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)

The hitherto-known species of Stenothoe are grouped by their distribution. A key for Mediterranean–Atlantic species is given. To the so-called Stenothoe gallensis group (until now, four species), four others are added: S. andamanensis, S. clavetta, S. himyara and S. senegalensis, all new to science. Knowledge of Stenothoe aucklandica, frecanda, macrophthalma, valida and verrucosa is amended. The studied material also yields both a new species of Parametopa (P. gorea n. sp.) and a new genus (Sudanea n. gen.) with S. inopinata n. sp. http://zoobank.org/urn:lsid:zoobank.org:pub:23C8BB82-5CC1-40B2-BED9-431AA55901B9


Introduction
The morphology of Stenothoidae is more defined by the lack of clear-cut characters than by easily distinguishable differences, and loss or reduction of characters may have occurred several times independently. Thus, studies on stenothoid phylogeny are difficult and even keys for distinguishing the members are quite tricky.
The genus Stenothoe Dana, 1852 is, next to Metopa Boeck, 1871, the most numerous one within the family Stenothoidae, and is widespread across the world. The name of the type genus as well as the family is said to allude to the narrow (stenos) mouthparts, and in fact the maxilliped has become slender with reduced plates, the first maxilla has in some members lost one article of the palp, the second maxilla has its plates often sitting not next to but upon each other and the mandibular pulp tends to be reduced or even lost.
During my regular stays at the Verona Museum (Italy), the Australian Museum (Sydney), the Amsterdam Museum (now transferred to Leiden) and finally during a shows an upwardly curved and characteristically sculptured last article on the third uropods in males. In the following decades, during the twentieth century, several species with similarly built third uropods were described as new and soon after synonymized with S. gallensis, a species then consequently labelled as 'cosmopolitan'. But, as in many other amphipod species, there are extremely similar species all over the world, based on their morphology, with small but constant differences.
Typing 'Stenothoe gallensis' for a search on the internet brings up citations from seemingly everywhere (Cuba by Ortiz 1978;Hawaii by Barnard 1955; Atlantic by Feeley and Wass 1971;Fox and Bynum 1975;Mediterranean by Krapp-Schickel 1976;East and South Africa by Walker 1909;K.H. Barnard 1916K.H. Barnard , 1925Schellenberg 1928;Griffiths 1973Griffiths , 1974aGriffiths , 1974bRed Sea by Monod 1937;K.H. Barnard 1937;Ruffo 1969; Japan by Nagata 1965), but also many indications about this species as 'invasor', 'non indigenous' or 'introduced'; there is even a citation by Winfield et al. (2006) from soft bottoms of the Sigsbee abyssal plain, Gulf of Mexico, at 3635 m depth.
A closer look at the morphology quickly indicates that this so-called cosmopolitan species must be a complex of several different species, and, before stating an invasion from somewhere, it has to be defined clearly which species lives where. Furthermore, there do exist already the species S. cattai Stebbing 1906, S. crenulata Chevreux 1908, S. irakiensis Salman 1985, S. dentirama Hirayama & Takeuchi, 1993or S. uncinifera Mateus & Mateus 1966, which later were all put in synonymy with S. gallensis because of the similarly structured third uropod in males, by the above-cited authors (see Figure 1). Given this overview, it seems necessary to look better at all these cited species with their slightly different characters (see Table 1 or species treated in detail below).   2) Gnathopod 1 male length ratio of propodus to carpus + merus together 3) Gnathopod 1 male propodus length to width 4) Gnathopod 2 male setae on palm 5) Gnathopod 2 male palm 6) Gnathopod 2 setae on dactylus 7) Gnathopod 2 female propodus palm 8) Gnathopod 2 male merus posterior margin 9) Coxa 2 male shape and posterior margin 10) Peraeopod 6, 7 basis posterior margin 11) Uropod 2 rami 12) Uropod 3 male ramus first article 13) Uropod 3 male length ratio peduncle to ramus 14) Uropod 3 male shape of ramus article 2 15) Uropod 3 male lenth ratio article 1: article 2 16) Uropod 3 male ratio of peduncle to article 1: article 2 17) Telson spine pairs Note: Character states in bold indicate main differences.

