Colobocephalus costellatus M. Sars, 1870

Ohnheiser, Lena Tina & Malaquias, Manuel António E., 2014, The family Diaphanidae (Gastropoda: Heterobranchia: Cephalaspidea) in Europe, with a redescription of the enigmatic species C olobocephalus costellatus M. Sars, 1870, Zootaxa 3774 (6), pp. 501-522 : 512

Colobocephalus costellatus M. Sars, 1870

( Fig. 9 A–G)

Colobocephalus costellatus M. Sars 1870: 54 –57, tab. 11 figs 7–14. Odhner 1939: 8 –10, fig. 1. Høisaeter 1986: 100. Høisaeter et al. 2001: 249. Høisaeter 2009: 78.

Diagnosis. Shell internal, transparent, wavy growth lines and thin spiral ridges. Body whitish translucent. Foot anteriorly lobed, cephalic shield with short stumpy tentacular lobes.

Type locality. Drøbak, Oslofjorden, Norway.

Material examined. Aurlandsfjorden, Hordaland, Norway, 1 spc (dissected), ZMBN 91084, H = 2.1 mm. Marstein, Bergen, 60°08’08’’ N, 005°00’00’’ E, Norway, 1 spc, ZMBN 82283, H = 1.4 mm.

Shell ( Fig. 9 D–F): Maximum H = 2.1 mm. Internal (see Remarks), thin; transparent; globose in shape, aperture wide with thin parietal callus covering umbilicus partly, spire elevated but apex obtuse; shell surface costellate with faint wavy growth lines and thin elevated spiral ridges.

Animal ( Fig. 9 A–C): Body whitish transparent, eyes visible, foot anteriorly indented with groove, cephalic shield lobed with short stumpy tentacles formed by the rolled up shield, larval kidney visible.

Radula: 2.1.0.1.2 ( G. O. Sars 1878, tab. 12, fig. 16). Rachidian tooth absent. Inner lateral teeth curved with broad base, inner edge smooth. Outer lateral teeth with broad base, curved, smooth.

Male reproductive system ( Fig. 9 G): Short unbranched prostate emerging from short tubular penial sheath.

Ecology. On shell sand, sand and pebbles between 55–336 m ( G. O. Sars, 1878; Høisaeter 2009; present study).

Distribution. Aurlandsfjorden (present study), Kopervik (59°15’ N), Oslofjorden, Korsfjorden south of Bergen ( Brown 1979; Høisaeter 2009).

Remarks. M. Sars (1870) described Colobocephalus costellatus to have an external shell. Odhner (1939) observed that the mantle of smaller specimens in fact do not completely cover the shell, but in adults the shell is totally enclosed by the mantle. The specimens studied here had the shells slightly exposed but this was due to damage of the mantle edges. The mantle is very thin and easily torn apart.

Several authors (e.g. Odhner 1939; Høisaeter 2009; Høisaeter et al. 2001) considered Colobocephalus costellatus and Colpodaspis pusilla distinct species, while others (e.g. Brown 1979; Thompson 1988) list them as synonyms because of anatomical similarities. The latter authors considered the different shape of the head and mantle to be either due to young age or damage, but Odhner (1939) showed that C. costellatus has distinct and consistent features. The shell in C. costellatus is sculptured with a globose spire, while C. pusilla has an almost smooth shell with a keeled angulated shoulder. Furthermore C. pusilla possesses a siphon formed by the mantle (not present in Colobocephalus ) and unlike Colobocephalus can retreat into its shell completely.

Figures

FIGURE 9. Colobocephalus costellatus. A. dorsal view of complete animal (automontage picture). B. ventral view of complete animal (automontage picture). C. lateral view of live animal (automontage picture). D. dorsal view of first whorls of shell (SEM). E. detail of protoconch (SEM). F. detail of shell sculpture from last whorl (SEM). G. male reproductive system. Abbreviations: f, foot; ga, genital aperture; m, mouth; mg, median groove; mt, mantle tissue; pr, prostate; ps, penial sheath; sh, shell; t, tentacle. Scale bars: A, B = 250 µm; D, G = 100 Μm; E, F = 20 µm.