Comprehensive DescriptionRead full entry
Hydrolagus alphussp. nov.
Figures 2, 3; Tables 1, 2
Holotype: CAS 201902, adult male, 419 mm TL, 249 mm BDL; Galapagos Islands , Ecuador , North end of Seymour Island ( 0°21'42"S , 90°15'0"W ), 648 m, 25 July 1998, J. E. McCosker (CAS) and Carole Baldwin (USNM) by vacuum hose aboard the Johnson Sea Link II (JSL dive #3113).
Paratype: CAS 86425, sub-adult female, 480 mm TL, 244 mm BDL, Galapagos Islands , Ecuador , Fernandina Island ( 0°14.641'S91°26.535'W - 0°14.820'S91°26.410'W ), 731.52 m, 17 Nov. 1995, J. E. McCosker (CAS), R. Grant Gilmore (HBOI) and Bruce Robison (MBARI) by vacuum hose aboard the Johnson Sea Link II (JSL dive #3958).
Additional specimens observed but not collected: 26 November 1995, Galapagos Islands , Ecuador , Isla Santiago, James Bay, one specimen seen and photographed at a depth of 907 m (JSL dive #3977) ; 23 June 1998, Galapagos Islands , Ecuador , Isla Fernandina, Cabo Douglas, one specimen observed at a depth of 630 m (JSL dive #2900) and sketched by J.E. McCosker .
Diagnosis. Assigned to the genus Hydrolagus ZBK based upon the absence of an anal fin. Hydrolagus alphus is distinguished by being medium in size (average PCL 321 mm) and uniform dark brown in color with a distinct white spot (4%-6% BDL) on the lateral side above the pectoral fins. Paired fins with bluish hue and white margins. The dorsal spine longer than triangular shaped first dorsal fin and extends beyond the origin of the second dorsal fin when depressed. Eyes large (40.8-44.5% HDL) and pectoral fins reaching to or beyond insertion of pelvic fins when depressed. Pelvic claspers small, not extending beyond distal edge of pelvic fin, and divided distally for one half their length with slender fleshy denticulate tips. The base of second dorsal fin is long, deeply depressed and light colored in the center, anterior and posterior regions dark, considerably greater in height than the white middle region.
Description. Measurements and body proportions of the holotype and paratype are presented in Table 1. Small to medium sized species with stout trunk, uniform in height until insertion of pelvic fins, then quickly tapering posteriorly and extending to a thin caudal filament. Eyes large (40.8-44.5% HDL) and slightly oval in shape. Snout blunt, tip protruding slightly, squared off towards the mouth.
First dorsal fin triangular and attached to the dorsal spine at or below midpoint. Both spine and fin depress into a groove situated on dorsal ridge. Dorsal spine longer than the dorsal fin, serrated along the anterior side from just below the distal tip to above the base. Posterior side of dorsal spine has two rows of long serrations extending from the tip to the midpoint. When the dorsal spine is depressed, the tip reaches beyond the origin of the second dorsal fin. Base of the second dorsal fin is long, extending from mid-body to caudal fin, anterior and posterior regions are dark colored with a small band of white coloration on the distal margins and distinctly lobed, with the middle region white, indented for 1/4 the length of second dorsal fin. Anterior portion is greater in height (7.2% BDL) then the posterior portion (4.4 % BDL) with the middle portion less than 2% BDL in height. Second dorsal fin is separated from the caudal fin by a membrane.
Caudal fin consists of a rounded dorsal and ventral lobe, with the ventral lobe being slightly greater in height. The ventral portion of the caudal fin base is longer than the dorsal caudal fin base and has a fleshy membrane extending onto the posterior body region. Caudal fins are white distally around the margins. Anal fin absent. Pectoral fins are large (39.8% BDL) and triangular reaching just below the first dorsal fin when depressed dorsally and to the insertion of the pelvic fins when depressed horizontally. Posterior margin of pectoral fins are convex near base, progressing concavely to distal tip, anterior margin slightly convex. Pectorals moderately dark in color, preserved specimens have a purplish-blue hue on fins with a white border around the margins. Pelvic fins are large, oblong with convex margins. Coloration of pelvic fins in preservative is the same as pectoral fins.
Oral and preopercular head canals extend ventrally sharing a common branch from the infraorbital canal. Trunk lateral line canal extends the length of the body until it reaches the caudal fin where it extends down ventrally onto the tail filament. Secondary sexual characteristics in males include a long frontal tenaculum (32.5 % HDL) moderately curved, ending in a bulbous tip with tenacular hooks starting below the dorsal surface and aligned vertically in rows with 6-8 denticles in each, getting larger ventrally, pre-pelvic tenacula with at least 4 hooks on the outer margin and pelvic claspers that are bifid, slender with wrinkled fleshy pads at tips and divided distally for one half of their length. Females with anal pads, not present in males.
Color. Uniform chocolate brown when live, with superficial bluish sheen depending on angle of light. Areas around the opercular, rostral, predorsal, and abdominal regions may be slightly darker. Medial, pectoral, pelvic, and caudal fins are dark slate or blackish grey with a slight bluish hue, and all the fins have a narrow white band on the terminal edge. The low median area between the anterior and posterior sections of the second dorsal is entirely white. The first dorsal, pectoral and pelvic fins show a narrow blackish band on the posterior edge of the fins adjacent to the white terminal band. The dorsal axial area of the pectoral region is usually whitish, and specimens have one well-defined white spot on their lateral sides above the pectoral fin, sometimes one or more white spots not as welldefined may occur in the abdominal or anterior caudal regions also. The pigment around the outer margins of the eye is a blackish brown, and the tapetum lucidum reflects a celadon green. Coloration of the main body in preserved specimens is a uniform brown to dark brown, with white margins on the tips of the fins and a distinct white spot (4%-6% BDL) on the lateral side, mid-body above the pectoral fin. Second dorsal fin is dark anteriorly and posteriorly, center region distinctly white in color.
