Taxonomic history of the genus Protatlanta
The first extant species of Protatlanta was introduced by Gray (1850), described from a specimen collected in the Atlantic Ocean. Gray (1850) identified this specimen as Atlanta lamanoni (a homonym of Steira lamanoni Eschscholtz 1825), after a species dubiously illustrated by Lamanon (1797) that is now considered to be a synonym of Atlanta peronii (Janssen, 2012c). Smith (1888) realised that Gray’s specimen was not A. lamanoni, but a new species and, in the description of specimens from the Challenger expedition, named this new species Atlanta souleyeti. In 1908, Tesch recognised that the conchiolin keel of this species required the introduction of a new intermediate genus between Atlanta and Oxygyrus, thus establishing the genus Protatlanta containing one species, Protatlanta souleyeti. Protatlanta souleyeti is a small species with an aragonite shell of up to 2 mm in diameter and a conchiolin keel. It is characterised by a rounded, slightly elevated shell spire that has no ornamentation (Smith, 1888; Seapy, 2011).
Issel (1911, 1915) described a further species and a variety of Protatlanta, Protatlanta sculpta Issel, 1911 from offshore of the Cape Verde Islands, Atlantic Ocean and Protatlanta sculpta var. mediterranea Issel, 1915 from offshore of Messina, Mediterranean Sea. Issel (1911) described the main features of P. sculpta as having a smaller juvenile shell than P. souleyeti, with brown colouration. The spire is described as being ornamented with irregular, wavy and frequently interrupted spiral lines, becoming just two lines in the final whorl of the juvenile shell. The final, adult whorl bears no ornamentation. Issel (1911) also suggested that the keel of P. sculpta inserts closer to the aperture than in P. souleyeti. Despite the clear morphological features that characterise P. sculpta, the species name P. sculpta has not been used since 1915 (Issel, 1915), having been assumed a synonym of P. souleyeti since this time. A number of authors have noted this ‘second form’ of Protatlanta, particularly in the Atlantic Ocean (Batten and Dumont, 1976; Richter and Seapy, 1999; Seapy 2011). Fossil specimens have even been described from the Pliocene of the Philippines (Janssen, 2007), although the name P. sculpta was not used.
Protatlanta sculpta var. mediterranea is described as having generally the same shape and ornamentation as P. sculpta from the Atlantic Ocean, but lacking the brown colouration. Unlike P. sculpta, this variety was recognised as a valid taxon until 1970 (Di Geronimo, 1970). We did not find any specimens matching this description from the Mediterranean Sea or elsewhere during this study. Therefore, we are unable to determine whether this is a valid variety of P. sculpta.
Two extinct species of Protatlanta with shell ornamentation have been described, both from the Miocene fossil record. Protatlanta rotundata (Gabb, 1873) was described from the Miocene of the Dominican Republic and Protatlanta kbiraensis Janssen, 2012b was described from the Langhian (15.97–13.65 Ma, Middle Miocene) of Malta and Italy. Descriptions and illustrations of specimens by Janssen (2007, 2012a) suggest that these species may have had spiral lines on the juvenile shell.
Here we investigate whether the morphological characteristics described by Issel (1911), and identified in specimens from recent plankton sampling, indicate a second species of Protatlanta. A combination of DNA barcoding of the CO1 gene, morphometrics and biogeography of specimens from the Atlantic Ocean are used to address the original description of P. sculpta to determine whether this species name should be revived. We follow an integrated taxonomic approach, combining molecular and morphological evidence (e.g. McManus and Katz, 2009; Padial et al., 2010), and consider a species to be different when it is genetically distinct and shows corresponding differences in morphological characters.