Phorcus Risso, 1826

Steiner, Susanne Affenzeller Nicole Haar Gerhard, 2017, Revision of the genus complex Gibbula: an integrative approach to delineating the Eastern Mediterranean genera Gibbula Risso, 1826, Steromphala Gray, 1847, and Phorcus Risso, 1826 using DNA-barcoding and geometric morphometrics (Vetigastropoda, Trochoidea), Organisms Diversity & Evolution 17 (4), pp. 789-812 : 809-810

Genus Phorcus Risso, 1826

Type species: Phorcus margaritaceus Risso, 1826 (by subsequent designation), accepted as Phorcus richardi ( Payraudeau 1826)

Phorcus is characterised by a subtriangular shell shape with whorls more rounded than in Steromphala and a convex base. The shells are either smooth or show coarser spiral striae than in Steromphala . The umbilicus narrows with age. In large species, it can be closed completely by a columellar fold. The upper and lower aperture attachment points are separated by at least a quarter turn of the whorl (α ≥ 90°).

Phorcus richardi ( Payraudeau 1826)

Original combination: Monodonta richardi Payraudeau 1826: p 138 , n278, Pl VII, Figs. 1 and 2 .

Type material: MNHN-IM-2000-28253 (Muséum National d’ Histoire naturelle Paris, France), two syntypes .

The shell of Phorcus richardi is depressed with a bloated last whorl as originally described ( Payraudeau 1826). It appears rather thin-walled, and the inside of the rhomboid aperture always shows a layer of bluish nacre ( Gofas et al. 2011; Nordsieck 1968; Payraudeau 1826) ( Fig. 12 ). The type material matches the original descriptions well and corresponds to the collected specimens ( Fig. 12 ).

Phorcus richardi is characterised by a distinct shell shape and an open, large, round umbilicus surrounded by a rounded, white umbilical atrium edged by a black and white pattern, which is unique for this species (compare Gofas et al. 2011; Nordsieck 1968). The shell is wider than high. The attachment points of the aperture are separated by nearly a half turn of the shell (α ≈ 180°). The aperture lip attaches above the whorl edge with a slight shoulder and suture.

Phorcus mutabilis ( Philippi, 1846)

Original combination: Trochus mutabilis Philippi, 1846: p 166 , n 201, Pl 26, Figs. 18–22

Type material not located.

The original description by Philippi (1846) distinguishes Phorcus mutabilis from Ph. turbinatus by the denticle at the base of the columella. This denticle is not present in all individuals identified as Ph. mutabilis by DNA-barcoding of the present material. One of the specimens genetically identified as Ph. mutabilis shows the typical shell shape of Ph. richardi ( Fig. 13a and d , black arrow). Future research including nuclear genetic markers could elucidate possible hybridisation or incomplete lineage sorting in these species. Furthermore, Ph. mutabilis shows two distinct morphotypes: the individuals from Crete closely resemble Ph. richardi ( Fig. 13c ) in shell shape; two of the Crete specimens (e.g. Fig. 13c ), however, have the columellar denticle and spiral lines typical for Ph. mutabilis but not for Ph. richardi (compare Philippi 1846). Specimens from Croatia rather resemble Ph. turbinatus ( Figs. 13b and 14 ), a similarity already mentioned in the original description. They also show similarities to Ph. articulatus ( Fig. 15 ).

In all Phorcus mutabilis specimens examined, the umbilicus is not as open as in Ph. richardi . It seems as if the umbilicus becomes smaller or is even completely closed as the shell grows (compare Philippi 1846).

Further research is needed to understand the mechanisms behind this high variability in shell morphology.

Phorcus turbinatus ( Born, 1778)

Original combination: Trochus turbinatus Born, 1778: 340–341 , n K.II.4

Type material: NHMW14002 ( Naturhistorisches Museum Wien, Austria), two syntypes .

Phorcus turbinatus is originally described having an ovate shell with smooth whorls and a denticle at the columellar base. The specimens collected for this study match the original description and the type material ( Fig. 14c ) in their overall shape and the lack of an umbilicus, which is closed by a columellar fold. They differ, however, in lacking a denticle, and the presence of prominent spiral ridges ( Fig. 14b ). These differences may be due to the relatively small size of the collected shells. They seem to be immature and probably lose the shell sculpture with age.

Phorcus turbinatus shows a compact shell with a convex base. The slit like umbilicus is closed with age by a columellar fold. The aperture lip attaches on the round whorl edge, leaving a small shoulder and shallow suture.

Phorcus articulatus ( Lamarck 1822)

Original combination: Monodonta articulata de Lamarck 1822: p 36 , n 17

Type material: INVE 51532 (Muséum d’ Histoire naturelle Genéve, Switzerland), four syntypes .

Phorcus articulatus is originally described as having a conical shell with very convex whorls. The umbilicus, round and small, is closed with age. The columella base is marked by a distinct, rounded denticle. The single specimen of Ph. articulatus in this study ( Fig. 15b ), as well as the syntypes (e.g. Fig. 15c ), matches the original description.

The shell shape of Phorcus articulatus is characterised by a very high spiral height to diameter ratio. The columella shows a distinct denticle. The aperture lip attaches below the rounded whorl edge, leaving each whorl bulbous and without shoulder or suture. The aperture is comparatively small for Phorcus species, with attachment point set apart by just a quarter of a shell turn (α ≈ 90°).

Figures

Fig. 1 Sampling sites for Mediterranean specimens of Steromphala and Phorcus species

Fig. 2 Landmark templates for representative a lateral and b ventral standardised views. Filled circles show fixed landmarks. Empty circles show semilandmarks, processed as sliding landmarks

Fig. 12 Phorcus richardi (Payraudeau 1826). a PCA plot of PC1 vs. PC2 of genus Phorcus. Phorcus richardi (maroon) is the most abundant species in the current sample. It separates well from Ph. articulatus and Ph. turbinatus. One group of Ph. mutabilis is not distinguishable from Ph.

Fig. 13 Phorcus mutabilis (Philippi, 1846). a PCA plot of PC1 vs. PC3 from genus Phorcus. Phorcus mutabilis (olive) splits into two morphotypes: one from Croatia and one from Crete. b Representative specimen of Ph. mutabilis (Croatia) from this study. c Representative specimen of Ph. mutabilis (Crete) from this study. d Specimen (black arrow) showing Ph. richardi morpholo- gy, but Ph. mutabilis COI barcode. From this study. Scale bars 5 mm

Fig. 14 Phorcus turbinatus (Born, 1778). a PCA plot of PC1 vs. PC2 for genus Phorcus. Phorcus turbinatus (petrol) separates from all other species. b Representative specimen of Ph. turbinatus from this study. c One syntype of Ph. turbinatus (NHMW 14002). Scale bars 5 mm

Fig. 15 Phorcus articulatus (Lamarck 1822). a PCA plot of PC1 vs. PC2 for genus Phorcus. Single specimen of Phorcus articulatus separates from all other species. b Representative specimen of Ph. articulatus from this study. c One syntype of Ph. articulatus (INVE 51532). Scale bars 5 mm