Understanding the life of a sandy beach polychaete of functional importance – Scolelepis squamata (Polychaeta: Spionidae) on Belgian sandy beaches (northeastern Atlantic, North Sea)

The cosmopolitan sandy beach polychaete Scolelepis squamata constitutes an important food resource for juvenile flatfish and wading birds in the northeastern Atlantic, thus playing an important role in sandy beach ecosystem functioning. However, its population dynamics and life history in this part of the world have gone widely uninvestigated. Eight beach transects on Belgian sandy beaches were sampled monthly from October 2003 until October 2004, in order to investigate seasonal trends in the species’ abundance, biomass, secondary production, and patterns in reproduction and zonation. Average density, modal density and modal biomass (ash-free dry weight) (mean average density ¼ 169 9 SE ind/m2; mean modal density ¼ 505 38 SE ind/m2; mean modal biomass ¼ 0.25 0.02 SE g/m2) did not exhibit major seasonal changes, whereas average biomass (0.081 0.005 SE g/m2) and individuals and biomass per strip transect (IST ¼ 16286 1330 SE ind/m; BMST ¼ 7.8 þ 0.7 SE g/m) did, peaking in May 2004. Production was calculated at 1.9 g/(m2*year) (size-frequency method, SFM) and 0.88 g/(m2*year) (mass specific growth rate method, MSGR) and mean annual biomass was 0.797 g/m2; resulting in a P/B ratio of 2.40/year (SFM) and 1.11/year (MSGR), which is intermediate to moderately low compared to other polychaete species. Gravid individuals were found from February until August and a single recruitment period was observed from July until September. An average sex ratio of 1.41 0.08 SE was calculated, with a female predominance. Highest densities (>200 ind/m2) were mostly found above 3 m above MLLWS and at a median grain size from 190 to 320 mm. Average modal or peak density along each transect was situated from 3.95 m up to 4.40 m above MLLWS, in contrast to some other studies where the species was restricted to mid-tidal levels. Significant differences in elevation of peak density were found between non-gravid (411 4 SE cm) and gravid (402 5 SE cm) animals, but none between gravid males and females. The portion of the tidal range that was occupied by the species in the vertical dimension, just failed to differ significantly between non-gravid and gravid individuals, while gravid males (48.2 5.6 SE cm) occupied a significantly wider range than gravid females (36.8 4.7 SE cm), corresponding with a horizontal range difference of 6e9 m cross-shore. The observed zonation pattern, together with a trend in the difference of peak abundance elevation between males and females, are hypothesised to be a result of a strategy of gravid animals to ensure co-occurrence in a narrower zone during reproduction. Observed differences between Brazilian, Moroccan and Belgian populations (e.g. density, biomass, productivity, P/B, life span, body width, period of presence of gravid individuals and recruitment) do not seem to concur with differences in morphodynamics and might be attributed to latitude-associated gradients like climate. Furthermore, the fact that Scolelepis squamata, as defined today, might be a superspecies, cannot be ruled out.