Acanthais brevidentata

Acanthais brevidentata is a predatory marine snail of the family Muricidae, common on rocky shores of the tropical eastern Pacific ^[1]. It is recognised by a short, sharp spine at the base of the outer lip of the shell ^[1].

In Peru there is a record from Máncora (Piura) of August 2018, with 2 specimens in a private collection ^[2]. In Ecuador it is one of the species most often seen on the rocky intertidal ^[3], and, together with two other muricids, it has been used to measure imposex, an effect of tin compounds on snails ^[4].

The shell has a moderately high spire with distinct sutures, no umbilical slit, and a short, broad siphonal canal that ends in an upturned notch ^[1]. The last whorl bears up to 5 spiral cords, with low, rounded nodes on the two central ones, and the whole surface is overlain by very fine spiral threads ^[1].

The feature that defines the genus is a short, sharp labral spine placed below the fifth spiral cord, at the end of a shallow groove ^[1]. The edge of the outer lip is crenulate and the inside carries 4 weakly elongate denticles, without lirae; the columella is straight and bears a more or less distinct central fold ^[1]. The operculum is D-shaped ^[1].

At first sight it resembles Acanthina, which also has a labral spine and strong cords, but it differs in the columella, the aperture and the sculpture, and in its rapanine anatomy ^[1]. Ocenebrines usually have 7 or more primary cords and a smooth columella ^[1].

Similar species

• Neorapana muricata: It also has a short labral spine below the fifth cord, but it lacks the central fold of the columella, has lirae on the inside of the outer lip and shows an umbilical slit ^[1].
• Stramonita biserialis: It looks alike in general form and, as in Acanthais, cords 2 and 3 carry a row of nodes. According to Vermeij and Kool, in typical Stramonita species the primary cords are narrow and angular, not low and round-topped, there are lirae on the inside of the outer lip, and the labral spine and the columellar fold are missing; in Stramonita populations with reduced cords these are hard to tell from the secondary ribs ^[1].

It is a common snail on the intertidal of rocky shores of the tropical eastern Pacific ^[1]. The authors of the genus examined 15 lots whose origin ran from northwestern Costa Rica to southern Ecuador ^[1].

In Costa Rica, a study of the rocky shore of Bahía Panamá (Guanacaste), visited on 13 December 2007 and again in August 2013, lists it among the intertidal organisms and recalls that muricids of this kind can reach high densities ^[5]. In mainland Ecuador it is among the most frequent species of the rocky intertidal in an inventory of the coast ^[3], and it was collected at 8 localities on the oceanic coast of Ecuador ^[4].

In Peru, an inventory of macromolluscs from Máncora records it on Las Pocitas beach, with specimens collected on 21 August 2018 ^[2].

The protoconch has more than 3 whorls, an outward-flaring lip and a sinusigeral notch, features that point to a planktotrophic larva, as in the other rapanines ^[1]. The study of the genus was based on 6 males and 6 females from Venado Island, Panama ^[1].

The anatomy is typical of rapanines. Females have an elongate, well-developed accessory boring organ, and most of those examined had a minute pseudopenis; males have a small, sinuous penis ^[1]. The radula has a rachidian tooth with a wide central cusp and well-developed marginal denticles ^[1].

Of 204 specimens collected by hand in Ecuador, 56% were females and 44% males ^[4].

It is a predatory gastropod of the rocky intertidal ^[5]. At Bahía Panamá it was found in the mid-littoral, together with the pulmonate limpet Siphonaria maura, the barnacle Chthamalus panamensis and bivalves of the genus Brachidontes ^[5].

In Ecuador the specimens were collected by hand on rocky areas, pier pilings, pipes and other hard surfaces, where species of this genus normally live ^[4]. In another Ecuadorian inventory it is among the species observed most often at the mid and low tidal levels ^[3].

Its labral spine is an exaggerated version of a crenulation on the edge of the lip, common in rapanines, and such spines arose at least 5 times independently in the subfamily ^[1].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

In Ecuador, 204 specimens from 8 localities on the oceanic coast were examined to measure imposex, the appearance of male sexual features in females, which the authors link to pollution by organotin compounds ^[4]. Of the females, 57.9% showed imposex, with a prevalence ranging from 0 to 100% depending on the locality ^[4].

Even so, the intensity was low: the maximum grade on the VDSI scale was I and the highest relative penis length index (RPLI) was 2.8%, lower than in the other two species studied, which reached 100% prevalence at all their localities ^[4].

The authors could not compare the sensitivity of the three species because they never occurred together at the same site, and they propose Thais kiosquiformis for estuaries and Thais biserialis for marine waters above 30 ‰ as the more suitable for monitoring ^[4].

The only Peruvian reference reviewed is an inventory of macromolluscs from Máncora, in Piura, with material gathered between 2017 and 2020. It records 2 specimens of this species on Las Pocitas beach, collected on 21 August 2018 ^[2].

The paper does not describe the substrate or tidal zone of the find, nor does it say whether access is difficult or restricted ^[2].