Pteropagurus inermis, Mclaughlin, Patsy A. & Rahayu, Dwi Listyo, 2006

Mclaughlin, Patsy A. & Rahayu, Dwi Listyo, 2006, A new genus with two new species of hermit crabs (Crustacea, Decapoda, Paguroidea, Paguridae) from an unique habitat, Zootaxa 1116, pp. 55-68 : 57-63

Pteropagurus inermis n. sp. ( Figs. 1–4 )

Type material. New Caledonia. Holotype ovigerous female (sl = 1.6 mm), MUSORSTOM 5, stn 301, 22°06.9'S, 159°24.6'E, 487–610 m, 12.10.1986 ( MNHN Pg 7633).

Paratypes. 2 males (sl = 1.2, 1.3 mm, both missing left chelipeds and one also missing right), 1 ovigerous female (sl = 1.2 mm, missing both chelipeds), New Caledonia, MUSORSTOM 5, stn 301, 22°06.9'S, 159°24.6'E, 487–610 m, 12.10.1986 ( MNHN Pg 7634); 1 male with rhizocephalan (sl = 1.6 mm, missing chelipeds), stn 330, 20°19.8'S, 156°48.4'E, 360–365 m, 15.10.1986 ( MNHN Pg 7635); 1 male (sl = 1.2 mm, missing chelipeds), BIOCAL, stn DW 51, 23°05.3'S, 167°44.9'E, 700 m, 31.08.1985 ( MNHN Pg7636).

Description. Shield ( Figs. 2 a, 3a) slightly broader than long, to longer than broad, weakly calcified; anterior margin between weakly developed rostral lobe and lateral projections concave; anterolateral margins sloping; posterior margin truncate. Rostral lobe broadly rounded, obsolete or sometimes produced only slightly beyond level of obsolete, unarmed lateral projections. Carapace lateral lobes elongate, reaching distal 0.2–0.3 of shield. Posterior carapace with moderate to very broad median plate; cardiac sulci reaching to posterior margin. Branchiostegites membranous, unarmed.

Ocular peduncles very short and stout, approximately 0.7 length of shield; corneal diameter 0.8 to 1.2 total peduncular length (including cornea). Ocular acicles subtriangular, reaching to or beyond proximal margin of ultimate peduncular segment, each with small submarginal spine; separated basally by more than basal length of one acicle.

Antennular peduncles overreaching distal margins of corneas by more than length of ultimate peduncular segment; ultimate segment with 1 long, stiff seta on distal margin; penultimate segment with few scattered setae; basal segment with small spine on dorsolateral margin, ventral margin produced distally as spiniform lobe.

Antennal peduncles overreaching distal corneal margins by approximately 0.5 length of ultimate segment. Fifth and fourth segments with few scattered setae; third segment with small spine at ventrodistal angle; second segment with dorsolateral distal angle produced, terminating in slender spine, dorsomesial angle unarmed or with small spine; first segment with hook­like spine at dorsolateral distal margin, ventrolateral margin unarmed. Antennal acicle reaching distal margin of fourth peduncular segment to midlength of fifth, slender, terminating in simple spine. Antennal flagellum with 1 or 2 minute setae every several articles (flagella present only on largest male).

Chelipeds ( Figs. 4 a–d) subequal in length, but right appreciably stouter; dactyl and fixed finger of right with small hiatus proximally. Dactyl of right cheliped ( Figs. 4 a, b) approximately 0.6 length of palm; dorsomesial margin rounded, dorsal surface weakly convex, all surfaces unarmed, but with few scattered, short setae mesially and ventrally; cutting edge with 3 low, broad, calcareous teeth, terminating in tiny corneous claw, slightly overlapped by fixed finger. Palm slightly longer than carpus, surfaces all smooth, rounded, entirely unarmed, but with few scattered short setae on mesial and ventral surfaces; fixed finger similarly unarmed, but with few scattered short setae; cutting edge with two broad, low calcareous teeth, terminating in tiny corneous claw. Carpus slightly longer than merus; dorsomesial and dorsolateral margins not distinctly delimited; all surfaces unarmed but with few scattered short setae. Merus laterally compressed; all surfaces unarmed; ventromesial and ventrolateral margins not distinctly delimited. Ischium unarmed.

Left cheliped ( Figs. 4 c, d) somewhat shorter than right, much slenderer; Dactyl slightly longer than palm, no hiatus between fixed finger and dactyl; surfaces rounded and unarmed, but with sparsely scattered, moderately long setae ventrally; cutting edge with row of minute corneous teeth, terminating in tiny corneous claw; palm with convex dorsal surface unarmed but with few scattered setae; dorsomesial and dorsolateral margins not delimited; fixed finger similarly rounded and unarmed but with several moderately long setae laterally; cutting edge with row of minute corneous teeth, terminating in tiny corneous claw. Carpus longer than palm but approximately equal to length of merus; dorsomesial margin only faintly delimited by sparse tufts of setae, dorsolateral margin not delimited; surfaces all unarmed but few scattered setae dorsodistally and ventrally. Merus with surfaces unarmed but with few scattered short setae; ventromesial and ventrolateral margins not delimited. Ischium unarmed.