Diagnosis
Length 5-6 mm. Coxa 2 rectangular, with parallel margins, in hyperadult males posterior margin somewhat excavate. Gn 2 male propodus with two palmar humps near dactylus insertion; palm straight, beset with long, dense setae; no remaining posterior margin; dactylus also densely beset with setae of about half the length of the ones on palm, but they can be worn out and lacking; merus posteriorly rounded, with shallow incisions where long setae are inserted. Gn 2 female propodus triangular, similar to the shape of Gn 1. U 2 rami spinose; U 3 peduncle spinose, ramus art. 1 with one distal spine; ratio peduncle: ramus subequal, with geniculation and sculpture of second article in males.

Redescription
Based on Walker's original description, completing it: Length of male 5-6 mm.
Peraeon. Peraeonite 4 not elongate, similar in length to peraeonite 3. Gnathopods 1-2 dissimilar in shape. Gn 1 subchelate; length of propodus = length of merus + ischium together; length of propodus = 3× length of carpus; merus enlarged, produced distally, the tip reaching or surpassing end of carpus; carpus triangular, l ≥ b; propodus 2× as long as broad, palm equal to remaining hind margin, group of defining setae is small and not strong; palm rounded, propodus on palmar corner widest, distally narrowing. Gnathopod 2 merus posterior margin in both sexes crenate, with a setule in each notch. Propodus posterior margin straight, palm in male reaching proximal end of propodus, thus no posterior margin remaining, palmar corner lacking, with two posterodistal humps (Walker:'double-pointed tooth'), the proximal higher, the irregular distal one with 6 intra-marginal setules; about 2.5-3× as long as broad, with few setae in female, dense row(s) of fine longer setae in males; dactylus reaching end of propodus in female, often surpassing it in male, both margins smooth and beset with setae. Peraeopods: P 3, 4: similar to each other and to S. marina. P 5 basis after Walker l.c, 'about half as wide as long, longer than the next 2', which must be an error; merus only little produced distally; propodus nearly as long as carpus + merus together; dactylus strong.
Uropods: U 1 reaching end of U 3; peduncle > rami, rami subequal, a spine in the middle of the inner one, 2-3 spines in the middle of the outer one. U 2 shorter than U 1 or U 3, rami subequal, spinose. U 3 peduncle 'longer than the ramus' in text of Walker; but in his illustration U 3 peduncle is equal to ramus art 1 + art 2, peduncle beset with 5-6 spines along the upper margin; ramus art 1 with one distal spine, ramus art 2 upwards bent in the middle and finely denticulate on the upper surface.
Telson concave on the upper side, oblong, with four spines increasing in size distally on the proximal half of each side.
Female. Gn 2 propodus palm convex, without palmar corner, but four nearly equidistant spines and some setae near the middle. U 3 last article straight and not rugose.

Remarks
The crucial characters after Walker are: length and shape of Gn 1 propodus, carpus, merus; Gn 2 two palmar humps near dactylus insertion; P 5 'basis wide, l:b = 2!' (a character which Walker withdrew later, as based on an error), U 3 ratio of peduncle and ramus subequal, geniculation and sculpture of second article in males.
On the Madras coast (India), Nayar (1959) found additional material of this species, and in 1967 again in the Gulf of Mannar (Sri Lanka), which he redescribed and illustrated. In 1985, Salman erected a new species Stenothoe irakiensis (thoroughly guarded by Jerry Barnard) from the Arabian Gulf, and it matches perfectly in all described details, except that he describes and illustrates a peduncular spur on U 1, not mentioned before; thus, it is given in synonymy here.
The material from East Africa, that I examined at the Berlin Museum, also matches S. gallensis perfectly. I do not offer an illustration of this species in the present paper, as two colleagues have detailed drawings ready for publication, and on the other hand the drawings by Nayar and especially by Salman (1985) give sufficient information.

