Eiconaxius acutifrons Bate 1888

Eiconaxius acutifrons Bate, 1888

(Figs 1–6)

Eiconaxius acutifrons Bate, 1888: 40, fig. 4, pl. 5, fig. 2d, g, h, i, k, l; Sakai & de Saint Laurent 1989: 16 (in part); Kensley 1996: 475 (list); Sakai, 2011: 270.

Axius (Eiconaxius) acutifrons: Borradaile 1903: 538 (list).

Iconaxius acutifrons. –– Balss 1925: 209 (list; in part).

Not Axius acutifrons. –– Faxon 1895: 103, pl. 28, fig. 2. = Eiconaxius albatrossae Kensley, 1996.

Not Axius (Eiconaxius) acutifrons. –– De Man 1925: 37, pl. 3, fig. 5–5e. = Eiconaxius indonesicus Sakai, 2011.

Not Axius acutifrons. –– Wicksten, 1982: 246, fig. 1; 1989: 313 (list). = Eiconaxius baja Kensley, 1996. Not Eiconaxius acutifrons. –– Hendrickx 1995: 157 (list).

Material examined. Lectotype (designated herein): male (cl 5.0 mm), Banda Sea, Indonesia, H. M. S. Challenger, stn 194A, 04°31.0’S, 129°57.20’E, 658 m, volcanic mud, 29 September 1874, NHM 1888.22.

Paralectotype: 1 ovigerous female (cl 6.9 mm), same data as lectotype.

Non-type: 1 male (cl 4.4 mm), Myojin Knoll, northern Izu Islands, 32°05.952’N, 139°52.050’E, 1074 m, RV Kairei, KR10-E02 cruise, ROV Kaiko 7000II, dive 486, 17 September 2010, JAMSTEC 083387; 2 males (cl 3.6, 3.9 mm), same data, JAMSTEC 83398, 83713.

Description. Lectotype (male). Rostrum (Fig. 1 A, B) narrowly triangular in dorsal view (about 1.9 times longer than basal width), apically narrowly rounded, directed forwards, reaching midlength of second segment of antennular peduncle; lateral margins each with 3 tiny obtuse teeth; apex upturned. Carapace (Fig. 1 A, B) with gastric region slightly convex, slightly sloping down to rostrum; cervical groove absent; median carina low, entire, not markedly broadened posteriorly, nor bifurcated; submedian carina absent; lateral carinae diverging posteriorly, reaching to about anterior one-third of carapace length.

Abdomen (Fig. 1 C) slightly depressed dorsoventrally. First abdominal somite with pleuron poorly defined ventrally. Second and third pleura produced posteroventrally into sharp tooth; fourth pleuron produced into subacute tooth ventrally, with tiny sharp tooth anteroventrally; fifth pleuron generally rounded, with tiny tooth on ventral margin. Sixth somite subequal to fifth somite, posterodorsal margin weakly denticulate; pleuron bluntly triangular; posterolateral process acuminate. Telson (Fig. 2 A) with greatest width at anterior 0.2; lateral margin bearing 6 (left) or 5 (right) tiny teeth, posteriormost tooth located at posterolateral angle; no movable spine at posterolateral angle; posterior margin nearly straight, bearing small median tooth.

Eye (Fig. 1 A, B) not reaching midlength of rostrum, subglobose; cornea not faceted. Antennular peduncle (Fig. 1 A, B) moderately stout; distal two segments subequal in length to first segment. Antennal peduncle (Fig. 1 A, B) moderately stout; distolateral prolongation of second segment acute, slightly overreaching distal margin of third segment of antennular peduncle; third segment with small tooth at ventromesial distal angle; antennal acicle large, acuminate, reaching beyond distal margin of fifth segment of antennal peduncle.

Third maxilliped similar to that of paralectotype.