Habitat. The adjacent substrate at sites of capture varied between slopes and ledges containing large volcanic boulders, cobbles, and gravels, frequently overlain by, or interspersed with, patches of sand and coarse silt. The rocky areas were completely devoid of algae, but often had sparse to dense encrustations of benthic invertebrates such as stony corals, sponges, crinoids, hydroids, gorgonians, and bryozoans. Holothurans and ophiuroids were also present. All four specimens of Hydrolagus alphus were caught or observed within three meters of the substrate.
Etymology. The specific name, alphus (Latin, adj.), means white spot on the skin and is in reference to a key characteristic found in this species.
Common name. We propose the English common name of “whitespot ghostshark” in keeping with the Latin translation and key characteristic.
Interspecific Comparisons. There is at least one additional species that occurs within the Galapagos Islands region in addition to Hydrolagus alphus . A new Hydrolagus ZBK species, Hydrolagus mccoskeri , was discovered during the same surveys of the Galapagos Islands as in this study (Barnett et al., 2006). Hydrolagus mccoskeri is distinct from H. alphus in coloration and fin sizes. Most notably, H. mccoskeri contains numerous white markings on the body different than the single white spot found on H. alphus . The second dorsal fin is uniform in height in H. mccoskeri with a marginal rise in the anterior portion, whereas in H. alphus the anterior and posterior regions of the second dorsal fin are considerably taller than the middle region, with the middle region white in H. alphus . The pectoral and pelvic fins are smaller in H. mccoskeri . Distribution of the two species also differs. Hydrolagus alphus occurs in deep pre-abyssal waters (about 600-800 m), while H. mccoskeri occurs in more shallow waters, at depths of 400 m (Barnett et al., 2006).
Hydrolagus alphus is distinct from Hydrolagus macrophthalmus ZBK , most notably in coloration and the appearance of a white spot on the lateral side. In addition, the size of the tail region from insertion of pelvic fins to origin of dorsal caudal fin, the structure (shape and size) of the second dorsal fin, eye length, snout morphology, lengths of lateral line canals in the head and secondary sexual characteristics also differ. Coloration of preserved Hydrolagus macrophthalmus ZBK is an even brown with no white markings or white coloration pattern. Hydrolagus alphus is also brown, but with a distinctive white spot found on the lateral side above the pectoral fin or abdominal region. The tail region from insertion of pelvic fins to origin of dorsal caudal fin in H. macrophthalmus ZBK is elongate and slender and always greater than 60% BDL. The tail region in H. alphus is short and stout and always less then 57% BDL. The second dorsal fins in H. alphus and H. macrophthalmus ZBK have anterior and posterior regions that are lobed and significantly taller than the middle region. However, the anterior region in H. alphus is 7.2-9.0% BDL as compared to 3.8-5.6% BDL in H. macrophthalmus ZBK , and posterior regions are taller in H. alphus , 4.4-5.7% BDL versus 2.2-3.8% BDL in H. macrophthalmus ZBK . The eyes are larger in H. alphus (greater then 40% HDL) than in H. macrophthalmus ZBK (less then 39.5% HDL). The snout of H. macrophthalmus ZBK is clearly more pointed at the tip, whereas in H. alphus it is blunt. Lateral line canals in the head, particularly IOA, OTM, and OCL of H. alphus are longer than in H. macrophthalmus ZBK (table 2). Lastly, secondary sexual characteristics differ between species in that the length of the frontal tenaculum is longer in H. alphus (32.5% HDL) than in H. macrophthalmus ZBK (16.5-20.3% HDL). The pre-pelvic tenacula in H. alphus contains four hooks on the outer margin whereas in H. macrophthalmus ZBK it contains three hooks. However, it is important to note that number of hooks on the pre-pelvic tenacula varies intraspecifically, so this feature alone is not sufficient to distinguish the two species.
The new species Hydrolagus alphussp. nov. may be restricted to the Galapagos as this is the only locality in which it is found. Endemism of fishes in the Galapagos Islands with low dispersal capabilities has been shown to be high (Grove and Lavenberg, 1997), and preliminary data from deepwater fishes (McCosker et al., 1997) also indicates a high degree of endemism. This suggests that Hydrolagus alphussp. nov. may likely be endemic to the Galapagos Archipelago as well, but only thorough deepwater surveys of the Pacific coasts of Central and South America will bear this out. Chimaeroids have features consistent with species that tend to be more geographically restricted. They are oviparous, depositing large egg cases in mud or attached to stones on the sea floor (Bigelow and Schroeder, 1953). This mode of reproduction in chimaeras is not affected by ocean currents which normally facilitate dispersal of eggs and larval fishes in the open ocean. In addition, chimaeroids have a morphology consistent with slow swimming. Their large wing-like pectoral fins and flapping motion do not promote speed or the ability to swim long distances, thus their potential for long-distance dispersal is limited.