Ecophysiological and Genetic Traits of the Baltic Clam Macoma balthica in the Baltic: Differences between Populations in the Gdansk Bay Due to Acclimatization or Genetic Adaptation?

Normal and irregular blunt clam‐shells were found. Blunt shells increased with depth, because of increasing anoxia. Minimal weight‐indices and reproductive stages were found at 40 m depth (near thermo‐haline stratification) probably due to spawning just before sampling. Clams from shallower stations were more ripe and heavier due to higher temperatures and re‐development than at deeper stations since temperatures were still below threshold levels for spawning (10 °C). Respiration was at ambient temperatures acclimated to standard rates. Consequently, clams from deeper stations (with lower ambient temperatures) had 2 times higher respiration at the same test‐temperature. Clams from the most shallow station (Puck Lagoon 5 m) showed the worst performance due to adverse hydrodynamics (stagnated water, anoxia). Heterozygotes showed better performance than homozygotes (heterosis). In 1995 genetic differentiation was found (Idh was diagnostic): clams below 30 m resembled Atlantic populations; shallow stations formed a distinct group (Baltic race). In later years all populations were Atlantic, probably due to inflow of North Sea water. Before 1996 ecophysiological differences between clams in the Gdansk Bay can possibly be related to genetic differentiation (adaptations of Atlantic vs. Baltic types), in later years differences were due to acclimation to ambient temperatures, salinity and oxygen levels.