Diagnosis: Female. Carapace clearly tapering proximally, almost parallel with only slightly rounded lateral margin, pereonites lateral margin almost parallel. Cheliped basis separated from pereonite- 1 by large gap ventrally. Antennule 4 -articled. Antenna 6 -articled, occasionally fusion line at midlength on article- 4. Labrum relatively massive, covered by minute setae, with some longer setae distally. Mandible molar as long as incisor, with a few tubercles gradually elongated into five spines. Labium inner lobe slender (twice as long as wide). Labium outer lobe naked. Maxilliped endites with two short setae and one large conical tubercle. Cheliped massive; carpus only about 1.5 times as long as wide, 0.5 times as long as propodus; propodus large, with well calcified inner margin and strongly serrated keel ventrally; dactylus with strong tubercle on inner margin; unguis large, strongly calcified, as long as dactylus. Pereopods 1–6 compact; coxa present; merus, carpus and propodus with strong biserrated setae and numerous microtrichia along article. Pleopod rami paddle-shaped, with setae aggregated distally. Uropod endopod 2 -articled, twice as long as exopod.
Male: Unknown.
Type species: Monstrotanais mirabilicheles Kudinova-Pasternak, 1981.
Gender of generic name: Masculine.
Genera included: Monstrotanais mirabilicheles Kudinova-Pasternak, 1981; Monstrotanais ingens n. sp.
Remarks: Monstrotanais was erected for one specimen of M. mirabilicheles Kudinova-Pasternak, 1981 in the manca-II stage that was found during dredging in the North Pacific (174 ° 36 ' E – 22 ° 25 ' N). The genus is well characterized by its compact pereopods, massive cheliped with large propodus and strongly calcified unguis, and strongly calcified serrated lower margins of chela.
In the classification of Paratanoidea by Larsen and Wilson (2002) Monstrotanais, along with twenty six other genera, was not included to any of the newly defined families (Anderson et al., 2006: <http://tidepool.st.usm.edu/tanaids/incertaesedis.html>). In the preliminary phylogenetic efforts toward revising the classification Monstrotanais clearly associates with Leptognathioides Bird & Holdich, 1984 and Singula (Błażewicz-Paszkowycz, 2005) sharing some peculiar characters such as: compact pereopods 1–3, a wide mandible molar with a few spines on the lower margin, a massive chela, the cheliped basis separated by a gap from pereonite- 1 ventrally, and a one-articled uropod exopod (Błażewicz-Paszkowycz & Larsen, study in progress). A similar set of characters can be observed in Subulella Holdich and Bird, 1986 and Pseudoleptognathia Sieg, 1986 although the first genus lacks compact pereopods and spines on the mandible molar, while the other has a rather slim chela and its cheliped basis reaches pereonite- 1 ventrally. Compact pereopods and a stout chela occurs also in Stenotanais Holdich & Bird, 1984 (incertae sedis), Leptognathiella Hansen, 1913 (Colletteidae) and Filitanais Kudinova-Pasternak, 1973 (Colletteidae) but they all have a pointed mandible molar. Nematotanais Bird & Holdich, 1985 also has compact pereopods 1–3; in the classification by Larsen and Wilson (2002) it was included in the Colletteidae but it bears some peculiar characters distinguishing it from all genera mentioned above as well as from the others colletteids (lack of mandibles, reduced spines/ setae on merus and carpus of all pereopods, and exopod uropod modified to a ‘nonarticulated projection’).