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Holotype: ♂ (13.0 mm, partially dissected, preserved in ethanol 96%), Dellys, Boumerdès, Algeria, 36°55'27.14"N, 3°53'42.30"E, 6 Aug 2009, intertidal seaweeds (ZMUC-CRU-20458).
Paratypes: ♂ (12.5 mm), ♀ (11.0 mm), Galite Islands, Bizerte, Tunisia, (approx. 37°31'27.21"N, 8°56'23.54"E), 5 Feb 1909, ‘on the shore' (Stephensen, 1915) (ZMUC-CRU-20228). 2♂ (10.5, 8.9 mm), Dellys, Boumerdès, Algeria, 36°55'27.14" N,3°53'42.30"E, 6 Aug 2009, intertidal seaweeds, (CIBIO-UP, SstDel5 and SstDel9). 3♂ (10.1, 9.9, 10.5 mm), 3♀ (1 ovig. 11.8 mm, 2 non-ovig. 9.1, 9.8 mm), Tighremt, Bejaïa, Algerie, 36°52'0.60"N, 4°51'25.29"E, 4 Aug 2009, intertidal seaweeds (CIBIO-UP, SstTit4, SstTit2, SstTit18, SstTit1, SstTit15, SstTit17, respectively). 2♂ (13.2, 7.9 mm), Sidi Khaled, Tigzirt, Tizi-Ouzou, Algerie, 36°53'48.52"N, 4°10'52.46"E, 28 Jul 2009, intertidal seaweeds (CIBIO-UP, SstTiz16, SstTiz17). 3 mancas (3.8, 4.1, 4.3 mm), Alboran Island, Spain, 35°56'58.06"N, 3°01'48.57"W, 12 Feb 2005, intertidal seaweeds (CIBIO-UP, SstAlb1-3).
The species is characterised by a smooth and domed cephalon, with a prominent dorsal boss in lateral view; pereonites smooth, lacking lateral tubercles; pereopods II–VII robust, with merus and carpus 1.2 and 1.1 times as wide as long, respectively; pleotelson margins parallel or subparallel, curving regularly towards distal extremity at one third of its length; pleotelson with three pairs of lateral sutures only visible in ventral view; appendix masculina long, extending beyond apical margin of the endopod by more than one fifth of its length, but not beyond apical spines of endopod.
Body elongate, five times as long as wide (Figure 1). No secondary sexual dimorphism observable. Length of specimens in type series: 4.3–13.2 mm. Colour light brown to pale yellow, lightly pigmented.
Cephalon 1.3 times as wide as long, posterior margin immersed in pereonite I, smooth (no signs of mid-dorsal tubercle) but domed, with a prominent dorsal boss in lateral view; eyes dark, triangular or round, on lateral edge of cephalon; supra antennal line straight, anterolateral angles acute. Pereonites smooth, without dorsal carina. Coxal plates small, present on pereonites II–VII and hardly visible in dorsal aspect. All pleonites medially fused, with three pairs of small antero-lateral sutures in ventral view only. Pleotelson 2.4 times as long as wide, approximately one third of total body length.
Antennule: peduncle of three articles, article 1 ovoid, articles 2–3 cylindrical, similar in size; flagellum bearing seven pairs of aesthetascs. Antenna: peduncle of five articles, article 1 reduced, article 2 as wide as long, articles 3–5 progressively longer; flagellum of 17 articles, the distal one with minute vestigial apical article bearing a brush of short setae; flagellum varying from 14 to 17 articles on type series.
Mandible: Right mandible incisor 4 toothed; lacinia mobilis with one or two incisors; spine row with seven curved serrate spines; molar process truncate, without tooth. Maxillule: inner lobe with three distal plumose spines, inner margin with thin simple setae; outer lobe 1.8 times longer than inner lobe, with eight stout spines, four of them serrate; outer margin with small simple setae. Maxilla: trilobate, endopod with seven recurved plumose spines and eight simple setae; inner and outer lobes of exopod with five and four pectinate spines, respectively. Maxilliped: palp 4–articulate; exopod round; endite with a single coupling hook, five spines and a few simple setae along the distal margin.
