At first glance, this photograph of curious clownfish (Amphiprion ocellaris) peeking out from the safety of their sea anemone home looks idyllic—a classic portrait of a cooperative relationship between two species. But a closer inspection reveals a far more macabre sort of arrangement.

Inside each clownfish’s mouth resides a hitchhiker, a parasitic crustacean called an isopod that feeds off the fish’s blood. While these free-riders aren’t often photographed, they’re relatively common reef residents: There are more than 300 described species of parasitic isopods in the family Cymothoidae, each of which attaches to a specific host fish. The species captured here by photographer Qing Lin is almost certainly a new addition to that list, since scientists have yet to officially describe an isopod that targets clownfish in the genus Amphiprion. Scientists are, however, aware of its existence. Over the past year, a research team from the Queensland Museum in Australia has frequently observed these “tongue-biters” infesting clownfish in Indonesia’s Lembeh Strait, and they hope to collect specimens for analysis in the near future.

Isopods like these first enter their host as juveniles through the fish’s gills. They then journey to the mouth cavity and attach to the fish’s tongue with hook-like appendages. “The tongue parasites feed directly from the tongue of the host fish by sucking the blood, much like female mosquitoes,” says Rick Brusca, an invertebrate zoologist at the University of Arizona. The isopod will stay attached long enough to complete its life cycle—anywhere from 60 days to 1 year, depending on the species. During this time, an adult will hold its embryos in a brood pouch until the young are developed enough to strike out on their own, in search of a new host.

As an isopod feeds, its body swells, often nearly filling the mouth of the host. Despite this nuisance, the clownfish is usually still able to feed normally, consuming its diet of algae, krill, copepod fragments, and other invertebrates. Indeed, scientists have found very few examples of cases in which parasitic isopods inflict any lasting harm on their hosts. However, several studies of other host species have demonstrated that in high-stress circumstances, including immersion in warmer-than-usual water, infested individuals have higher rates of mortality than parasite-free fish. This is yet more evidence that climate change will bring complex and often unpredictable changes to the ecosystems we rely on. For now, these incognito crustaceans are merely uninvited, but tolerated, company for the clownfish in Lembeh Strait—visitors that eat their fill and stay too long, but eventually leave their hosts in peace.

Lembeh Strait in North Sulawesi, Indonesia

Qing Lin

Qing Lin took up diving in 2002 and began experimenting with underwater photography in his spare time in 2006. Lin was also appointed an official photographer for the 2008 Beijing Olympic Games. His isopods-in-clownfish photo was a winner in the 2017 underwater photographer of the year competition. Lin is based in Toronto, Canada.

Katie Jewett

Katie Jewett

Katie Jewett is a science writer, producer, and communications manager at the Stanford Center for Ocean Solutions where she loves learning something new every day about our planet. Previously, she spent winters in the Colorado Rockies and summers living and working on the water.

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