Acmaeopleura Stimpson, 1858

Naruse, Tohru, 2015, Description of a new genus and a new species of gaeticine crab (Crustacea: Brachyura: Varunidae) from the Ryukyu Islands, and a review of Acmaeopleura Stimpson, 1858, and Sestrostoma Davie & N. K. Ng, 2007, Zootaxa 3925 (2), pp. 211-228 : 215-218

Acmaeopleura Stimpson, 1858

Acmaeopleura Stimpson, 1858: 105 ; 1907: 130; Balss 1922: 152; Tesch 1918: 106; Sakai 1939: 662; Sakai 1965: 195; Crosnier 1965: 42; Sakai 1976: 642; Dai & Yang 1991: 518.

Diagnosis. Carapace subovate, wider than long, dorsal surface convex longitudinally ( A. parvula ), region poorly defined. Orbital margins entire. Infraorbital margin ending mesially with triangular inner orbital tooth, wide gape between tooth and lateral angle of front. Eyes oval, almost filling orbit. Third maxilliped rectangular; ischium as long as merus; ischium, merus each as long as its width, border between ischium, merus horizontal; ischium with broad, shallow concavity on proximal margin medially; merus lateral margin straight to weakly auricular, with concave anterior margin, carpus attached to concave margin; propodus attached to distal end of carpus, dactylus developed from subdistal portion of outer surface of propodus; carpus, propodus trigonal-pyramid in cross-section, propodus thicker than carpus, dactylus flat, linguiform, distal end of dactylus reaching short of proximomesial corner of merus when folded; long setae on distoanterior margin of propodus, distal end of dactylus, distal end of long setae, when palp folded, reaching anterior thoracic sternum. Exopod with long flagellum. Male chelipeds equal; female chelipeds almost symmetrical, smaller than those of male. Ambulatory legs moderately stout, P3 longest; no marginal spine or tooth. Thoracic sternum wide; anterior sternal plate with wide, shallow depression on sternites 2, 3. Lateral end of sternite 2 widely, weakly produced anteriorly, projection fitting concavity of third maxilliped ischium when folded. Sternites 3, 4 indiscernible. Male sternoabdominal cavity moderately wide; cavity, telson reaching distal third level of cheliped bases. Penis sternal. Vulvae apart from each other, placed posterior to base of third maxilliped ischium, adjacent to suture 5/6 on thoracic sternite 6, very short, wide sternal cover developed from posterolateral corner; operculum present. Male abdomen moderately wide, third to sixth somites with sutures visible but functionally fused. G1 stout, straight, distally setose, with distal, short chitinous beak.

Species included. Acmaeopleura parvula Stimpson, 1858 [type species]; A. hichiro Karasawa & Tanaka, 2005 ; A. rotunda Rathbun, 1909 (?).

Remarks. Acmaeopleura parvula is the type species of Acmaeopleura Stimpson, 1858 (ICZN 1925, Opinion 85; ICZN 1956, Direction 37). The type specimen of A. parvula Stimpson, 1858 , collected from Oushima (=Amami-Ohshima Island, Ryukyu Islands) (male, 4.8 × 5.3 mm; Stimpson 1858; 1907), was almost certainly lost during the Great Chicago Fire in 1871 (see Evans 1967; Deiss & Manning 1981; Vasile et al. 2005). No detailed redescription of A. parvula had been made since the original description ( Stimpson 1858; 1907). The specimens examined herein agree to the original description, except for the male abdomen which was described as “penult joint [sixth abdominal somite] pentagonal”. This description of the male abdomen is most probably an error as brachyuran sixth abdominal somites are in general symmetrical, and its proximal and distal margins are horizontal to articulate with fifth somite and telson, respectively. To stabilize the genus itself and its subfamilial position, a male specimen from Kagoshima (RUMF-ZC-2939) is designated as a neotype of A. parvula .

Davie & N.K. Ng (2007) distinguished Acmaeopleura balsii Shen, 1932 , A. depressum Sakai, 1965 , and A. toriumii Takeda, 1974 , from A. parvula by the characters of the third maxilliped, which are modified for suspension feeding, and the presence of a medial sulcus on the anterior thoracic sternal plate. They established a new genus, Sestrostoma , for the three species, and referred it to their new subfamily Gaeticinae . They restricted Acmaeopleura Stimpson, 1858 , for A. parvula Stimpson, 1858 [type species] and A. rotunda Rathbun, 1909 (which also includes the fossil A. hichiro , see Karasawa & Tanaka 2005) and left the genus in Varuninae . A close examination of all Sestrostoma species has revealed that the characters emphasized by Davie & N.K. Ng (2007) are variable. The anterior sternal plate is widely and shallowly depressed on thoracic sternites 2 and 3 in S. balssi and A. parvula ( Figs. 1 b, d, 4c), widely and deeply depressed on the sternites 2 and 3 in S. toriumii , and widely and shallowly depressed only on the sternite 2 in S. depressum ( Fig. 1 c). The dactylus of the third maxilliped is slightly elongated and reaching the proximomesial corner of the merus ( S. depressum , Fig. 1 c; S. toriumii ) and the distomesial corner of the ischium ( S. balssi , Fig. 1 b), but that of A. parvula is not elongated and reaches the proximal quarter of the mesial margin of the merus ( Figs. 1 d, 5a). The long setae of the palp of the third maxilliped reach the thoracic sternite 2 ( S. depressum ) and the thoracic suture 2/3 ( S. balssi , S. toriumii and A. parvula ). These indicate that the characters distinguished the two genera are not reliable. Acmaeopleura and Sestrostoma also share unique characters that are probably related to suspension feeding. The third maxilliped dactylus is attached to a subdistal portion of the outer surface of the propodus, and long setae are present on the distoanterior margin of the propodus and the distal end of the third maxilliped dactylus ( Fig. 2 a, b, d).

