Nocturnal surface activity of endobenthic nemertines on tidal flats  <B>Nocturnal surface activity of endobenthic nemertines on tidal flats</B><P><B>Martin Thiel</B><SUP>1</SUP><B>, Walter Nordhausen</B><SUP>2</SUP> <B>& Karsten Reise</B><SUP>3</SUP><P> 1. <I>Darling Marine Center, University of Maine, Walpole, ME 04573, USA</I><BR> 2. <I>Scripps Institution of Oceanography, La Jolla, CA 92093-0202, USA</I><BR> 3. <I>Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-25989 List/Sylt, Germany</I><B><P> Abstract</B><BR> Nocturnal activity of organisms living in intertidal mud flats was previously thought to be a result of predator avoidance behaviour. However, many organisms (e.g. nemertines) which have been reported active during night low tides are known to be ignored as prey items by shorebirds.<BR> Nemertines, like some other endobenthic predators, forage at the sediment surface. During their reproductive periods they can be observed on exposed tidal flats searching for mating partners. In this study we examined their diel and seasonal activity pattern in detail.<BR> In laboratory experiments, the nemertine <I>Lineus viridis</I> appeared at the sediment surface at night, and was found in highest numbers at night low tide. On exposed tidal flats, many nemertines were active during night low tides, but none at day low tides. The nemertines <I>L. viridis</I> and <I>Amphiporus lactifloreus</I> were observed only in low numbers at the sediment surface in the late summer; during their reproductive periods in March and December, respectively, both nemertine species were active at the sediment surface. Immigration into mussel clumps by both nemertine species was highest during their respective reproductive periods. <I>L. viridis</I> foraged during nocturnal low tides in summer, and during its reproductive period (March) followed the traces of other individuals. No such behaviour, either foraging or mating, was observed for <I>A. lactifloreus</I>.<BR> We hypothesize that for endobenthic predators like nemertines which forage at the sediment surface, resource location via chemoreception is facilitated at low tide. Additionally prey escape is restricted during low tide. Desiccation stress, competition with fast visual predators and predation are the most important factors selecting against diurnal low tide activity.<BR> The role of endobenthic predators in structuring soft-bottom communities may be based on their temporal activity pattern. Non-predatory infauna facing both high epi- and endobenthic predation in some areas may have to emigrate episodically or permanently.<I><P> Keywords</I>: nemertines, endobenthic predators, diel rhythm, intertidal, chemoreception.<P>