Genus Stenothoe Dana, 1852 In Krapp-Schickel (2006b), a key to the genus Stenothoe was presented, dividing the (at that time) 45 valid species by the characters of a carinate body (one species), prehensile peraeopods (one species), naked telson (10 species) or spinose telson (33 species). While the group with naked telson contains rather well-defined species, the last group still contains species with questionable status. The situation of Stenothoe mediterranea Ledoyer [valid species or subspecies of S. marina (Bate)] is still not clearly defined; I count it here as a valid species. In the present study, I define S. quingtaoensis Ren as a junior synonym of S. haleloke Barnard, and also S. irakiensis Salman as a junior syn. of S. gallensis Walker. S. cattai was seen partially as a synonym of S. gallensis but is herewith revalidated. At the beginning of this study, knowledge had grown to 51 species considered valid; four additional new ones are here described. Here is the actual situation: S. adhaerens Stebbing, 1888, S. allinga Barnard, 1974, S. andamanensis n. sp., S. antennulariae Della Valle, 1893, S. aucklandica Stephensen, 1927, S. bosphorana Sowinsky, 1898, S. brevicornis Sars, 1883, S. cattai Stebbing, 1906, S. coutieri Chrevreux, 1908, S. cavimana Chevreux, 1908, S. clavetta n. sp., S. crassicornis Walker, 1897, S. crenulata Chevreux, 1908, S. dentirama Hirayama & Takeuchi, 1993, S. divae Bellan-Santini, 2005, S. dollfusi Chevreux, 1887, S. eduardi Krapp-Schickel, 1976, S. elachista Krapp-Schickel, 1976, S. elachistoides Myers & McGrath, 1980, S. estacola Barnard, 1962, S. frecanda Barnard, 1962, S. gallensis Walker, 1904, S. garpoorea Krapp-Schickel, 2009c, S. georgiana Bynum & Fox, 1977, S. haleloke Barnard, 1970 (syn. S. qingtaoensis Ren, 1992), S. hansgeorgi Krapp-Schickel, 2006b, S. himyara n. sp., S. inermis Ledoyer, 1979, S. kaia Myers, 1985, S. macrophthalma Stephensen, 1931, S. magellanica Rauschert, 1998, S. mandragora Krapp-Schickel, 1996b, S. marina (Bate, 1857), S. marvela Bellan-Santini, 2005, S. (m.?) mediterranea Ledoyer, 1973, S. megacheir (Boeck, 1871), S. menezgweni Bellan-Santini, 2005, S. miersi (Haswell, 1879), S. microps Sars, 1895, S. minuta Holmes, 1905, S. moe Barnard, 1974, S. monoculoides (Montagu, 1813), S. nonedia Barnard, 1974, S. penelopae Krapp-Schickel, 2006b, S. pieropan Krapp-Schickel, 1996b, S. quabara Barnard, 1974, S. richiardi Chevreux, 1895, S. senegalensis n. sp., S. sivertseni Stephenseni, 1949, S. stephensen Reid, 1951, S. symbiotica Shoemaker, 1956, S. tenella Sars, 1883, S. tergestina Nebeski, 1880, S. valida Dana, 1852, S. verrucosa Krapp- Schickel, 2009c. Two further species wait to be published. Within the Mediterranean, five species belong to the group of species with telson lacking spines: S. cavimana Chevreux, 1908, S. elachista Krapp-Schickel, 1976, S. mandragora Krapp-Schickel, 1996b, S. monoculoides (Montagu, 1813) and S. pieropan Krapp-Schickel, 1996b. Nine species have a spinose telson: S. antennulariae Della Valle, 1893 (until a short time ago thought to be a Mediterranean endemic, but now found also on the Dutch coast, in litt.), S. bosphorana Sowinsky, 1898 (until now seen as Mediterranean endemic), S. cattai Stebbing, 1906 (until recently called ‘ S. gallensis ’), S. dollfusi Chevreux, 1887, S. eduardi Krapp-Schickel, 1976, S. marina (Bate, 1857) with S. mediterranea Ledoyer, 1973, S. tergestina Nebeski, 1880 and S. valida Dana, 1852. From the Atlantic Ocean, the following four species have a telson without spines: S. brevicornis Sars, 1883, S. cavimana Chevreux, 1908, S. elachistoides Myers & McGrath, 1980 and S. monoculoides (Montagu, 1813). Twenty-four species have a spinose telson: S. antennulariae Della Valle, 1893, S. cattai Stebbing, 1906, S. coutieri Chevreux, 1908, S. crassicornis Walker, 1897, S. divae Bellan-Santini, 2005, S. dollfusi Chevreux, 1887, S. eduardi Krapp-Schickel, 1976, S. frecanda Barnard, 1962, S. georgiana