Abstract
Aplacophorans are common inhabitants of the deep-sea, where many places remain unexplored regarding their biodiversity. Filling a gap in knowledge about these animals from the South Atlantic, Scutopus variabilis sp. nov. (Caudofoveata, Limifossoridae) is described; further, species distribution modelling (SDM) was performed to elucidate the distribution patterns of Atlantic species of Scutopus. The type materials of S. megaradulatusSalvini-Plawen (1972) and S. chilensisSalvini-Plawen (1972), were examined and a search was performed for specimens of Scutopus held in museum collections. Scutopus variabilis sp. nov. has a slender and highly variable body form and a very distinct suture line is present midventrally. Two dominant types of trunk sclerites were observed by Scanning Electron Microscopy (SEM): one elongated with lateral margins slightly concave in medial portion, and another longer, with narrower base; its radula bears up to eight rows of heavily sclerotized teeth bearing 12–16 small denticles. The species occurs in a wide bathymetric range (40–1300 m), being more abundant at the edge between the continental shelf and upper slope. Outside the areas from where these samples were obtained, suitable areas for S. variabilis sp. nov. were found in the Southern Caribbean Sea (from where S. megaradulatus is recorded) and in the Brazilian Northern coast; the Gulf of Mexico and the Brazilian Northeastern coasts were found as unsuitable. Species of Scutopus appear to exhibit different patterns of geographical distribution: the European S. ventrolineatusSalvini-Plawen (1968) and S. robustusSalvini-Plawen (1970) are known as widely distributed, while non-European representants, the American S. megaradulatus, S. chilensis and S. variabilis sp. nov., and the Japanese S. schanderiSaito and Salvini-Plawen (2014) and S. hamatamiiSaito and Salvini-Plawen (2014) have more restricted distributions. However, clear and definite patterns of distribution of some of these species are probably blurred by sampling bias, for the European area is better studied. In the Atlantic, the SDM showed that species of Scutopus occur in a way that overlapping is minimized. Great sampling efforts combined with detailed descriptions based on SEM have revealed an interesting, abundant and up to now undescribed Brazilian deep-sea malacofauna.
Highlights
S. ventrolineatus by its long, undivided, cylindrical body which coils up spirally when it is disturbed, its radula composed by multiple rows of paired teeth, each tooth bearing median denticles, an undivided or homogeneous, post-oral shield, and the presence of a long midgut sac or digestive gland, and by its sclerites that ventrally are positioned in a divergent way forming a clear external ventral sutural line
Apart from this, the radula of Psilodens and Scutopus is quite distinct from the one of Limifossor: in the former genera, each tooth is inwardly curved, like a sickle, the ones of Scutopus having a variable number of denticles distributed all along or in a part of the concave inner side of each tooth, while in Psilodens there are no denticles; in Limifossor each tooth consists of broad plates that bear a main lateral projection like a long hook-shaped stylet, and one or two small additional projections which form a jagged cutting inner edge (SalviniPlawen, 1968, 1977, 1992; Ivanov, 1981; Scheltema, 1981)
Differences among these genera occur in the shape of the oral shield, that is undivided in species of Scutopus and bipartite in Limifossor and Psilodens (Salvini-Plawen, 1968, 1977, 1992; Ivanov, 1981; Scheltema, 1981)
Summary
Ubiquitous among the deep-sea organisms, aplacophorans form a molluscan group of particular phylogenetic interest (e.g., Kocot et al, 2011; Smith et al, 2011; Vinther et al, 2012, 2017; Mikkelsen et al, 2018, 2019), ecological importance (Scheltema, 1987, 1997; Scheltema and Ivanov, 2009), and biogeographical relevance (Scheltema, 1985; Ivanov and Scheltema, 2008; Corrêa et al, 2014, 2018; Bergmeier et al, 2017, 2019; Cobo and Kocot, 2020) They are generally characterized by their wormlike bodies covered by sclerites, and formed by two clades: Solenogastres, which are epifaunal and bear a ventral pedal furrow, and Caudofoveata, whose species are infaunal, without a ventral locomotory organ, and have an oral shield (that is absent in solenogasters). For the Brazilian coast (Southwestern Atlantic), in particular, eight species are known: Chevroderma turnerae Scheltema, 1985 recorded by Scheltema (1985) and Ivanov and Scheltema (2008), and Spathoderma bulbosum Ivanov and Scheltema, 2008 by Ivanov and Scheltema (2008); Falcidens targatus Salvini-Plawen, 1992 and F. acutargatus Salvini-Plawen, 1992 studied by Corrêa et al (2014); Claviderma amplum Ivanov and Scheltema, 2008, C. crassum Ivanov and Scheltema, 2008 and C. virium Corrêa, Miranda and Passos, 2018 investigated by Corrêa et al (2018); and F. australocaudatus Passos, Corrêa and Todt, 2016 described by Passos et al (2018). Passos et al (2019) have summarized all the records of aplacophorans from Brazilian waters up to that time, pointing out that most of them are from restricted oil-rich areas, there remaining large parts of its coast in which these molluscs were never reported (as its southernmost and northernmost portions), and that there are many other species to be investigated
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