Shell morphometrics in four species of Gadilidae (Mollusca, Scaphopoda) in Southwestern Atlantic Ocean, Brazil

Six morphometric shell direct measurements of the Gadilidae species Gadila acus (Dall, 1889), Cadulus braziliensis Henderson, 1920, Cadulus parvus Henderson, 1920 and Polyschides tetraschistus (Watson, 1879) were carried out and used to build three morphometric indices in according to the proposition of SHIMEK (1989): LI, relating total shell length to length from the dorsal aperture to the maximum width; WI, relating maximum shell diameter to apertural diameter; and Ws, whorl expansion rate (sensu RAUP 1966). Direct measurements and morphometric indices were tested as discriminating tools among the four species. None of the morphometric indices, alone, were able to discriminate among all species. Among then, Ws is not an effective discriminating morphometric index because it fails in all pairwise comparisons, LI and WI are fairly efficient ones, since they have got effective discriminations in three out six pairwise comparisons. The combined use of direct measurements ones was more efficient than the indices ones when dealing with relatively small samples, as ours. Although both descriptors can be combiningly used; since the direct measurements descriptors are easier to obtain and understand than the more complex indices, they should be recommended as main tools.