Medaeops Guinot 1967
Creators: name: Ng, Peter K. L.
name: Mclay, Colin L.
Publisher: name: Zenodo
Year: 2007
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Xanthidae
Medaeops

description: Medaeops Guinot, 1967
Remarks. The generic placement of the new species is uncertain, being close to both Medaeops Guinot, 1967, and Monodaeus Guinot, 1967. Medaeops was established for two species formerly placed in Medaeus Dana 1852: M. granulosus (Haswell, 1882) (type species by designation) and M. neglectus (Balss, 1922) as well as a new species, M. edwardsi Guinot, 1967. A clear diagnosis of the genus was unfortunately not given. The most important characters she employed in characterizing Medaeops and separating it from Monodaeus Guinot, 1967, were the relative lengths of the ambulatory legs and cheliped fingers both being shorter in Medaeops. Other characters stated were the relative prominence of the carapace regions, the degree to which the thoracic sternum was swollen, the arrangements and sizes of the articles of the antennules and antennae, and the form of the G1, which narrows distally and is quite strongly curved, with sometimes a sub-distal patch of long setae. There have been only two major publications dealing with Medaeops since 1967: Serène (1984) and Davie (1997). Serène (1984) did not discuss the genus at any length and his key (Serène 1984: 91) basically separated Medaeops from the allied Paramedaeus and Monodaeus by its members possessing relatively lower carapace regions, proportionately more elongated cheliped fingers, the merus of the fifth ambulatory leg being relatively short (about three times longer than broad), a semicircular telson, and a relatively straighter G1. Davie (1997) described two new species (M. merodontos and M. gemini) and provided a new key (Davie 1997: 357), but remarked it was difficult to distinguish Medaeops from Monodaeus because most of the characters used by Guinot (1967) were variable. He noted that the primary reasons why he referred his two new species to Medaeops and not Monodaeus were because the meri of their ambulatory legs were relatively shorter and the fingers of the chelipeds are neither long nor incurved. Monodaeus species are known from the western Indian Ocean (Guinot & Macpherson 1988).
While most of the species now included in Medaeops appear to be related, there is no suite of existing characters that are really shared by all species. The type species, M. granulosus, is the most widespread member of the genus and is actually the easiest species to diagnose. It is the only member of the genus with distinctly carinate ambulatory legs, especially along the margins of the meri, and its G1 is the stoutest, with the proximal two-thirds straight and stout, and the distal third more slender and lined with long setae nearer the tip (Guinot 1967: Fig. 40). The other species of Medaeops have the meri of their ambulatory legs granulated or serrated, but never carinate. The G1s of M. neglectus and M. edwardsi are also relatively stout with long setae on the distal-most parts, but they do not have the characteristic form of the G1 of M. granulosus, being gradually tapering throughout their length (Serène 1984: Figs. 52, 53). The G1s of M. merodontos and M. gemini are very slender (closely resembling that of the new species described here, M. serenei) and gently tapering to a sharp tip, with the distal-most parts having scattered short setae at best (Davie 1997: Figs. 7 g, h; 8g, h). As it stands, it would be more parsimonious to restrict Medaeops to M. granulosus. This of course begs the question as to what to do with the remaining Medaeops species.
In general carapace form, the other Medaeops species (M. neglectus, M. edwardsi, M. merodontos, M. gemini and M. serenei, new species) are actually closer to Monodaeus species. As noted by Guinot (1967), Serène (1984), Guinot & Macpherson (1988) and Davie (1997), the fingers of the chelipeds of Monodaeus species are generally very long and curved, although this is not always the case, those of M. tuberculidens (Rathbun, 1911) are relatively short as well (see Guinot & Macpherson 1988: Pl. 2C). Similarly, the proportions of the legs are also not a reliable generic character as it does vary (see Guinot & Macpherson 1988). While M. neglectus and M. edwardsi may be included in Monodaeus, this is not the case for the other three species. Medaeops merodontos, M. gemini and M. serenei can easily be distinguished from Monodaeus species in the form of the G1: long and slender, with the distal-most margins possessing short setae versus relatively stouter with a stocky basal part and numerous long setae on the distal-most margins in Monodaeus.
Even though the present new species significantly broadens the generic boundaries of Medaeops even further, especially with regards to its relatively long ambulatory legs, we choose to include the new species in this genus rather than in Monodaeus. A future revision of these two genera should clarify the characters that are more reliable. While morphological characters are needed for the identification of the species, molecular characters may be more helpful in finding natural generic groups among these xanthids.
description: Published as part of Ng, Peter K. L. & Mclay, Colin L., 2007, Two new species of deep-water xanthid crabs of the genera Euryxanthops Garth & Kim, 1983, and Medaeops Guinot, 1967 (Crustacea: Decapoda: Brachyura: Xanthidae) from New Zealand, pp. 37-50 in Zootaxa 1505 on pages 43-44, DOI: 10.5281/zenodo.177158