancho’, others seine netting and setting traps along the shores, and some cast netting from the dock. In ‘El Rancho’, net after net, all we found were the generalist C. beltrani and the introduced cichlid, Mayaheros. Sadly, ‘El Rancho’ was foreshadowing the bleak outcome of our field expedition on the current abundance and diversity of pupfishes in Chichancanab.
After fish sampling, our plan for the evening was to conduct sperm motility experiments with the pupfish species that we had sampled in the field, at our cabañas. Matt C. Kustra, a postdoctoral researcher from UC Berkeley’s Miller Institute, is a fish sperm expert who studies how natural and sexual selection interact to create new species. One of Kustra’s goals in Chichancanab was to measure pupfish species sperm performance (a relation between the sperm motility, velocity and trajectory) to understand potential incompatibilities leading to the speciation process in the Laguna.
Our collaborators Maribel Badillo and Fernando Mesh, from UNAM’s Laboratorio de Conservación Biológica, brought from Sisal the Yucatán pupfish, C. artifrons, the outgroup species to the Chichancanab radiation, a lineage separated by 100,000 years. Alongside the specimens we had sampled at ‘El Rancho’ that morning and afternoon, Kustra and I began the sperm motility assays in an improvised but cozy lab set up in his cabaña.
The procedure for collecting sperm involves gently squeezing an alive anesthetized 1.5 inches adult pupfish to collect less than one microliter of sperm that will be later serially diluted to analyze the movement and trajectory of individual sperm cells in waters with varying osmolarities and concentrations of metals and ions on the microscope tracking system. The distant relative, C. artifrons, inhabits marine and brackish waters and is endemic to the coasts of the Yucatán Peninsula, whereas C. beltrani from Chichancanab come from freshwater to brackish waters with high concentrations of gypsum precipitates.
We observed higher motility trajectories and velocities in the sperm cells obtained from C. beltrani swimming in waters collected from Chichancanab and a lower motility in seawater sampled from Sisal. Conversely, C. artifrons sperm exhibited a higher motility in seawater yet reduced motility in water from the Laguna. Led by Kustra further experiments measuring sperm motility, in vitro fertilization, and pre- and post-zygotic incompatibilities between C. beltrani and C. artifrons are ongoing in the Martin lab at UC Berkeley.
In the field, we spent the next three days sampling sites that were previously surveyed in 2022. Much like at ‘El Rancho’, only the dominant generalist C. beltrani and the introduced Mayaheros, along with Poecilia and Astyanax, were found in ‘La Playa’ and ‘San Diego’. Although I have been working with pupfishes for the last three years, this was my first pupfish expedition, and thus, one of my main goals in the Yucatán was to observe pupfishes in their natural habitat. Amid turbid waters filled with algae, beds of Cladium and Eleocharis grasses, and the rare salt-tolerant mangrove (Rhizophora mangle), I not only had the opportunity to see adult C. beltrani in Chichancanab but also captured a video of an adult male in breeding colors visiting various territories within the Laguna, alongside the presence of the introduced Mayaheros and Astyanax fishes.
Wading into research findings
Subsequent stomach content analyses led by Dr. Carlos Grácida and his team from Instituto Tecnológico Nacional, strongly suggest that Mayaheros is a significant predator of the often-smaller Cyprinodontiformes pupfishes. However, today, the threats to this spectacular adaptive radiation of pupfishes are not only represented by the introduction of predatory fish—potentially more appealing to local communities due to their size and food value—but also by the simultaneous threat posed by water contamination in the laguna. At ‘La Playa’ we observed that most Mayaheros sampled, regardless of size, showed evidence of ectoparasitic skin cysts. And sadly, little is known about the numbers and incidence of these parasites and how they may affect the health and life cycle of the Chichancanab pupfishes.
After four days of sampling pupfishes in Chichancanab, it was now our turn as a group to support our collaborators’ expedition in search of rare troglobitic fish – species that inhabit the dark depths of the natural water-filled caves. Equipped with oxygen tanks and wetsuits for the cold underground waters, Arroyave and Erick Sosa, expert certified cave divers, successfully collected two different species of troglobitic fish from two water-filled caves.