Distribution
Indian Ocean: from East Africa to South China Sea. Stebbing, 1906 ( Figure 2; Table 1)

Diagnosis
Length 3-4 mm. Gn 2 male propodus palm with dense but short setae, dactylus with few very short setae; carpus posterior margin not much rounded. Gn 2 female propodus with small triangular protuberance. U 3 many spines along upper margin of ramus art 1; art 2 only distally upwardly bent, never geniculate, little sculptured.

Remarks about various authors
The description of Stebbing's new species of 3 mm is very sparse; the hint to Stenothoe valida 'partim' described by Della Valle (1893) is not very helpful, as on p. 566 in Della Valle Gn 2 male propodus is described (and illustrated in t. 58) with one distal tooth, exactly as S. valida should have, but not the new species. Stebbing erected this species after the description and illustration of Probolium polyprion (name preoccupied) by Catta 1876, but Catta himself synonymises his species with Probolium megacheles of Heller, 1866, which is again a synonym of Stenothoe valida (see Krapp-Schickel 1974 who checked the type material). Looking finally at the quite detailed illustrations, the most helpful character for this species is not illustrated: U 3 ramus article 1 has in S. valida only distal, but no marginal spines (always also marginal ones in the Mediterranean-Atlantic material, see Krapp-Schickel 1976, p. 15, fig. 14-16, 1993Lincoln 1979, p. 202, fig. 92). Gn 1, although illustrated with a rectangular propodus instead of a regularly rounded one, has a short merus and triangular carpus, which never is the case in S. valida; thus, it should correspond to the Mediterranean material until now called S. gallensis.
Stebbing offers only 'Mediterranean' as type locality, but the type locality of Catta's species is Marseille, and the material was taken from the keel of a ship coming from Pondicherry (India) and the Cape of Good Hope. Perhaps this fact is one of the reasons that S. cattai has been given so quickly in synonymy with S. gallensis, a species described from Ceylon. Otherwise, S. valida is well known as preferring harbours, and thus would have ecologically fitted perfectly.
The description in Chevreux & Fage (1925, p. 132, fig. 131) corresponds well with the one by Lincoln (1979, p. 202, fig. 92) except for Gn 1 where the carpus is shorter in L. (l = b, vs l > b in Ch&F). Gn 2 female shows a triangular acute hump in the middle of the convex palm after Ch&F (p. 132, fig. 131), which is shown in K-S for 'juv. male'. But all authors report many spines along the upper margin of U 3 ramus art 1, a unique character of this species within the S. gallensis group.

Diagnosis
Length 3 mm. Gn 2 male basis anteriorly widened; palm with dense but short setae, similar to those on dactylus inner margin.
Pleon. Uropods: U 1 reaching end of U 3; with prominent peduncular spur; peduncle > rami, rami subequal, spinose. U 2 shorter than U 1 or U 3, rami subequal. U 3 peduncle equal to ramus, ramus art 1 ≥ art 2, art 1 beset with 2-3 spines along the upper margin, with pair of distal spines, ramus art 2 upwards bent in the middle and regularly sculptured on the upper surface. Telson concave on the upper side, oblong, with three spines on the proximal half of each side.
Female: Gn 2 propodus palm convex, without palmar corner, but four nearly equidistant spines. U 3 last article straight and not rugose.

