First North Pacific records of the pointy nosed blue chimaera, Hydrolagus cf. trolli (Chondrichthyes: Chimaeriformes: Chimaeridae)

Observations

Hydrolagus cf. trolli from dive T-0142 (Fig. 2a) was the first observation of these unknown bluish Hydrolagus specimens in the Northern Hemisphere, and the first of four observations from Monterey Bay, California. Specimen from trawl T-0142 was first reported by Lundsten et al. 2009, and recorded off the coast of central California at the Davidson Seamount on 9 May 2000 at approximately 2064 m. A similar specimen was observed near the same location on dive T-1102 (Fig. 2b) at the Davidson Seamount, on 20 June 2007, at approximately 1641 m. Both specimens from dives T-0142 and T-1102 have striking similarities including: a short, pointed snout, with preopercular and oral canals that share a common branch from the infraorbital canal, bluish-grey body coloration, a concave first dorsal fin with keeled spine, large, triangular pectoral fins, and broad pelvic fins. Additionally, the dorsal, pectoral, and pelvic fins of these specimens all have a pale blue margin on the distal edge.

The third observation of this Hydrolagus species from Monterey Bay, CA was identified from dive T-0215 in the Monterey Canyon, on 5 October 2000, at approximately 1679 m (Fig. 2c). A fourth specimen was observed on dive T-1075 (Fig. 2d), also in the Monterey Canyon on 24 January 2007 at approximately 1658 m. Specimens from dives T-1075 and T-0215 had similar morphological features to the Davidson Seamount specimens. The specimen observed on dive T-1075 had preopercular and oral canals sharing a common branch from the infraorbital canal on the left hand side, while these canals branched separately on the right side of the head. This could not be observed in the specimen from dive T-0215 as the majority of the photos were from a dorsal view.

During a subsequent survey of seamounts off southern California, another large, bluish Hydrolagus specimen was observed. This specimen, also identified as H. cf. trolli, from dive T-0664 was recorded on 2 May 2004 on the San Juan Seamount at approximately 1629 m (Fig. 2e). The Hydrolagus specimen from this dive had a short pointed snout, large, triangular pectoral fins, and a bluish-grey coloration, consistent with those specimens observed on the Davidson Seamount.

A sixth observation of a H. cf. trolli specimen was during surveys off the western coast of the Hawaiian Islands. This was the first observation of H. cf. trolli from the central North Pacific. Hydrolagus cf. trolli from dive T-0296 was recorded on 16 April 2001 at approximately 1641 m (Fig. 2f). Although this Hydrolagus specimen was identified as H. cf. trolli it had several subtle differences from specimens T-0142 and T-0215 such as a short, blunt snout, and larger eyes. However, this specimen exhibited a few similar features to specimen T-0664. These similarities include: a keeled dorsal spine, longer than the height of the primary dorsal, and a short caudal filament.

Observations from all surveys show H. cf. trolli occurring over rocky substrates, sometimes over high relief outcrops. These observations suggest individuals typically occur over hard-bottom habitats or rocky, and mixed substrate patches with vertical relief. These observed substrate associations are in contrast to Hydrolagus melanophasma, which usually associates with soft bottom habitats (James et al. 2009). However, some eastern Pacific Hydrolagus species such as Hydrolagus mccoskeri, are known to associate with high relief habitats (James et al. 2009; Barnett et al. 2006).

Description

Hydrolagus trolli is a highly distinctive chimaera species, often identified by a combination of the following characteristics: an even blue-gray to pale blue color, a pointed snout, a dark margin around the orbit with dark shadows along edges of the lateral line, and preopercular canal and oral canals usually sharing a common branch (Didier and Séret 2002; Compagno and Dagit 2006). Hydrolagus trolli is a large, although slender bodied species with a narrow head that evenly tapers to a whip-like tail. A caudal filament is present, although short and blunt. The pectoral fins of H. trolli are large and triangular, usually tapering to a point on the distal edge. The pelvic fins are broad, and square along distal edge. The first dorsal fin is triangular, with a concave distal edge (Didier and Séret 2002). The dorsal fin spine is curved anteriorly, with two small rows of serrations on the distal 1/3 of the posterior surface. The fin spine is usually just shorter than the height of dorsal fin in juveniles, and slightly longer in adults. The second dorsal fin is elongate, sloping, relatively even in height, and is connected to the dorsal caudal fin by a narrow piece of skin. The caudal fin is rounded with dorsal and ventral lobes nearly equal in height, though ventral lobe is slightly longer (Didier and Séret 2002). Males have a deeply curved frontal tenaculum, which is distally upturned with spines along the dorsal edge; and pelvic claspers that have fleshy, pale, distal lobes, divided for 1/3 their length, with tips usually extending beyond distal edge of pelvic fin (Didier and Séret 2002).

Comparisons

Hydrolagus cf. trolli is the third species of Hydrolagus observed in the eastern North Pacific, and is the second species observed from the central North Pacific. Hydrolagus cf. trolli is morphologically distinct from the other two eastern North Pacific Hydrolagus species, Hydrolagus colliei and Hydrolagus melanophasma, in coloration. The overall brownish-red color and white spots of H. colliei, and overall dark, black coloration of H. melanophasma are easily distinguished from the even blue color of H. cf. trolli (Ebert 2003).

The only Hydrolagus species currently known from the central North Pacific is Hydrolagus purpurescens (Gilbert 1905) that was described from Hawaiian waters. It closely resembles H. cf. trolli specimens, but differs from it in body coloration, pectoral fin shape, and dorsal spine length and shape. Hydrolagus purpurescens is thought to be more widespread throughout the central and western North Pacific, though similar bottom depth ~1130 m. Despite possible overlapping geographically with H. cf. trolli these two species are morphologically different in several aspects. The head of H. purpurescens is robust and deeply compressed, with a snout that is high and compressed, while the observed specimens had pointy snouts (Gilbert 1905; Garman 1911). The pectoral fins of H. purpurescens are large, and broad (Gilbert 1905; Garman 1911), while our specimens had pectoral fins more pointed, and triangular. Hydrolagus purpurescens has a straight dorsal spine that is longer than the height of the first dorsal fin, with no serrations (Gilbert 1905; Garman 1911). Dorsal fin spines of our H. cf. trolli specimens were keeled, except for the specimens observed on dives T-0142, and T-0215 where the fin spines could not be observed in detail. Finally, the even black-purple to purplish-plum coloration of H. purpurescens is distinct from the grey and blue coloration of our H. cf. trolli specimens. All the observed H. cf. trolli specimens have characteristics that more closely resemble H. trolli than H. purpurescens.

Remarks

The presence of Hydrolagus cf. trolli increases the number of known Hydrolagus species to three off California, and to two species off the Hawaiian Islands. Our specimens cannot yet be confirmed as Hydrolagus trolli until morphometric data and or DNA samples from preserved specimen have been collected and analyzed. However, these observations by ROVs suggest that even in relatively well-known areas much remains to be elucidated on the Chondrichthyan fauna from these regions.