Community structure and predation marks on Astarte longirostra d’Orbigny (Bivalvia) from the Southwest Atlantic off Tierra del Fuego

Astartids are a major component in marine ecosystems, playing important roles in trophic networks. At the southern Southwest Atlantic, Astarte longirostra lives partially buried in the sediment. Muricid and naticid gastropods feed on clams, drilling a hole on their valves. Drillholes made by predatory gastropods are useful tools for evolutionary and ecological studies, since they provide direct, preservable evidences of predation. The aim of this study was to analyse the community structure of and predation pressure on A. longirostra from the continental shelf off Tierra del Fuego (Southwest Atlantic), covering an area of 120 km². Bottom samples were collected with a 13.94 dm² Van Veen grab during July 2014. Complete valves were separated from the sediment under a stereomicroscope. The valve length and drillhole diameter were measured with a micrometer eyepiece. Astarte longirostra valves occurred from 42 to 92 m. Predation pressure was assessed in 12 samples with at least 20 valves each, which were collected from 74 to 92 m, as clams were less frequent at shallower depths. The densities of this clam were variable (14–409 ind.m^-2). Of the 4,292 disarticulated valves analysed, 959 had drillhole marks. Due to their irregular shape, 11.6% of the drillholes could not be attributed to predatory gastropods. The remaining 88.4% had rounded drillholes attributable to muricids and naticids, which exert strong predation pressure on clams. The predation rate was high (34.2% to 68.2%; pooled rate=44.4%). Predation did not differ between left and right valves (Fisher´s Test, p=0.67). The correlation between drillhole diameter and valve length was positive and highly significant, meaning that larger predators prefer larger clams. The size of the valves ranged from 0.62 to 5.85 mm. The size--frequency distribution was bimodal, with modes in 2–2.49 mm and 3.5–3.99 mm, suggesting the existence of two cohorts. Drillholes were significantly more frequent in the central sector of the valves. Of the seven species of predatory gastropods found, the naticid Falsilunatia patagonica was the most frequent.