The two genera are, however, still distinguishable from each other. Acmaeopleura differs from Sestrostoma by its relatively thick cephalothorax with high sidewalls ( Figs. 3 a, 4a) (relatively thin with low sidewalls in Sestrostoma, Shen 1932 : fig. 98; Sakai 1965: fig. 26, pl. 91(4); Takeda 1974: fig. 2), less produced lateral end of the thoracic sternite 2 ( Figs. 1 d, 3b, 4c) (distinctly produced in Sestrostoma , Fig. 1 b, c), less concave proximal margin of the third maxilliped ischium ( Figs. 1 d, 3b, 4c, 5a) (more concave in Sestrostoma , Fig. 1 b, c), and proportionally short and stout ambulatory legs ( Figs. 3 a, 4a) (proportionally slender in Sestrostoma, Shen 1932 : fig. 98; Sakai 1965: pl. 91(4); Takeda 1974: fig. 2). These clear differences certainly keep Acmaeopleura and Sestrostoma as separate genera. Due to close similarities in other important characters, e.g. the third maxilliped, the anterior portion of the cephalothorax, male abdomen and G1, the present study transferred Acmaeopleura to Gaeticinae .

Acmaeopleura rotunda Rathbun, 1909 , has been known only from the holotype (juvenile male, 2 mm long and 1.75 mm wide) from the south of Koh Kut, Thailand. The figure of the third maxilliped of A. rotunda ( Rathbun 1910: fig. 10) shows a roundly produced lateral margin of the third maxilliped merus and a distinctly concave posterior margin of the third maxilliped ischium, characters similar to those of Sestrostoma balssi , S. depressum and Proexotelson tokoroi n. sp. ( Figs. 1 b, c, 8, 9a). Rathbun (1910) also noted that the legs are more slender when compared with the drawing of A. parvula ( Stimpson 1858, pl. 11, fig. 4). In this regard, A. rotunda is closer to Sestrostoma and Proexotelson n. gen. than Acmaeopleura . The dactylus of the third maxilliped of A. rotunda , however, bears short setae only ( Rathbun 1910: fig. 10), not long setae as in Sestrostoma and Proexotelson n. gen. species, although this may be due to size-related variation or the quality of the drawing. Rathbun (1910) described that the surfaces of the carapace and chelipeds are granulated, which is not seen in Acmaeopleura , Sestrostoma and Proexotelson n. gen. It is possible that A. rotunda is not even a gaeticine species. In any case, it is impossible to assess its precise systematic position without examining the specimen. The present study tentatively retains A. rotunda within Acmaeopleura . Rathbun (1909; 1910) described the carapace of A. rotunda about as long as broad, subcircular, with measurements of 2 mm long and 1.75 mm wide. This unique anteroposteriorly long carapace of A. rotunda inidcates that A. rotunda is, at least, distinct from all gaeticine species.

Figures

FIGURE 1. Anterior thoracic sternum and third maxillipeds. a, Gaetice depressus (De Haan, 1835) (RUMF-ZC- 1814, male, 24.0 × 29.3 mm); b, Sestrostoma balssi (Shen, 1932) (CBM ZC 8146, male, 9.2 × 12.1 mm); c, Sestrostoma depressum (Sakai, 1965) (CBM ZC 7012, male, 5.0 × 6.1 mm); d, Acmaeopleura parvula Stimpson, 1858 (RUMF-ZC- 2939, male, 6.9 × 8.5 mm). 2 / 3, suture between thoracic sternites 2 and 3; G 1, male first gonopod; ms, median sulcus; S 2 l, lateral end of thoracic sternite 2; S 3, 4, thoracic sternite 3 and 4.

FIGURE 2. Palp of left third maxilliped in anterior view. a, Gaetice depressus (De Haan, 1835) (RUMF-ZC- 1814, male, 24.0 × 29.3 mm); b, Sestrostoma balssi (Shen, 1932) (CBM ZC 8146, male, 9.2 × 12.1 mm); c, Proexotelson tokoroi n. sp. (holotype, RUMF-ZC- 3669, male, 3.0 × 3.8 mm); d, Acmaeopleura parvula Stimpson, 1858 (neotype, RUMF-ZC- 2939, male, 6.9 × 8.5 mm); e, Gopkittisak angustum Komai, 2011 (RUMF-ZC- 3673, female, 5.5 × 10.0 mm). Ca, carpus; Da, dactylus; Pr, propodus; Pr-d, distal end of propodus.

FIGURE 3. Acmaeopleura parvula Stimpson, 1858 (neotype, RUMF-ZC- 2939, male, 6.9 × 8.5 mm). a, habitus; b, anterolateral part of cephalothorax; c, right chela, outer view.