Netherlands Journal of Sea Research 13 (3/4) : 536-546 (1979) OSMOTIC RELATIONS OF THE COELOMIC FLUID AND BODY WALL TISSUES IN ARENICOLA MARINA SUBJECTED TO SALINITY CHANGE by R. E. WEBER* (Institute of Biologr, Universityof Odense, Denmark) and D. H. SPAARGAREN (Netherlands Institute for Sea Research, Texel, The Netherlands) CONTENTS I. Introduction .......................... 536 II. Material and Methods ..................... 538 III. Results and Discussion ..................... 539 IV. Summary ........................... 544 V. References ........................... 545 I. INTRODUCTION The burrowing lugworm Arenicola marina, one of the most conspicuous inhabitants of northern European intertidal flats, is an osmoconformer capable of tolerating considerable variations in ambient salinity. Despite scattered, undocumented references to lugworms exposed to salinities down to the horohalinicum, the most dependable records (cf. OCLESB'Z, 1973) indicate that the lower limit for Arenicola marina in the field is about 40% sea water (which corresponds to a salinity of about 14~o) although the species is clearly capable of surviving greater osmotic stress for short periods. This limit corresponds by and large with that below which successthl reversal of osmotic adaptation can no longer be achieved (FREEMAN• SHUTTLEWORTH, 1977a). Mean- while behavioural adaptations appear to play an important part in protecting Arenicola from extreme, short-termed osmotic stress in nature; decreasing the salinity of overlying water to 55% sea water or below stops the irrigation activity of burrowed worms (SHuMWAY ~; DAVENPORT, 1977). The osmotic conformity of the coelomic fluid and the blood of Arenicola (cf. SCHLIEPER, 1929; BALYAEV, 1957; KROGH-RAsMUSSEN & * This work was carried out partly at the Department of Zoophysiology, University of Aarhus, Denmark.