(FIGS 5C, 8B, D, 9C, 10C; TABLE 1)
Alternative species name: trigonostomum-setigerum
Trigonostomum setigerum Schmidt, 1852: 500, t. 47, fig. 13; Diesing, 1862: 229; Graff, 1905: 113–114, t. 3, figs 17–21; Micoletzky, 1910: 173; Southern, 1912: 3, 9; Graff, 1913: 303–305, figs 263, 264; Southern, 1915: 34; Meixner, 1924a: 202–203; 1924b: 89, 90, 92, 93–94, 96, 99–101, 102, figs 1, 2, 5; 1925: 256; 1926: 577; Steinböck, 1933: 10–11; Southern, 1936: 45, 57; Meixner, 1938: 25, 114, fig. 23; Westblad, 1952: 30–31; Ax, 1959: 97; Riedl, 1959: 319–322, fig. 6; Den Hartog, 1964: 375; Karling, 1978: 231, figs 27, 28; 1986: 209–210, figs 39, 40, 47,48; Ax & Armonies, 1990: 100.
Trigonostomum setigerum setigerum Graff, 1905: 113, t. 3, figs 19–21; 1913: 305.
Trigonostomum setigerum album Graff, 1905: 114, t.3, figs 17, 18; 1913: 305.
Trigonostomum setigerum lunulatum Graff, 1905: 114; 1913: 305.
Spiroclytus nisus Schmidt, 1857: 356, 365, t. 3, fig. 8; Diesing, 1862: 225.
Spiroclytus euryalus Schmidt, 1857: 356, 365, t. 3, fig. 8.
Spiroclytus setigerus Claparède, 1863: 15.
Vortex ornatus Uljanin, 1870:, 18, t. 4, fig. 15.
Hyporhynchus setigerus Graff, 1882: 338–339, t. 9, figs 6–14, t. 11, fig. 27; Pereyaslawzewa, 1893: 267, t.4, fig. 29, t. 10, figs 60a- e, 63d; Fuhrmann, 1898: 459; Sekera, 1901: 81; Sabussow, 1905: 488.
Known distribution: Mediterranean Sea (Schmidt, 1852,, 1857; Graff, 1905; Sabussow, 1905; Micoletzky, 1910; Meixner, 1925; Steinböck, 1933; Riedl, 1959), Adriatic Sea (Meixner, 1925, 1926), Black Sea (Uljanin, 1870; Pereyaslawzewa, 1893; Graff, 1905; Ax, 1959), Sea of Marmara (Ax, 1959), northern Atlantic Ocean (Fuhrmann, 1898; Graff, 1905; Southern, 1912, 1915, 1936; Karling, 1978), North Sea (Meixner, 1924b), southern Atlantic Ocean (Westblad, 1952).
New localities: France, Corsica, Port de la station Stareso, on algae, 10 April, 9 May and 19 October 1982; Punta Reveletta, on algae, 11 May and 22 October 1982. France, Banyuls, Ile Gros, on green algae near the jetty behind the station, 20–23 June 2000. Greece, Perea, east side of the beach, coarse-grained detritus-rich sand mixed with shell gravel; on green and red algae, 22 July 2002; on green algae and seagrasses, ± 2 m deep, 31 July 2002. Greece, Nea Michaniona, flat exposed beach, on Enteromorpha sp. and seagrasses, 22 July 2002. Greece, Aghias Triada, flat beach, on green algae and seagrasses ± 2.5 m deep, 22 July 2002 and 6 August 2002 (type locality). Greece, Nea Fokea (Kassandra Peninsula), heavily exposed beach, on algae, 28 July 2002. United Kingdom, Plymouth, Wembury, tidepool, 21 July 1949, Westblad (coll. SMNH). Kenya, Tiwi, on algae, 6 October 1991.
Material examined: Several specimens studied alive. Neotype (LUC no. 230) from Greece. Whole mounts from Banyuls (2), Bermuda (SMNH, no. 46460), Corsica (5), Falkland (SMNH, nos. 46429, 46430, 46440), Greece (26), Kenya (1), Plymouth (SMNH, no. 46452) and South Georgia (SMNH, no. 46448). Serially sectioned specimens from Falkland (SMNH, nos. 46431– 9, 46441–4), Plymouth (SMNH, nos. 46449–51), South Georgia (SMNH, nos. 46445–7) and Greece.
Diagnosis: Trigonostomum species with coiled copulatory organ, with two whole spires. Stylet 284– 498 Mm long, enveloped by the mantle over its entire length. Mantle distally split into two spiny plates with terminal hook. Bursal appendage 64–161 Mm long, with two tubules, proximally curved over 360∞.
Remarks: Graff (1905, 1913) recognized three subspecies, based on the pattern of pigmentation: T. setigerum setigerum (with dorsal stripe), T. setigerum lunulatum (with a rostral spot between the eyes) and T. setigerum album (without pigmentation). The validity of these subspecies was questioned by Southern (1912). Observations on a large number of live animals from one population in Greece showed that the three subspecies occurred sympatrically and that the difference between these three is not always clear. For example, specimens with a rostral spot (T. s. lunulatum of Graff, 1905) showed a large variation in size of the spot, making the difference with T. s. album very small. There is also variation in the length and width of the dorsal stripe: some individuals have a short dorsal stripe, which is broader between the eyes, while in other specimens the dorsal stripe is long and very broad over its whole length, giving it a network-like appearance. Based on the occurrence of all forms in one population (Greece, Thessaloniki), the lack of other distinctive features and the large variation of the three ‘forms’, the three subspecies are synonymized with T. setigerum.