Hymenosomatoidea, MacLeay, 1838

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 : 224-225

publication ID	https://doi.org/ 10.11646/zootaxa.3665.1.1
publication LSID	lsid:zoobank.org:pub:8358B363-BEE3-416D-96CA-8614E38B61D5
persistent identifier	https://treatment.plazi.org/id/03BB9C75-FF36-FF72-FF78-FD26FD95FA17
treatment provided by	Felipe
scientific name	Hymenosomatoidea
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Treatment

Affinities between Hymenosomatoidea View in CoL and Inachoididae

The Inachoididae Dana, 1851 View in CoL , studied by Drach & Guinot (1982, 1983), was hypothesised as closely related to Hymenosomatidae View in CoL by Guinot & Richer de Forges (1997), who based their conclusion on similarities between the two families.

The main inachoidid features are: (1) flattening of the body, particularly in Leurocyclus View in CoL , Paradasygyius View in CoL and Paulita View in CoL , and, not so extreme, however, as in Hymenosomatidae View in CoL ; (2) expansion of pleurites 5–8 beyond each side of the carapace, their exposed latero-external parts being calcified and ornamented in the same way as the carapace ( Fig. 47G View FIGURE 47 ; see for example species of Euprognatha Stimpson, 1871 View in CoL , figured by Santana & Tavares 2008); (3) insertion of the carapace into a gutter (gouttière de sertissage of Drach & Guinot 1982: 716; 1983: 38), which marks the separation between the covered internal portion of the pleurites 5–8 and their uncovered latero-external portion; (4) integration of the first abdominal somite into the cephalothorax; (5) absence of a true branchiostegite; (6) a thoracic sternum/pterygostome junction at the sternite 4 level, which varies from incomplete to complete, with intermediate states and sternal extensions posterior to each pereopod, from P1 to P4; (7) the broad thoracic sternum displaying pattern 5, subpattern 5e, with a median line along sternites 7, 8, and a raised median plate on sternite 7 connecting to the thick sella turcica ( Fig. 56K View FIGURE 56 ) (see Thoracic sternum in the Eubrachyura; Evolution of the thoracic sternum in the Eubrachyura and its patterns); (8) short, only lateral but deep thoracic sternal suture 3/ 4, which ends as a perforation of the sternal surface, and sutures 4/5–7/8 interrupted; (9) male abdomen with all somites free except for somite 6, which is fused with the telson (pleotelson); (10) male gonopore opening far from suture 7/8, in a posteriormost location ( Figs. 31D View FIGURE 31 , 50C, E View FIGURE 50 ); (11) female abdomen having, as in the male abdomen, a maximum of six elements, somites 5 and 6 being fused to the telson (pleotelson), with the formation of a large, discoid plate in ovigerous females ( Guinot & Richer de Forges 1997: figs. 11E, 12E); (12) development of a brood cavity limited by a high sternal ridge, closed like a box, the abdominal margin being tightly joined to its edge, thus the need of a branchiosternal canal for the oxygenation of the eggs; (13) axial skeleton with pleurites almost horizontal, regularly connecting medially, and fused to the carapace by pillars ( Fig. 47H, I View FIGURE 47 ) so that it is difficult to detach the carapace without breaking it (see Evolution of the axial skeleton in the Eubrachyura and its patterns). A true branchiostegite posterior to P2 is absent in Inachoididae View in CoL .

The gymnopleurity of Inachoididae , with a dorsal exposure, calcification, and ornamentation of pleurites 5– 8 as parts of the carapace, is totally different from that of Raninoidea . The laterally exposed thoracic pleurites 5–7 in the Raninoidea ( Fig. 38B, C View FIGURE 38 ) are a consequence of a lifting of the carapace (a specialisation for burying), thus extending the body’s depth, and form a specialised plate for the respiratory current.

Different states of the thoracic sternum/pterygostome junction are found in Inachoididae depending on the extension of sternite 4. Consequently, the Milne Edwards opening is variously shaped, the developed mxp3 coxa being differently exposed ( Figs. 48A View FIGURE 48 , 49C, E View FIGURE 49 ). The junction is incomplete, e.g., in Paradasygyius depressus , Collodes leptocheles , Pyromaia tuberculata , and Leurocyclus Rathbun, 1897 , whereas it is complete, with entirely separated Milne Edwards openings, in other species, e.g., Paulita tuberculata , Batrachonotus fragosus Stimpson, 1871 , Arachnopsis filipes Stimpson, 1871 , Euprognatha rastellifera Stimpson, 1871 , E. bifida Rathbun, 1893 , and Anasimus latus ( Guinot & Richer de Forges 1997: 488, figs. 11C, 12C, D, 13A, B, 14A, B; Guinot 2012).