Major (right) cheliped (Fig. 1 D, E) massive. Ischium with 3 tiny to minute spiniform denticles on ventral margin; dorsal margin unarmed. Merus strongly compressed laterally; dorsal margin arcuate, bearing 3 minute denticles in distal half (distalmost denticle terminal in position), otherwise nearly smooth; ventromesial margin sharply carinate, with 2 tiny subterminal denticles followed by 5 minute denticles; laterodistal projection rounded. Carpus cup-shaped, much higher than long, ventral angle with minute denticle. Chela slightly longer than carapace (including rostrum). Palm 1.1 times longer than high; dorsal margin sharply carinate, terminating distally in small acute tooth, further bearing 2 tiny denticles in distal 0.4; lateral surface convex, glabrous, with some scattered low tubercles in distal 0.3; ventrolateral carina sharp, extending nearly to tip of fixed finger, margin upturned; mesial surface also with some scattered low tubercles in anterior 0.3. Fixed finger nearly straight but distally slightly upturned; opposable margin with 1 prominent triangular tooth at midlength and row of several minute denticles in distal half, and with small, shallow hiatus proximally. Dactylus 0.9 times as long as palm, terminating in slightly curved calcareous claw, dorsal margin sharply carinate, unarmed; lateral surface slightly elevated along midline, shallowly sulcate along dorsal carina; opposable margin with 1 large blunt cusp proximally, otherwise unarmed, no prominent notch or excavation distal to cusp.

Minor (left) cheliped (Fig. 1 F) subequal in length to major cheliped. Ischium with 2 tiny denticles on dorsal and ventral margins respectively. Merus strongly compressed laterally; dorsal margin generally convex, sharply carinate, bearing 3 minute denticles distal to midlength; ventromesial margin sharply carinate, with 2 minute denticles and 1 small subterminal tooth; laterodistal projection subacute. Carpus cup shaped, much higher than long, ventral angle unarmed. Chela subequal in length to carapace (including rostrum); ventral margin gently sinuous. Palm slightly becoming higher distally, nearly as long as high; dorsal margin sharply carinate, bearing 1 minute denticle subdistally and 1 tiny spiniform tooth distally; lateral surface convex, smooth; ventrolateral carina sharp, extending nearly to tip of fixed finger; mesial surface smooth. Fingers forming deep excavation defined by sharp longitudinal carina on mesial side. Fixed finger almost straight but distally slightly upturned, with row of small, triangular or rounded teeth over entire length; dactylar articulating margin with 2-spined cusp laterally; lateral face flanked by blunt upper ridge along cutting edge and ventrolateral carina concave. Dactylus about 1.5 times as long as palm, terminating in slightly curved calcareous claw, dorsal margin sharply carinate; lateral surface with blunt longitudinal carina on midline; cutting edge unarmed.

Second to fourth pereopods of similar length, moderately stout. Second pereopod (Fig. 2 B) unarmed on ischium to carpus; chela about 1.4 times longer than carpus, with scattered tufts of setae; fixed finger deflexed, with row of minute corneous spinules on cutting edge; dactylus about 0.5 times as long as palm, setose, also with row of minute corneous spinules on cutting edge. Third pereopod (Fig. 2 C) unarmed on ischium to carpus; propodus about 1.5 times longer than carpus, with 9 sets of spinules on lateral surface ventrally (each set consisting of a single spinule or paired spinules) and single spinule at ventrodistal margin; dactylus strongly compressed laterally, suboval, terminating in clearly demarcated claw, with about 9 accessory spinules on flexor margin and 2 submarginal spinules proximally. Fourth pereopod (Fig. 2 D) similar to third pereopod; propodus with 5 sets of spinules (each set consisting of 2 or 3 transverse row of spinules); dactylus 10 accessory spinules on flexor margin. Fifth pereopods shorter than preceding ones; propodus with grooming apparatus consisting of oblique row of stiff setae and slender spinules on distal part; dactylus with 10 accessory spinules on flexor margin.

First pleopod absent. Second pleopods not preserved. Third to fifth pleopods slender, biramous, each with appendix interna. Uropodal exopod (Fig. 2 E) with lateral margin serrated with 15 minute to tiny acute teeth on lateral to posterior margin (posteriormost tooth strongest), mesial margin regularly convex; endopod with lateral margin serrated with 8 tiny acute teeth (posteriormost tooth strongest).

