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

publication ID	https://doi.org/ 10.11646/zootaxa.4179.2.7
publication LSID	lsid:zoobank.org:pub:A890332A-41BC-4AAC-8415-4245E947E0DF
DOI	https://doi.org/10.5281/zenodo.6057852
persistent identifier	https://treatment.plazi.org/id/03F2480A-FFB9-BC76-FF42-C9E0A5C7FD46
treatment provided by	Plazi
scientific name	Ankerius
status	gen. nov.

Treatment

Ankerius View in CoL 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 View FIGURE 1 , 2 View FIGURE 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 View FIGURE 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 View FIGURE 5 A). The space between the third maxillipeds is slightly wider in Gustavus Ahyong & Ng, 2009 ( Fig. 5 View FIGURE 5 C). In Gandoa , the third maxillipeds are positioned further apart, leaving about half the buccal cavity exposed ( Fig. 5 View FIGURE 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 View FIGURE 3 D, 5D) whereas that of Gandoa is rounded ( Fig. 5 View FIGURE 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 View in CoL gen. nov. superficially resembles the pinnotherid Tetrias Rathbun, 1898 View in CoL (with two species: T. scabripes Rathbun, 1898 View in CoL ; and T. fischerii (A. Milne-Edwards, 1867)) View in CoL (at present their position in the Pinnotheridae View in CoL 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 View in CoL gen. nov., however, has a quadrate and smooth carapace ( Figs. 1 View FIGURE 1 , 2 View FIGURE 2 A) (transversely rectangular and distinctly granular in Tetrias View in CoL , 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 View FIGURE 1 , 2 View FIGURE 2 A, 4) (distinctly shorter and stouter in Tetrias View in CoL , 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 View in CoL of P2–P4 is relatively short, it is still longer than that of Ankerius View in CoL gen. nov. More importantly, the ambulatory dactyli of P2–P5 of all aphanodactylids (including Ankerius View in CoL gen. nov.) are all subqual in length. The ambulatory dactyli of P2–P4 are proportionately longer in Tetrias View in CoL , and only that of P5 is short and comparable in length to that of aphanodactylids (unpublished data). The most important difference between Tetrias View in CoL and Ankerius View in CoL gen. nov. (and all aphanodactylids) is that the third maxilliped in Tetrias View in CoL 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 View FIGURE 5 D). Tetrias View in CoL 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 View in CoL was phylogenetically separated from the main Pinnotheridae View in CoL , and they suggested it might be related to Aphanodactylidae View in CoL . They also indicated Tetrias View in CoL may be allied with Parapinnixa Holmes, 1895 View in CoL , 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.