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The embryonic development of the deposit-feeding Baltic amphipod Monoporeia affjnis (Lindstrom) was studied from fertilization to hatching. Severe types of aberrant embryonic development are described and the normal variation of the aberrations is estimated. Between 2 and 6% malformed, 0 and 5 % undifferentiated and 0 and 6% dead eggs and embryos were observed at different sites in the coastal area of the northern Baltic proper and the Bothnian Sea without local contaminant &scharges. The embryonic development of M. affinis was described to facilitate the use of the embryo variables in field studies of sediment toxicity. The merits of the different embryo variables as biomarkers in effect monitoring were evaluated. Gravid females were sampled in the receiving waters of 2 different types of pulp mills and an aluminum smeltery. Fecundity (eggs per female) and frequency of malformed, enlarged, undifferentiated and dead embryos per female were determined. Frequency of malformed embryos was higher at industrially affected stations as compared to reference areas (p < 0.0001, p < 0.0001 and p = 0.003 respectively). The highest frequencies (15.3%) were recorded close to the alurninum smeltery. In reference areas the frequency vaned between 1.5 and 4.9. The number of enlarged embryos with oedema was higher near the pulp m d s (p = 0.03, p > 0.0001), particularly the pulp mill with a bleaching stage, where 25 to 40% embryos per female were affected. Background frequencies of enlarged embryos (0.7 to 3.5%) were observed outside the alurninum smeltery. UnMferentiated eggs and dead eggs did not correlate to the exposure situation, suggesting these variables are of lunited value in effect monitoring of xenobiotics. The frequency of malformed embryos was increased also when fecundity, reproduction success and developmental stage of embryos were unaffected. Thus, the variable is suggested to be particularly toxicant-sensitive and by analysing the number of malformed embryos of M. affinis an early warning of affected populations could be provided.

Malformations in embryos of the deposit-feeding amphipod Monoporeia affinis in the Baltic Sea