Xanthichthys greenei, a new species of triggerfish (Balistidae) from the Line Islands

Abstract Xanthichthys greenei sp. n. is described from six specimens, 97-154 mm standard length (SL) collected from mesophotic coral ecosystems (90-100 m) at Kiritimati (Christmas Island), Line Islands, part of the Republic of Kiribati in the Central Pacific. Of the six species of Xanthichthys, it is most similar to the Atlantic Xanthichthys ringens and the Indo-West Pacific Xanthichthys lineopunctatus, sharing with these species the character of three pigmented cheek grooves. It is distinctive in its low body scale row count (33-35, other Xanthichthys species with 39 or more), small size (maximum SL 154 mm, other species over 225 mm), and color pattern of scattered dark spots sub-dorsally and no other spots or lines on body.


Introduction
In their revision of the triggerfish genus Xanthichthys Kaup in Richardson (1856), Randall et al. (1978) recognized five distinct species within this genus: the Atlantic X. ringens (Linnaeus 1758); the Indo-Pacific X. lineopunctatus (Hollard 1854), X. caeruleolineatus Depth of the caudal peduncle is the least depth. Eye diameter is the maximum fleshy diameter. Interorbital width is the least bony width. Interdorsal space is the distance from the posterior edge of the first dorsal spine at its base to the origin of the first ray of the second dorsal fin (excluding the basal scaly sheath). Measurement of the longest ray of the second dorsal and anal fins exclude the basal scaly sheath. Bases of the second dorsal and anal fins were measured from the anterior margin of the first ray at the upper end of the basal scaly sheath to the posterior edge of the last ray. Caudal concavity is the horizontal distance between the distal tips of the longest and shortest caudal rays.
Tissue samples intended for DNA sequencing were removed from collected specimens prior to formalin fixation in the field, but these tissue samples were subsequently lost in transit. Description of this species was postponed for eight years in anticipation of obtaining additional specimens from Kiritimati or other nearby localities, but following exploration of similar habitats in French Polynesia, the Cook Islands, and Johnston Atoll without observing this species, we have elected to describe this new species without molecular analysis.
The holotype has been deposited at the Bernice Pauahi Bishop Museum fish collection, Honolulu (BPBM); and paratypes have been deposited at BPBM, the California Academy of Sciences fish collection, San Francisco (CAS), and the U.S. National Museum of Natural History, Washington, D.C. (USNM).

Description
Proportional measurements expressed as a percentage of standard length (SL). Data in parentheses apply to paratypes, when different from the holotype (see also Dorsal and ventral profiles of head strongly convex; three longitudinal slightly diagonal darkly pigmented grooves on head following border of scale rows, running from behind and below corner of mouth, converging slightly as they pass posteriorly, and nearly reaching gill opening and upper pectoral base; a groove running anteriorly and diagonally downward from front of eye, extending approximately half the distance to tip of snout; deep medial groove behind first dorsal fin extending approximately half distance from last spine to origin second dorsal; pelvic flap not developed, pelvic terminus barely movable, small, slightly over one half orbit diameter.
Color in life as in Figs 1, 2; pearly gray, some paratypes with light brownish cast, slightly whiter ventrally on body, faint light blueish cast to head; irregular honey-brown spots sub-dorsally on body in several irregular rows commencing anteriorly at the origin of the first dorsal spine, becoming an irregular row prior to the origin of the second dorsal and ending posteriorly with a few scattered spots on the caudal peduncle; honey-brown diffuse bar below the anterior sub-dorsal spots and immediately posterior to gill opening, running from mid-orbit to slightly below mid-anterior pectoral base, opercular membrane darkly pigmented; three cheek grooves pigmented pale blue, dark on some paratypes, a weak groove above the three pigmented grooves faintly pigmented on some paratypes, groove extending anteriorly and diagonally downward from eye with faint blue pigment on some specimens; dark brown line below lower lip, thickening and extending diffusely beyond corner of mouth; first dorsal fin light tan brown becoming dark distally, first dorsal fin membrane basally tan, dark brown distally, second dorsal and anal fins with light brown rays, light blueish membranes; caudal fin with reddish upper and lower margins, a broad reddish crescent posteriorly in fin not extending to upper and lower margins, a light blueish area forming a diffuse crescent in the center of the fin; pectoral fin not pigmented. When viewed alive underwater the overall color appears light blue-gray.
Color in alcohol tan, paler ventrally; pale brown spots dorsally from origin of dorsal fin to caudal peduncle, more spots anteriorly; grooves in cheek brown; opercular membrane brown, faint brown bar posterior to gill opening from mid-orbit to mid pectoral base; brown line below lower lip extending diffusely beyond corner of mouth; anterior nostril in a small white spot; first dorsal spine light brown, darker distally on a b c d  some specimens, membrane of first dorsal fin brown, darker posteriorly; second dorsal and anal fins pale with light brown rays; caudal fin pale yellow; pectoral fins translucent.

