Predator gastropods in benthic communities from north Patagonian shallow waters

GREGORIO BIGATTI, CARLOS SANCHEZ ANTELO AND PABLO E. PENCHASZADEH

Kobe Japan

Simposio; An international Nearshore Biodiversity symposium. NaGISA World Conference; 2006

Census of Marine Life

Odontocymbiola magellanica and Adelomelon ancilla (Neogastopoda:Volutidae) occur along the South Atlantic coast from 35°S, down to the Magallanes Strait reaching the Chiloé Island in the Pacific coast of Chile (42°S) and also occurs in the Malvinas Islands  (Weaver & Dupont 1970; Castellanos 1970; Castellanos & Landoni 1992; Gallardo & Penchaszadeh 2001). In Norpatagonic gulfs, these sympatric species inhabits mixed bottoms of gravel and sand, being reached by SCUBA  diving at depths between 5 and 20 m during low tide and near the shore; which  makes them vulnerable if exploited as a fishery resource. Bottoms in Golfo Nuevo and Golfo San Jose have a  low mollusk’s densities. In some places patches of the bivalves Aulacomya atra, Protothaca antiqua and Eurhomalea exalbida are found. Also the scallop Aequipecten tehuelchus  was dispersedly present. The algal assemblage is dominated by Codium vermilara and Dictyota dichotoma in addition to other small algal species (Casas et al, 2004), mainly coinhabited  by the gastropods Buccinanops globulosus,  Natica isabeleana and Tegula patagonica among others. The analysis of  prey obtained in situ indicated that O.magellanica  and A. ancilla are  top predator in the benthic community, its diet consisting mainly of gastropods and bivalves. Gastropod ingest for O magellanica mainly included Tegula patagonica and Natica isabeleana, while Trophon geversianus and Buccinanops globulosus were consumed in lower percentages. Only in one case, was predation on Crepidula sp. registered. The main bivalve dietary item was the venerid Protothaca antiqua, followed by Ensis mancha and Eurhomalea exalbida; the pectinid Aequipecten tehuelchus and the mytilid Aulacomya atra showed less relevance in diet composition. A. ancilla  mainly consumes  bivalves (90 % of the cases), follow by gastropods and in some isolated cases sea urchins. Bivlaves consumed where Protothaca antiqua Aulacomya atra, and Eurhomalea exalbida; gastropods comprised Tegula patagonica and Natica isabeleana; the green sea urchin  Arbacia dufresnei  was consumed in less than 2 % of the studied cases. In some cases the field observations for both benthic predators included animals feeding on carrion: dead mollusks with degraded tissues . No rests of animal tissues were found in the stomachs analyzed. Feeding was related with the fauna composition of the bottoms in which these animals were found. In sandy bottoms, where aggregation of O. magellanica occurred but no other mollusks were found, intraspecific cannibalism was registered in 4.7% of the studied cases while  A. ancilla  did not present cannibalism cases. The observations done in natural environment revealed that in the first step of capture, O. magellanica and A. ancilla  strongly engulfed the prey with their foot, creating a cavity which is closed but not isolated from the environment. After some time prey are narcotized by a liquid produced in the salivary and accessory salivary glands and introduced inside this cavity by the proboscides. The effect is that the bivalves open their adductor muscles or in the case of gastropods, they lose reaction in the retraction of the foot muscles. Prey narcotization leads to lower muscular speed reaction, enabling the predator gastropods to feed on the prey tissues by means of its radula. This organ is very sharp and tears the tissues of the living prey Field observations suggest that narcotization is produced due to the constant release of salivary liquid into the cavity generated by the foot, where the prey are immobilized. In this cavity, water is probably not abundant, consequently enabling a proportionally higher  concentration of salivary liquid.