Hydrolagus mccoskeri, Lewis A. K. Barnett, Dominique A. Didier, Douglas J. Long & David A. Ebert, 2006

Lewis A. K. Barnett, Dominique A. Didier, Douglas J. Long & David A. Ebert, 2006, Hydrolagus mccoskeri sp. nov., a new species of chimaeroid fish from the Galápagos Islands (Holocephali: Chimaeriformes: Chimaeridae)., Zootaxa 1328, pp. 27-38 : 30-36

Hydrolagus mccoskeri sp. nov.

Galápagos Ghost Shark

(Figs. 1-3)

Holotype. CAS 86558, juvenile female, 381 mm TL, 274 mm PCL, 211 mm BDL, Southeast of San Cristóbal Island, Galápagos, (01º5.981'S, 89º12.235'W), 396.24 m, coll. by John E. McCosker (CAS), R. Grant Gilmore (HBOI) & Bruce Robison (MBARI), 17 Nov. 1995 (JSL dive 3934).

Paratype. CAS 223971, juvenile female, 227 mm TL, 138 mm PCL, 107 mm BDL, off Isla Española, Galápagos, (01º18.5'S, 89º45.5'W), 505.97 m, coll. by John E. McCosker (CAS), and John Ross (Smithsonian Magazine), 6 Jul. 1998 (JSL dive 3094).

Diagnosis. A species of Hydrolagus distinguished from congeners by the following combination of characters: head small with short, blunt snout; second dorsal fin only slightly indented along its length; preopercular and oral lateral line canals branching from the same node off the infraorbital canal and sharing a short common branch; dorsum medium brown with numerous narrow, sharply delineated irregular circular and elongate white blotches; ventrum white to tan with extremely fine brown mottling.

Description. Holotype and paratype morphometric measurements are presented in Table 1. Head small (HDL 31-32 % BDL), with rounded, short snout and large eyes (EYL 9-13 % BDL). Body depth (dorso-ventral height of body at a given distance from snout tip) remains similar from head to origin of pelvic fins, from which point it rapidly tapers distally to pelvic fin tips (when depressed posteriorly), subsequently transitioning into a whiplike tail. Skin firm and robust, not deciduous as in some other Hydrolagus (Didier 2002). Post-anal pad present, but inconspicuous in the juvenile type specimens.

Tooth plates light yellow in preserved specimens, with white tritors. Vomerine tooth plates small and incisor-like with 5 tritors visible on the right and 5-6 on the left. Palatine tooth plates with 2-3 tritors along the oral surface originating at the posterior edge and terminating prior to the anterior edge. Small protrusions are present along the anterior labial edge. Mandibular tooth plates incisor-like and large (three times wider than vomerine tooth plates), with 5 tritors per side.

Lateral line canals on head are open grooves with especially wide dilation of canals on the snout. Preopercular and oral lateral line canals branch from the same node off the infraorbital canal and share a short common branch (Fig. 4). Lateral line of the trunk and tail is fairly straight, with very small asymmetrical undulations and a sigmoid curve at the point where it meets the canals of the head.

First dorsal fin preceded by a highly robust dorsal spine, which is triangular in cross section. Two narrowly spaced columns of serrations are present on the posterolateral edges of the distal 2/3-3/4 of spine. Spine only slightly curved posteriorly, with the majority of curvature occurring in the distal 1/3 of spine. First dorsal fin nearly triangular, with a straight posterior margin and slightly convex anterior margin. Second dorsal fin slightly indented along its length with the anterior lobe higher than posterior lobe. Dorsal and ventral caudal fin lobes elongate, with a longer ventral caudal fin lobe extending anteriorly. Anterior portion of the ventral caudal fin lobe merges into a fleshy ridge. The ridge extends anteriorly an additional 26% of body length and is nearly aligned vertically with the midpoint of the second dorsal fin. Both caudal fin lobes are approximately equal in height. Caudal filament moderately stout. No anal fin present.

Anterior margin of pectoral fin slightly convex, with increasing curvature toward an acutely pointed distal tip. Posterior margin straight, becoming concave at distal 1/4 of length. Pectoral fins, when depressed posteriorly, reach past the pelvic fin insertion and distal tip of pelvic fin lobe. Pelvic fins large and nearly triangular. Anterior margin slightly convex, particularly at distal 1/3. Posterior margin nearly straight, but very slightly convex. Distal tip slightly rounded to acutely pointed.

Coloration. Life color of the holotype is an overall medium gray on dorsal and lateral sides extending to near the ventrum, with slight superficial silvery sheen; snout and oral region slightly lighter gray; ventrum light gray to whitish, with underside of caudal region from the pelvic insertion to the terminal end of the tail whitish, except for the dark gray caudal filament; first dorsal fin dark gray with slightly lighter gray areas in membranes between proximal ceratotrichia; second dorsal fin with white proximal and distal margin bordering a medial dark gray stripe; pectoral and pelvic fins darkish gray on lateral portion, slightly lighter gray on medial portion, with whitish posterior margin; dorsum between the eyes and the anterior caudal region is profusely marked with irregular but well-defined spots, vermiculations, and stripes of a silvery white; pigment around the outer margins of the eye is dark grey and the tapetum lucidum reflects an emerald-green.

Color of preserved holotype is medium brown on the dorsum with numerous irregular, finely delineated rounded to elongate white blotches. Rostrum is a uniform medium brown dorsally and uniform tan from snout tip to mouth. Coloration of the head and trunk ventral to the dorsal edge of gill opening is a uniform white to tan with fine brown mottling, with finer mottling at the head, creating a uniform light brown appearance. On the tail, this lighter ventral coloration begins below the lateral line.

