Development of mantle organs, feeding, and locomotion in postlarval Macoma balthica (L.) (Lamellibranchiata)

Abstract

Postlarval M. balthica is well adapted for interstitial life in a particulate sediments, although metamorphosis to the adult form and function is not complete until a shell length of 2 mm is reached.Spatfall at 300–310 μ shell length is followed by a plantigrade stage in which the ciliated plantiform foot is used as an organ of feeding, locomotion, and rejection of pseudofaeces. The inhalant current is produced by the ciliation of the foot and inner demibranch, and enters through the pedal gape. This is already separated from the lumen of the inhalant siphon by the cruciform apparatus. Food sorting in the early postlarvae is exclusively by the palps, which are well developed in the midline to overhang the mouth, and already have simple sorting ridges on their inner surfaces.Siphon development proceeds by infolding of the fusions of the mantle edge around the siphonal apertures. In early postlarvae the pseudofaeces are transported to the pedal gape by the mantle ciliary tract, and swept from the mantle edge by the foot. At approximately 1 mm shell length, rejection of pseudofaeces occurs via the inhalant siphon, which only gradually takes over its adult function as exclusive route of the inhalant current at between 1 and 2 mm.