Ankerius, Ng, Peter K. L., Rahayu, Dwi Listyo & Shih, Hsi-Te, 2016

Ng, Peter K. L., Rahayu, Dwi Listyo & Shih, Hsi-Te, 2016, Ankerius aenigmaticus, a new genus and new species of aphanodactylid crab symbiotic with polychaetes from the Red Sea coast of Saudi Arabia (Crustacea: Decapoda: Brachyura: Aphanodactylidae), Zootaxa 4179 (2), pp. 271-278 : 271-273

Ankerius gen. nov.

Type species. Ankerius aenigmaticus gen. et sp. nov., by present designation.

Etymology. Named after fellow carcinologist Arthur Anker, who collected the specimen. Gender masculine.

Diagnosis. Carapace as long as wide, deeply vaulted, dome shaped, dorsal surface smooth, region indistinct. Front entire, triangular in frontal view, slightly deflexed medially, anterolateral margin arcuate, entire, finely cristate, pubescent; anterolateral margin separated from gently converging posterolateral margin. Orbital margin entire, unarmed; eyes stout, mobile. Antennule folding obliquely, filling fossa; antenna not excluded from orbit, basal segment stout. Epistome wide, medially triangular, bifurcated apex. Third maxilliped relatively slender, covering about 50% of buccal cavity; ischium approximately as long as merus; merus subquadrate, anterolateral angle rounded; exopod relatively narrow, with long flagellum. Outer surface of palm with rows of strong, sharp spines, obscured by long, plumose setae. Merus, propodus of ambulatory legs relatively long; dactylus elongated, distinctly longer than greatest height; posterior margin of merus with row of strong, sharp spines; propodus with row of spines on distal margin bracketing dactylus; dactylus with bifurcate claw, movable spines on flexor margin. Female abdomen broad, 6 free somites, telson; somites separated by deep, relatively broad suture. Vulva rounded, positioned near suture between sternites 4, 5.

Remarks. Ankerius gen. nov. has several atypical characters for Aphanodactylidae . One of the diagnostic features of the family is the possession of a short ambulatory dactylus (e.g., see Ahyong & Ng 2009: fig. 1E). This character, together with the normal third maxilliped structure (without an enlarged and/or modified palp), easily distinguishes aphanodactylids from all other pinnotheroids ( Ahyong & Ng 2009). The ambulatory dactylus is proportionately longer ( Figs. 1 , 2 A, 4) in Ankerius gen. nov. than in other aphanodactylids, about 1.5 to twice the length of those seen in other genera. It is nevertheless still shorter than the condition in most pinnotheroids. Most aphanodactylids (with the exception of Gandoa , cf. Ng & Rahayu 2016) also have several short spines on the distal edge of the ambulatory propodus, bracketing the short dactylus; this character is present in Ankerius gen. nov. ( Fig. 4 I, J). Considering the very wide diversity in carapace, however, the morphologies of the cheliped and ambulatory leg of known aphanodactylids (cf. Ahyong & Ng 2009; Naruse & Maenosono 2012; Ng & Rahayu 2016), Ankerius gen. nov. is readily accommodated in this family.

The wide gap between the third maxillipeds of Ankerius gen. nov. is distinctive. In most species of aphanodactylids (notably Selwynia Borradaile, 1903 , Uruma Naruse, Fujita & Ng, 2009 , and Takedactylus Naruse & Maenosono, 2012 ), the third maxillipeds cover about 80% of the buccal cavity, leaving just a narrow gap between them ( Fig. 5 A). The space between the third maxillipeds is slightly wider in Gustavus Ahyong & Ng, 2009 ( Fig. 5 C). In Gandoa , the third maxillipeds are positioned further apart, leaving about half the buccal cavity exposed ( Fig. 5 B). The condition in Ankerius gen. nov. is generally similar to that of Gandoa except that the relatively more slender ischium of the third maxilliped has the inner margin distinctly sloped ( Figs. 3 D, 5D) whereas that of Gandoa is rounded ( Fig. 5 B).

The shape of the third maxillipeds of Ankerius gen. nov. and the prominent gap between them is similar to the condition seen in members of Grapsidae MacLeay, 1838 , Gecarcinucidae MacLeay, 1838 , and Sesarmidae Dana, 1851 ; members of which have a rhomboidal gap between the third maxillipeds even when closed. None of these taxa, however, have the third maxillipeds positioned as far apart as in Ankerius gen. nov.. In any case, the form of the carapaces, orbits, mouthparts, ambulatory legs and many other features of Grapsidae , Gecarcinucidae , and Sesarmidae are very different from Ankerius gen. nov.. With regards to the other thoracotreme families, members of Varunidae H. Milne Edwards, 1853 , Macrophthalmidae Dana, 1851 , and Xenograpsidae N.K. Ng, Davie, Schubart & Ng, 2007 , have mouthparts superficially similar to those of Ankerius gen. nov., but they do not have distinctly gaping third maxillipeds, and their carapace, chelipeds, and ambulatory legs are very different. In any case, none of the above families have members that are known to be symbiotic with polychaete worms.