Remarks
This species has been synonymized with S. gallensis and is herewith revived, belonging to the S. gallensis group. Difference to the above described S. gallensis: length only 3 mm; Cx 2 posteriorly somewhat excavated, distally narrowing, but this character probably changes with size; Gn 2 male basis anteriorly widened, sometimes with crenulate margin, merus posterior margin regularly rounded and densely beset with short setae; length of setae on dactylus and propodus similar; U 2 subequal; U 3 male ramus art 2 claw-like, regularly curved and sculptured; U 3 ratio peduncle ≤ ramus, ramus art 1 > art 2. J.L. Barnard (1955, p. 3-5, fig. 1) illustrates material of what he called S. gallensis collected from Hawaii which seems very similar, but the length of males is given as 5 mm (vs 3 mm in Chevreux for S. crenulata), Cx 2 has a rectangular shape and no excavation on posterior margin, male Gn 2 merus has an acute posterodistal corner, and he does not mention nor illustrate the remarkably long peduncular spur on U 1 of S. crenulata, also not described by Chevreux, but always well visible in the present material.
The material of S. crenulata collected from Puerto Rico by Shoemaker (1935, p. 237 fig. 2) has no body length indicated, Cx 2 is drawn as regularly rectangular, and the setae on Gn 2 propodus are longer than the ones on the dactylus.
From Barbados and from Curaçao came two much smaller males, also illustrated here. The characters mentioned above are mixed, thus probably depending on allometry: Length is 2 mm, Gn 2 male is very similar to the drawing by Barnard (1955, fig. 1; but body length less than half), Gn 2 merus is also strikingly acute distally, but the setae on the propodus are much longer than illustrated in Barnard, while the ones on the dactylus are extremely short and much less dense. Cx 2 is slightly excavate (vs straight in Barnard), U 1 has a long peduncular spur (not illustrated in Barnard).
More than 20 years ago, I collected quite a lot of stenothoids from Bali (Indonesia): in the same samples there is S. frecanda Barnard, 1962 (described from California) and material very close to the original description of S. crenulata: Cx 2 is posteriorly slightly excavated or rectangular, Gn 2 male setae on the dactylus as long and dense as on propodus inner margin or somewhat longer. The difference between S. frecanda and S. crenulata is not very strong, besides the shape of U 3, but U 1 in S. frecanda is never straight, but bent upwards in the articulation between peduncle and ramus, and U 2 rami are clearly unequal.
I think all material discussed here matches S. crenulata Chevreux morphologically.

Diagnosis
Length 2.5-3.2 mm. Gn 2 sexually little dimorphic: Gn 2 palm with many short setae and few long ones; dactylus few short setae or naked. U 3 last article scarcely curved, in male strongly sculptured, in female smooth; length of peduncle subequal ramus article 1 subequal article 2.

Remarks
This species clearly belongs to the 'Stenothoe gallensis group' as it has a strongly sculptured last article in the male U 3, although the authors did not discuss the other members at all when comparing it with other Stenothoe. An important and easily differentiating character is the Gn 2 in males which is not or scarcely sexually different from the female. Type locality Andaman Islands, Indian Ocean.

Etymology
The specific epithet is built as an adjective from the locality of Andaman Islands.    Cx 2 male about as long as wide, posteriorly excavate. Gn 2 merus posterior margin smooth, distally acutely ending and often surpassing carpus. Propodus in male with straight and densely setose palm, distally with two humps; dactylus setose. Peraeopods: P 5 basis l: b = 3; merus posterodistally not much lengthened or widened; with nearly rectangular bases on P 6-7 in males, hind margin less widened than in females, without posterodistal lobe.
Ovigerous female. Gn 2 merus distally rounded, propodus with only a few setae, palm with characteristic shallow excavation; dactylus naked. U 3 second art of ramus straight, without grooves and not thickened, distally scarcely upwards bent.

Remarks
This species belongs to the S. gallensis group, and differs in some easily visible morphological details from Stenothoe gallensis from Sri Lanka: first of all in the body length (only half as long), then in the Gn 2 of the females (shallow excavation on palm of propodus) and in the shape of U 3 (last article not upwards bent or 'geniculate', but straight and distally thickened).
The description of 'Stenothoe gallensis' by Ledoyer (1986, p. 973-74, fig. 384) matches well with the present species, but Gn 2 female does not show the excavation on the palm of propodus and U 3 male is not thickened distally. Both could depend on immature material.