Paralectotype (female). Generally similar to lectotype except for sexual characters. Rostrum (Fig. 3 A) relatively broader than in lectotype (about 1.4 times longer than wide); lateral margins only faintly denticulate. Abdomen (Fig. 3 C) proportionally broader than in males. Telson with 4 tiny teeth on lateral margin. Third maxilliped (Fig. 3 D) moderately slender for genus; basis with 1 tiny spine on ventral margin distally; ischium bearing row of tiny denticles on mesial crest (crista dentate); propodus subequal in length to carpus; dactylus distinctly shorter than propodus; exopod flagellar, reaching midlength of merus; epipod curved, marginally minutely denticulate; podobranch about three-fourths length of epipod, simple. Major cheliped (Fig. 3 E) with merus bearing 1 minute denticle distally on dorsal margin, ventral margin serrated with 11 minute denticles; palm with 5 minute denticles on distal half of dorsal margin, lateral surface smooth; cusps on opposable margins of fingers much weaker than those in lectotype. Minor cheliped (Fig. 3 F) with ischium bearing 2 small denticles; merus with 3 tiny denticles on ventral margin; fixed finger with less acute and less conspicuous teeth on opposable margin. Uropodal exopod with lateral margin serrated with about 20 minute teeth; endopod with lateral margin serrated with 12 tiny acute teeth.

Myojin Knoll material. Generally similar to lectotype (Figs 4, 5). Rostrum with 3–4 low teeth on lateral margin (these teeth sharply pointed in 2 small specimens) (Fig. 4 C, D). Sixth abdominal somite with 4 denticles on posterodorsal margin (Fig. 4 G). Telson with 5–7 small teeth on lateral margin (Fig. 4 G). Third maxilliped as figured; ischium with crista dentata consisting of row of minute, sharply pointed denticles (Fig. 5 B). Major cheliped (Fig. 5 C) with ischium bearing 0–1 denticle on dorsal margin and 0–2 denticles on ventral margin; merus with 1 or 2 minute denticles on dorsal margin, ventral margin with 0–2 minute denticles; palm with 3–5 denticles on dorsal margin (in small specimen, these denticles acute, conspicuous). Minor cheliped (Fig. 5 D) with merus bearing 2 dorsal and 3 ventral denticles; palm with 1 minute denticle on dorsal margin; fixed finger with row of acute to subacute teeth on opposable margin. Male second pleopod with appendix masculina slightly shorter than appendix interna, bearing several stiff setae (Fig. 4 F). Uropodal exopod with lateral margin serrated with 11–14 tiny acute teeth; endopod with lateral margin serrated with 6–8 teeth.

Variation. As is apparent from the above description, the armature of the ventral margin of the merus of the major cheliped is variable, although this variation seems to be size-related. The numbers of denticles are as follows: 11 in the largest, female paralectotype (cl 6.9 mm), seven in the male lectotype (cl 5.0 mm), two in the non-type male from the Myojin Knoll (JAMSTEC 083387; cl 4.4 mm), and one in the male from the Myojin Knoll (JAMSTEC 83398; cl 3.9 mm). The third specimen from the Myojin Knoll (JAMSTEC 83713; cl 3.6 mm) has both chelipeds removed for DNA extraction by the second author, and thus the count of the denticles could not be recorded. The number of serrations on the lateral margin of the uropodal exopod shows similar pattern of sizerelated variation (six to up to 20).

Furthermore, the palm of the major cheliped is provided with low, tiny tubercles on the lateral and mesial surfaces in the male lectotype, whereas the surfaces of the palm are nearly smooth in the other specimens, including the female paralectotype. The rostrum is relatively broader in the female paralectotype than in the male specimens; and teeth on the opposable margin of the fixed finger of the minor cheliped are more acute in the male specimens than in the female paralectotype. It is still unclear whether these differences are attributable to sexual dimorphism.

