Garthiellinae, Mendoza & Manuel-Santos, 2012

Mendoza, Jose Christopher E. & Manuel-Santos, Marivene R., 2012, Revision of Garthiella Titgen, 1986 (Crustacea: Decapoda: Brachyura: Xanthidae), with description of a new subfamily and a new species from the central Philippines, Zootaxa 3446, pp. 32-48 : 33-37

Garthiellinae subfam. nov.

Type genus. Garthiella Titgen, 1986 , by present designation.

Diagnosis. Carapace transversely subhexagonal, subovate, width about 1.5–1.6 times length; dorsal surface depressed, variably covered with conical or round granules, increasing in size toward anterior, lateral regions; regions well defined, 2M partially divided longitudinally, 3M distinct, entire, 4M indistinct, 1P flat. Front approximately 0.3 times carapace width, not advanced beyond orbits, bilobed, lobes divided by U-shaped median incision. Anterolateral border divided into 4 teeth (excluding external orbital angle) with denticulate margins. Posterolateral border slightly longer than anterolateral. Basal antennal article subrectangular; outer angle prolonged partly into wide orbital hiatus, but not blocking it from antennal flagellum. Pterygostome with small, narrow, accessory plate abutting against lateral margin of antennal basal article. Endostomial ridges developed only posteriorly. Male thoracic sternum granular; short median line exposed on sternite 4, continuing uninterrupted into sterno-abdominal cavity, absent at sternites 5, 6, present at sternites 7, 8; small part of sternite 8 visible between abdominal somite 2 and coxo-sternal condyle of P5. Chelipeds subequal in both sexes, profusely covered with conical granules; fingertips pointed. Ambulatory legs with spinose anterior margins, with scattered setae; dactylopropodal lock absent; no conical teeth on distal flexor margin of dactylus. Male abdomen with somites 3–5 fused, tip of telson anterior to level of coxo-sternal condyles of P1. G1 curved, tapering, with several, long, stiff, subterminal setae; conical spines on distal half; distal tip bifurcate. G2 approximately one-third length of G1.

Remarks. Garthiella aberrans ( Rathbun, 1906) , the type and until recently, only, species of the genus Garthiella , has been traditionally classified under Chlorodiinae Alcock, 1898 (sensu Serène, 1984) (= presently Chlorodiellinae Ng & Holthuis, 2007) on account of its superficial resemblance to species of Pilodius . As earlier pointed out by different authors (e.g., Rathbun 1906; Serène & Nguyen 1958; Serène 1984; Titgen 1986), however, the tips of the fingers of the chelae of G. aberrans are pointed rather than cupuliform as seen in all other chlorodiellines. The fingertips of G. sikatuna sp. nov. are also pointed. The cupuliform (spoon-tipped) fingers are a shared morphological feature for Chlorodiellinae, but are also seen in Etisinae and Liomerinae and in some xanthine and zosimine genera such as Cataleptodius Guinot, 1968 , Leptodius A. Milne-Edwards, 1863 , Macromedaeus Ward, 1942 , and Zozymodes Heller, 1861 (see Rathbun, 1930; Serène, 1984). It appears to be a feature that independently evolved on several occasions among the xanthids, being probably closely associated with grazing on periphyton. There are other morphological features common to chlorodiellines which are absent in Garthiella : the dactylo-propodal locking mechanism in the ambulatory legs, and the calcareous, conical, subdistal tooth (or teeth) on the ambulatory dactyli ( Clark & Galil 1993; Ng & Yang 1998; Clark & Ng 1999; Ng & Holthuis 2007). Garthiella also has a small portion of thoracic sternite 8 visible externally (not visible in Chlorodiellinae) and a G1 with a bifurcate distal tip (simple and unilobate, never bifurcate, in Chlorodiellinae). It has been shown that morphological characters of the thoracic sternum and G1 can be quite useful in delineating taxa at the generic and suprageneric levels (see Guinot 1967a, b, 1968, 1976, 1979; Lai et al. 2011).

