Living fossil

Basal lineage of eels discovered after millions of years

Not all species discoveries are equal in scientific impact.  Every species has something unique to teach us if we are clever enough to learn its lesson.  From an expanded concept of the diversity of a lineage to evolutionary novelties and interesting details of morphology, natural history, speciation, evolution, or geography, each species is unique.  Yet among the 18,000 or so species discovered each year a few are profoundly important, rewriting our knowledge of great branches in the tree of life.

One such species was reported in 2011 by Dr. David Johnson of the Smithsonian Institution and several colleagues.  It involved the discovery of a so-called “living fossil,” a species with attributes only previously seen among extinct species.  In this case, it was an eel so remarkable that it warranted a genus and family of its own.  Found in a fringing-reef cave at 25 meters depth in the Pacific Ocean Republic of Palau, this little eel, under 180 mm in length, rewrote science’s understanding of the order Anguilliformes—the true eels.

Video of living Protanguilla palau. Source: G. David Johnson, Hitoshi Ida, Jiro Sakaue, Tetsuya Sado, Takashi Asahida, Masaki Miya CC BY 3.0, Wikimedia Commons.

The new species was named Protanguilla palau and placed in a new family Protanguillidae.  Based on its morphology, this species is the most basal lineage of eel.  That is, this species appears to be the sister-group to all other living eels.  Molecular phylogenetic studies agree with the assessment that this is a basal lineage of true eels with a history comparable to that of all eels combined—said another way, it represents a lineage that has existed for about 200 million years and that presumably arose in the early Mesozoic!

The name “living fossil” is thrown around without a single or consistent definition, but rare discoveries such as this one clearly qualify.  This is, in intriguing ways, a peek at what some of the earliest eels may have looked like.  As Johnson and colleagues write, “extremely long-lived or geologically long-ranging taxa with few morphological changes aid in forming a picture of ancient forms of life.”  Not only does this species show features more primitive than observed in Recent eel species, it has some features which are more primitive than seen in even in the oldest known fossil eels.

Figure 1 from Johnson et al. (2012): Protanguilla palau: (a) Holotype, NSMT-P 98249 female, 176 mm SL. (b–g) Paratype USNM 396016 juvenile, 65 mm SL: (b) whole specimen; (c,d) head in lateral and ventral view, respectively; (e) close-up of tubular gill opening, left side in ventral view; (f) alizarin red-stained body scales along lateral midline (lateral-line scales are forming in alcian blue-stained areas); (g) USNM 396051, 150 mm SL, alizarin red-stained, close-up of lace-like, tubular lateral-line scale.

Eels are a distinctive group of fishes represented by 19 families, 146 genera, and 819 species.  Their fossils show up first in the Cretaceous, about 100 million years ago, but their history is much deeper.  Most have lost pelvic fins and have the dorsal, anal and caudal fins fused, giving them that instantly recognizable eel profile.

Morphological details are best appreciated by ichthyologists and others familiar with fish structure.  Some are unique among all living eels.  Some shared only with Cretaceous fossils. And others unique even among living and extinct eels combined.  Among its unexpected structures is the presence of gill rakers which are primitive to teleost fishes but absent in all other living and fossil eels;  and fewer than 90 vertebrae, contrasted with between 98 and 200 in most eels — and even more than 300 in some.

To collect one small eel and suddenly open a window into a lineage that diverged in the early Mesozoic and that had remained unknown to ichthyologists for centuries is a major discovery by anyone’s measure. 

This amazing species is convincingly that most primitive living eel.  While its morphological details are most easily appreciated by ichthyologists and others familiar with fish morphology, this most peculiar species is described in exquisite detail in the original publication which is cited in my newsletter.  

Reference

Johnson, G. David, Hitoshi Ida, Jiro Sakaue, Tetsuya Sado, Takashi Asahida and Masaki Miya (2012) A “living fossil” eel (Anguilliformes: Protanguillidae, fam. Nov.) from an undersea cave in Palau.  Proceedings of the Royal Society B 279: 934-943. (published online 17 August 2011)