Etymology
The specific name alludes to the shape of the third uropod of the male, similar to a little club, pin or bludgeon; the Latin word 'clavetta' is used as noun in apposition.  Peraeon. Coxae. Cx 1 in adult male and female distally with some incisions, Cx 2, 3 on posterodistal corner irregularly finely dentate; Cx 2 in male about twice as long as wide, anteriorly shorter and rounded; Cx 3 rectangular, more than twice as long as wide, Cx 4 smooth, shield-like, trapeze-shaped, wider than long.
Gnathopods: Gn 1 propodus twice as long as wide, carpus triangular, merus on posterior margin beset with spines. Gn 2 female similar to Gn 1, merus less prominent, without spines and distally acute; propodus with smooth palm and strong defining spines on palmar corner. Gn 2 male propodus up to three times as long as Gn 1 propodus, in adult males propodus ratio l:b about 3, distally narrowing and near dactylus insertion 1-2 teeth, posterior margin as well as dactylus inner margin densely beset with long setae; carpus triangular, shorter than wide; merus longer than wide, posterior margin smooth, with short setae.
Peraeopods: P 5 basis linear. P 6, 7 basis ovoid and subequal, about twice as long as wide, posterior margin with incisions; merus posterodistal corner somwhat widened and lengthened, reaching about one third of carpus.
Pleon. Uropods: U 1 naked peduncle (vs S. gallensis peduncle with spines) with distal spur, rami somewhat unequal, with few spines. U 2 similar to U 1, but without spur and shorter; U 3 in female peduncle with three strong spines, about the same length as ramus article 1, while the claw-or dactylus-like art 2 is shorter. U 3 male peduncle is longer than ramus (vs in S. gallensis subequal to ramus), with 3-4 spines, art 1 is longer than art 2 and has two distal spines. The last article of the ramus has a very peculiar shape, being rounded and thickened proximally and abruptly narrowed in about half of the length (vs S. gallensis, whereas art 2 is sculptured, but not abruptly narrowed), with the distal half about one third of the width, strongly sculptured and finger-like rounded ending. Telson: triangular, with three pairs of spines.

Distribution
Bermuda, Atlantic Ocean.

Remarks
There is no doubt that Kunkel described the here found new species, under the name of Stenothoe valida; his illustration of U 3 fits perfectly in its unusually thickened and abruptly narrowing last article. Barnard (1955, p. 5) had already surmised that this species is a new one. However, in the text, Kunkel writes that the second article of the ramus is longer than the first one (which is not the case in our material, and the opposite is illustrated in his fig. 5), and it is not clear what he illustrated with 'ep 3'; it may be the second coxa. Kunkel even mentions the serration on some coxae ('ventral margin very finely serrate'), a unique character in the 'gallensis group', to which it clearly belongs.

Type locality
Port Sudan, Red Sea.

Etymology
Many living beings found in the Red Sea receive the specific name 'erythraeus, -a, -um' (classical Greek for 'red'), stressing the colour of the earth or sandstone in the region. But there is also another explanation, going back to the population of the Himyares living there, allegedly also meaning 'the red ones'. The epithet is used as an adjective.

Diagnosis
Length 1.5-2 mm. Male Cx 1, 2 and 3 without serration on distoposterior margin. U 3 male ramus art 2 proximal half about as wide as long or wider, then abruptly narrowing and bluntly ending like a thumb.

