By Laura Newcomer
Described by researchers in 1852, photographed once in 1959, and not seen since— this 52-year-old photo in National Geographic provides the only evidence of the existence of a rare species of sea slug published in more than a century. A question by a diver with an affinity for sea slugs prompted a desire to learn the fate of this denizen of the deep.
In 1959, NG photographer Dr. Paul Zahl wrote and photographed a story on the marine life of Hawaii’s Kaneohe Bay. His focus was nudibranchs (pronounced nOO-dee-brangks)— soft-bodied marine gastropod mollusks beloved by divers for their psychedelic colorations (warnings to would-be predators that the slugs are poisonous and not to be messed with, despite averaging only one inch in length).
During his assignment, Zahl photographed species Chromodoris petachialis— a yellow-bodied slug with bright red spots— and according to experts it hasn’t been seen since, despite being hunted by researchers and amateur divers alike.
Scientists aren’t willing to declare the species extinct, however. Various explanations for the lack of sightings exist: populations may persist in rarely-dived waters or in small numbers that go undetected; the species’ primary geographic range remains undiscovered or the species has migrated; the species is nocturnal; petechialis is actually a color form of another species of Chromodoris… in short, the verdict is still out.
Nudibranchs have disappeared for years in the past without going extinct. Trapania tartanella was described in Naples, Italy in 1886 and not recorded again until 100 years later in northern Spain. There’s a good chance, say experts, that petachialis could make a similar reappearance.
Perhaps hope lies in Zahl’s photograph. It’s rare for nudibranchs to be pictured together, as they’re typically solitary creatures who come together only once a year— in summer, when hundreds of sea slugs gather for group mating rituals. If we’re lucky, the two slugs* pictured here liked each other well enough not only to have hung out in spite of their cohort’s solitary tendencies, but also to have carried on the species’ legacy all those decades ago.
*Nudibranchs are hermaphroditic, meaning that any individual slug can reproduce with any other slug and both will lay eggs.
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