A review of Atlantic deep-water species in the genus Talassia (Caenogastropoda, Vanikoridae)

. A review was done on all species of the genus Talassia (family Vanikoridae Gray, 1840), which are known from an upper bathyal depth range in the Atlantic Ocean. Four new species are proposed: Talassia mexicana sp. nov. from the Gulf of Mexico, T. laevapex sp. nov. and T. flexisculpta sp. nov. from off Mauritania and T. rugosa sp. nov. from off Angola. Empty shells of the new species were found in sediment samples collected in habitats associated with deep-water corals. The new species were compared with the type species Talassia coriacea (Manzoni, 1868) and the deep-water species T. tenuisculpta (R.B. Watson, 1873), T. dagueneti (de Folin, 1873) and T. sandersoni (A.E. Verrill, 1884). Particularly the shape and sculpture of the protoconch show regional differences. Other specific characteristics are macro- and micro-sculpture of the teleoconch.


Introduction
This article is a contribution to the biodiversity of Gastropoda Cuvier, 1798 (Mollusca) in the upper bathyal depth range of the Atlantic Ocean. The study reviews Atlantic upper bathyal species in the genus Talassia Warén & Bouchet, 1988. The genus Talassia was erected by Warén & Bouchet (1988) in the family Vanikoridae Gray, 1840. To date, only six species are known in Talassia (MolluscaBase 2022): the type species Talassia coriacea (Manzoni, 1868), T. tenuisculpta (R.B. Watson, 1873), T. dagueneti (de Folin, 1873), T. sandersoni (A.E. Verrill, 1884), T. macrostoma (Thiele, 1925) and T. philippeswinneni Rolán & Swinnen, 2011. Talassia coriacea lives on the shelf from the Canary Islands to the southern Bay of Biscay (Warén & Bouchet 1988;Segers et al. 2009;Naturalis 2022). Talassia dagueneti and T. tenuisculpta live on the upper bathyal slope and shelf in the northeastern Atlantic and into the western Mediterranean Sea (Warén & Bouchet 1988;Bouchet & Warén 1993;Segers et al. 2009;Naturalis 2022), and T. sandersoni lives on the upper bathyal slope in the northwestern Atlantic (Rosenberg 2009). Talassia macrostoma was described from shallow water of the Agulhas Bank, South Africa and T. philippeswinneni was described from the sublittoral in Senegal (Rolán & Swinnen 2011). In our study, we only concentrate on deep-water species and the latter T. macrostoma and T. philippeswinneni have not been reviewed. Four new species of the genus Talassia are proposed herein: Talassia mexicana sp. nov. from the Gulf of Mexico, T. laevapex sp. nov. and T. flexisculpta sp. nov. from off Mauritania, and T. rugosa sp. nov. from off Angola.

Material and methods
The samples studied herein were gathered during cruises by German and Dutch research vessels over the period 1978-2018. The Atlantic species in Talassia were sampled during cruises with the RV Victor Hensen (VH97), RV Meteor (M61/1, M61/3, M122, M151), RV Poseidon (POS316, POS346), RV Sonne (SO175), RV MS Merian (MSM16/3, MSM20/4), RV Tydeman (CANCAP/3) and RV Pelagia (64PE182, 64PE229, 64PE320). The objectives of most of these cruises included studying biological, oceanographic and geological aspects related to deep-water coral (DWC) habitats. New species of the genus Talassia were encountered in three areas: the Gulf of Mexico (Matos et al. 2017), off Mauritania (Ramos et al. 2017;Wienberg et al. 2018) and off Angola (Hebbeln et al. 2020). In 2007, the RV Poseidon cruise POS346 concentrated on the southern coral mound chain off Mauritania (Westphal 2007); sampling of two new species was conducted by box core. In 2010, the RV MS Merian cruise MSM16/3 sampled in and near DWC habitats on the coral mound chain as well as in canyons of the Mauritanian upper margin (Westphal et al. 2014;Wienberg et al. 2018); sampling was done by bottom grabs and box cores. In 2012, the RV MS Merian cruise MSM20/4 sampled DWC habitats off the Bahamas, around Florida and off Yucatan (Hebbeln et al. 2012(Hebbeln et al. , 2015Matos et al. 2017); samples with a new species were taken by box core and bottom grab. In 2016, the RV Meteor cruise M122 visited DWC mound chains off Namibia and Angola (Hebbeln et al. 2016(Hebbeln et al. , 2020. During M122, shells of a new species were only found in sediment taken by a bottom grab off Angola. The seabed sediment samples were sieved retaining fractions larger than 0.5 mm. Subsequently, the fractions were washed with fresh water and dried on board. The molluscs were separated from the sieved fractions at Senckenberg am Meer (SaM) under low magnification binocular microscopes. Selected specimens were prepared for imaging using a scanning electron microscope (SEM: VEGA3-Tescan); a gold coating was used to improve image quality. An incident electron energy of 10 KeV was used and micrographs were taken using both, secondary electrons as well as back-scatter electrons.
The shells from the Gulf of Mexico and West Africa were compared with species known from the northern Atlantic and the western Mediterranean. Important references included Warén & Bouchet (1988), Bouchet & Warén (1993, Segers et al. (2009) and Rolán & Swinnen (2011). Shells from known species were taken from the reference collection at Senckenberg am Meer (SaM), Wilhelmshaven.
The holotype and most paratypes of the new species are stored in the Senckenberg Museum Frankfurt (SMF). Two paratype shells are kept in the Naturalis Biodiversity Center, Leiden, The Netherlands. The remaining material (including a few paratypes) is retained in the reference collection at SaM.
Coordinates have been converted to decimal degrees to facilitate incorporation into Geographic Information Systems and databases. The four decimals used suggest a spatial accuracy of about 10 m; the actual areal accuracy of sea bottom sampling is estimated to be 5-50 m.

