Acute and cronic response to a change in salinity in the euryhaline polychaete Pygospio elegans (Claparède)

Estuaries are thought to be very harsh environments because their physico-chemical parameters, such as salinity, temperature and oxygen, can fluctuate substantially. The distribution of species living in an estuary is largely determined by how well they can cope with such fluctuations. The spionid polychaete Pygospio elegans is common in boreal estuarine habitats that show strong salinity fluctuations, living in habitats ranging from fully marine salinities to brackish environments with salinities as low as 5. In this study we investigated the abilities of P. elegans to cope with an acute as well as a long-term change in salinity. Specimens originating from a salinity of about 15 were exposed to salinity 15 as control and 5 and 30 as low and high salinity treatments. In acute exposure experiments, we measured body volume and tissue water content as well as gene expression of seven genes related to volume- and osmoregulation within 4 h of exposure. In a long-term experiment, we monitored survival, growth and reproduction as well as potential changes in DNA methylation within 6 weeks after a gradual salinity change. In response to abrupt exposure to a hyposmotic medium (salinity 5) we observed increased body volume that could not be fully restored, increased mortality, and no clear change in gene expression. Hence, P. elegans might be a weak cell-volume regulator that cannot cope well with sudden drops in salinity. In contrast, a gradual, long-term change in salinity seemed to be less stressful, although worms at salinity 5 showed slightly increased mortality and reduced or delayed maturity. Overall, there were fewer detrimental effects of exposure to a higher salinity. While the tested salinities fall within the tolerance range of P. elegans from this population, decrease in salinity seems to be a stronger stress.