Pereopods I–VII ambulatory (Figure 2), robust, with merus 1.2 times as wide as long, and carpus 1.1 times as wide as long, terminating in a biungulate dactyl with simple setae; pereopod I with simple spines on inner surface of propodus, and weak setation on ventral margin; pereopods II–VII subsimilar; pereopods II and VI with 8–12 palmate setae on distal margin of propodus.
Ventral penis smooth. Pleopods I–II rami with plumose marginal setae (Figure 3); pleopod II with long appendix masculina, extending beyond endopod by more than one fifth of its length, but not beyond its apical spines, apex distal inner margin serrated, with five minute spines; pleopods III–V 1.1 times longer and 1.2 times wider than I–II, without setae. Uropod: uniramous, with small plumose seta on lateral distal angle of peduncle.
The epithet honours Knud Hensch Stephensen (1882–1947), former curator of the crustacean collections at the ZMUC, who first noticed that some specimens he placed in Stenosoma acuminatum were likely to be a new species from the Mediterranean (Stephensen, 1915).
The material from Thor campaigns in 1908–1810, originally described by Stephensen (1915) fits in well with the present description of Stenosoma stephenseni sp. n. (see figures from Stephensen, 1915: 15–16). In particular, the male appendix masculina (also drawn in Stephensen’s figures) leaves no doubt on the taxonomic status of both specimens.
There are three sympatric species with which Stenosoma stephenseni sp. n. can be confounded: Stenosoma nadejda (Rezig, 1989), Stenosoma mediterraneum (Rezig, 1989) and Stenosoma capito (Rathke, 1837). Stenosoma stephenseni sp. n. can be easily distinguished from all three species, as these have a mid-dorsal tubercle on the cephalon, one pair of lateral tubercles on the first two (Stenosoma capito) or three (Stenosoma nadejda and Stenosoma mediterraneum) pereonites, and more slender pereopods, with carpus and merus at least 1.5 times as long as wide. The appendix masculina does not extend beyond the apical margin of the endopod in Stenosoma nadejda (see Rezig 1989: 72), and extends beyond the apical margin of the endopod by 0.05 and 0.14 of its length in Stenosoma mediterraneum and Stenosoma capito, respectively. However, in the latter two species, the appendix masculina reaches the tip of the apical spines of the endopod (see Rezig 1989: 49, 65), whereas in Stenosoma stephenseni sp. n. it does not (Figure 3B).
As discussed below, the inclusion of Stephensen’s specimens labeled “Stenosoma acuminatum” in Stenosoma stephenseni sp. n. has implications for the distribution of Stenosoma acuminatum. According to (Naylor (1972, 1990), Stenosoma acuminatum ranges from the southwest coasts of Britain to the Mediterranean, Adriatic and Black Sea. However, no factual information (reference, site/date) is given for the presence of this species in the Mediterranean. Stephensen’s (1915) record remains as the only published and verifiable record of Stenosoma acuminatum in the Mediterranean.
After the description of Stenosoma acuminatum by Leach (1814), many authors opted to synonymise it with Stenosoma appendiculatum (Risso, 1826) or Stenosoma capito (Rathke, 1837). White (1847: 95), in his “List of the specimens of Crustacea in the collection of the British Museum”, listed a single specimen of Idotea acuminata from England (Leach’s own Stenosoma acuminatum from Devon, see also White 1850) and three specimens from Tripoli (unknown collector). As Leach never mentioned any material other than the one from Devon in his descriptions of Stenosoma acuminatum (Leach, 1814, 1815), the specimens from Tripoli must have been acquired later.
Bate and Westwood (1868: 394) re-described Idotea acuminata from the British Isles, basing their drawings and description on Leach’s specimen, but included “Idotea capito” from the Black Sea (attributed to Rathke 1937) in the list of synonyms. Thus, although they did not mention explicitly the Mediterranean, their popular reference clearly led the unaware reader to infer the presence of Stenosoma acuminatum in that region. Gourret (1891) corrected the error of Bate and Westwood (1868), but subsequent authors acknowledged their synonymy (e.g. Carus 1885; Stebbing 1893; Gerstaecker 1901; Monod 1923), always referring the presence of Stenosoma acuminatum in both the Mediterranean and the Atlantic. Yet, none of these works added a single new record of Stenosoma acuminatum from the Mediterranean, data being copied from earlier literature without further checking of taxonomic consistency. For example, Carus (1885) lists Idotea acuminata from the Mediterranean, synonymising it with “Stenosoma acuminatum Leach, Idotea capito Rathke, Leptosoma lanceolatum Risso, Idotea lanciformis Risso”, and ranging from “Mare Brittanicum” (data taken from Leach, 1814), “Pontus Euxinus” (Black Sea, data taken from Rathke 1837), Nice (data taken from Risso 1816), and Lissa, Lesina and Curzola (Croatia, Adriatic) which are records of Idotea capito (=Stenosoma capito) from Heller (1866).
