Blackwater divers dive in the black of night. Descending to nearly 100 feet (30 m), divers sweep with lights to illuminate tiny organisms, many of which have risen from the deep ocean to feed under the cover of darkness. Most of these divers are also photographers, and as they encounter beautiful and bizarre creatures, they are able to capture mesmerizing photographs of organisms few people in the world have seen alive. Larvae of the Bony-eared assfish are one of the most sought-after species due to their beauty.
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Revealing the Family Ties of Cusk-Eels
Deep in the ocean, cusk-eels lurk along the muddy bottom. With bulbous heads and elongated tails, some are homely creatures with an almost ghost-like appearance. One species, the Bony-eared assfish (Acanthonus armatus), is literally named after the rear end of a donkey. It was, therefore, quite a shock when scientists discovered that a tiny fish with beautiful billowing and fanning fins was the larval stage of the assfish.
Working with blackwater photographers, scientists are beginning to piece together relationships between the seemingly unrelated adult fishes and their larvae. A larval fish can appear completely different from its adult form, to the point where it is near impossible to connect the two just by sight. The Bony-eared assfish is only one species, and it wasn’t until 1981 that the larva and adult were linked. But for many cusk-eels, the larval stage is still a mystery. That was the case for the Gargoyle cusk (Acanthonus myersi) until photographers snapped a photo of what they thought was a colorful baby Bony-eared assfish . After further investigation, the larva was a devilish orange and yellow and had more long fin rays than expected.
In a recent study, Smithsonian scientists have now confirmed, the colorful larva is actually a baby Gargoyle cusk. This discovery is reminiscent of a 1960s observation that the adults of Bony-eared assfish and Gargoyle cusk have similar anatomies. The evolutionary relationships of the two were questionable at the time, though now that the two larvae are known to also look alike, scientists dug a little deeper. After comparing the two species’ anatomy and genetics in this most recent study, it appears that the two are, in fact, close relatives. A third species, an Indian Ocean cusk-eel known as the Spiny blind brotulid (Acanthonus hextii), appears to also be a close relative, with similar larval and adult anatomy. Thanks to the eagle eyes of blackwater scuba divers and photographers, we now know these three species of cusk-eels are all assfishes.
Blackwater Photography Reveals an Unseen World
Credit: © Dennis Whitestone
The Sought After Assfish
Credit: © Steven KovacsThe larval Bony-eared assfish is anything but ugly. The fanned and billowing fins give this fish an ethereal or out-of-this-world appearance. This was the cusk-eel larva photographers were familiar with. Unlike adult cusk-eels that live in some of the deepest depths of the ocean, larval assfishes live near the surface, where scuba divers can easily swim.
Adult Bony-eared Assfish
Credit: Photograph courtesy of Ocean Exploration TrustThe adult Bony-eared assfish looks very different than its larval stage. It’s large, bulbous head gives it a gangly appearance and its coloration is lackluster due to the depths where it lives. It has a big spine that pierces through its skin on the snout, giving it a unicorn-like horn.
The Ghoulish Look-alike
Credit: © Steven KovacsLarval Gargoyle cusk looks strikingly similar to the Bony-eared assfish; however, the orange- tinged fins hint they are not the same. Initially, photographers and scientists believed this fish was the same as the Bony-eared assfish since a larval Gargoyle cusk had yet to be discovered.
Adult Gargoyle Cusk
Credit: Photograph courtesy of Ocean Exploration TrustAn adult Gargoyle cusk photographed a mile (1888 m) deep in the Caribbean. Gargoyle cusks are thought to live in tropical waters around the globe and are a rare site for deep-sea ocean explorers.
To the Museum Shelves
Credit: Courtesy Matt GirardIn the 1960s, scientist Daniel Morris Cohen noted that the Bony-eared assfish, Gargoyle cusk, and this Spiny blind brotulid had very similar anatomies, but he was unable to verify their evolutionary relatedness. Nearly 60 years later, the similarity between the larvae of Bony-eared assfish and Gargoyle cusk prompted scientists to investigate this connection once again. This is one of the preserved specimens used to determine whether the Spiny blind brotulid is also closely related to the Bony-eared assfish and Gargoyle cusk. A photograph of a live Spiny blind brotulid does not yet exist.
An Elusive Cusk Eel
Credit: Courtesy Matt GirardThough the larvae of the Bony-eared assfish and the Gargoyle cusk have been photographed alive, the larvae of the Spiny blind brotulid remain elusive except for this museum specimen. After scientists searched historical fish collections, a single larva of Acanthonus hextii was identified almost 50 years after it was collected from the eastern Indian Ocean.
Three Assfishes
Credit: Courtesy Matt GirardThe three related cusk-eels side by side. After examining their anatomies and comparing their genetics, scientists hypothesize these three species should be grouped together as close relatives in one genus, rather than three separate ones as they have been historically classified. (Acanthonus myersi, top, Acanthonus armatus, middle, Acanthonus hextii, bottom)