Description
Length 1.5-2 mm.   Head. Antennae: A 1 three times longer than head, about as long as head + peraeonites and clearly shorter than A 2. A 1 flagellum about 14 arts, A 2 flagellum about 11 arts.
Mouthparts: Md with lacking palp; Mx 2 plates in riding position; Mxp long and narrow, IP and OP vanishing.
Peraeon. Coxae: Cx 2 in both sexes longer than wide, anteriorly rounded; Cx 3 rectangular, more than twice as long as wide, Cx 4 smooth, shield-like and trapezium-shaped, wider than long. Gnathopods: Gn 1 propodus twice as long as wide, carpus triangular, merus on posterior margin beset with spines. Gn 2 female similar to Gn 1, merus less prominent, without spines and distally acute; propodus with smooth palm and strong defining spines on palmar corner. Gn 2 male propodus twice as long as Gn 1 propodus, in adult males propodus ratio l:b somewhat more than 2, distally near dactylus insertion no teeth or incisions, neither posterior margin nor dactylus inner margin beset with dense long setae; carpus triangular, shorter than wide; merus longer than wide, distally acute, posterior margin smooth.
Peraeopods: P 5 basis linear. P 6, 7 basis ovoid and subequal, less than twice as long as wide, posterior margin with small incisions; merus posterodistal corner widened and lengthened, reaching about one third to one half of carpus.
Pleon. Uropods: U 1 peduncle with spines, with distal spur, rami somewhat unequal, with few spines. U 2 similar to U 1, but without spur and shorter; U 3 in female peduncle with three strong spines, about the same length as ramus article 1 and the claw-or dactylus-like art 2. U 3 male peduncle is shorter than ramus, with one distal spine; art 1 is subequal art 2 in length and has two distal spines. The last article of the ramus U 3 shows a very peculiar shape being circularly rounded proximally and abruptly narrowed in about half of the length, with the distal half of about one third of the width, strongly sculptured and thumb-like rounded ending. Telson: triangular, with two pairs of spines.

Remarks
This quite small species has an astonishingly similar U 3 to the Atlantic new species S. clavetta, seemingly a useful convergent structure within the S. gallensis group (may be for fixing the body somewhere?). Otherwise, there are important differences, such as the length (S.c. twice as big), the Gn 2 in males (without special teeth or incisions in S.h.) and A 2 < A 1 in S.h., vs subequal in S.c.

Distribution
Red Sea.

Etymology
After the type locality.
Pleon. Uropods: U 1 not reaching end of U 3 (vs reaching end of U 3), with long peduncular spur. U 3 peduncle subequal to ramus (S.g. longer, Reid: the ramus is only a little, if at all, longer than the peduncle), with two robust spines along the upper margin and a pair on the distal margin (vs 5-6 spines); ramus art 1 with two distal spines (vs 1), ramus art 2 proximally much thicker than distally, with many coarse gutters, tip acutely uncinate (S.g.: upwardly bent in the middle and finely denticulate on the upper surface; Reid: serrated dorsally, proximal swelling ventrally).
Remarks Reid (1951, p. 228) reports ovigerous females with 2 mm length and others with 5 mm length; thus, he clearly did not have only one species in his samples called 'S. gallensis'. After the illustration of his S. cattai (p. 231, fig. 28), he probably had S. valida females (see below) in his collection of West African amphipods (sampling locality 11°54ʹ N, 17°14ʹ W, off Bathurst, Gambia), but the material of his 'S. gallensis' matches the here-described animals from Senegal. The most striking differences from S. gallensis or from S. valida are the body length, the shape of Gn 2 female, basis and merus of P 7, the shape of U 2, U 3. Differences from S. clavetta: the serration on the coxae and the shape of U 3. In the present material, there is some crenulation but never the incisions of S. clavetta. The material could belong to Stenothoe uncinifera Mateus and Mateus (1966, p. 177-178, fig. 3-6), but only one incomplete female was found and the description is poor and shows an artifact, ? damaged or malformed U 3 (sampling locality Ponta da Mina, île Principe). The shape of basis and merus P 7 is similar to that of S. valida, but otherwise the species belongs to the S. gallensis group.