Neither Miers (1881) nor Collinge (1917) helped in eliminating this confusion. Miers (1881) united all described Stenosoma species (except Stenosoma carinatum) under a single species: Idotea acuminata. However, he mentioned that “This is a very variable species, and I have been obliged to unite under one name several types that have usually been considered distinct”. He correctly placed the specimens from Tripoli belonging to the collections of the British Museum in the variety “appendiculata”, which he synonymised with Stenosoma appendiculatum (Risso, 1826). Collinge (1917) who did not examine any British specimens of Stenosoma acuminatum, copied literally the description of Miers (1881), along with its presumed distribution (Mediterranean, Adriatic, Black Sea and Atlantic, up to Scotland). These inaccuracies made their way into popular references (Naylor 1972, 1990), and although some authors questioned the presence of Stenosoma acuminatum in the Mediterranean (Amar 1957; Prunus and Pantoustier 1976; Junoy and Castelló 2003), the record of Stephensen (1915) has always been there to attest to the contrary.
By including the two specimens from the campaigns of the Thor (Stephensen 1915) in Stenosoma stephenseni sp. n. the only published and verifiable record of Stenosoma acuminatum in the Mediterranean is eliminated. Other published records (e.g. Graeffe 1902; Argano and Campanaro 2011) should be checked if collections are available. These are likely to be misidentifications of Stenosoma appendiculatum, as is the case of the unpublished record of A. Dohrn from Naples (1957-06-16), labeled “Synisoma acuminata Leach”, and deposited at the Stazione Zoologica Anton Dohrn. Specimens can be observed online (see movie for CRU072 at http://szn.i.hosei.ac.jp/HTML/Prep_list.php?Family=Idoteidae&ListType=icon). Their pereon margins are clearly serrated (triangular coxal plates) and the pleotelson shape is like an ink pen nib, two features characteristic of Stenosoma appendiculatum.
A note on Idotea angustata Lucas, 1849 During this work, the description of Idotea angustata Lucas, 1849 came to our attention. This species was described from Algiers (Algeria), and judging from its original description, clearly belongs to the genus Stenosoma, together with Idotea carinata Lucas, 1849. Carus (1885) included Lucas’ record in his list of the Mediterranean fauna, but since then Idotea angustata has never been used as a valid name again. Some authors synonimised it with Stenosoma acuminatum (e.g., Miers 1881), others with Stenosoma capito (Monod, 1925; Kussakin, 1982). Both the drawing and the description of Idotea angustata bear some similarities with Stenosoma stephenseni sp. n. but also with three other sympatric species: Stenosoma mediterraneum (Rezig, 1989), Stenosoma nadejda (Rezig, 1989), and Stenosoma capito (Rathke, 1837)
Lucas refers that “La tête est légèrement gibbeuse” [the head is slightly convex] and that “Les organes de la locomotion sont courtes et assez robustes” [the organs of locomotion are short and rather robust], but the lack of any reference to the presence/absence of lateral tubercles in the first pereonites, and the exact shape of the pleotelson and the protuberance of the cephalon make this description ambiguous. Hence the name Idotea angustata which, according to the rules of the ICZN, is available from Lucas (1849), could be either a junior subjective synonym of Stenosoma capito (Rathke, 1837) or a senior subjective synonym of Stenosoma meditarraneum (Rezig, 1989) , Stenosoma nadejda (Rezig, 1989) or Stenosoma stephenseni sp. n. According to Rezig (1989), Lucas’ specimens were deposited at the MNHNP, but they could not be found there and currently there is no indication as to their present whereabouts (Danièle Defaye, pers. comm.). Unless these material is found, Stenosoma angustata (Lucas, 1849) has to be treated as a nomen dubium.
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