Stenorhynchus Lamarck 1818

Assignment of Stenorhynchus Lamarck, 1818, to Inachoididae

The family Inachoididae, comprising 10 genera (see above), is here expanded to receive an additional genus, Stenorhynchus Lamarck, 1818, traditionally assigned to Inachidae (e.g., Rathbun 1925; Monod 1956; Garth 1958; Yang 1976; Powers 1977; Manning & Holthuis 1981; Williams 1984; Felder 1973; Poupin 1994; Hendrickx 1999; Cruz & Campos 2003; García-Guerrero & Hendrickx 2004; Almeida et al. 2007a; Felder et al. 2009; Hultgren et al. 2009; Wicksten 2011). The original spelling Stenorynchus (Lamarck 1818: 236 ) was emended to Stenorhynchus by Desmarest (1823) and subsequently cited with this spelling by most carcinologists, e.g., H. Milne Edwards (1834: 278) and Dana (1852: 83). The spelling Stenorhynchus was validated later, at the same time that Cancer seticornis Herbst, 1788, was selected as its type species, by subsequent designation in an Opinion of the International Code of Zoological Nomenclature (Anonymous 1966) (see also Rathbun 1897; Garth & Holthuis 1963).

Stenorhynchus, known by only four species (Ng et al. 2008: 113), presents the main characters of Inachoididae that differentiate the family from Inachidae (see below). Pleurites 5–8 in Stenorhynchus are exposed beyond the sides of the carapace as is typical in Inachoididae, but the pleurites are relatively small and narrow, instead of expanded and salient as in other inachoidids. The carapace rests on a setting gutter. Abdominal somites 1 and 2 are integrated into the cephalothorax of males (the first somite substantially encroaching on the posterior carapace margin), whereas in adult females somites 1–4 and a portion of somite 5 are in the prolongation of the carapace, and thus visible dorsally. The dorsally exposed pleurites 5–8 and first abdominal somites are entirely smooth, like the dorsal surface of the carapace. This is in contrast to most other inachoidids, where the dorsally exposed abdominal somites are granular or tuberculate, like the carapace. The sternum/pterygostome junction is complete in Stenorhynchus, and sternal extensions from P1–P4 connect the sternum to the laterally exposed pleurites 5–8.

Unusual characters, such as the unpaired, long rostrum (longer than the carapace) and the narrow, unsculpted carapace are present in all four species of Stenorhynchus. This contrasts with the relatively short rostrum, broadly triangular or widened carapace, and areolated and ornamented dorsal surface found in other inachoidid taxa. The thoracic sternal suture 3/4 of Stenorhynchus is only slightly visible laterally, thus not marked by a deep depression and pocket as in other inachoidids. Stenorhynchus is also characterised by the absence of an orbit, having an unprotected eye, and by a small postocular spine situated along the long “neck” that is distant from the eyestalk. In contrast, the eyes are partially protected in most inachoidids by a triangular postocular process (close to the eyestalk). The male gonopore practically opens at the extremity of the coxo-sternal condyle, whereas in other inachoidids it is situated on the dorsal border of the condyle, or partially condylar and partially coxal.

Behaviour and associated morphology support the removal of Stenorhynchus from inachids. Inachids are generally covered by hooked setae used in active decorating for camouflage (Wicksten 1993). In comparison, the carapace of Stenorhynchus species is practically naked, and decoration behaviour is absent, evidence that the genus is much closer to other inachoidids than to inachids. Inachoidids do not decorate at all, or only weakly so, as in Inachoides laevis Stimpson, 1870, where a short pubescence on its body and appendages allow for decoration with algae (Sánchez-Vargas & Hendrickx 1987: 162, table 1). The inachoidid Pyromaia tuberculata may be found with materials attached to the body (Schejter et al. 2002; Luppi & Spivak 2003). Stenorhynchus seticornis was observed walking or resting with legs spread under strong currents, allowing for pereopod setae to passively trap particles that individuals picked off for later consumption (Barr 1975). The same species also takes refuge with other marine invertebrates, being commonly found in association with sessile organisms such as stony and soft corals, sponges, gorgonians, and crinoids. It is also often found associated with the long-spined urchin Diadema antillarum ( Philippi, 1845 ), the urchin spines providing increased protection (Joseph et al. 1998; Hayes et al. 1998; Hayes 2007). Individuals were also seen interacting with sea anemones and using them as protection from predation (Barr 1975; Herrnkind et al. 1976; Williams 1984; Hayes et al. 1998; Cobo 2002; Okamori & Cobo 2003; see also Medeiros et al. 2011). Wicksten (2011: 252) noticed that in nature and in aquaria Stenorhynchus, as in inachids, can crowd together (“huddle”) with their appendages tightly folded against the body. However, unlike inachids, Stenorhynchus does not show the inachid defensive (“stockade”) behaviour that involves distal spines on the meri of pereopods 2–5 forming a circle of spines, along with the sharp rostrum.