Distribution
Senegal, Atlantic Ocean.
Herewith ends the description of various members of the Stenothoe gallensis group.
As the above-described material is quite close to the morphology of S. valida Dana (which has straight and unsculptured U 3 articles also in males, and thus does not belong to the S. gallensis group), a description of this species is herewith included:

Remarks
Two characters are quite helpful for the determination: the widened and extremely lengthened merus of P 6, P 7 reaching the end of carpus, and male Cx 3 with margins not parallel, but rather triangular or even trapezium-shaped, not very different from Cx 4, while females have a rectangular Cx 3 like all other Stenothoe members. The first character is repeated everywhere in the literature, but the second one is barely described, though illustrated in Chevreux (1935, pl. 16), Barnard (1953, pl. 15;1970, fig. 165) and Krapp-Schickel (1976, fig. 21).
Three differences to the original description of S. valida Dana are (1) very large round dark-pigmented eyes; (2) the body length (here 3-4.5 mm; in S. valida the original description reports '3-4 lines', which means up to 7.5-8 mm); and (3) the shape of Gn 2 female. For this last character, Dana (1853, p. 924) writes: 'palm nearly straight', and the margins of propodus and dactylus are densely beset with setae like in male; here Gn 2 propodus hind margin is regularly rounded, the propodus is medially the widest and shows a triangular hump. But all this variation could depend on allometry.
Not only in the samples of Reid, but also in the material studied here from Senegal, the two stenothoids S. senegalensis n. sp. and S. valida were living together. For distinguishing them it is helpful to look at the following characters: ovigerous females of S. senegalensis less than 3 mm, in S. valida more than 3 mm; U 3 male in S. senegalensis uncinate, in S. valida always straight; U 2 in S. senegalensis with subequal rami, in S. valida rami clearly unequal; Gn 2 female in S. senegalensis with rounded and smooth palmar margin, in S. valida with triangular hump or more elevations near dactylus insertion; eyes in S. senegalensis of normal size, in S. valida large and dark pigmented; P 7 in S. senegalensis basis always longer than wide, in S. valida strongly widened, merus in S. senegalensis never reaching carpus, in S. valida much widened, lengthened and usually reaching carpus.
Three specimens found at the Andaman Islands fit quite well the description of the morphology of S. valida; however, they are only about 3 mm and P 6, 7 basis is still wider; Gn 2 propodus of ovigerous females has a slightly different shape, not being widest in the middle, but towards the proximal end. But the three specimens are too few for judging if these character states are stable or variable.
I have the strong suspicion that not only has Stenothoe gallensis erroneously been called cosmopolitan, but Stenothoe valida too may well be a group of morphologically extremely similar species living in different regions of the world. I give here all the citations I know about this species, ordered by geographical region. Maybe they can be helpful in this 'detective' task.

Distribution
Until now seen as cosmopolitan, which is questionable.
Here follow some other members of Stenothoe for clearing up discovered taxonomic problems or streamlining the species' distinctions.

Material examined
Auckland Islands, Feb. 1973 Jim Lowry coll., sandy bay (1 slide); Auckland Islands red algae on rock, 0-2 m, Feb. 1973 Jim Lowry coll. (2 slides), both deposited at the AMS. Falkland Islands det. Schellenberg (1 slide), deposited at SNM. All the studied material matches the original description of S. aucklandica except the unequal rami of U2, which are defined for S. aucklandica var. falklandica Schellenberg, 1931 from the Falkland islands as different from those of S. aucklandica, but are equal also to the Falkland material studied here, and the reported ovoid bases on P 6, 7, which are here nearly circular. Thus, the variety S. a. falklandica is withdrawn.

Distribution
Auckland and Falkland Islands. Material examined I checked and illustrated this single specimen, deposited at SNM, with more details than in the original description.
Type locality North Atlantic.

Distribution
After 1931, I am unaware of any recorded specimens other than the single holotype, found at 60-65°N, 0-45°W at 425 m depth in the bathyal of the North Atlantic.

Material examined
Three specimens Nagasaki, deposited at SNM. One specimen East China Sea, deposited at SNM.
Type locality Southeast Indonesia.

Remarks
Morphologically, this species is astonishingly similar to the above-cited S. macrophthalma, besides the much smaller size of 3.5 mm, Cx 3 not rectangular but trapezium-shaped and A 2 peduncle art 5 short. In the present specimens, the name-giving 'warts' on Gn 2 palm are always present, but never the ones on A2 peduncle; thus, the shape of antenna 2, described and photographed in the original paper (Krapp-Schickel 2009c), could be an anomaly or depend on allometric growth.