Coloration in life. Body and appendages entirely whitish translucent. Eyestalk orange at base of cornea; cornea opaque.

Distribution and habitat. Banda Sea, Indonesia, 658 m (Bate 1888), and Myojin Knoll, Izu Islands, Japan, 1074 m (this study). The newly collected specimens represent the second record of this species, as well as a new record for Japanese waters.

Ecology. The newly collected specimens were found to be associated with farreid sponge (Hexactinellida) collected at the inside slope of the Myojin Knoll Caldera located about 400 m from hydrothermal vent fields (Fig. 6 A). Two colonies of the sponge were collected, of which one and two shrimps (both males) were found inside the internal cavity respectively.

Remarks. Species of Eiconaxius Bate, 1888 are known to be associated with deep-water hexactinellid sponges (e.g., Ortmann 1891; Faxon 1893, 1896; Bouvier 1925; de Man 1925; Kensley 1996; Komai 2011). Valid species and their geographical ranges are summarized in Table 1. The genus is widely distributed in the Indo-Pacific, Gulf of Mexico and the Caribbean Sea, but the known range of each species is restricted to a rather narrow area. Species of the genus are morphologically similar (Kensley 1996). Differences between species are generally minor, and assessment of variation is sometimes difficult because of the paucity of material. In fact, most species are still represented only by type material and few by subsequently reported specimens (see Sakai 2011). Kensley (1996) summarized characters of possible diagnostic significance.

The newly collected specimens from the Myojin Knoll, Japan, agree well with the type material except for the size-related variation in the number of ventral serrations on the merus of the major cheliped, as noted above.

The reexamination of the type material has revealed that Bate’s (1888) illustration is not altogether accurate. In particular, the rostrum of the male specimen (here designated as the lectotype) was figured as more elongate than in reality and with spinose lateral margins. Diagnostic characters of E. acutifrons, reassessed here, include: rostrum relatively narrow (1.4–1.9 times longer than wide), lanceolate; lateral margin of rostrum slightly to distinctly dentate; rostral median carina smooth, not bifurcate or hardly broadened posteriorly on gastric region of carapace; second and third abdominal pleura distinctly produced into an acute tooth, fourth pleuron also subacutely pointed ventrally; dactylus of major chela with cusp at midlength and fixed finger with opposing proximal cusp; and palm of major cheliped with minute denticles on dorsal margin, whereas that of minor cheliped unarmed or with one minute denticle. These characters are also shared by E. consobrinus and E. kermadeci compared below.

Eiconaxius consobrinus can be distinguished from E. acutifrons by the different structure of the chelipeds (de Man 1925). In E. consobrinus, the opposable margin of the fixed finger of the major cheliped has a deep notch just proximal to the prominent cusp arising at the midlength; the dactylar articulating margin of the palm of the minor cheliped is unarmed; and the dactylus of the minor cheliped is 1.1 times as long as the palm. On the other hand, in E. acutifrons, there is no prominent notch just proximal to the middle cusp on the opposable margin of the major cheliped; the dactylar articulating margin of the palm of the minor cheliped bears a bi-spined cusp on the lateral side; and the dactylus of the minor cheliped is about 1.5 times as long as the palm.

Eiconaxius kermadeci can be differentiated from E. acutifrons by the more distinctly delimited lateral rostral carinae, and the different configuration of the major chela. In E. kermadeci, there is a prominent hiatus between the fingers of the major chela, and the proximal part of the cutting edge of the fixed finger is armed with several strong teeth (Bate 1888). As shown above, in E. acutifrons, there is no such a hiatus between fingers of the major chela and no strong proximal teeth on the cutting edge of the fixed finger.

