Mystery of unusual fish larva solved

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A larval fish, which intrigued scientists after an unidentified photo appeared in a research paper, has been linked to a new species of sea bass.

Most marine fishes have a pelagic larval stage that drifts in the surface or near-surface currents of the ocean ― an environment very different from the one they inhabit as adults. Two different environments often require two different body shapes and appearances, resulting in larvae that look very different from the adults of the same species.

The larva at the centre of this study first came to the attention of Smithsonian scientists from a photograph without identification in another research paper. The team recognised it as a member of the sea bass family Serranidae but were intrigued by its seven very elongate dorsal-fin spines.

"This feature isn’t known in any Atlantic sea bass larvae, but it is similar to one species of Indo-Pacific sea bass," said David Johnson, a zoologist at Smithsonian’s National Museum of Natural History. "We initially thought the larva must have been caught in the Indo-Pacific Ocean, but we were wrong." The fish larva in the photo was in fact caught in the Florida Straits.

The team obtained the preserved larval fish for further study and were met with an immediate mystery — a DNA sequence from the specimen did not match any known fish species. That, along with unique morphological features, led the scientists to begin describing the larva as a new species despite the absence of adults.

Meanwhile, in a separate project, Smithsonian scientists were using a manned submersible to explore the deep-reef fish species off of Curacao in the southern Caribbean. Among the fish collected were 'golden basses', which the team identified as Liopropoma aberrans based on general colour pattern; however, genetic analyses revealed more than one species.

Combining this new genetic information with available DNA barcoding data for all western Atlantic sea bass specimens yielded an unexpected discovery. The larva from the Florida Straits is the pelagic stage of a cryptic new species of Liopropoma from southern Caribbean deep reefs. The mystery was solved, and a new species of sea bass — now known as Liopropoma olneyi — was discovered.

The team named the new species in honour of a deceased colleague, John E. Olney, who studied and taught courses about marine fish larvae.

"This was one of those cases where all the stars were properly aligned," said Carole Baldwin, a zoologist at Smithsonian’s National Museum of Natural History. "We discover a new species of sea bass on Curacao deep reefs that just happens to be the missing adult stage of a larval fish from Florida, which we only knew existed because it was included as 'decoration' in a scientific publication. What a great little fish story!"

The team’s research is published in PLOS ONE.

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