Plagusiidae, Dana, 1851

GUINOT, DANIÈLE, TAVARES, MARCOS & CASTRO, PETER, 2013, Significance of the sexual openings and supplementary structures on the phylogeny of brachyuran crabs (Crustacea, Decapoda, Brachyura), with new nomina for higher-ranked podotreme taxa, Zootaxa 3665 (1), pp. 1-414 : 135-137

Families Plagusiidae and Percnidae

The male gonopore is sternal. It opens very close, seemingly joined, to the P5 coxa, as in Plagusia depressa ( Fig. 23B; see H. Milne Edwards 1836–1844, Atlas , pl. 23, fig. 3d; Guinot 1979a: 209, fig. 52E), P. squamosa (see N.K. Ng, Davie, Schubart & Ng 2007: fig. 4C), Guinusia dentipes (see Karasawa & Kato 2001: fig. 2.17, as P. dentipes ), Davusia glabra (see Guinot 1979a: pl. 18, fig. 9, as Plagusia glabra ; 2007: 29). The sternal gonopore also opens close to P5 coxa in Percnon planissimum (see Guinot 1979a: pl. 23, fig. 2; Rodríguez 1992: fig. 11E; Karasawa & Kato 2001: fig. 2.18). This proximity of the gonopore to the P5 coxa led Brocchi (1875: 81, as Acanthopus planissimus ) to assume that the gonopore was coxal, with the ejaculatory duct connected to the appendage and not to the sternum. Our dissections demonstrate, nonetheless, a direct connection between the gonad and the sternal gonopore ( Fig. 33) so the condition is truly sternal in both families. The gonopore is large and opens far from and markedly below suture 7/8, on the sternal region just above the P5 coxo-sternal condyle, which is surrounded by a conspicuously elongated episternite 7.

The configuration is rather similar in Plagusia , Percnon , Davusia and Guinusia , and varies only in the details. In the species of these genera that were examined the outer portion of sternite 8 remains laterally exposed and shows as a distinct plate, similar to the preceding sternites. Sternite 8 was erroneously described as being completely concealed by the abdomen although a small anterior portion is laterally visible ( Guinot 1979a: 209, pl. 18, fig. 9, as Plagusia glabra ). In Plagusia , Percnon , Davusia , and Guinusia , episternite 7 is posteriorly elongated so that it is close to or even reaches the narrow expansion of sternite 8 in the proximity of the P5 coxo-sternal condyle. Episternite 7 and the portion of sternite 8 bearing the gynglyme for the P5 coxa are joined in Plagusia , Davusia , and Guinusia , but are more distant in Percnon . This posterior area of sternite 8 remains partially exposed dorsally, a small portion being slightly dorsally visible (e.g., in Plagusia depressa and P. squamosa ); but it may be smaller, hardly visible (e.g., in P. immaculata , Guinusia chabrus ), and not visible at all in Davusia glabra . In Percnon , the posterior portion of sternite 8 laterally forms a raised, rounded plate, only partially concealed by the abdomen, a small exposed portion remaining visible on each side at the limit of abdominal somites 2 and 3, abdominal somites 3–5 being fused.

In Plagusia and Guinusia , the penis consists of a large, oval, sclerotised proximal portion, and a thick papilla; in Daviusa, the proximal portion is smaller, elongated, the papilla is short, pointed. In Percnon , the sclerotised proximal portion is much reduced and the papilla is foliaceous. The tribe Davusiini Števčić, 2011 , established within Plagusiidae Plagusiinae , by being simply based only on the diagnosis of Davusia (as type genus), without consideration of the other genera, is an oversimplification, the Plagusiinae not being diagnosed and differentiated. It is possible that Plagusiidae consists of several groups of genera, but a rigorous taxonomic revision of the family, such as that of Schubart & Cuesta (2010), must first be undertaken.