Four species of Eiconaxius have been recorded the northwestern Pacific: E. farreae, E. kensleyi, E. mortenseni, and E. rubrirostris (Ortmann 1891, Sakai 1992, Komai et al. 2010, Komai 2011). Of these, E. farreae is similar to E. acutifrons in the median carina on the carapace hardly bifurcate posteriorly and the relatively narrow, lanceolate rostrum. Nevertheless, E. farreae differs from E. acutifrons in the less produced and less acuminate posteroventral angles of the second and third abdominal pleura and the usually unarmed dorsal margin of the palm of the major cheliped. The lateral margins of the rostrum are more strongly upturned in E. farreae than in E. acutifrons.

Sakai (2011: 271) incorrectly cited the character of the male second pleopod in his diagnosis of E. acutifrons as “Male Plp 2 biramous, endopod with appendix interna, but without appendix masculina. [From Bate, 1888, pl. 5, fig. 2q].” According to Bate (1888: captions to the plate 5), the illustrated pleopod was the third pleopod, normally lacking an appendix masculina. In the descriptive text of the pleopods in the account of E. acutifrons, Bate (1888: 42) stated that “In the male these branches are smaller and less important, and the inner, instead of carrying one, supports two stylamblides.” This statement clearly indicates that the male second pleopod bore two appendices, i.e., appendices interna and masculina. No second pleopods of the male lectotype are now preserved, but the three additional male specimens all carry appendix masculina on the second pleopod.

Eiconaxius indonesicus was established by Sakai (2011) for specimens previously referred to E. acutifrons from Indonesia (De Man 1925), although he did not actually examine the types of E. acutifrons or De Man’s (1925) specimens. Sakai (2011) cited the shape of the rostrum and armature of the major chela in differentiating E. indonesicus from E. acutifrons. Comparison between the present material of E. acutifrons and the extensive description of de Man (1925) reveals the following morphological differences: the rostrum is broader in de Man’s (1925) specimens than in the types and additional specimens of E. acutifrons; the rostral apex forms a distinct point in de Man’s (1925) specimens, whereas there is no distinct point in the types and additional specimens of E. acutifrons; and the concavity distal to the proximal cusp on the opposable margin of the dactylus of the major cheliped is more prominent in de Man’s (1925) specimen than in the type specimens and additional specimens of E. acutifrons. On the other hand, no significant difference in the spination of the dorsal margin of the major cheliped is seen between the type specimens of E. acutifrons and the specimens of de Man (1925). Because of the abovementioned differences, we suppose at present that the taxon E. indonesicus might be a valid species.

Kensley (1996) described a new species, E. albatrossae, on the basis of specimens from Pacific Panama, referred by Faxon (1895) to E. acutifrons. The type series consists of the following specimens: a single male from the Albatross station 3359 is the holotype; two males and four ovigerous females from the Albatross station 3358, and three males and two ovigerous females from Albatross station 3359 are paratypes (Kensley 1996: 475). Sakai (2011) proposed a new taxon E. faxoni for the same series of specimens, though he did not explicitly fix holotype or syntype (s). Consequently, Sakai’s taxon name is unavailable according to Article 16.4 and 72.3 of the ICZN (1999). Eiconaxius albatrossae is also similar to E. acutifrons, but the two species can be differentiated by the following characters (Kensley 1996): the rostrum is relatively broader in E. albatrossae than in E. acutifrons; the median gastric carina is distinctly bifurcated posteriorly in E. albatrossae, but not bifurcated in E. acutifrons; the pleura of the second to fourth abdominal pleura are more strongly produced in E. albatrossae than in E. acutifrons; the lateral margin of the telson bears more numerous teeth in E. albatrossae than in E. acutifrons (11 versus five to seven); and denticles on the dorsal margin of the palm of the major cheliped are more conspicuous in E. albatrossae than in E. acutifrons.

During this study, we have also reexamined two specimens from California (deposited in the LACM, one male and one female, 32°25’N, 119·60’W to 32°050’N, 119°65’E, 1200 m, 6 September 1978, coll. P. Gregory), identified as E. acutifrons by Wicksten (1982), and found that they actually represent E. baja Kensley, 1996. Eiconaxius baja is immediately recognized by the strongly upturned rostrum and the presence of strong setose tubercles on the lateral surface of the palms of chelipeds (Kensley 1996).