Mussel seed Mytilus galloprovincialis from two origins (rocky shore and collector ropes) was cultivated on a raft (submerged culture) in the Ría de Arousa (north-west Spain) from approximately 20 mm to 60 mm shell length. Absorption efficiencies (AEs) and absorption rates (ARs) of the individuals were recorded in situ throughout cultivation time from November to July (1995–1996). Absorption efficiencies varied according to a multiple model that highlighted the significant effect of quality of natural seston (f as relative organic content), time of culture and origin of mussel seed. Initial AE values were 42 and 34% of the ingested organic material for collector ropes and rocky shore mussels (P<0.01), respectively. These differences remained significant for the first two months of the culture with higher values for collector ropes mussels. A significant relationship AE vs quality of natural seston (f) was observed according to an asymptotic equation: AE=0.82[1[minus sign]e−5.01(f−0.21)] for both sources of seed (r2=0.90). Absorption rate (mg organic matter g−1 h−1) followed a seasonal pattern with low values in winter and higher values in spring–summer as a consequence of better nutritional quality of food resources. Nevertheless, exceptional peaks of organic material from sedimentary origin in January caused an increase of ingested/absorbed organic material despite its low nutritional quality. Differences in AR values between origins of mussel seed referred to the first month under culture, with higher values for collector ropes individuals. Original habitat of mussel seed usually employed in mussel culture in Galicia is responsible for long-lasting effects through physiological changes that the individuals from the rocky shore habitat presented when placed under raft-submerged culture.