Seasonal trend in burrow depth and tidal variation in feeding activity of Nereis diversicolor

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Abstract
Ragworms Nereis diversicolor live in burrows which, if deeper than 5 to 10 cm, offer a safe refuge from most of their predators. Burrow depth must be ca 15 cm before they live out of reach of all predators. Burrow depth increases with body size, levelling off at ca 15 cm, when the depth refuge is reached. Worms live at greater depths in sand than in mud, but this effect disappears if worms with identical body condition (expressed as deviation from mean body weight for a given body length) are compared. Seasonal variation in burrow depth is correlated with sea temperature and to a lesser extent with day length and body condition. Feeding worms are present near or at the surface. Filter feeding occurs in the first 2 h after exposure (at least if a water film still remains at the surface) and surface feeding begins after that. The total time spent at the surface doses not exceed an average of 50 s per tidal cycle. Feeding must be an extremely risky activity, because predators readily catch most N. diversicolor while present near or at the surface.