Daily Growth Rates as Indicated by Valve Ridges in Postlarval Giant Scallop (Placopecten magellanicus) (Bivalvia: Pectinidae)

Abstract

Postlarval giant scallop (Placopecten magellanicus) were examined for daily growth ridges and growth rates by marking the dissoconch shell with Alizarin red dye. The surface of the left valve of postlarvae was composed of concentric ridges, each consisting of a series of irregularly shaped raised nodules. Ridges were clear and distinct in newly settled scallop between ≈0.25 and 2.0 mm shell height. The shell of postlarvae >2 mm was pigmented and ribbed and ridges were no longer distinguishable. Estimated age was significantly correlated with actual age, suggesting that growth ridges were produced daily, under the environmental conditions of Passamaquoddy Bay. Mean growth rate ranged from 32 to 57 μm∙d−1 and was proportional to size and age, but growth of individual scallop showed no coherence in their daily growth patterns. The short-term growth ridges in postlarval giant scallop can be used to determine age and can be applied to comparative growth, mortality, and recruitment studies of newly settled individuals <2.0 mm (≈40 d old postsettlement). The high accuracy and precision of age determination for postlarval scallop differs from studies of short-term internal growth increments of bivalve shells and larval fish otoliths.