Distribution
Pacific Ocean: Indonesia, East China Sea.

Remarks
As far as I know, no other localities were cited until now besides the Californian coast from Monterey Bay to South California shelf, 64-92 m depth.    Among the material from Senegal, I found still another species, formerly placed within the genus Stenothoe: Parametopa Chevreux, 1901 Included species: P. alaskensis (Holmes, 1904); P. crassicornis Just, 1980;P. kervillei Chevreux, 1901.

Etymology
Collected on the coasts of Gorée, Italian Gorea, a small island off the coast of Senegal.

Diagnosis
Gn 1 propodus subchelate, merus reaching end of carpus; Gn 2 propodus anterior and posterior margin parallel, palm oblique, with several incisions and humps; basis with many robust spines on anterior margin; U 1-3 and T strongly spinose.
Head. Antennae subequal in length, length of A 1 ped. art 1 = art 2, art 3 about one third; flagellum with 14 arts. A 2 peduncle art 4 somewhat longer than art 5, flagellum with eight arts. Mouthparts: Mx 1 palp with one art; Mx 2 lobes in riding position; Md palp lacking; Mxp narrow, OP vanishing.  Peraeon: Gn 1 subchelate, merus reaching end of carpus, distally not acute but rounded ending; propodus ovoid, palm oblique, about half length of propodus. Cx 2 anterior margin regularly convex, posterior margin concave; Gn 2 basis on anterior margin beset with many strong spines; carpus triangular, length subequal to width; propodus with nearly parallel margins, palm oblique, with many more or less deep incisions and humps; dactylus about half length of propodus. Cx 3 narrow, with parallel margins, distally slightly serrate, Cx 4 much wider than deep, ovoid. Peraeopods: P 3 > P 4; P 3 basis with many setae anteriorly; P 5 basis rectilinear, dactylus ≥ half length of propodus; anterior margin of merus, carpus and propodus with many spines; P 6, 7 basis widened, merus on both margins with spines, posterior one widened and lengthened, but not reaching half length of carpus.

Distribution
Senegal, Atlantic O.
Key to Parametopa species Within the present material from the Red Sea, I found a stenothoid with Gn 1 similarly long as but wider than Gn 2: Sudanea n. gen.

Diagnostic characters
Mandible without palp. Gn 1 about as long as but clearly wider than Gn 2; Gn 2 with parallel margins. P 6,7 basis widened. T without spines.

Remarks
The narrow second gnathopod with parallel margins strongly reminds of the genus Chucullba J.L. Barnard 1974, where both gnathopods show this shape. The propodus of Gn 1 is quite similar to Gn 2 of Sandrothoe Krapp-Schickel, 2010, where also the urosome is matching; but in Sandrothoe the basis of P 6, 7 is rectangular.

Type locality
Port Sudan, Red Sea.

Etymology
The epithet reflects the region inhabited by the species, at Port Sudan.
Peraeon. Gn 1 subchelate, merus reaching end of carpus, distally not acute but rounded ending; propodus ovoid to triangular, palm oblique, about half length of propodus; palmar corner defined by one strong spine. Cx 2 anterior margin irregularly convex, posterior margin straight; Gn 2 basis on anterior margin naked; carpus triangular, longer than wide, rounded; propodus with totally parallel margins, palm short, oblique, beset with short spines; palmar corner well defined by one srong spine; dactylus about one third length of propodus. Cx 3 with parallel margins, smooth; Cx 4 deeper than wide. Peraeopods: P 3 > P 4; P 5 basis rectilinear, dactylus ≥ half length of propodus; anterior and posterior margins of merus, carpus and propodus with few spines; P 6, 7 basis widened, merus on posterior margin with spines, posterior margin neither widened nor lengthened.

Distribution
Port Sudan, Red Sea.