They first found the elusive and hermaphroditic Obscure Swamp Eel, Ophisternon aenigmaticum at a depth of 98 feet below the water’s surface in a cenote found within a stunning system of seven interconnected lagoons, three hours south of Chichancanab. Associated with their burrowing habits, all species within the genus Ophisternon show a tendency toward eye size reduction; however, among the neotropical species of the genus, only O. infernalis (a strictly troglobitic, blind swamp eel endemic to the cenotes and submerged caves of the Yucatán Peninsula) and the newly described O. berlini (from Costa Rica) are completely blind.
Excited by the latter discovery and eager to find more troglobitic fish, we traveled the next day to the tourist hub of Tulum. Our sampling site this time—of course—was nothing like the idyllic, crystal-clear cenotes often visited in around Tulum.
Crossing through mosquito-infested Mayan lands, with 100% humidity and temperatures approaching 100°F, we arrived at our next sampling site in the middle of the jungle: a muddy pool with no shoreline—the ominous Darth Vader cenote. At an approximate depth of 160 feet, several specimens of the endemic Typhliasina pearsei, or Dama Blanca in Spanish and sak kay in Mayan, were brought to the surface from the depths of the Darth Vader’s profundities. Believed to be one of the top predators in the Yucatán aquifer, the Dama Blanca is completely eyeless and scaleless and possesses translucent and highly vascularized skin covered by enlarged sensory papillae and cavities.
A disappearing species
After a week of fieldwork and a morning encounter with the Dama Blanca, we started wrapping up and making our way back to Cancún, preparing for our upcoming return trip—we were about to leave behind the slow-paced, tranquil sin afán life we had briefly been a part of. But before heading out, we naturally had to make one last stop, hoping to find a sampling site closer to Chichancanab for C. artifrons, the outgroup species of the radiation. Dressed in our wet and muddy field gear and speaking a loud Spanglish, we certainly stood out among locals, and among the international tourists enjoying the warm crystal-clear waters of the private lagoon in their chic swimsuits. Of course, any remaining inconspicuousness about ourselves vanished the moment we erupted in excitement after confirming the presence of pupfishes at the shore.
In the blink of an eye, my first expedition as a field ichthyologist was coming to a bittersweet end. I could list countless lessons that made up for the sweet side of the journey. Above all, the highlights of the fieldwork were undoubtedly the fish and the people we worked with and interacted with. I feel grateful to have had such an immense opportunity to spend a week among fish, learning from local and international expert tropical fish nerds.
The return to the lab
A successful fieldwork experience requires extensive planning and coordination—but it also demands sincere, respectful, and meaningful communication among all involved. Importantly, as privileged visiting researchers from one of the world’s top universities, the fish specimens collected during this expedition will enrich scientific understanding not only at UC Berkeley and the Museum of Vertebrate Zoology but also at local laboratories in Mexico, where our valued collaborators—and now friends—are based. Collected samples were evenly shared among research teams according to each group’s work, reflecting the lab’s continued commitment to equitable and transparent collaboration. Without the diverse expertise, dedication, and contributions of our local collaborators, this fieldwork would never have succeeded. International, equitable, and diverse collaboration enriches not only our research at UC Berkeley but also empowers and gives opportunities to less advantaged research communities around the globe.
And the bitter side? First and foremost, confirming the possible extinction in the wild of the Laguna Chichancanab specialist pupfishes (C. labiosus and C. simus). These unique endemic species offer critical insights into how genomic mechanisms shape adaptive craniofacial morphologies, revealing evolutionary processes that parallel genetic conditions in humans. For example, many of the genes that the Martin lab has previously identified to be adaptive in San Salvador Island pupfishes, are associated with a variety of craniofacial syndromes. Studying pupfish adaptive radiations thus holds immense promise to understand the relationship between the genetics of adaptation, craniofacial developmental biology, and human genetic syndromes.
This pupfish expedition was my first one, but certainly not the last. As a researcher, I aim to run an independent research plan to understand the genomic basis of craniofacial and sensory adaptations using pupfish adaptive radiations as ‘naturally occurring evolutionary mutant’ systems. This trip further underscored in me the critical importance of fostering and nurturing collaborations with diverse groups of people—individuals with different life stories, areas of expertise, and skill sets that make groundbreaking scientific discoveries possible. In a world with constant species richness decline, we cannot afford to lose our surrounding intellectual richness.
M. Fernanda (feña) Palominos is a National Institute of Dental and Craniofacial Research Pathway to Independence Awardee (K99/R00) and postdoctoral fellow in the Martin Lab. Originally from Chile, when not in the lab, the computer, or the fish room, feña enjoys biking and dancing.
Read this story in Spanish.