Abbreviations for morphological terms
H = height of shell Ha = height of shell aperture Hp = height of exposed protoconch W = width of shell Wp = maximum width of exposed protoconch

Remarks
The aperture of the shell is relatively large (about 45% of shell height, Fig. 1) when compared to other NE Atlantic species (typically less than 40%). The spiral sculpture has fine, rugose and continuous ridges covering the teleoconch (Fig. 3). The species is found on the shelf of the Canary Islands, the Salvage Islands, Madeira and the western and northern Iberian Peninsula down to 200 m (Segers et al. 2009;Naturalis 2022). Talassia coriacea has been found sympatrically with T. dagueneti and T. tenuisculpta on the shelf of Madeira (Segers et al. 2009 Segers et al. 2009; this study). The outline of the shell is more slender (Fig. 6) than that of T. coriacea ( Fig. 1) and the spiral sculpture of the teleoconch has finely dotted lines; the flexuous axial riblets are equidistant (Fig. 7).

Remarks
Talassia tenuisculpta was described from off Madeira; a syntype is shown by Warén & Bouchet (1988: fig. 44; National Museum of Wales). Four additional syntypes are in BMNH (reg. 1875.5.27.29, not seen). The species is distributed from the Bay of Biscay to Madeira, the Azorean Seamounts and Islands and in the western Mediterranean Sea in 21-948 m (Segers et al. 2009; this study). The outline of the shell is more slender (Fig. 8) than that of T. coriacea (Fig. 1)

Remarks
Talassia sandersoni was described from off Cape Hatteras (Verrill 1884) and is found off the Atlantic coast of the USA and into the eastern Gulf of Mexico; in NMNH the syntypes (USNM 34447) are stored as well as two more lots from off Cape Hatteras and six lots from off Florida (Rosenberg 2009; this study). The syntype (USNM 34447) was depicted by Warén & Bouchet (1988: fig. 48
VAriAbility. Number and strength of axial riblets and micro-sculpture is variable. Spiral cordlets sometimes faint or irregular, more frequently distinct and regular. Apical angle 29°-30°. Known range of adult height 3.6-4.1 mm.

Distribution
Gulf of Mexico, off Yucatan and off western Florida, known depth range 507-640 m.

Remarks
The radula and the soft parts of the new species are unknown. Only empty shells were found in or near coral debris with silty or muddy sand.

Differential diagnosis
This species is the largest in Talassia; adult size is more than 3.5 mm whereas all other species measure less than 3 mm. The protoconch is also largest with a diameter exceeding 0.4 mm; all other known congeneric species have a smaller protoconch diameter. Talassia sandersoni from off the SE USA is smaller, has 9-10 spiral lines on the last ¼ whorl of the protoconch and the sculpture of the teleoconch lacks the axial ribs (Warén & Bouchet 1988: fig. 48; Figs 12, 16-17, 19). The protoconch of the new species is smooth (Figs 23-24, 26) and the teleoconch has sharp axial ribs (Figs 22, 25, 29-30). All hitherto known NE Atlantic species (Talassia coriacea, T. tenuisculpta, T. dagueneti) have a protoconch with spiral cordlets composed of raised dots and dashes; the new species has a smooth protoconch. Talassia dagueneti shows more flexuous riblets on the teleoconch (Warén & Bouchet 1988;Figs 4, 6). Talassia coriacea has a large pyriform aperture (Warén & Bouchet 1988 ; Fig. 1).
VAriAbility. The growth lines on teleoconch vary in roughness. Occasionally, umbilical slit is nearly closed. The observed range for apical angle is 34°-36°. No variability observed in adult height (2.2 mm).

Remarks
The radula and the soft parts of the new species are unknown. Only empty shells were found in or near coral debris with silty or muddy sand.

