Sinum cymba

Sinum cymba is a marine snail of the family Naticidae, accepted in the World Register of Marine Species under the authority Menke, 1828 ^[1]. A Chilean anatomical study notes that it is one of the largest species of its family and gives its range as Ecuador to Valparaíso ^[2]. The systematic list of Peruvian marine molluscs gathers Peruvian localities running from Punta Capones to Pisco ^[3].

The authors of an anatomical study point to it as one of the largest naticids of the family ^[2]. In Chile 28 naticids are recorded, three of them in the genus Sinum: Sinum praetenius, Sinum antarcticus and this species ^[2].

Females and males differ in size. In the 31 males and 21 females collected in La Herradura Bay, in northern Chile, males weighed 287.4 g on average and measured 31.8 mm in peristomal length, while females weighed 367.1 g and measured 35.5 mm ^[2].

Sex can be told by eye from the right groove of the propodium, the front part of the foot: in the female it is rough and dark in colour, yellow or orange, and forms the ovipositor; in the male it is smooth and of a single colour ^[2]. The authors propose using that trait to sex specimens easily ^[2].

The systematic list of Peruvian marine molluscs gives its distribution as Ecuador to Chile and notes Peruvian localities such as Punta Capones, Banco Máncora, Zorritos, Paita, the islands of Lobos de Tierra and Lobos de Afuera, Chicama, Huacho, Ancón, Callao, Pucusana and Pisco; the same list places it down to the infralittoral zone, between 8 and 90 m depth, on sandy, muddy and rocky bottoms ^[3]. The 2009 anatomical study summarises the range as Ecuador to Valparaíso ^[2].

The malacology collection of the Natural History Museum of Los Angeles County holds 29 preserved Peruvian specimens of the species, published in GBIF and consulted in 2026; those that carry a year were collected between 1935 and 1974, in the departments of Tumbes, Piura, Lambayeque, La Libertad, Lima and Ica ^[4].

The Chilean animals of the 2009 study were collected by breath-hold diving in La Herradura Bay, Coquimbo (29°58' S), and the authors note that they are usually found isolated in the wild ^[2].

The sexes are separate: no hermaphroditism, gonadal parasitism or imposex was found in any of the specimens analysed, and all were mature ^[2]. That last phenomenon, the development of a penis in females, has been detected in other gastropods of the same bay ^[2].

In the female, the gonopore opens behind the right tentacle and leads directly into the ovipositor of the foot ^[2]. The capsule and albumen glands form a compact structure, and the ovary is pale orange ^[2]. Its tubules average 370.2 µm in diameter; mature oocytes are spherical and average 90.4 µm, and in every tubule of every female there were oocytes with and without yolk ^[2].

The female also has a bursa copulatrix and a sac-shaped seminal receptacle, about 0.5 mm wide and 2 mm long, where she stores sperm; this could let her fertilise her eggs without mating at that moment, useful in a species that is often found isolated ^[2]. The authors note that these two structures had not been described in other naticids ^[2].

In the male, the penis lies behind the right tentacle, is cream-coloured, shaped like a flattened tube and averages 17.6 mm; the sperm is thread-like and about 137 µm long ^[2]. The testis tubules hold all stages of sperm formation at once, an organisation the authors relate to a continuous reproductive cycle, described earlier for the species in another study ^[2]. The vas deferens is closed, unlike what has been described for Lunatia nitida and Polinices mellosus, which for the authors places Sinum as a more terminal group within Naticidae ^[2].

The animals were found in low numbers and have nocturnal habits, so they were collected only sporadically between 2006 and 2009 ^[2]. They are usually found isolated in the wild, and that condition has been proposed as an advantage of being able to keep sperm to fertilise without a simultaneous mating ^[2].

The authors of the only Chilean study cited here state that its low numbers and nocturnal habits meant it was collected only sporadically, by breath-hold diving, in La Herradura Bay between 2006 and 2009 ^[2]. The IMARPE list, published in 1997, gives it down to the infralittoral zone, between 8 and 90 m, and records Peruvian localities such as Punta Capones, Banco Máncora and Paita, in the north, and Pucusana and Pisco, further south ^[3].