Bynum & Fox, 1977, S. macrophthalma Stephensen, 1931, S. marina (Bate, 1857), S. marvela Bellan-Santini, 2005, S. megacheir (Boeck 1871), S. menezgweni Bellan-Santini, 2005, S. microps Sars, 1895, S. minuta Holmes, 1905, S. richardi Chevreux, 1895, S. stephensen Reid, 1951, S. symbiotica Shoemaker, 1956, S. tenella Sars, 1883, S. tergestina Nebeski, 1880 and S. valida Dana, 1852, plus two new species, S. clavetta and S. senegalensis (see below); one additional species is in prep. (Krapp-Schickel and Vader 2014). From the Pacific Ocean, eight species are known (all with spinose telson): S. crenulata Chevreux, 1908, S. dentirama Hirayama & Takeuchi, 1993, S. estacola Barnard, 1962, S. frecanda Barnard 1962, S. garpoorea Krapp-Schickel, 2009c, S. haleloke Barnard, 1970 (syn. S. qingtaoensis Ren, 1992), S. kaia Myers, 1985, S. verrucosa Krapp-Schickel, 2009c. In addition, one new species from Chile, also with spines on the telson, is ready to be published. From Australia– New Zealand, the following seven species have a spinose telson: S. allinga Barnard, 1974, S. aucklandica Stephensen, 1927, S. miersi (Haswell, 1879), S. moe Barnard, 1972b, S. nonedia Barnard, 1974, S. penelopae Krapp-Schickel, 2006b, S. quabara Barnard, 1974. Only one species from this region, S. hansgeorgi Krapp-Schickel, 2006b, has a naked telson. From the Indian Ocean, only two species have been reported until now: S. gallensis Walker, 1904 with spines on the telson, and S. inermis Ledoyer, 1979 with a smooth telson; S. andamanensis n. sp. from Andaman Sea and S. himyara n. sp. from the Red Sea are added here, both with spines on the telson. In Ruffo, 1938 S. monoculoides and S. spinimana (syn. of tergestina) are cited, the first with naked, the latter with spinose, telson; however, no material could be checked at the Verona collection. From the Subantarctic, the following three species are reported, all having a spinose telson: S. adhaerens Stebbing, 1888, S. magellanica Rauschert, 1998, S. sivertseni Stephensen, 1949. At the moment the genus Stenothoe contains 55 species plus two new species in preparation. The great majority, similar to the type S. valida Dana, 1852, show a clear sexual dimorphism: their gnathopods within one sex are quite different in size and shape, gnathopod 1 merus is strongly lobed, the inner plates of the maxilliped are very small and the telson is submarginally beset with strong spines. The other, much smaller group, similar to S. monoculoides (Montagu, 1813) and nearly exclusively distributed in the Atlantic–Mediterranean region, shows less or even no sexual dimorphism at all: first and second gnathopods are quite similar in shape, gnathopod 1 merus is not or little lobed, the inner plates of the maxillipeds are well visible and the telson is naked or has only tender marginal setae. I tried already several times to split the genus into at least two groups, but there is not one character which does not show some variability; even the arrangement of the plates in the second maxillae (a character often very difficult to see, but striking within the stenothoids) is not clearcut in tandem- or riding position, but has also many transitions. It is, especially in this family, so difficult to make groups, as one has to expect convergences everywhere: stenothoids like to live with or even inside other animals, or buried between sand grains in the interstitium; thus several characters lose their function for very different reasons. In any case, all species treated below belong to the first group.

Published as part of Krapp-Schickel, Traudl, 2015, Minute but constant morphological differences within members of Stenothoidae: the Stenothoe gallensis group with four new members, keys to Stenothoe worldwide, a new species of Parametopa and Sudanea n. gen. (Crustacea: Amphipoda), pp. 2309-2377 in Journal of Natural History 49 (37) on pages 2310-2312, DOI: 10.1080/00222933.2015.1021873, http://zenodo.org/record/4000077