The authors have considered the option of classifying Garthiella in Xanthinae MacLeay, 1838 (sensu Ng et al., 2008). The subfamily Xanthinae has not been satisfactorily defined by way of diagnostic morphological characters, however, and has been traditionally utilized as a ‘dumping ground’ for those species which are clearly xanthids but do not possess the diagnostic characteristics of any of the other subfamilies (see Serène 1984; Davie 2002; Poore 2004). Some workers have implied that there are distinct, monophyletic groups within Xanthinae (viz. Guinot 1967a, 1968, 1971) or have formally established new subfamilies and ‘tribes’ apart from or within Xanthinae (viz. Števčić 2005; Mendoza et al. 2012), suggesting that there is unacknowledged diversity within this taxon. Nonetheless, the apparent consensus is to leave Xanthinae as it is, with the understanding that it is polyphyletic and that adequate scrutiny of the diverse morphologies of its numerous component taxa is required to validate, refine or overturn these subdivisions (see Ng et al. 2008). That having been said, Garthiella can be clearly separated from the type genus of Xanthinae ( Xantho Leach, 1814 ) by the relatively shorter, wider thoracic sternum and longer abdomen in the male (longer, narrower thoracic sternum and shorter male abdomen in Xantho ; see Lai et al. 2011: fig. 7f); more anterior position of the sterno-abdominal press-button, which is closer to suture 4/5 (more posterior, closer to suture 5/6, in Xantho ; see Lai et al. 2011: fig. 7k); absence of prominent slits or recesses on the anterior region of the sternoabdominal cavity for receiving the tips of the G1 (present in Xantho ; see Lai et al. 2011, fig. 7k); median line continuing into the posterior region of sternite 4 within the sternoabdominal cavity (median line absent on posterior region of sternite 4 in Xantho ); small portion of sternite 8 visible externally (hidden in Xantho ); abdominal somites 1, 2 proportionately shorter and wider (longer and narrower in Xantho ); and the relatively shorter and stouter G1, with simple subterminal setae (G1 longer and more slender, without subterminal setae, in Xantho ).

Lai et al. (2011), in the most recent phylogenetic analysis of the Xanthidae , included both species of Garthiella , and their resulting tree (Lai et al. 2011: fig. 1) derived from base sequences of four genes (mitochondrial 12S and 16S sub-units, cytochrome oxidase I, and histone H3) showed Garthiella in a clade (Chl 2) with strong statistical support and was distinct from the chlorodielline clade (Chl 1), which contained representatives of the genera Chlorodiella , Pilodius and Cyclodius . Furthermore, this clade, Chl 2, was in a statistically well-supported basal position relative to most of the Xanthidae , and even to clades containing species that have been classified in Panopeidae Ortmann, 1893 (e.g., Panopeus spp. ) and Pseudorhombilidae Alcock, 1900 . Xanthinae sensu Ng et al. (2008) is also shown to be polyphyletic, with 10 separate clades and with the type genus, Xantho , forming its own unique clade (Xan 4). There are indications, as pointed out by Lai et al. (2011), that these “xanthine” clades have unique morphological features that could be candidates for synapomorphies, and could pave the way for their recognition as valid subfamilies apart from Xanthinae . They further stated (Lai et al. 2011: 437):

“One fundamental change to the current systematics is the assignment of taxa to the

Xanthinae . From our findings, only the Atlantic genus Xantho appears to belong to this subfamily (Xan 4)… Most xanthines clustered into a ‘ Leptodius ’ group (Xan 3) with others scattered throughout the tree in 10 widely dispersed clades: Xanthias (Xan 2, Zos 1, Eux 3), Demania /Liagore (Xan 1), Garthiope (Xan 6), Nanocassiope (Xan 9), Euryxanthops (Eux

3) and Lachnopodus (Xan 10). More importantly, this segregation of the Xanthinae based on genetic markers is well supported by adult and some zoeal characters.”

This analysis provides additional support for the removal of Garthiella from Chlorodiellinae, and further emphasizes the need for Garthiella to be classified in a suprageneric taxon of its own, either as a distinct subfamily within Xanthidae or a distinct family within Xanthoidea . The authors of the present work have opted to be conservative, establishing Garthiellinae subfam. nov. for Garthiella , but keeping it within Xanthidae sensu lato.

It is also important to note that the clade, Chl 2 in Lai et al. (2011), contained Medaeus danielita Mendoza & Ng, 2010 , as well as both species of Garthiella , whereas other species of Medaeus Dana, 1851 ( M. ornatus , type species, and M. elegans ) were positioned in a separate, well-supported clade, together with species of Lachnopodus Stimpson, 1858 , Metaxanthops Serène, 1984 , and Paraxanthias Odhner, 1925 (Lai et al. 2011: fig. 1). Garthiella aberrans and G. sikatuna sp. nov. are similar morphologically to M. danielita and M. aztec Davie, 1997 , particularly in the general carapace shape (transversely subhexagonal, with regions moderately defined), the front (relatively broad and poorly projecting), the carapace anterolateral margins (with 4 anterolateral teeth), the general proportions of the pereiopods (moderately long, slender ambulatory legs, chelipeds) and the general form of the G1 (moderate in length, curved, with stiff, spiniform, subterminal setae, and bilobate distal end). Garthiella spp. , however, do not have some features on the carapace seen in M. danielita , and diagnostic for Medaeus (sensu Davie 1997): the well-defined 4M region and the transverse ridge on the 1P region. This suggests that M. danielita (and possibly M. aztec , to which it is most similar; see Mendoza & Ng 2010) may be more aptly classified in Garthiellinae subfam. nov., although it is apparently not congeneric with G. aberrans or G. sikatuna sp. nov., and it may be necessary to eventually establish a new genus for it. The revision of the genus Medaeus is the subject of a separate work by the first author.