In terms of larval development, the zoeae of Stenorhynchus, unlike those of inachid genera, have no lateral carapace spines and exhibit antennae, maxillules, and a telson fork that are distinctive; in addition, the megalopa develops functional uropods (Yang 1976; Rice 1980, 1983, 1988; Paula 1997; Paula & Cartaxana 1991 ). Larval characters of Stenorhynchus suggest its placement between the two larval groups recognised within Inachidae but remained unamed (Rice 1980: 308), supporting its placement “somewhere on the monophyletic line between the primitive and the advanced Inachinae ” (Paula & Cartaxana 1991: 121). Webber & Wear (1981: 370, 380) stated that the larvae of two inachoidids, Anasimus and Pyromaia “fall quite neatly between the Stenorhynchus species and Inachus, Achaeus and Macropodia ”, thus supporting the view that Stenorhynchus fills such a gap (see Guinot & Richer de Forges 1997: 490). Larval studies by Marques & Pohle (2003: 76, 78, figs. 1, 2) demonstrated the inadequate position of Stenorhynchus among the Inachidae and supported the genus as basal to a clade of inachoidids. These results are congruent with the morphology of adults that provide evidence for the inclusion of Stenorhynchus in Inachoididae and to the establishment of a separate inachoidid subfamily to receive it (see below). In a molecular study (Hultgren et al. 2009: 444, figs. 1, 2) Stenorhynchus, assumed to be an Inachidae, was found to have an uncertain status, forming an “idiosynchratic clade” with the tychid Pitho lherminieri (Shramm, 1867) and the epialtid Menaethius monoceros (Latreille, 1825). The inclusion of data from true inachids and other inachoidids should shed further light on genetic relationships.

The Inachoididae is mostly a New World family, formerly known exclusively from the Atlantic and Pacific coasts of the Americas. The only exception is the invasive species Pyromaia tuberculata, the “fire crab” native to the Pacific coast of North America that is now successfully established in several distant regions (Sakai 1976; Webber & Wear 1981; Wear & Fielder 1985; Morgan 1990; Furota 1996; Melo 1996; Furota & Furuse 1988; Flores et al. 2002; Furota & Kinoshita 2004; Poore 2004; Ahyong 2005; Galil et al. 2011; Doi et al. 2011; Ahyong & Wilkens 2011; Tavares et al. 2011; Brockerhoff & McLay 2011). With its inclusion in Inachoididae, Stenorhynchus becomes the only inachoidid genus occurring outside the Western Hemisphere, namely in the eastern Atlantic. The distribution of the four species of Stenorhynchus includes: the western Atlantic from North Carolina to Argentina for the common “yellowline arrow crab” or “arrow crab” S. seticornis (Herbst, 1788) (Rathbun 1925 pro parte: 13, pls. 2, 3; Williams, 1984 pro parte: 304; Goeke 1989: 625, figs. la–b, d–g, 2, 3; Poupin 1994: 44; Melo 1996: 190; Braga et al. 2005); the western Atlantic from Martha’s Vineyard to the Gulf of Mexico, West Indies, and Suriname for the “red arrow crab” S. yangi Goeke, 1989 (Goeke 1989: 631, fig. lc; Poupin 1994: 45, pl. 5a; Felder et al. 2009: 1076); the eastern Pacific from the Gulf of California to Chile, Revillagigedo Is., Galápagos Is. and Clipperton Atoll for the “Pacific (or “Panamic) arrow crab” S. debilis (Smith, 1871) (Rathbun 1925: 18, pls. 4, 5; Garth 1946: 366, pl. 63, fig. 1; Garth 1958: 130, pl. B, fig. 7, pl. 9; Takeda & Okutani 1983: 123; Hendrickx 1995: 130; 1999: 44, figs. 26, 27; Hernández Aguilera 2002: 314; Poupin et al. 2009: 181, fig. 191; Poupin 2010: 72); and the eastern Atlantic (Madeira, Canary Is., Cape Verde Is., and numerous west-African localities from Western Sahara to Angola) for the “eastern Atlantic arrow crab” S. lanceolatus (Brullé, 1837) (Capart 1951: 81, fig. 25; Monod 1956: 567, figs. 838, 839; Manning & Holthuis 1981: 304, fig. 78b).

Stenorhynchus seticornis represents a rare case in Brachyura, where ovigerous females occur throughout the year and of copulation frequently involving ovigerous females (Barr 1975; Cobo 2002; see also Okamori & Cobo 2003). This rare reproductive strategy is also found in Hymenosomatidae, considered closely related to Inachoididae (Guinot 2011b). For example, it has been observed in the sub-Antarctic hymenosomatid Halicarcinus planatus, a species able to re-mature its ovaries while ovigerous (Diez & Lovrich 2010).