Comparison of intermittent and continuous exposures to cadmium in the blue mussel, Mytilus edulis: Accumulation and sub-lethal physiological effects

• Target organs for Cd toxicity in M. edulis are the same for intermittent and continuous exposure.
• Animals from the intermittent exposure accumulated less Cd than those from the continuous profile.
• The hemolymph showed dynamic increases and decreases with the exposure profile.
• Despite differences in Cd accumulation, biological effects were similar.

Little is known about the bioaccumulation responses of shellfish to metals during intermittent compared to continuous exposure. There is also the concern that the toxicity of intermittent events may not be the same as that from the steady-state continuous exposures. The aim of the present study was to determine whether there was any difference between cadmium (Cd) accumulation, or Cd-dependent biological responses, in tissues of blue mussels (Mytilus edulis) during intermittent compared to continuous Cd exposure. Tissues and hemolymph were collected from M. edulis exposed for 14 days to either control (no added Cd, only seawater), or 50 µg/l Cd as CdCl2 in continuous or intermittent profile (2 day exposure, 2 days in clean seawater alternately); and sub-lethal responses examined using a suite of assays including total glutathione, thiobarbituric acid reactive substances (TBARS), neutral red retention, total hemocyte counts, hemolymph Na+ and K+, plasma glucose and histopathology. A time-dependent accumulation of the Cd was observed in tissues of mussels after continuous exposure, while the intermittent exposure showed step-wise changes in the hemolymph and gonad. Tissue Cd concentration in the continuous exposure was significantly increased (≥2 fold) for most tissues compared to the intermittent exposure. No clear differences were seen between the continuous and intermittent exposure for most end points measured apart from a 2 fold significant increase in hemocyte infiltration in the digestive gland of the continuous exposure compared to the intermittent exposure. Overall, the data showed that the Cd accumulation was generally greater in the continuous exposure regime, but despite this, most of the biological responses being similar in both regimes.