Sexual dimorphism in shells of the southwestern Atlantic gastropod Olivella plata (Ihering, 1908) (Gastropoda: Olividae)

Most known cases of sexual dimorphism in molluscs are based on size differences, where female shells are usually larger than males (e.g. Morse, 1876). Despite some striking examples of this, such as the cephalopod Argonauta or the puelche oyster (Pascual et al., 1989), for gastropods the differences are not quite as marked. For example, in the common southwestern Atlantic nassariid, Buccinanops monilifer (Kiener, 1834), male size varies between 20.3 and 36.8 mm shell length, while mature females range between 31.0 and 49.5 mm (Simone, 1996: 97). This pattern is seen in species of Eulimidae (Waren, 1983), Cypraeidae (Griffiths, 1961; Schilder & Schilder, 1962), Cassidae (Clench & Abbott, 1943) and Strombidae (Abbott, 1949). In the latter family, Lambis is an interesting case in which males are normally smaller than females, but they also possess two smaller knobs on the shoulders of the last whorl, whereas females have a longer, single, high knob. Additionally, spines of males are shorter and flatter (Ueno, 1997). Other known cases of sexual dimorphism in gastropods include differences in radular teeth between males and females (Arakawa, 1958; Maes, 1966; Fujioka, 1982, 1984; Matthews-Cascon et al., 2005). A different expression of sexual dimorphism is the development of distinctive shell features that are generally related to reproduction or spawning. Such is the case in the turrids Aforia circinata and Gemmula lordhoweensis, where a tertiary apertural notch is present only in mature females (Shimek, 1984; Kantor & Sysoev, 1991). Colton (1905) recorded larger apertures and smaller columellar angles in males of Strombus pugilis. According to Beu (1998), some Bursidae have differently digitated or flared apertural margins depending on sex and egglaying capability. The whelk Buccinum undatum shows some sexual differences in the shape of the aperture (ten HallersTjabbes, 1979). Arakawa & Hayashi (1972) also recorded sexrelated shape differences in the shell of Ficus subintermedia. This paper describes a remarkable case of structural sexual dimorphism observed during a taxonomic revision of the genus Olivella from Argentine waters. Live specimens of Olivella plata (Ihering, 1908) were collected (December 2002, 2003, 2004 and March and December 2005) from the sandy infralittoral off Punta Villarino on the Golfo San Jose (428240S–648150W) at about 1–3 m during low tide, and at Punta Pardelas (428370S–648150W), Golfo Nuevo. Both localities are on the Valdes Peninsula, northern Patagonia, Argentina. Some of the specimens were frozen to observe expanded soft tissues, and others were relaxed with MgCl2. Several specimens were dissected and their penes critical-point dried and mounted for SEM observation. All the material is housed in the malacological collection at the Museo Argentino de Ciencias Naturales, Buenos Aires (MACN). Specimens of the two sexes were recognized by the presence of a penis in males and ventral pedal gland in females. The ventral pedal gland is visible to the naked eye (Fig. 1F). In addition, mature females are easily distinguished because they carry their own egg capsules attached usually to the columellar callus area and sometimes to the dorsal part of the shell (Fig. 1F). After the capsules are gone, a shallow, circular scar is present and visible in the position where each capsule was attached to the shell, facilitating recognition of females (Fig. 1I). Female shells possess a wide, vertical, anterior groove, adjacent to the parietal callus and the columellar pillar structure. The groove runs approximately at right angles to the apertural plane. Therefore, it is clearly visible in lateral view and less so in apertural or adapertural views. The groove curves adaxially at the tip of the curved pillar. It grows at the same rate as the shell, as shown by the growth lines visible at the base (Fig 1D, G, I, J). Male shells are easily distinguished because there is no groove, but a continuation of the parietal callus which apparently infills the groove described for the female shells (Fig. 1A– C, E, H). Adult shells can easily be separated by sex, but juveniles are more difficult. Olivella plata showed no shell differences at sizes as large as 4.7 mm, but males can be identified because of penis development. Shells smaller than this are not externally distinguishable by shell or soft anatomy. Shimek (1984), and later Kantor & Sysoev (1991), described the presence of a third apertural notch in the aperture of turrids (Aforia circinata from the Bering Sea and the Australian Gemmula lordhoweensis ). Shimek (1984) suggested a copulatory or oviposition role of this exclusively female structure and Kantor & Sysoev (1991) also concluded that it is possibly connected with breeding. Olivella species are known to lay egg-capsules in at least two different ways. Marcus & Marcus (1959) described the reproduction of Olivella minuta (as O. verreauxii ) in which the female attaches each capsule to any available hard substrate, such as shells of the bivalve Donax. Other species, such as O. plata and O. formicacorsi, lay the capsules over the parietal callus of their own shells (Borzone, 1995). The existence of the groove described here and present only on females of O. plata, could help the animal to glue the capsules to the apertural side. However, O. formicacorsi lays egg capsules in the same way, but has no groove present on the shell. I am grateful to Yuri Kantor (Moscow) for sending information about sexual dimorphism in turrids. A. Beu (New Zealand) also provided essential data on the same topic. J. C. Tarasconi (Porto Alegre) kindly made available Olivella specimens from his extensive collection. F. Scarabino (Montevideo) provided beneficial criticism and an updated bibliography. This work was supported in part by the Project PICT No. 14419 from the National Agency for Scientific and Technological Promotion, Argentina and Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), in which the author is member of the ‘Carrera del Investigador Cientifico’.