Eptatretus strickrotti n. sp. (Myxinidae): first hagfish captured from a hydrothermal vent

Colonization of the ocean floor by jawless vertebrates across three mass extinctions

BACKGROUND

The deep (> 200 m) ocean floor is often considered to be a refugium of biodiversity; many benthic marine animals appear to share ancient common ancestry with nearshore and terrestrial relatives. Whether this pattern holds for vertebrates is obscured by a poor understanding of the evolutionary history of the oldest marine vertebrate clades. Hagfishes are jawless vertebrates that are either the living sister to all vertebrates or form a clade with lampreys, the only other surviving jawless fishes.

RESULTS

We use the hagfish fossil record and molecular data for all recognized genera to construct a novel hypothesis for hagfish relationships and diversification. We find that crown hagfishes persisted through three mass extinctions after appearing in the Permian ~ 275 Ma, making them one of the oldest living vertebrate lineages. In contrast to most other deep marine vertebrates, we consistently infer a deep origin of continental slope occupation by hagfishes that dates to the Paleozoic. Yet, we show that hagfishes have experienced marked body size diversification over the last hundred million years, contrasting with a view of this clade as morphologically stagnant.

CONCLUSION

Our results establish hagfishes as ancient members of demersal continental slope faunas and suggest a prolonged accumulation of deep sea jawless vertebrate biodiversity.

Review of the hagfishes (Myxinidae) from the Galapagos Islands, with descriptions of four new species and their phylogenetic relationships

Hagfishes are an ancient group of benthic marine craniates that are found in deep or cold waters around the world. Among the 83 valid species, four are described from the Galapagos Islands: Eptatretus bobwisneri, E. grouseri, E. mccoskeri and Rubicundus lakeside. During a recent expedition to the archipelago, six species of hagfishes were collected, including four undescribed species of the genera Eptatretus (Eptatretus goslinei sp. nov.) and Myxine (Myxine greggi sp. nov., M. martinii sp. nov. and M. phantasma sp. nov.). In this paper, we provide a review of the eight species of hagfishes from the Galapagos Islands, including new diagnoses and an identification key for all species. Myxine phantasma is remarkable in that it is the only species of Myxine known to completely lack melanin-based pigments. Our species delineations were based on both morphological and molecular analyses. A phylogenetic hypothesis based on molecular data suggests that Galapagos hagfishes arose from multiple independent colonisations of the islands from as many as five different ancestral lineages. The large number of endemic hagfishes in the geologically young Galapagos Islands suggests that there is much global hagfish diversity yet to be discovered.

A new species of hagfish, Eptatretus wandoensis sp. nov. (Agnatha, Myxinidae), from the southwestern Sea of Korea

Four specimens of the five-gilled white mid-dorsal line hagfish, Eptatretuswandoensissp. nov. were recently collected from the southwestern Sea of Korea (Wando). This new species has five pairs of gill apertures, 14–18 prebranchial slime pores, 4 branchial slime pores, a dark brown back with a white mid-dorsal line and a white belly. These hagfish are similar to Eptatretusburgeri and Eptatretusminor in having a white mid-dorsal line, but can be readily distinguished by the numbers of gill apertures (5 vs. 6–7), gill pouches (5 vs. 6), and prebranchial slime pores (14–18 vs. > 18), as well as the body color (dark brown back vs. gray or brown pale). In terms of genetic differences, Eptatretuswandoensis could be clearly distinguished from E.burgeri (0.9% in 16S rRNA and 8.5% in cytochrome c oxidase subunit I sequences) and E.minor (4.5% and 13.9%).

Hagfish: Champions of CO2 tolerance question the origins of vertebrate gill function

The gill is widely accepted to have played a key role in the adaptive radiation of early vertebrates by supplanting the skin as the dominant site of gas exchange. However, in the most basal extant craniates, the hagfishes, gills play only a minor role in gas exchange. In contrast, we found hagfish gills to be associated with a tremendous capacity for acid-base regulation. Indeed, Pacific hagfish exposed acutely to severe sustained hypercarbia tolerated among the most severe blood acidoses ever reported (1.2 pH unit reduction) and subsequently exhibited the greatest degree of acid-base compensation ever observed in an aquatic chordate. This was accomplished through an unprecedented increase in plasma [HCO₃⁻] (>75 mM) in exchange for [Cl⁻]. We thus propose that the first physiological function of the ancestral gill was acid-base regulation, and that the gill was later co-opted for its central role in gas exchange in more derived aquatic vertebrates.

A new species of hagfish (Myxinidae: Eptatretus) from Papua New Guinea

A new species of seven-gilled hagfish Eptatretus astrolabium (Myxinidae) is described from a 400 mm total length female trapped 1 km east from Planet Rock, Astrolabe Bay, Papua New Guinea, at c. 500 m depth. This is the first hagfish species reported from the waters around New Guinea. It can be distinguished from other hagfishes by a combination of characters including seven pairs of gill apertures, three-cusp multicusps on the anterior and posterior rows of cusps, 10 posterior unicusps, 52 total cusps, 18-19 prebranchial pores, five branchial pores, 48-49 trunk pores, 83-84 total pores and no nasal-sinus papillae.

Review of the Australian hagfishes with description of two new species of Eptatretus (Myxinidae)

This paper revises and updates taxonomic and distributional information about hagfishes (Myxinidae) from Australia. It covers five species of the genus Eptatretus: Eptatretus cirrhatus known from eastern Australia and also distributed around New Zealand, Eptatretus longipinnis endemic to South Australia, Eptatretus strahani originally described from the Philippines and reported here as a new record from Western Australia and two new species described herein as Eptatretus alastairi and Eptatretus gomoni, both from Western Australia. Eptatretus alastairi is distinguished from all congeners by the unique combination of the following characters: six pairs of gill pouches; three-cusp multicusps on the anterior and posterior rows of cusps; anterior unicusps 9-12; posterior unicusps 8-11; total cusps 48-56; prebranchial pores 13-16; branchial pores 5-6; trunk pores 50-55; tail pores 11-13; total pores 83-88; two bilaterally symmetrical nasal-sinus papillae in the dorsal surface of the nasal sinus. Eptatretus gomoni is distinguished from all congeners by the unique combination of the following characters: eight pairs of gill pouches; three-cusp multicusps on the anterior and two-cusp multicusps on the posterior row of cusps; anterior unicusps 10-11; posterior unicusps 9-10; total cusps 50; prebranchial pores 12-13; branchial pores 7-8; trunk pores 57-58; tail pores 14-15; total pores 91-93; no nasal-sinus papillae. An identification key for the Australian species of Eptatretus is also provided.

Diversity of bile salts in fish and amphibians: evolution of a complex biochemical pathway

Bile salts are the major end metabolites of cholesterol and are also important in lipid and protein digestion, as well as shaping of the gut microflora. Previous studies had demonstrated variation of bile salt structures across vertebrate species. We greatly extend prior surveys of bile salt variation in fish and amphibians, particularly in analysis of the biliary bile salts of Agnatha and Chondrichthyes. While there is significant structural variation of bile salts across all fish orders, bile salt profiles are generally stable within orders of fish and do not correlate with differences in diet. This large data set allowed us to infer evolutionary changes in the bile salt synthetic pathway. The hypothesized ancestral bile salt synthetic pathway, likely exemplified in extant hagfish, is simpler and much shorter than the pathway of most teleost fish and terrestrial vertebrates. Thus, the bile salt synthetic pathway has become longer and more complex throughout vertebrate evolution. Analysis of the evolution of bile salt synthetic pathways provides a rich model system for the molecular evolution of a complex biochemical pathway in vertebrates.