Medial portion of paired fin lobes uniform medium brown, fading to off-white with fine brown mottling at the fin bases and lateral portion of fin lobes. Paired fin webs light brown to grey-brown, darkest at anterior margin, fading to a translucent posterior margin. Second dorsal fin and dorsal caudal fin lobe are uniform medium brown, bordered by a light colored proximal margin and a translucent distal margin. Ventral caudal fin lobe is uniform medium brown, fading to tan anteriorly. Coloration of preserved paratype is similar to holotype, with a few exceptions: coloration of paired fin webs is uniform, without a darkened anterior margin; hue of the body is darker, particularly on the ventrum; color pattern of second dorsal fin and dorsal caudal fin lobe is much less distinct and the hue is light brown instead of medium brown.

Etymology. The Latin name is designated in honor of Dr. John E. McCosker, a renowned ichthyologist who collected and graciously provided the type specimens. The common name is an allusion to the known distribution of the species.

Distribution and Ecology. Nine specimens were observed from four sites in the Galápagos archipelago: Cabo Douglas, Fernandina Island (01º17.534'S, 91º38.852'W) , Isla Española (01º18.5335'S, 89º45.4037'W) , Marchena Island (00º24.0'N, 90º26.5'W) and San Cristóbal Island (01º5.981'S, 89º12.235'W) , at 396.24-505.97 m depth.

Individuals were found alone or in groups along steep slopes, within a few meters of the seafloor. Habitat was composed of igneous boulders, cobbles, and pebbles.

The observed bathymetric range is shallow compared to many other Hydrolagus , and may reflect incomplete deep-water sampling in the region. However, the two most similar congeners in morphology and distribution, H. novaezealandiae and H. colliei , are also found in relatively shallow depths (Eschmeyer et al. 1983; Anderson et al. 1998), suggesting that the observed depth distribution of H. mccoskeri may accurately represent its true bathymetric range.

We suggest H. mccoskeri is likely endemic to the Galápagos archipelago. There is a high degree of endemism in this geographic region due to its isolation from the South American continent (Grove & Lavenberg 1997; McCosker 1998). The Galápagos archipelago has never been closer to the South American continent than today (Cox 1983). The expanse of deep water extending thousands of kilometers to the east and west, the East Pacific Barrier (Ekman 1953), has effectively contained many species within this small area. Given this species' apparently shallow bathymetric range and limited vagility, we hypothesize that this new species of Hydrolagus represents another of these species limited in its distribution.

Comparison. Hydrolagus mccoskeri is the third species of Hydrolagus described from the eastern Pacific. It is easily distinguished from its nearest geographic relatives, H. macrophthalmus and H. colliei . Hydrolagus macrophthalmus is an even brown color with no distinct color pattern (de Buen 1959; Quaranta et al. in prep.). Hydrolagus colliei is characterized by a reddish-brown color with distinct white spots, a second dorsal fin that is deeply indented, almost separated into two parts, and oral and preopercular lateral line canals that originate separately from the infraorbital canal (Didier & Rosenberger 2002). The body of H. colliei anterior to the external opening of the gastrointestinal tract is longer than in H. mccoskeri (Table 1; SVL 67-93% BDL, and TRL 38-55% BDL).

An additional variant of Hydrolagus , collected from the Galápagos archipelago during this survey, is easily distinguished from H. mccoskeri by a longer trunk length (TRL), deeper body (D1P1, D2P1), shorter second dorsal fin (D2B) and caudal dorsal margin (CDM), shorter head length (HDL), and greater height of the anterior lobe of the second dorsal fin (D2AH) (Quaranta et al. in review). Coloration is dark brown to black, with a white blotch on the midpoint of the second dorsal fin, occasionally with a few large oval white blotches on the lateral sides of the body, but not as extensive as in H. mccoskeri . This morphotype is likely a new species, however, its similarity to H. macrophthalmus is being addressed in a subsequent study (Quaranta et al. in review).

Hydrolagus mccoskeri most closely resembles H. novaezealandiae , a species endemic to the coast of New Zealand. However, the white markings on the dorsum of H. mccoskeri are more numerous, less continuous and more sharply delineated. The oral and preopercular lateral line canals of H. mccoskeri branch from the same node off the infraorbital canal and share a short common branch, whereas in H. novaezealandiae and H. colliei , they branch from separate nodes off the infraorbital canal. There are also distinct morphological differences that differentiate these species (Table 1). In particular, H. mccoskeri has a shorter, blunter head and snout (POB 11-12% BDL), and the origin of the second dorsal fin is further anterior. Additionally, the majority (86%) of lateral line canals along the head are greater in length in H. mccoskeri .

Remarks. Determination of chimaeroid taxonomy is difficult because of extensive interspecific similarity in morphology and poor condition of type specimens captured by traditional fishing gear. This study demonstrates the benefits of using submersibles as a method of taxonomic research, allowing identification of a new species from limited type material while yielding accurate and precise information on live body color, behavior, distribution, and habitat associations that would be otherwise unavailable from deep waters. The condition of the type specimens facilitated the analysis of characters considered to be conservative and robust for the distinction of chimaeroid species. These characters include color pattern, shape of the fins and body (Didier 1998), and lateral line morphology (Didier & Nakaya 1999). The type of branching of the oral and preopercular lateral line canals from the infraorbital canal has been found to be particularly useful in the taxonomy of chimaeras (Didier & Nakaya 1999; Didier & Séret 2002).