HEXAPODIDAE Miers, 1886

Guinot, Danièle, 2006, Rediscovery of the holotype of Paeduma cylindraceum (Bell, 1859) and description of a new genus of Hexapodidae (Decapoda, Brachyura), Zoosystema 28 (2), pp. 553-571 : 555-557

Family HEXAPODIDAE Miers, 1886

Hexapodinae Miers, 1886: 275 . — Ortmann 1894: 690. — Alcock 1900: 293, 329. — Borradaile 1907: 468, 485. — Tesch 1918: 150, 237. — Monod 1956: 340, 361. — Balss 1957: 1658. — Imaizumi 1959: 276. — Serène 1964: 270; 1968: 93. — Glaessner 1969: R527. — Gordon 1971: 106. — Sakai 1976: 522, 553. — Collins & Morris 1978: 977. — Crane 1981: 3. — Dai & Yang 1991: 393, 417.

Hexapodidae – Guinot 1978: 212; 1979: 16, 43, 114, 145, 155, 167, 215, 220, 261, 267. — Manning & Holthuis 1981: 166. — Glaessner & Secretan 1987: 9. — Saint Laurent 1989: 154, footnote, 157. — Beschin et al. 1994: 191. — Guinot & Richer de Forges 1997: 496, table 1. — Ng 1998: 1061, 1081. — Guinot & Bouchard 1998: 660. — Huang et al. 2002: 651. — Schweitzer et al. 2000: 55. — Von Sternberg & Cumberlidge 2001: 332. — Martin & Davis 2001: 75. — Schweitzer & Feldmann 2001: 330, 331. — Schweitzer 2003: 1107; 2005: 289. — Karasawa & Kato 2003: 129. — Guinot & Quenette 2005: 334.

EXTANT GENERA (SOME INCLUDE FOSSIL SPECIES). — Hexapinus Manning & Holthuis, 1981 (type species by original designation: Hexapus latipes De Haan, 1835 ); Hexaplax Doflein, 1904 (type species by monotypy: Hexaplax megalops Doflein, 1904 ); Hexapus De Haan, 1835 (type species by subsequent designation, International Commission of Zoological Nomenclature: Cancer sexpes Fabricius, 1798 ); Lambdophallus Alcock, 1900 (type species by monotypy: L. sexpes Alcock, 1900 ); Latohexapus Huang, Hsueh & Ng, 2002 (type species by original designation: L. granosus Huang, Hsueh & Ng, 2002 ); Paeduma Rathbun, 1897 (type species by monotypy: Amorphopus cylindraceus Bell, 1859 ); Hexalaughlia n. gen. (type species by present designation: Thaumastoplax orientalis Rahbun, 1909 ); Parahexapus Balss, 1922 (type species by monotypy: P. africanus Balss, 1922 ); Pseudohexapus Monod, 1956 (type species by monotypy: Hexapus (Pseudohexapus) platydactylus Monod, 1956 ); Spiroplax Manning & Holthuis, 1981 (type species by original designation: Thaumastoplax spiralis Barnard, 1950 ); Stevea Manning & Holthuis, 1981 (type species by original designation: Hexapus williamsi Glassell, 1938 ); Thaumastoplax Miers, 1881 (type species by monotypy: T. anomalipes Miers, 1881 ); Tritoplax Manning & Holthuis, 1981 (type species by original designation: Hexapus stebbingi Barnard, 1947 ).

FOSSIL GENERA. — Goniocypoda Woodward, 1867 (type species by monotypy: G. edwardsi Woodward, 1867 ); Palaeopinnixa Via, 1966 (type species by original designation: P. rathbunae Schweitzer, Feldmann Tucker & Berglund, 2000 ; originally Pinnixa eocenica Rathbun, 1926 , see Schweitzer et al. 2000).

Globihexapus Schweitzer & Feldmann, 2001 (type species by monotypy: G. paxillus Schweitzer & Feldmann, 2001 ) was recently removed to the Pinnotheridae ( Nyborg 2002; Schweitzer 2005).

REMARKS

Hexapodid crabs were often regarded as having sternal male genital openings ( Barnard 1950: 283, key; Balss 1957: 1658). The family was assigned either to the Thoracotremata (or Catometopa) ( Guinot 1978 provisionally; Schram 1986; Schweitzer et al. 2000; von Sternberg & Cumberlidge 2001) or to the Heterotremata ( Guinot 1979; Saint Laurent 1989; Guinot & Richer de Forges 1997; Guinot & Bouchard 1998; Martin & Davis 2001), the latter implying a true coxal condition of the male openings.

Hexapodids are considered as highly modified and specialized. Many species are known to live in annelid tubes or on hydroids. Tesch (1918: 238) commented that their commensalistic mode of life “has brought about not only the cylindrical shape of the body, but also the disappearance of the posterior legs, which, by the fact that they are inserted at a higher level than the preceding pairs, perhaps would rather impair the animal’s moving up and down in the tubes of Annelids and Hydrozoa”.

The P5 were presumed to be absent in all Hexapodidae , except in Paeduma . Thus, the corresponding sternite 8 was supposedly also lacking. Sternite 8 was seen as a triangular piece remaining visible dorsally ( Barnard 1950: 300, fig. 56g, k, under “sternite 5”). A line dividing it into two parts has been observed in Hexapus stebbingi Barnard, 1947 ( Gordon 1971: figs 1, 2). The hypothesis that a portion of the reduced sternite 8 may have an appendicular origin was formulated by Guinot (1979: 115, fig. 32). In fact, sternite 8 is present, although markedly reduced and concealed under the abdomen, with only a very small exposed portion. P5 vestigial coxae were clearly indicated, but not figured, by Saint Laurent (1989: 154, footnote). Such a vestigial coxa was confirmed by several dissections of Hexaplax megalops Doflein, 1904 by Guinot, Tavares & Castro (unpublished data), and it is supposedly characteristic to all Hexapodidae . The presence of a vestigial P5 coxa, from which the penis emerges, has necessitated a new interpretation of the hexapodids, which are no longer hexapods in term of anatomy (at least the males) but are clearly true decapods. Male Hexapodidae actually have five pairs of legs, with an extremely reduced P5 which lacks all its articles except for the vestigial coxa, which is concealed under the abdomen.

Genera within the Hexapodidae are essentially distinguished by the shape of the eyes and mxp3, the shape and degree of fusion of the male abdomen, the development of sternal grooves on the thoracic sternum, the frequent presence of a stridulating apparatus (in the family at least two different shapes of one of its two components), and the structure of the G1. Some characters, such as the G1, show such a wide range of variation that subfamilies could be considered.

In addition to the dorsal location and reduction of sternite 8 and the partial loss of P 5 in males, the Hexapodidae show other characteristics that are not found in most other Eubrachyura Saint Laurent, 1980: loss of the exopodite of pleopods of somite 2 in females, persistence of structures for the abdominal maintaining in adult females, and female abdomen not markedly sexually dimorphic. Guinot & Quenette (2005: 334, fig. 29b) discussed the possible relationships of the Hexapodidae with other eubrachyuran families characterized by reduced P5, in particular the Retroplumidae Gill, 1894 , females of which show the same features (loss of the exopodite of pleopods of somite 2 in females, persistence of structures for the abdominal maintaining in adult females) as the Hexapodidae .