Monsamnis, Richard, Jasmine, Grave, Sammy De & Clark, Paul F., 2012

Richard, Jasmine, Grave, Sammy De & Clark, Paul F., 2012, A new atyid genus and species from Madagascar (Crustacea: Decapoda: Caridea), Zootaxa 3162, pp. 31-38 : 32-33

Monsamnis gen. nov.

Type species. Monsamnis carpolongus sp. nov., by present designation and monotypy.

Diagnosis. Carapace without supraorbital tooth, suborbital margin not produced, rostrum short and unarmed, exopods present on all pereiopods (fifth vestigial or reduced), carpus of second pereiopod elongated 7–9 × long as broad with anterior excavation, uropod diaeresis with a single spine, appendix masculina long and armed with numerous spiniform setae, female carrying ca. 14 large eggs, 1.4–1.5 × 1.00– 1.1mm in size.

Etymology. The genus name is formed by an arbitrary combination from the Latin mons meaning mountain and amnis meaning stream; for the habitat in which these crustaceans were collected; gender masculine.

Remarks. The traditional characters used to distinguish atyid subfamilies (série sensu Bouvier, 1925) are the relative development of the exopods on the pereiopods, branchial formulae, pigment presence and reduction in eyes, number of spines on the uropodal diaresis, as well as the shape and proportions of the chelipeds (see Holthuis, 1986, 1993). Four subfamilies are recognised in the traditional classification, three of which were originally termed “série” in Bouvier (1925), viz. caridienne, caridellienne, paratyienne; with the série typhlatienne added by Holthuis (1965) (see also Monod and Cals, 1970). All four were formally elevated to subfamily level in Holthuis (1986) (série caridienne as Atyinae ), although their usage since then has not been consistent in systematic literature (e.g., Sanz and Platvoet, 1995; Sket and Zakšek, 2009), resulting in them not being recognised in De Grave et al. (2009). A growing body of literature, primarily molecular based (e.g., Page et al., 2007, 2008; Sket and Zakšek, 2009) has shed considerable doubt on some of the characters used by Holthuis (1965), especially the presence and number of exopods as well as the branchial formulae, both of which are perhaps better interpreted as environmental adaptations, such as low oxygen environments and a troglobitic lifestyle. As the true phylogenetic significance of these characters will not be resolved for some time, the relationship of Monsamnis gen. nov. to the majority of other atyid genera is relatively unclear.

Based on characters (presence of exopods, branchial formula, absence of carapacial teeth) examined for the present study, the new genus is related to the troglobitic and anchialine genera, traditionally included in the subfamily Typhlatyinae. Page et al. (2008) already provided support to recognise a série typhlatienne, if perhaps not in a formal Linnean hierarchy, but justifying contrasting the new genus to those genera currently included in that subfamily. Recent molecular phylogenetic work (Von Rintelen et al. in review) has re-emphasised that the “série” proposed by Bouvier (1925) are indeed natural groupings, although in a somewhat modified composition, and a série typhlatienne is indeed recognised.

According to Holthuis (1986), this subfamily (série) comprises of Antecaridina , Stygiocaris , Typhlopatsa , Typhlatya and Spelaeocaris . The status of the monotypic genus Spelaeocaris (type species Spelaeocaris pretneri Matjašič, 1956 ) remains problematic. Monod and Cals, (1970) continued to separate this genus from Typhlatya on the basis that the third and fourth pereiopodal exopodites are reduced, with the fifth being absent. This contrast to Typhlatya in which the exopodites are well developed on the third and fourth pereiopod, and well developed or absent on the fifth (see Jaume and Bréhier, 2005 for an overview). However, Sanz and Platvoet (1995) re-assigned S. pretneri to Typhlatya , as they interpreted the reduction in exopod size from P1 through to P5 as a convergent character, given that similar reduced exopods on P3–4 and absence on P5 are also present in Typhlatya monae Chace, 1954 and Typhlatya pearsei Creaser, 1936 . Jaume and Bréhier (2005) agreed and maintained the synonymy of both genera. However, Sket and Zakšek (2009) using molecular data from previous studies combined with morphological information of European cave dwelling shrimp taxa, recognised that Spelaeocaris was allied to Troglocaris (série paratyienne). Further, in that study Spelaeocaris was formally recognised as a subgenus of Troglocaris and three new species described in it, as well as a single unassigned specimen, indicative of higher species richness than previously believed ( Sket and Zakšek, 2009). Although this may not be the final position of Spelaeocaris , here Sket and Zakšek (2009) are followed and this genus is not considered part of Typhlatyinae (série typhlatienne).

Considering the remaining genera in Typhlatyinae (série typhlatienne), Monsamnis gen. nov. appears to occupy an intermediate position between those with a well-developed exopod on the fifth pereiopod and a spiniform, well-developed sub-orbital angle (viz. Antecaridina , Stygiocaris , Typhlopatsa ) and Typhlatya which lacks a produced sub-orbital angle (exopods on fifth pereiopod present, reduced or absent). As the exopod on the fifth pereiopod varies from present to absent in Typhlatya (see Jaume and Bréhier, 2005), it is tempting to suggest Typhlatya could be the sister taxon to Monsamnis gen. nov., a hypothesis refrained upon in the present study, as current biogeographical understanding of this genus would counter argue. Instead the morphological differences between all genera are simply outlined below.

Monsamnis gen. nov. is similar to the anchialine genus, Antecaridina , in having a short unarmed rostrum, absence of carapacial teeth and exopods present on all pereiopods. The new genus differs from Antecaridina by the possession of well developed eyes (degenerate in Antecaridina ) and the long carpus of the second pereiopod (vs. 4 × long as broad, see Holthuis 1965). Monsamnis gen. nov. is similar to Typhlopasta Holthuis 1956, in having a short unarmed rostrum and no supraorbital teeth, but differs conspicuously from Typhlopatsa in having a well developed eye (vs. degenerate) and in having a long carpus on the second pereiopod (vs. 5 as long as broad). Typhlatya and the Australian genus, Stygiocaris are relatively closely related to each other ( Page et al., 2008); both can easily be separated from Monsamnis gen. nov. in having degenerate eyes (vs. well developed).

Two further, perhaps distantly related genera are Halocaridina and Halocaridinides , which are anchialine taxa with eyes lacking any clear cornea, although they are relatively large. Additionally, their diaresis harbours numerous spines and no exopods are present ( Fujino and Shokita, 1975; Holthuis, 1973) facilitating the distinction with Monsamnis gen. nov.

Another two, quite possibly unrelated, genera are known from Madagascar; Caridina H. Milne Edwards, 1837 and Parisia Holthuis, 1956 ( Holthuis, 1965; Gurney, 1984; Cai, 2005). Monsamnis gen. nov. is similar to Caridina , in not exhibiting a supra-orbital tooth and possessing slender pereiopods. However, the new genus differs from Caridina by possessing exopods (vs. absent) on all pereiopods and a long carpus on the second pereiopod, being 7– 9 × long as broad (vs. 3.5–7.1 × long as broad in the Madagascan species) and being excavated (vs. absent) anteriorly. Further, in Caridina the uropod diaeresis is armed with more than one spine ( Holthuis 1965; Richard and Clark 2009). Monsamnis gen. nov. can be separated from the troglobic genus Parisia , in having exopods on all pereiopods (vs. absent) as well a well-developed, cornea.