GROWTH DISRUPTIONS IN THE ANTARCTIC SCALLOP, ADAMUSSIUM COLBECKI

We found that the epibenthic Antarctic scallop, Adamussium colbecki, exhibits multiple growth disruptions on its valves. Disruptions include raised and thickened regions and distinct alterations in growth lines that either have an uneven edge or demark a change in growth. We do not know what causes these growth disruptions, as they are different from shell repair related to predatory damage. As a first approach to understanding how they may form, we compared Adamussium valves from two Antarctic Ross Sea localities (Ice Cliff in Explorers Cove with semi-permanent sea ice; Ferrar Glacier, with periodic sea-ice melt out) to determine if growth disruptions varied by site. We also examined whether growth disruptions were more common in younger than older parts of Adamussium valves based on size classes (size class 1, 0-10mm, to size class 10, 90-100 mm) and if scallops in shallower water (15 m) had more growth disruptions than deeper water (24 m).

Significantly more growth disruptions occurred at the Ferrar Glacier site than the Ice Cliff site (p = 0.007). At both sites, growth disruption by size class had a bimodal distribution, with modal peaks at 10-20 mm and 60-70 mm. Top and bottom valves each had similar bimodal distributions in growth disruptions by size class. Significantly more growth disruptions occurred in the younger valve region (< 30 mm; p = 0.40). The number of growth disruptions was significantly different with depth (p = 0.0001): scallops from Ferrar had the most disruptions at 24 m, whereas scallops from Ice Cliff had most growth disruptions at 15 m.

The incidence of growth disruptions was more common at the Ferrar Glacier site, a site with icebergs and seasonal sea ice. Growth disruptions occurred more frequently in the younger parts of the scallop shells, suggesting vulnerability to environmental perturbations. Growth disruptions decreased with depth at Ice Cliff, but increased with depth at Ferrar. We speculate that environmental factors, such as anchor ice, debris from melting sea ice, icebergs, seasonal freshwater input from melting ice, among other ice-related factors, may influence scallop growth especially at different depths and stages of maturity. If these disruptions are directly attributable to ice, they can be used to determine the ubiquity of ice dynamics in the past using molluscan fossils.