Appearances can deceive, in more ways than one, as exemplified here by the truly remarkable case of two very different fishes that turned out to be one and the same. Let me explain.
In 1965, a small but spectacular fish called Kasidoron edom was described in a Bulletin of Marine Science paper by C. Richard Robins and Donald P. de Sylva from the University of Miami's Institute of Marine Science (click here to access this paper). Sole member of a totally new genus and taxonomic family (until a second, similar species, K. latifrons, was recorded in 1969, from the western Indian Ocean), it became known as the siphonophore fish.
This was due to its astonishing pelvic fins. These were greatly modified, the third ray in each one having transformed into a long, multi-branched tree-like organ dubbed the pelvic tree, hanging underneath its body, terminating in a series of luminous(?), leaf-like sacs, and closely resembling the tentacular appendages of those superficially jellyfish-like composite creatures the siphonophores (exemplified by the famous Portuguese man-o'-war Physalia).
Known at that time only from waters of around 6-165 ft depth, about 150 miles east of Florida's Cape Canaveral and northeast of Bermuda, this 1.25-in-long velvet-black fish attracted appreciable interest, on account of its conjoined pelvic fins' unique, extraordinary structure. This was assumed to be a device for warding off predators, as they would be likely to mistake its harmless form for the deadly stinging tentacles of genuine siphonophores.
After a time, however, the remainder of this fish's anatomy began to receive attention too, and researches ultimately disclosed that in spite of its distinctive appearance the siphonophore fish was not a new species at all.
On the contrary, it was unmasked as the hitherto-unknown juvenile form of an odd little species called the gibber fish (aka gibberfish) Gibberichthyes pumilus, which had been formally described and named in 1933. It had been discovered in the waters around Bermuda and the Bahamas.
Previously known only from four specimens, this deepwater denizen attains a total length of 4.5 in, and inhabits the western North Atlantic, as well as the South Pacific waters close to the Samoan Islands. With a very large head, a deep, laterally flattened body, and perfectly normal fins lacking any vestige of its juvenile's astounding tentacle-impersonating pelvic tree, the gibber fish is placed within a taxonomic family of its own, most akin to the squirrelfishes and slimeheads.
Moreover, as the second siphonophore fish species, K. latifrons, has also been reclassified as a gibber fish, it is now known as G. latifrons.
Interestingly, a very similar scenario of extreme metamorphosis from juvenile to adult was more recently revealed with another enigmatic, highly distinctive little fish that had long puzzled ichthyologists – the so-called hairy fish Mirapinna esau. You can read about its own very intriguing history here on ShukerNature, and also here in an early cryptozoology article of mine reproduced on ShukerNature.
This ShukerNature blog article is expanded from my book The Encyclopaedia of New and Rediscovered Animals.