Extreme size difference in Akera bullata of the same age

摘要

In March 2010 a dense population of the gastropod Akera bullata was discovered in a settling pond receiving outflow from a marine aquarium at Dunstaffnage Marine Laboratory near Oban in west Scotland (Craik 2012). In September 2011 the pond was drained and renovated without warning, and its contents were unfortunately lost. However, about one hundred medium-sized Akera were rescued from the diggings and the resulting population was maintained in captivity at the aquarium in tanks of flowing sea water from September 2011 to May 2017. The ultimate aim was to reestablish the pond population. During this time the Akera bred successfully in the tanks and large numbers of eggs and small young were regularly returned to the pond. For years they did not survive there, probably because the pond had not redeveloped the silty bottom and the strong growths of filamentous green algae that seem to be needed by growing Akera. However, by May 2017 a self-maintaining population had appeared in the pond. All the captive Akera were then moved to the pond and captive rearing in aquarium tanks was stopped. The pond population was still numerous at the time of writing (March 2018) although individuals were much smaller than in 2010-2011.