Second and third pereopods ( Figs 4 e–g) similar in armature but unequal in length, second appreciably shorter. Dactyls straight in dorsal view, slightly curved in lateral view; slightly shorter (second) to 1.3 longer than propodi (third); dorsal surfaces each with few scattered setae; ventral surfaces with 5–8 small corneous spines (often imperceptible in unstained specimens) and few scattered setae. Propodi 1.3 (third) to 2.0 (second) length of carpi; surfaces all unarmed but with occasional seta dorsally. Carpi with dorsal margins usually unarmed, occasionally third microscopically serrate, other surfaces unarmed but with occasional seta. Meri unarmed, but each with 1 or 2 setae dorsally and ventrally. Ischia unarmed. Fourth pereopods ( Fig. 2 c) semichelate; propodal rasp with single row of corneous scales.

Sternite of third pereopods in males very broad anteriorly and extending posteriorly as moderately broad, terminally blunt plate ( Fig. 3 b), narrower and only slightly produced posteriorly in females ( Fig. 2 b); anterior lobe subrectangular in both sexes. Sternites of fourth and fifth pereopods very widely separated

Males with coxae of fifth pereopods approximately equal, right with stout, long sexual tube appearing as posterior coxal extension ( Figs 3 a, c) directed to the exterior and curving over anterior dorsal portion of pleon; coxa of left with very short tube projecting from elevated gonopore. Female with single left gonopore; without paired first pleopods, noneyed eggs approximately 0.6 mm diameter, numbering 25–30.

Uropods with 1 (larger male) or 2 (female holotype) rows of scales. Telson ( Figs. 2 d, 3d) with median concavity or tiny median cleft; terminal margin unarmed or with few minutes bristles.

Etymology. The specific name is from the Latin inermis meaning unarmed, reflecting the unarmed nature of the chelipeds and appendages of this species.

Color. Unknown.

Habitat. Pteropod shells.

Distribution. Known only from the environs of New Caledonia.

Var ia t io n. Pteropagurus inermis n. sp. exhibits considerable intraspecific variability, some, but not all of which can be attributed to sexual dimorphism. The first example of variability can be seen in the corneal diameters of the holotype and the paratypes, both male and female. The corneal diameter of the holotype is only 0.8 of the peduncular length, whereas those of the paratypes range from 1.0–1.2 of that length. These differences can only be attributed to variability in corneal dilation, as animal size did not appear to be a factor. A variation that might or might not be indicative of sexual dimorphism is the difference between the holotype female and allotype male (the remaining two paratypes from the same locality were not extracted from their shells). In the former, the undelineated posterior lobes of the telson are separated by a very slender median cleft; the terminal margins are rounded and unarmed. The telson of the male allotype similarly has the lateral margins lacking transverse sutures, but the posterior lobes are separated by a moderately broad concavity and the terminal margins are armed with a few spiniform bristles on each lobe. One attribute that is clearly sexually dimorphic is the development of the sternite of the third pereopods (thoracic sternite XII). In the females of P. i n e r m i s, and P. s p i n a as well, the posterior lobe of the sternite of the third pereopods is moderately well developed, but not noticeably, or only slightly, produced posteriorly ( Fig. 2 b). In contrast, the male third sternite is markedly developed with a pronounced subtriangular, posteriorly directed lobe ( Fig. 3 b) that is nearly contiguous with the sternite of the fourth pereopods.

Figures

FIGURE 1. Pteropagurus inermis n. sp., paratype male (sl = 1.2 mm) (MNHN Pg 7634) in original pteropod shell. a, dorsal view; b, dorsolateral view; c, ventral view. Scale equals 1 mm.

FIGURE 2. Pteropagurus inermis n. sp., a – d, holotype ovig. female (sl = 1.6 mm) (MNHN Pg 7633), e – h, paratype male (sl = 1.3 mm) (MNHN Pg 7634). a, shield and cephalic appendages (aesthetascs omitted); b, sternites of second and third pereopods (thoracic sternites XI & XII) (ventral view); c, left fourth pereopod (lateral view); d, telson; e, right maxillule; f, right maxilla; g, right first maxilliped; h, right third maxilliped. Scales equal 0.5 mm.

FIGURE 3. Pteropagurus inermis n. sp., paratype male (sl = 1.3 mm) (MNHN Pg 7634). a, shield and cephalic appendages (aesthetascs omitted); b, sternites of second and third pereopods (thoracic sternites XI & XII) (ventral view); c, coxae and sternite of fifth pereopods (ventral view); d, telson. Scales equal 0.5 mm.

FIGURE 4. Pteropagurus inermis n. sp., holotype ovig. female (sl = 1.6 mm) (MNHN Pg 7633). a, carpus and chela of right cheliped (dorsal view); b, carpus and chela of right cheliped (lateral view); c, carpus and chela of left cheliped (lateral view); d, chela of left cheliped (dorsal view); e, left second pereopod (lateral view, setae omitted); f, right second pereopod (mesial view); g, left third pereopod (lateral view, setae omitted). Scale equals 1 mm.