Let us ruin your day with this tongue-eating parasite - Niko Zlotnik

Key Concepts

• Cymothoa exigua (tongue-eating louse): A parasitic isopod that replaces the tongue of fish.
• Cymothoids: A family of fish parasites to which Cymothoa exigua belongs.
• Isopods: An order of crustaceans, including pill bugs and cymothoids.
• Basihyal: A bony structure in fish that functions as a tongue.
• Protandrous hermaphroditism: The ability of an organism to change sex from male to female.

Cymothoa Exigua: The Tongue-Eating Louse

• Discovery: In 2013, a man in Northern Ireland found a Cymothoa exigua in a sea bass.
• Classification: Cymothoa exigua is a crustacean isopod belonging to the cymothoid family of fish parasites.
• Diversity: The cymothoid family includes nearly 400 species with various parasitic strategies. Some attach to the head, others suck blood from gills, and some burrow into flesh.

The Parasitic Lifecycle

• Host Selection: Tongue biters seek a fish host for life, which can last up to three years.
• Initial Stage: They swim for days using yolk reserves, sensing light changes and chemical cues from fish.
• Attachment: They hook onto the fish's gills using sharp limbs and crawl inside.
• Transformation: Once inside, they molt, their eyes recede, and swimming structures are lost. Their reproductive organs mature.
• Sex Change: If the fish doesn't have a tongue biter, the isopod transitions from male to female.
• Female Development: The female grows larger, her testes shrink, ovaries activate, and a brood pouch forms.
• Tongue Replacement: The female clamps onto the fish's basihyal (tongue-like structure) and sucks blood.
• Basihyal Atrophy: Over time, the basihyal withers away due to the isopod's feeding.
• Functional Replacement: The isopod functionally replaces the basihyal, aiding in moving food and water.

Reproduction and Social Structure

• Female Dominance: Only mature female tongue biters attach to the basihyal, and only one at a time.
• Male Role: If the tongue is occupied, other isopods remain male until an opportunity arises.
• Sex Reversal: If the female dies, a nearby male can transform into a female.
• Mating: Males typically stay in the gills but may enter the mouth to mate.
• Reproduction: Females carry hundreds of offspring in a pouch until they are ready to find their own hosts.

Impact on Fish Hosts

• Survival: Fish can survive for years with a cymothoid parasite.
• Negative Effects: Despite survival, the parasite can cause localized damage, slower growth, anemia, and increased susceptibility to environmental stressors.
• Parasitism Rates: High parasitism rates are found in preferred hosts like snappers, trout, and croakers.
• Examples: Surveys of Mediterranean striped sea breams and South African largespot pompanos found nearly half the fish carried cymothoids.
• Fish Farms: Parasitism rates can be even higher in fish farms.

Human Interaction

• Removal: Cymothoids are typically removed from fish in fish farms.
• Accidental Consumption: They occasionally end up in grocery stores and home kitchens.
• Health Risks: Eating a tongue biter poses no health risks.
• Unpleasant Surprise: However, it is not a pleasant experience.

Conclusion

Cymothoa exigua is a fascinating example of parasitism, demonstrating a complex lifecycle and significant impact on its fish hosts. While harmless to humans if consumed, its presence highlights the intricate relationships within marine ecosystems and the potential for unexpected discoveries in our food.