The effect of body size on digestive chemistry and absorption efficiencies of food and sediment-bound organic contaminants in Nereis succinea (Polychaeta)

Abstract

We investigated the hypothesis that absorption efficiencies for organic matter (OM) and hydrophobic organic contaminants (HOC) correlate with body size in the deposit feeding polychaete Nereis succinea (Frey and Leuckart). Gut passage time (GPT) in N. succinea is approximately five to ten times shorter in juveniles than in adults. Since shorter GPT is likely to diminish the efficiency of intestinal digestion and solubilization, one would expect juvenile worms to have significantly diminished absorption efficiencies compared to adults. To test this hypothesis, we measured absorption efficiencies (AE) for radioactively labeled phytoplankton ( Pseudonitzschia sp.), sediment OM, and three sediment-bound hydrophobic organic contaminants (tetrachlorobiphenyl [TCBP], hexachlorobenzene [HCB] and benzo(a)pyrene [BaP]) over a gradient of body size spanning 10–110 mm. We furthermore measured gut pH and gut fluid surface tension (surfactancy) in relation to body size, to examine whether small worms might compensate for shorter GPT by having comparatively more aggressive gut conditions. Absorption efficiencies were measured in pulse-chase feeding experiments over 5–48 h. Live phytoplankton was absorbed with AEs of 55–95%, while bulk sediment OM was absorbed with AEs of only 5–18%. Sediment-bound TCBP, HCB and BaP were absorbed with AEs of 55–92%. AEs were commonly higher in larger worms, and linear and Ivlev-type regressions of AE onto body size explained more than 59% of the AE variance in any treatment. AEs for phytoplankton and OM correlated strongly with depuration time, i.e. the time until first detection of non-radioactive “chaser”-feces. In contrast, no time dependency of AE was detected for HOCs, albeit over a narrower range of depuration times. Gut fluid pH of N. succinea ranged between pH 5.8 and 7.7, and was on average slightly higher (closer to seawater pH) in larger individuals. Surface tension, measured as drop contact angle, was greatly diminished relative to seawater in all worms, with greatest differences found in large worms. In contrast to AE measurements, only 10–20% of the variance in gut surfactancy and pH data was explained by body size, suggesting that differences in gut chemistry (pH and surfactancy) play a subordinate role in explaining the higher AEs in adult worms. We conclude that gut chemistry is likely to set upper and lower limits on absorption efficiency of food and sediment-bound HOCs in N. succinea, while body size, and in the case of food uptake, differences in GPT, probably account for most of the variance within this range.