Ankerius gen. nov. superficially resembles the pinnotherid Tetrias Rathbun, 1898 (with two species: T. scabripes Rathbun, 1898 ; and T. fischerii (A. Milne-Edwards, 1867)) (at present their position in the Pinnotheridae is uncertain; Palacios-Theil et al. 2016). This is particularly with regard to the general carapace shape, and the spiny chelae and ambulatory legs (cf. A. Milne-Edwards 1873: pl. 18 fig. 3, 3a; Rathbun 1918: text-fig. 114a, c, pl. 39 figs. 4, 5; Tesch 1918: pl. 18 fig. 1, 1b). Ankerius gen. nov., however, has a quadrate and smooth carapace ( Figs. 1 , 2 A) (transversely rectangular and distinctly granular in Tetrias , cf. A. Milne-Edwards 1873: pl. 18 fig. 3; Rathbun 1918: text-fig. 114c, pl. 39 figs. 4; Tesch 1918: pl. 18 fig. 1), and the ambulatory legs are relatively more slender and longer ( Figs. 1 , 2 A, 4) (distinctly shorter and stouter in Tetrias , cf. A. Milne-Edwards 1873: pl. 18 fig. 3; Rathbun 1918: text-fig. 114c, pl. 39 figs. 4, 5; Tesch 1918: pl. 18 fig. 1, 1b). While the ambulatory dactylus of Tetrias of P2–P4 is relatively short, it is still longer than that of Ankerius gen. nov. More importantly, the ambulatory dactyli of P2–P5 of all aphanodactylids (including Ankerius gen. nov.) are all subqual in length. The ambulatory dactyli of P2–P4 are proportionately longer in Tetrias , and only that of P5 is short and comparable in length to that of aphanodactylids (unpublished data). The most important difference between Tetrias and Ankerius gen. nov. (and all aphanodactylids) is that the third maxilliped in Tetrias is typically “pinnotherid”-like, with the ischium and merus reduced in size, the palp (carpus, propodus, and dactylus) very large, and the dactylus prominent (cf. Tesch 1918: pl. 18, fig. 1a; Rathbun 1918: text-fig. 114b; Campos 2006: fig. 2E, F)). This contrasts with the typical third maxilliped structure in aphanodactylids in which the ischium and merus are relatively large and the palp reduced (e.g., Fig. 5 D). Tetrias species have been known to be associated with clams and polychaete worms although they have also been found free-living (see Tesch 1918: 271). The genus is now being revised by T. Naruse and colleagues (see Naruse et al. 2011).

In their recent molecular reappraisal of Pinnotheroidea, Palacios-Theil et al. (2016) showed that Tetrias was phylogenetically separated from the main Pinnotheridae , and they suggested it might be related to Aphanodactylidae . They also indicated Tetrias may be allied with Parapinnixa Holmes, 1895 , and aphanodactylids. It is clear that to ascertain their phylogenetic relationships, a detailed genetic study of aphanodactylids will need to be done and compared with other thoracotremes in the future.

Figures

FIGURE 1. Ankerius aenigmaticus gen. et sp. nov., female holotype (7.2 × 7.0 mm) (UF), Red Sea coast of Saudi Arabia. Colours in life. Photograph: A. Anker.

FIGURE 2. Ankerius aenigmaticus gen. et sp. nov., female holotype (7.2 × 7.0 mm) (UF), Red Sea coast of Saudi Arabia. A, overall habitus; B, frontal view of cephalothorax; C, ventral view of cephalothorax showing thoracic sternum and vulvae.

FIGURE 3. Ankerius aenigmaticus gen. et sp. nov., female holotype (7.2 × 7.0 mm) (UF), Red Sea coast of Saudi Arabia. A, frontal view of cephalothorax showing orbit, antennule, antenna, epistome and endostome; B, front, orbit and antenna; C, epistome and endostome; D, left third maxiliped; E, female abdomen; F, left chela; G, merus of right cheliped. Scales = 1.0 mm.

FIGURE 4. Ankerius aenigmaticus gen. et sp. nov., female holotype (7.2 × 7.0 mm) (UF), Red Sea coast of Saudi Arabia. A, B, left P 2; C, D, left P 3; E, F, left P 4; G, H, left P 5; I, J, propodus and dactylus of left P 4. A, C, E, J, lateral view; B, D, F, I, mesial view. Scales: A – H = 1.0 mm; I, J = 0.5 mm.

FIGURE 5. Buccal cavity and third maxillipeds. A, Selwynia laevis Borradaile, 1903, holotype male (7.1 × 5.6 mm) (CUMZ I. 63872), Hulule Atoll, Maldives, coll. J. S. Gardiner, 1900; B, Gandoa brevipes (H. Milne Edwards, 1853), female (7.2 × 5.0 mm) (ZSM 1277 / 1) [holotype of Voeltzkowia zanzibarensis Lenz, 1905], Kokotoni, Zanzibar, coll. July 1889; C, Gustavus mecognathus Ahyong & Ng, 2009, paratype female (13.9 × 8.0 mm) (ZRC 2010.0252), SW Cocos Barrier, Guam, near small pass on large terebellid worm, coll. G. Paulay, 20 March 2000; D, Ankerius aenigmaticus gen. et sp. nov., female holotype (7.2 × 7.0 mm) (UF), Red Sea coast of Saudi Arabia.