The inachoidid sternum is connected to the carapace by sternal extensions between the P1 and P2; moreover, there are extensions between the P2 and P3, P3 and P4, and P4 and P5, extensions that connect the sternum to the laterally exposed pleurites 5–8 ( Drach & Guinot 1982: 720, figs. 1, 3–5; 1983: figs. 1–6; Guinot 1984b: fig. 1, pl. 1, figs. A, B, D, pl. 2, figs. A, C; Guinot & Richer de Forges 1997: fig. 13C). Such sternal extensions do not exist in Hymenosomatoidea , where the interlocking of the carapace with the cephalothorax is considerably modified.

The axial skeleton of the Inachoididae ( Fig. 47G – I View FIGURE 47 ), with both a dorsoventral partition and a lateral compartment, is similar to that of Hymenosomatidae . Additionally, vertical pillars in the anterior region connect the axial skeleton (i.e., the dorsal edges of the pleurites) to the internal surface of the carapace in taxa with a flattened carapace (e.g., Leurocyclus , Paradasygyius and Paulita ), and as well as in those with a thicker body (e.g., Anasimus , Collodes ). This “fusion” is an exceptional disposition in Brachyura ( Drach & Guinot 1982: figs. 5, 6; 1983: figs. 4, 7, 8; Guinot 1984b: 380). Such a pleurites/carapace fusion is absent in Hymenosomatoidea , where the flattening of the body characterising most genera is the most extreme condition known among the Brachyura . The dorippid and inachoidid dispositions are features that have not evolved convergently but have most probably been inherited from a close common ancestor. The hymenosomatoid and inachoidid peculiarities cannot be considered homoplasic, because such important similarities are part of a common groundplan.

The numerous similarities between Inachoididae, Hymenosomatoidea, and Dorippoidea are being interpreted here as a synapomorphy relation (and not a homoplasy relation). This hypothesis is supported by other traits. The gonopore of Inachoididae is coxal as in some (although few) dorippids ( Fig. 15 View FIGURE 15 ), in contrast to the coxosternal condition of the more derived Dorippoidea ( Figs. 16 – 23 View FIGURE 16 View FIGURE 17 View FIGURE 18 View FIGURE 19 View FIGURE 20 View FIGURE 21 View FIGURE 22 View FIGURE 23 ) and the peculiar condition of all Hymenosomatoidea ( Fig. 29A, B View FIGURE 29 ). The inachoidid gonopore opens, however, far from suture 7/8, thus in a posteriormost location, a strong similarity with hymenosomatoids (see Modalities of penis protection: “ sternitreme” protection). The gutter inside of which the carapace lies and involving pleurites 5–7 in Dorippoidea ( Figs. 46A, B View FIGURE 46 , 47A, B View FIGURE 47 ) and the gutter involving pleurites 5–8 in Inachoididae are both reminiscent of the “hymenosomian rim”. Whether this reflects common descent more than convergence needs to be still investigated. The uniform G1 of Inachoididae corresponds to a plesiomorphic state: rather straight, simple, with a slender tip, often with a subterminal lobe ( Garth 1958: pl. B, figs. 8, 9, pl. E, figs. 1–7, 9; Williams 1984: fig. 241b, c, e–g, i, m, n). It is comparable to that of Odiomarinae (basal Hymenosomatoidea ) ( Guinot 2011a) and some Dorippidae . The G2 is very short in Hymenosomatoidea and Inachoididae , whereas it is longer than the G 1 in Dorippoidea .

Deep grooves are often present on the dorsal surface of the carapace in Inachoididae , which are reminiscent of the deep grooves in Dorippidae , resembling a “human face”. For example, the carapace of Paulita is somewhat similar to that of a dorippid. The parallel grooves that deeply cut the carapace dorsal surface of P. tuberculata ( Lemos de Castro 1949: figs. 8–11; Takeda & Okutani 1983: fig. p. 133) perhaps mirror an original metamery, thus denoting a plesiomorphic condition.