The importance of bacteria and microalgae in the diet of the deposit-feeding polychaete Arenicola marina

The distribution of bacteria and chlorophyll a in sediment around and within Arenicola marina while feeding on sandy sediment at the shallow Bregnor Bay, Denmark, was examined throughout an annual cycle. Specimens inhabiting homogenized sediment were compared with worms from unmanipulated sediment. Although the feeding pocket was difficult to locate, a low mean grain size of sediment in the gut of A. marina compared with surrounding unmanipulated sediment indicates selective feeding on particles <500 mu m. It appears that A. marina only occupies one feeding pocket for short time in the densely populated (similar to 80 ind. m(2)) sandy sediment at Bregnor Bay. Digestion and assimilation of bacteria and chlorophyll a (i.e. microalgae) during passage of sediment through the gut of A. marina occur primarily in the stomach. Due to a rapid 50-100% regrowth in the hindgut of A. marina, assimilation of bacteria must be quantified as the difference in numbers between esophagus and stomach. Assimilation efficiency was similar in homogenized and unmanipulated sediment (32-53% for bacteria and 20-67% for chlorophyll a). However, the concentration of microorganisms in worm guts from homogenized sediment was up to 50% higher than in unmanipulated sediment due to artificially high subsurface levels. The strong seasonal variation of chlorophyll a at the sediment surface was almost completely damped in the hindgut of A. marina probably due to the presence of a constant fraction of 'indigestible' microalgae. Assimilation of chlorophyll a may therefore be controlled by the excess chlorophyll a over the indigestible level. Based on rate measures and conversion factors from the literature the carbon budget of A. marina at Bregnor shows that bacteria only covers 3-7% of the requirement, while microalgae contributes 42-370%. The supply of bacteria and microalgae fully covers or exceeds the demand for carbon in summer and autumn. However, these two food sources only account for about half of the demand in winter, where worms may supplement the food by assimilation of digestible detritus.