Differential diagnosis
All hitherto known NE Atlantic species have a protoconch sculpture with spiral lines composed of raised dots and dashes. Talassia dagueneti shows an axial sculpture of regular raised flexuous riblets on the teleoconch and finely aligned spirals of raised beads on the protoconch (Warén & Bouchet, 1988;Fig. 5) whereas T. laevapex sp. nov. has finer irregular growth lines (Figs 38-39, 42-43) and a smooth protoconch (Figs 33, 35, 37). Talassia tenuisculpta has similar fine growth lines and spiral lines as the new species but its protoconch has a clear spiral sculpture of raised dotted lines (de Folin 1873; Warén & Bouchet 1988;Figs 9-10). Talassia coriacea is larger and has a large pyriform aperture (Warén & Bouchet 1988 ; Fig. 1). Talassia philippeswinneni has a protoconch with shallow micro-pits and its teleoconch has spiral cords (Rolán & Swinnen 2016: 120-124, figs 1-4). Talassia coriacea and T. philippeswinneni live on the shelf whereas the new species is found on the upper bathyal slope.

Etymology
The specific epithet refers to the flexuous axial sculpture.
VAriAbility. Sculpture of the teleoconch shows variability in strength of axial ribs. In a subadult shell (Fig. 44), the umbilical slit is nearly closed. Observed range of adult height 2.4-2.5 mm.

Remarks
The radula and the soft parts of the new species are unknown; only empty shells were found in or near coral debris with silty or muddy sand. Talassia flexisculpta sp. nov. was found sympatrically with T. laevapex sp. nov.

Description
Elevated shell with rounded whorls and flattened apex, irregular axial sculpture of raised ribs and spiral cordlets, flexuous lip and smooth protoconch, cream white. Holotype dimensions: height 3.2 mm, width 1.6 mm, apical angle 36°.

Distribution
The species is only known from the type locality.

Remarks
The radula and the soft parts of the new species are unknown.

Differential diagnosis
Talassia dagueneti has a similar teleoconch sculpture but its axial sculpture is more regular and lacks the spiral cordlets and its protoconch has a stronger spiral sculpture and a more elevated outline (Warén & Bouchet 1988;Figs 4-7) whereas T. rugosa sp. nov. has a more conical outline, an irregular sculpture with spiral cordlets and a smooth protoconch with a flattened outline. Talassia mexicana sp. nov. from the Gulf of Mexico is larger and it has a more regular and stronger axial sculpture.

Discussion
One new species of the genus Talassia is described from the Gulf of Mexico and three are described from western Africa. All American and western African species show a predominantly smooth protoconch. Conversely, the three hitherto known species from the NE Atlantic and western Mediterranean Sea all show protoconchs with spiral sculptures. The two American species have spiral cordlets on the teleoconchs; the NE Atlantic species lack distinct spiral features.
Talassia tenuisculpta, T. dagueneti, T. sandersoni, T. mexicana sp. nov., T. laevapex sp. nov. and T. flexisculpta sp. nov. have been found in association with DWC habitats. The food source of these species must be present in these habitats. A link between Talassia rugosa sp. nov. and coral habitats needs to be confirmed.
Talassia laevapex sp. nov., T. flexisculpta sp. nov. and T. rugosa sp. nov. have been found in upper bathyal oxygen minimum zones (OMZ) off western Africa (Westphal 2007;Westphal et al. 2014;Hebbeln et al. 2016Hebbeln et al. , 2020Ramos et al. 2017;Wienberg et al. 2018). Off Cape Blanc (NW Africa), the southward flowing Canary Current collides with the northward flowing Mauritanian current thereby forming the SW flowing Northern Equatorial Current (NEC). The bathyal water mass south of the NEC has a dissolved oxygen concentration of less than 1.5 ml / l (Wienberg et al. 2018). Higher oxygen levels are found in the water mass to the north of the NEC (Wienberg et al. 2018). Hence, it is likely that this hydro-oceanographic separation causes a distinct development of NE Atlantic and West African bathyal species.
The reference collection at SaM contains shells of Talassia dagueneti from the Porcupine Basin. This submarine structure is the northern extent of its distribution range. Talassia dagueneti shows significant variability in outline and sculpture over its distribution area (Warén & Bouchet 1988;Bouchet & Warén 1993).
Similarly, many shells of Talassia tenuisculpta have been found on seamounts south of the Azores. This area currently is the western extent of its distribution area. The species has relatively little variability; it appears to have a more southern range when compared to T. dagueneti, including the Lusitanian Seamounts and island slopes off Madeira and the Canary Islands. The syntype figured by Warén & Bouchet (1988: fig. 44) is a slender subadult specimen. We consider the species distinct from T. dagueneti based on the sculpture of the teleoconch.
Few descriptions of soft parts and radula have been reported upon (for example by Warén & Bouchet 1988). Therefore, differentiation of species within Talassia is predominantly based on shell morphology. Actually, most species are solely known from empty shells. A study on DNA sequences could verify the current specific split when adequate soft parts become available. Soft parts could also be used to determine the food source of Talassia, for example by analysis of stomach contents or by stable isotope studies.
to CANCAP/3 samples. Serge Gofas, Thierry Backeljau and an anonymous referee improved the manuscript by giving many constructive suggestions.