[[Hydrolagus]] Hydrolagus is one of two genera found within the family Chimaeridae. Species in this family are typically characterized as having short fleshy snouts that are bluntly pointed at the tip, elongate bodies tapering to a whiplike tail with an elongate filament, and are uniform brown, gray, or black in color with the possibility of white spots or stripes (Didier, 2004). There is very little difference between Hydrolagus and Chimaera, the other genus within this family, other than the presence (in Chimaera) or absence (in Hydrolagus) of an anal fin. The anal fin is located anterior to the ventral caudal fin and is only separated from it by a small notch and can be very subtle, making it sometimes difficult to distinguish the two genera (Bigelow and Schroeder, 1953; Didier, 1995, 2004; pers. obs.). Comprising 17 of the 24 known species within this family, species of Hydrolagus are found in every ocean, except the Arctic and Antarctic (Didier, 2004), with the greatest diversity known from the western Pacific. The only species of Hydrolagus known from eastern South Pacific is Hydrolagus macrophthalmus (de Buen, 1959), a poorly known species described from two specimens collected from off Valpraiso, Chile. The holotype (Figure 1), which was thought to be lost due to a tsunami that partially destroyed the museum it was previously contained in, was rediscovered and it is now part of the collection at the Museo Nacional de Historia Natural in Santiago, Chile (MNHNC P. 7282). There have also been specimens of H. macrophthalmus collected from off Peru and these are used for comparisons in this study (Chirichigno, 1968; Didier, unpubl. data). Morphometrics and morphological features confirm that Hydrolagus alphus sp. nov. is different from Hydrolagus macrophthalmus. This is the second new species of chimaeroid from the Galapagos Islands (Barnett et al., 2006) and adds a third species to the diversity of Hydrolagus in the eastern tropical Pacific.

Published as part of Kimberly L. Quaranta, Dominique A. Didier, Douglas J. Long & David A. Ebert, 2006, A new species of chimaeroid, Hydrolagus alphus sp. nov. (Chimaeriformes: Chimaeridae) from the Galapagos Islands., pp. 33-45 in Zootaxa 1377 on page 34