Stylostomum ellipse ( Dalyell 1853 ) Lang, 1884

Faubel, A. & Warwick, R. M., 2005, The marine flora and fauna of the Isles of Scilly: Free-living Plathelminthes (‘ Turbellaria’), Journal of Natural History 39 (1), pp. 1-45 : 23-24

Stylostomum ellipse ( Dalyell 1853) Lang, 1884

( Figure 6 )

Planaria ellipsis Dalyell, 1853: 101–102 , pl. XIV: 9–16.

Stylochus roseus M. Sars in Jensen, 1878: 75, pl. VIII: 1–3.

Stylostomum υariabile Lang, 1884: 585–588, pl. 8: 3, 4, 6.

Stylostomum sanguineum Hallez, 1893: 180–182 , pls III: 10, IV: 12–14.

Stylostomum antarcticum Hallez, 1905: 126 .

Stylostomum punctatum Hallez, 1905: 126 .

Material examined

Voucher specimen: site 8, one specimen, sagittally sectioned series of a sexually immature specimen. Same specimen in live observation stored on CD-ROM: 2003.6.6.4.

Geographical distribution. Arctic Ocean: Norway (Spitzbergen: Eisfjord); North Sea: Scotland (Firth of Forth: St Andrews); Skagerrak: Norway (Oslofjord: Hallangspollen), Sweden (Gullmarfjord, Halse Fjord); English Channel: England (Plymouth Sound, Falmouth); Irish Sea: Isle of Man (Port Erin), Ireland (Co. Dublin: Malahide shore); NE Atlantic: Ireland (Co. Donegal: Lough Swilly, Co. Mayo: Clew Bay, Co. Galway, Co. Kerry: Valentia Harbour); SE Atlantic: South Africa (Cape Town); SW Atlantic: Falkland Islands (Port William, Berkeley Sund), Tierra del Fuego (Cape Virgenes, Ushuaia); Mediterranean Sea: Tyrrhenian Sea: Gulf of Naples; Adriatic Sea: Gulf of Trieste ( Lang 1884; Plehn 1896; Micoletzky 1910; Bock 1913; Marine Biological Association of the United Kingdom 1957; Faubel 1984).

Description

Length of body 3.4 mm, width 1.4 mm; terminal ends broadly rounded, tapering slightly anteriorly, with smooth dorsal surface ( Figure 6 ). Colour yellowish white in incident light; in transmitted light, margins transparent but central body dark yellowish brown when gut diverticula full of food. Marginal tentacles less prominent forming mere blunt projections of the margin, with eye clusters at their base (8–10) and a few eyes (four) in between. Cerebral eyes in bilateral clusters above brain, each with 9–11 eyes ( Figure 6 ). Two characteristic pairs of eyes at the posterior margin of brain. Cylindrical pharynx immediately behind brain; intestinal trunk extending from proximal pharynx to hind end with nine pairs of intestinal lateral branches; frontal median branch of intestine extends dorsally of the pharynx to the anterior end (not drawn in Figure 6 ). Sucker in mid-body. Specimen not sexually mature.

Remarks

To date five valid species of the genus Stylostomum are known. Of these only Stylostomum ellipse is known from European Seas. All the other species were recorded from the Southern Ocean (Iles de Kerguelen: S. frigidium Bock, 1931 ) and Pacific Ocean. S. ellipse is obviously highly variable in colour depending on uptake of food. That variability led Lang (1884) to describe S. υariabilis as a synonym of S. ellipse . The characteristic determination features of S. ellipse are the frontal branch of the intestine, the typical arrangement of eyes, especially the two characteristic pairs of eyes at the posterior margin of the brain ( Lang 1884), and the tentacles formed as blunt marginal projections. In these characters the juvenile polyclad specimen recorded at site 8 agrees well with S. ellipse .

Figures

Figure 6. Stylostomum ellipse. Dorsal view in squash preparation. Scale bar: 500 Mm.