Discovery of a New Living Fossil
The Palauan Primitive Cave Eel, Protanguilla palau
by Stephanie Guzik, (Volunteer Science Writer)
In February of 2009, research diver Jiro Sakaue descended into a dark fringing reef cave in the Pacific Ocean Republic of Palau, where he encountered and easily collected with his hands a small (44 mm; 1.7 inches), unusual eel-like fish. It had a shorter body and relatively larger head than is typical of true eels (Anguilliformes).
Sakuae returned to the cave on several occasions and, to date, has collected a total of ten specimens, ranging from small juveniles like the first one to a mature female (179 mm; 7 inches). A close-up video that he shot of the mature female shows its beautiful live coloration – a reddish brown body with black, iridescent, ribbon-like dorsal and anal fins that are tipped in brilliant white.
Sakaue brought the fish to his previous academic advisor, Hitoshi Ida, who consulted with other ichthyologists, including Dave Johnson of the Smithsonian's Department of Vertebrate Zoology. Despite some initial uncertainty about the true connection of this fish to other fishes, it was eventually determined—unequivocally— to be a previously unknown member of the Anguilliformes based on both morphological and molecular evidence. The initial uncertainty about its relationships stemmed from some unusual features not present in any living eels that, together with its DNA, show it to be an ancient independent lineage that warrants recognition as a living fossil.
This conclusion is the basis for a new publication that appeared online on 17 August, 2011 in the Proceedings of the Royal Society B, authored by a team of researchers including Dave Johnson, Jiro Sakaue, Hitoshi Ida and their colleagues Tetsuya Sado, Takashi Ashida and Masaki Miya.
The newest member of Order Anguilliformes
This new genus and species of eel, Protanguilla palau, was the basis for erecting a new family, Protanguillidae. It was also given the common name Palauan primitive cave eel and the Palauan name Ngkelelaruchel, an ancient fish god of Palauan legend.
In Johnson's detailed comparative analysis of the skeletal anatomy in living and fossil eels, he found that Protanguilla has a unique blend of features. On one hand, it shares characteristics of living eels, but on the other hand, it exhibits characteristics from fossil eels of the Cretaceous period. It even has some features that are primitive with respect to both living and fossil eels.
Among the list of notable features that place Protanguilla palau within the order of true eels are the fusion of two bones in the skull, the ethmoid and vomer. This fused bone structure, called the ethmovomer, is a defining characteristic of the Anguilliformes, including Cretaceous eels. The new eel also has the characteristic basketweave pattern of body scales and delicate, lace-like lateral-line scales that are found in a few living eels but not known in the Cretaceous forms.
The most primitive known member of the Anguilliformes
One particularly important characteristic that Protanguilla shares with Cretaceous eels is the presence of a separate premaxilla in the skull. The premaxilla is one of two upper jaw bones that are present in most bony fishes, including Cretaceous eels, but notably absent from the upper jaws of living eels. The presence of this bone (and also a few others) in Protanguilla indicates that this species is primitive with respect to living eels.
Furthermore, Protanguilla has features that are primitive even in comparison to fossil eels. For example, it has a full complement of toothed gill rakers-- structures that are common and widespread among bony fishes, including the closest relatives of eels (e.g, tarpons and bonefishes) but are notably absent from all known eel species, living and fossil. Also, with the exception of two highly specialized deep-sea forms, Protanguilla is the only known eel with fewer than 90 vertebrae (most eels have well over 100).
DNA analysis supports morphology for placement of Protanguilla
In addition to Johnson's detailed comparison of the skeletal anatomy of Protanguilla, co-author Masaki Miya and his team conducted a DNA analysis (called a phylogenetic analysis) of the mitochondrial genome sequence of Protanguilla and compared it to representatives of all 19 living eel families and other bony fishes. From this comparison, Miya's team confirmed that Protanguilla represents one of the most basal, independent lineages of the true eels. Furthermore, the DNA analysis estimated that Protanguilla diverged from all other known anguilliforms around 200 million years ago—more than 100 million years prior to the date of the earliest known fossils.
A living fossil with a limited distribution
The discovery of Protanguilla palau is an exciting addition to the Order Anguilliformes. To date, ten specimens of Protanguilla palau have been found, all in the same fringing reef cave in Palau. Perhaps this species will be found in other locations in the future.
Its long, independent evolutionary history (estimated to date back to the early Mesozoic era), its retention of primitive morphological features, and its apparently restricted distribution warrant its recognition as a "living fossil" and have generated a level of excitement reminiscent of the discovery of another living fossil fish, the coelacanth, in the late 1930's.
All known eels and their close relatives have a distinctive larval form called "leptocephalus" that may drift in the open ocean for as long as three months before transforming into the adult form. However, as this beautiful, primitive eel remained undiscovered until just two years ago, it may indeed have a very restricted distribution. In light of this possibility, the living fossil Protanguilla palau is the newest, and certainly an exceptional, example of the critical necessity of sustained conservation efforts now and into the future.
To read full scientific article click on the link below:
Johnson, G. David, Hitoshi Ida, Jiro Sakaue, Tetsuya Sado, Takashi Asahida, and Masaki Miya. Published online 17 August 2011. A 'living fossil' eel (Anguilliformes: Protanguillidae, fam. nov.) from an undersea cave in Palau. Proceedings of the Royal Society B, 1-10.
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