Family PLAGUSIIDAE Dana, 1851

<em>Percnon affine</em> [from A. Milne Edwards 1873: pl. 14 fig. 5]

Percnon affine [from A. Milne Edwards 1873: pl. 14 fig. 5]

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Family PLAGUSIIDAE Dana, 1851

Compiler and date details

May 2012 - Peter Davie, Queensland Museum, Brisbane

Plagusinae Dana, J.D. 1851. On the classification of the Crustacea Grapsoidea. American Journal of Science and Arts 12: 283-290 [288] [spelling corrected to Plagusiinae by Miers, E.J. 1878. Revision of the Plagusiinae. Annals and Magazine of Natural History 5 1: 147–154 (147), Name No. 377 on Official List of Family-Group Names in Zoology].
Type genus:
Plagusia Latreille, 1804.
Secondary source:
Miers, E.J. 1878. Revision of the Plagusiinae. Annals and Magazine of Natural History 5 1: 147-154.
Euchirograpsini Števčić, 2005.
Davusiini Števčić, 2011.

Introduction

The Plagusiidae traditionally has been treated as a subfamily of the Grapsidae, comprising only two genera, Plagusia and Percnon, which seem to be clearly united by the unusual cleft front, fused male abdominal segments 3–5 or 3–6, and third maxilliped with a reduced exopod which lacks a flagellum. To these character states can be added an unusual abdominal press-button locking mechanism with a rim-like sternal tubercle (see Guinot & Bouchard 1998), and the suture between sternites 5/6 having a raised rim adjacent to the press-button. Sternberg & Cumberlidge (1998) conducted an extensive cladistic study of grapsoid crabs, and concluded that Plagusia and Percnon form a distinctive monophyletic clade; they raised the subfamily formally to full family status.

Euchirograpsus and Miersiograpsus are here transferred to the Plagusiidae from Varuninae and Grapsinae respectively. The change for Euchirograpsus was originally suggested by Cuesta & Schubart (1997) based on larval morphology, and later supported by DNA evidence (Schubart et al. 2000). Re-examination of specimens (in conjunction with P.K.L. Ng) has shown that Euchirograpsus and Miersiograpsus both have male abdominal segments 3–6 fused, a character state found only in Plagusiidae. Also there are shallow grooves on the frontal margin that appear to mark incipient antennular clefts. This, along with the larval and DNA evidence, is sufficient reason to recognise this group at family level. However, Euchirograpsus and Miersiograpsus retain a number of what must be considered as plesiomorphic grapsid features, such as the relatively broader front that overhangs the epistome, and the third maxilliped having a normal, long, grapsid-type exopod bearing a well-developed flagellum.

Interestingly, Guinot & Bouchard (1998: 664) also recognised that Euchirograpsus is unusual because of its type of push-button abdominal locking mechanism: '… a specially acute button and a socket posteriorly defined by a strongly calcified border, which suggests that the genus is misplaced in the Varuninae'.

Evidence from DNA (Schubart et al. 2000) and larval studies (J. Cuesta pers. comm.) also suggest that Percnon is significantly different, and perhaps doubtfully placed in the Plagusiidae. This suggestion has subsequently gained traction, and the Percnidae was given separate family status by Števčić, 2005. This is followed here.

Diagnosis

Carapace subcircular to quadrate; surface smooth and glabrous to strongly tuberculate and tomentose. Front of two types: a) narrow, not overhanging epistome although may be armed with projecting spines; divided into three lobes by deeply cleft antennular fossae, such that antennules visible in dorsal view, or b) broader and more grapsid-like, overhanging epistome; shallow grooves marking incipient antennular clefts, but not obvious in dorsal view. Anterolateral margins with one to several teeth or spines behind exorbital angle. Lower border of orbit curved, continued as ridge to meet prominent anterior border of buccal cavern. Antennal flagellum short. Third maxillipeds not completely closing buccal cavern; merus and ischium without an oblique setose crest; exopod slender, with or without flagellum; palp articulating near antero-external angle of merus. Male abdomen entirely covering sternum between last pair of legs; segments 3–6 or 3–5 fused, but sutures may still be evident. Abdominal locking mechanism of press-button type, corneous rim or constricted apex present in Plagusia and Percnon; suture between sternites 5/6 with raised rim adjacent to press-button.