Open the mouth of a snapper laid on a market table in Mazatlán and, in rare cases, a pale crustacean may be clamped inside, its plated back filling the space beneath the palate. For more than a century, an eastern Pacific specimen with that blunt head, flattened body, and seven pairs of hooked legs was likely to receive one famous name: Cymothoa exigua.
That certainty broke in 2025. José Salgado-Barragán and Mayra Ixchel Grano-Maldonado examined 32 mouth-dwelling isopods, 27 females and five males, taken from two snapper species offered for sale in Mazatlán and described them in Systematic Parasitology as a new species, Cymothoa facimar.
The discovery did more than add a name to a catalogue. It showed that two remarkably similar animals had been sharing the same regional story, and that older records once folded into C. exigua might need to be opened, measured, and judged again.
The second tongue-eater was hiding in a market
The specimens did not come from a submarine expedition or a remote reef survey. They came from the ordinary movement of fish through Mazatlán, where commercial catches are carried ashore, placed on ice, and sold. The scientists worked with animals removed from the oral cavities of fish passing through local markets.
The hosts were Lutjanus peru, the Pacific red snapper, and Lutjanus aratus, the mullet snapper. Both belong to a commercially important family whose members are handled by fishers, vendors, cooks, and diners long before a taxonomist sees them. In this case, a market counter became a sampling station.
Back in the laboratory, the isopods were compared as bodies rather than as horror stories. Their outlines, head shapes, trunk plates, appendages, and tail structures were examined across males, females, and egg-bearing females, because small anatomical differences are what separate one cymothoid species from another.
Salgado-Barragán, based at the National Autonomous University of Mexico’s Mazatlán unit, and Grano-Maldonado, of the Autonomous University of Sinaloa’s Faculty of Marine Sciences, gave the species a name tied to the place that helped produce the work. Facimar honours FACIMAR, the Spanish abbreviation used by the faculty.
The name has since entered the formal marine species record as Cymothoa facimar Salgado-Barragán and Grano-Maldonado, 2025. A creature that had been physically present in fish mouths was now visible to science as a separate species.
The differences were written in body segments
At first glance, C. facimar looks built from the same unsettling blueprint as C. exigua. The head is tucked into a broad, plated body, the legs end in gripping hooks, and the rear narrows into a shield and paired tail appendages. Similarity at that scale is precisely why the new species remained hidden.
The clearest contrast appears in adult females. The 2025 description reports that C. facimar females are generally larger and more slender than C. exigua, a distinction especially pronounced in egg-bearing individuals. Those are not dramatic differences to the naked eye, but taxonomy often turns on proportions.
The researchers also found differences in the sculpturing across the back, the shapes of the sixth and seventh trunk segments, and the outline of the cephalon, the hard head region. Marine bodies can diverge into astonishing forms, as deep-sea fish demonstrate, but here evolution had written the dividing line in a few millimetres of shell.
The paired structures at the rear, called uropods, provided another clue. Each uropod has inner and outer branches, and their relative proportions helped distinguish C. facimar from C. exigua. To a shopper, both animals would be a pale louse in a fish’s mouth; to a specialist, the tail branches tell different histories.
The new species also resembles Cymothoa excisa, a western Atlantic relative, but differs in the relative length of its fourth trunk segment. Such comparisons usually rely heavily on adult females because cymothoids pass through juvenile, male, transitional, and female stages, changing form as they mature. The result is a group in which sex and age can disguise the very traits used for identification.
The replacement-tongue story began with two fish
The famous replacement-tongue story began with a much smaller sample. In November 1979, Richard Brusca and Matthew Gilligan examined 37 spotted rose snappers collected near Guaymas, Sonora, and found two with female C. exigua attached inside the mouth and more than 90 percent of the normal tongue missing. Their paper appeared in Copeia in 1983.
The isopods held their position with seven pairs of hook-like legs. Yet both fish appeared to be in good condition, with full intestines and body fat, and one had a piece of fish measuring roughly 20 by 50 millimetres in its stomach. Whatever damage had occurred, the hosts had continued to feed.
A snapper’s tongue is not a protrusible muscular organ like a human tongue. It is a fixed extension of the branchial skeleton that helps manipulate prey and press it against teeth on the roof of the mouth. Brusca and Gilligan therefore proposed that the attached isopod might serve as a mechanical substitute after the tongue tissue degenerated.
The important word is proposed. The researchers did not witness the initial attachment, watch the parasite cut blood vessels, measure the fish’s feeding performance before and after infection, or follow either host for the remainder of its life. Their evidence made the replacement idea plausible, but it did not turn every later detail in popular retellings into an observed fact.
The 2025 C. facimar paper is narrower still: it is a taxonomic description based on specimens from oral cavities. It does not demonstrate that the new species removes a tongue in the same way, feeds in the same way, or performs the same mechanical role. The safest description is that scientists found a second eastern Pacific tongue-dwelling Cymothoa, not that every part of the C. exigua story can automatically be transferred to it.
One new species makes old maps less certain
Cymothoid taxonomy has always been difficult. A major review of the family notes that body form changes with attachment site, females can be far larger than males, and variation within a species can be mistaken for a difference between species. Historical descriptions were often brief, based on few specimens, or separated from reliable host data.
A 2024 global review of Cymothoa records shows how widely names, hosts, and localities have accumulated across warm seas. The new Mexican species does not automatically invalidate every old identification of C. exigua. It does mean that records based mainly on geography, host identity, or a quick visual match deserve re-examination where preserved material still exists.
That re-examination may happen in museum jars, university collections, or fresh market samples. A label written decades ago can survive unchanged while the standards used to distinguish species become sharper, leaving older specimens capable of answering a question their collectors never thought to ask.
The Sinaloa coast is especially likely to yield more surprises because its fish-parasite diversity remains incompletely sampled. A separate 2025 survey of commercial catches from three Sinaloa localities reported five parasitic isopod taxa across 16 host species, including an unidentified potential new Cymothoa. The coastline was not biologically empty; it had simply been examined unevenly.
A fish market became a biodiversity survey
Fish markets compress a large stretch of ocean into a few rows of tables. Boats return from different grounds, species arrive from different habitats, and parasites that would be difficult to locate underwater are delivered still attached to their hosts. For taxonomists, the market can function as a rough, human-made net cast across an entire fishing region.
Marine biology has seen this pattern before: an ordinary catch becomes scientifically extraordinary only after someone looks closely. The coelacanth rediscovered in a South African fish landing in 1938 is the famous version, but most discoveries are quieter, involving measurements, preserved appendages, and a name added to a database. C. facimar entered science not with a monster rising from the deep, but with 32 small bodies removed from two familiar food fishes.
Somewhere on the Sinaloa coast, another snapper is being lifted from ice with its mouth closed around an animal no buyer can see. Open it and the pale plates may look exactly like the tongue-eating louse already famous around the world. Under a lens, the curve of a trunk segment and the proportions of two tiny tail branches may be enough to make one old name split into two.