Euchirograpsus H. Milne Edwards, 1853 , shows an elongated episternite 7 (as in many grapsoid genera), which reaches the narrow expansion of sternite 8 receiving the P5 coxo-sternal condyle. There is a complete junction along episternite 7 and sternite 8; the male gonopore is far from suture 7/8 and partially encircled by sternite 8. When the abdomen is closed, a small portion of sternite 8 may be anteriorly exposed and laterally visible on each side at the limit of abdominal somites 2 and 3, as a narrow ( E. liguricus ) or hardly discernible ( E. americanus ) portion, or not at all visible ( E. timorensis ). The male gonopore is large. The penis is well developed, with a long, weakly sclerotised or unsclerotised proximal portion, and an elongated papilla (N.K. Ng & Martin 2010: 232, 239, 244, figs. 5, 7). In Miersiograpsus Türkay, 1978 ( Türkay 1978), also included in the Plagusiidae ( Davie 2002: 438; Poore 2004: 514; Ng, Guinot & Davie 2008: 218; Schubart & Cuesta 2010), the disposition is similar to that of Euchirograpsus , sternite 8 being exposed dorsally with a small portion visible on each side at the limit of abdominal somites 2 and 3. Zoeae of E. americanus show close relationships with those of Plagusiidae ( Wilson 1980) .

Percnon has been recently placed in its own family, Percnidae Števčić, 2005 , whereas Plagusia , Davusia , and Guinusia , together with Euchirograpsus and Miersiograpsus , were grouped in the Plagusiidae ( Schubart & Cuesta 2010) , instead of the subfamilial ranks recognised by Ng, Guinot & Davie (2008: 218). Molecular analyses using 16S rDNA sequence data of Euchirograpsus americanus , Percnon gibbesi and Plagusia squamosa by Wetzer et al. (2009: 486, 487, fig. 1) similarly did not support the monophyly of the family Plagusiidae , Percnon not being allied to the other two genera. As a confirmation of their first results (Schubart, Diesel, Neigel & Felder 2000a; Schubart et al. 2006), recent studies combining molecular and larval data by Schubart & Cuesta (2010) concluded to the separation of the Percnidae .

A large and prominent press-button for the abdominal-locking mechanism is characteristic of Plagusia and Percnon , with only a few variations. Its corneous aspect is due to a scaly paving on the upper portion, and this microstructure seems to be typical ( Guinot 1979a: 144, pl. 18, figs. 9–11, pl. 23, figs. 2, 3; Guinot & Bouchard 1998: 680, fig. 26F; Bouchard 2000: 189, fig. 46A–D). Associated to this strong sternal prominence, a deep, rimmed socket persists and functions throughout life in plagusiid females ( Guinot & Bouchard 1998: 680, fig. 26C), only a sternal scar remaining visible in post-puberty females. Such a persistence of the button in large females does not occur in the Percnidae . The microstructure of the locking system, which improves contact between the involved surfaces and increases the efficiency of the locking mechanism, is probably a conservative feature and seems to constitute a reliable taxonomic character, although its needs to be compared in both the Plagusiidae and Percnidae . The study of the microstructure of the press-button needs to be extended to a larger sample of species of these two groups to determine at which taxonomic rank this particular structure adequately applies. The press-button of Euchirograpsus , however, is particularly acute, corresponding to a socket posteriorly defined by a strongly calcified border.

Adult morphology had traditionally placed the species of Plagusia ( Dawson 1987) and those of the highly derived Percnon in a single family ( Crosnier 1965; Davie 2002). Schubart & Cuesta (2010) recognised homoplasy relation (convergence) between the two distinct families, Plagusiidae and Percnidae , which share, however, incipient or deep antennular cleft in the frontal margin ( Crosnier 1965: figs. 130, 131, pl. 7; Poupin 1994b: fig. 60) and a similar microstructure of the press-button. Molecular data show no affinity of Plagusiidae with the Gecarcinidae ( Schubart & Cuesta 2010: 293, figs. 1, 2).