Atlanta ariejansseni, Wall-Palmer, Deborah, Burridge, Alice K. & Peijnenburg, Katja T. C. A., 2016

Wall-Palmer, Deborah, Burridge, Alice K. & Peijnenburg, Katja T. C. A., 2016, Atlantaariejansseni, a new species of shelled heteropod from the Southern Subtropical Convergence Zone (Gastropoda, Pterotracheoidea), ZooKeys 604, pp. 13-30 : 17-19

Taxon classification Animalia Littorinimorpha Atlantidae

Atlanta ariejansseni sp. n. Figures 4, 5, 6

Type locality.

AMT24 station 28, 41.48°S, 33.86°W. Specimen collected on the 27th October 2014 at 02:59-03:48 local time at a water depth of 0-228 m.

Holotype.

Figure 5 j–l. Housed at the Naturalis Biodiversity Center, Leiden, accession number RMNH.5004155. For specimen dimensions, see Table 4. Collected by Alice K Burridge.

Paratypes.

Figure 4 a–i and k. See Table 4 for details.

Additional material.

See Table 1.

Diagnosis.

Atlanta species with a spire of 3 ¼ to 3 ½ whorls. The spire is moderately high, rounded and with deep sutures and covered in small, low projections approximately arranged in lines.

Description.

Shell small and transparent, with adult shells ranging from 2012 to 3059 μm in diameter excluding the keel and 2237 to 3370 μm including the keel in examined material. The shell inflates at 3 ¼ to 3 ½ whorls and has a total of 4 ½ to 4 ¾ whorls. The keel begins at 3 ¾ whorls and inserts between the final whorl and the spire for around ¼ whorl. The keel is tall and gradually truncated with a yellow-brown keel base. The keel often has a slightly undulating shape. The soft tissue varies greatly in colour among individuals from mottled white to orange-pink and dark grey (Fig. 5). Some specimens were observed to have a pearlescent lustre to the shell surface.

The spire is moderately high, well-visible in apertural view, with deep sutures, giving the whorls a rounded appearance (Fig. 6). The spire surface is ornamented with numer ous low projections in the form of punctae roughly arranged in 9-12 spiral rows over the surface of whorls 2-4 (Fig. 4). These low projections can vary in their spatial coverage, from closely spaced to sparse (Fig. 4 g–h). This gives the spire a rough appearance under a light microscope. The projections are clearly visible using SEM (Fig. 4). No other species of atlantid has been found with this type of micro-ornamentation in the inner spire. Juvenile specimens have approximately six fine lines of small projections running around the side of the shell, although these are not always obvious under light microscopy. Around the base of the juvenile shell the projections can become so closely positioned that they become irregular, frequently interrupted spiral lines in some specimens (Fig. 4j)

The operculum is type c, the radula is type I (Fig. 5 m–n) and the eyes are of type a ( Seapy et al. 2003), with no transverse slit (Fig. 5h and l).

Discussion.

The rounded spire, number whorls, opercular, radula and eye type all suggest that Atlanta ariejansseni belongs within the Atlanta inflata group of Richter and Seapy (1999). The most morphologically similar species are Atlanta californiensis and Atlanta selvagensis . Atlanta californiensis has the same number of whorls in the spire and the same overall adult shape as Atlanta ariejansseni , but it does not have any shell ornamentation. Atlanta californiensis also has much shallower spire sutures than Atlanta ariejansseni . Atlanta selvagensis is a slightly smaller species that does show shell ornamentation of the spire in the form of spiral lines that are frequently interrupted and highly variable; however, the ornamentation of Atlanta ariejansseni can clearly be distinguished from that of Atlanta selvagensis . Molecular results presented here also confirm that the two species are closely related, but separated by a K2P genetic distance of 0.14. No molecular data is available for Atlanta californiensis .

Previous publications have identified Atlanta ariejansseni as Atlanta gaudichaudi ( Howard et al. 2011) and Atlanta helicinoidea ( Pilkington 1970). However, these two species are also morphologically different from Atlanta ariejansseni . Although Atlanta helicinoidea belongs to the Atlanta inflata group, the spire has an extra whorl and the ornamentation is much coarser than that of Atlanta ariejansseni . Atlanta gaudichaudi is described as having no shell ornamentation, although some authors show this species with a single spiral line on the spire ( Seapy et al. 2003). However, Atlanta gaudichaudi does not have the low projections that are found on the spire of Atlanta ariejansseni . DNA barcoding also shows that these two species are not closely related, with an average K2P genetic distance of 0.25.

Distribution.

All specimens were found between 37°S and 48°S latitude, in a narrow circumtropical band located in the Southern Subtropical Convergence Zone. Specimens were collected from the epipelagic layer (upper 372 m) using oblique plankton tows in the Atlantic and Pacific oceans. For a summary of biogeography and sampling information, see Fig. 1 and Table 1.

Etymology.

Named after Arie Janssen, Naturalis Biodiversity Center, Netherlands, in recognition of his commitment and longstanding contributions to holoplanktonic gastropod research.