Diagnosis
Dorsal rays III, 29; anal rays 25; pectoral rays 13-14 (usually 13); body scale rows 33-35; head scale rows 17-18; gill rakers 36; body depth 2-2.22 in SL; color in life pearly gray with honey-brown spots sub-dorsally on body in several irregular rows between the first dorsal spine and the caudal peduncle; opercular membrane darkly pigmented with three cheek grooves pigmented pale blue; first dorsal fin light tan brown becoming dark distally, first dorsal fin membrane basally tan, dark brown distally, second dorsal and anal fins with light brown rays, light blueish membranes; caudal fin with reddish upper and lower margins, a broad reddish crescent posteriorly in fin not extending to upper and lower margins.

Etymology
Named for Brian D. Greene, a member of the deep diving team that discovered species, in recognition of his efforts to collect the type specimens.

Distribution
Currently known only from Kiritimati (Christmas Island) in the Line Islands. Further exploration of mesophotic coral ecosystems in the tropical Pacific may extend the known range of this species, although it was not observed during exploratory dives within the mesophotic depth range in the Hawaiian Islands, Johnston Atoll, Caroline Islands, Cook Islands, French Polynesia, or elsewhere in the Pacific.

Ecology
Relatively abundant within its depth range and habitat (coral rubble and holes adjacent to deeper drop-offs, below a thermocline) at several localities on the west and north coasts of Kiritimati (Christmas Island). Always observed near the reef substratum, where it would retreat to shelter when approached.

Revised Key to the Species of Xanthichthys
Based on the key included in Randall et al. (1978).

1
Cheek with three prominent slightly diagonal dark brown or blue grooves (a shorter narrower groove above and below the main three sometimes dark on X. lineopunctatus), extending from just behind and below corner of mouth nearly to gill opening; body with dark longitudinal lines and/or spots

-
Cheek with five or six grooves (darkly pigmentd only on X. mento; poorly developed on X. auromarginatus); body without dark longitudinal lines and spots except males of X. mento with a small light blue spot in scale centers that may be brown in preservative (in addition, a concentration of dark pigment corners of the scales of some specimens of X. mento, giving a dark-spotted effect)  Table 2 compares Xanthichthys species using counts from Randall et al. (1978) and counts we have made for X. greenei. The six known Xanthichthys species can be separated into two groups according to the number of cheek grooves, 3 or 5-6. X. ringens, X. lineopunctatus and X. greenei all have three pigmented cheek grooves and share similarities in color pattern. Color of the caudal fin, soft dorsal and anal fin is nearly identical in these three species. However in X. greenei the body spots of X. ringens and the spots and lines of X. lineopunctatus are reduced to a few spots dorsally. Another difference is that the base of the second dorsal and anal fins of X. greenei is not darkly pigmented as it is on the other two species. X. greenei can also be separated from these two species and from all other Xanthichthys species by its low body scale row count, which has no overlap with any other Xanthichthys. Moreover, it appears to be a small species, and the size of the holotype at 154 mm SL, as large as any we observed underwater, is considerably smaller than the maximum size recorded for the next smallest species (X. auromarginatus at 227 mm). While X. greenei has only been recorded from Kiritimati, we would expect it to be present in suitable habitat throughout the northern Line Islands, possibly the southern Line Islands (but the junior author has noted faunal differences between the northern and southern Line Islands), and perhaps the Phoenix Islands. Although the habitat of X. greenei is below normal scuba diving depths and the species could be missed by divers, Balistidae can be readily caught by hook and line from deep water by local fishermen. Since this species has not been recorded from French Polynesia or other heavily fished areas in the Central Pacific (despite comparable observations at similar depths and habitats using mixed-gas rebreathers), we are inclined to doubt its presence there.