Feeding and growth kinetics by the planktotrophic larvae of the spionid polychaete Polydora ciliata (Johnston)

We studied the effect of food concentration on the feeding and growth rates of different larval developmental stages of the spionid polychaete Polydora ciliata. We estimated larval feeding rates as a function of food abundance by incubation experiments with two different preys, presented separately, the cryptophyte Rhodomonas salina (ESD=9.7µm) and the diatom T. weissflogii (ESD=12.9µm).

Additionally, we determined larval growth rates and gross growth efficiencies (GGE) as a function of R. salina concentration.P. ciliata larvae exhibited a type II functional response. Clearance rates decreased continuously with increasing food concentration, and ingestion rates increased up to a food saturation concentration above which ingestion remained fairly constant.

The food concentration at which feeding became saturated varied depending on the food type, from ca. 2µg C mL−1 when feeding on T. weissflogii to ca. 5µg C mL−1 when feeding on R. salina. The maximum carbon specific ingestion rates were very similar for both prey types and decreased with increasing larval size/age, from 0.67d−1 for early larvae to 0.45d−1 for late stage larvae.

Growth rates as a function of food concentration (R. salina) followed a saturation curve; the maximum specific growth rate decreased slightly during larval development from 0.22 to 0.17d−1. Maximum growth rates were reached at food concentrations ranging from 2.5 to 1.4µg C mL−1 depending on larval size.

The GGE, estimated as the slope of the regression equations relating specific growth rates versus specific ingestion rates, were 0.29 and 0.16 for early and intermediate larvae, respectively. The GGE, calculated specifically for each food level, decreased as the food concentration increased, from 0.53 to 0.33 for early larvae and from 0.27 to 0.20 for intermediate larval stages.From an ecological perspective, we suggest that there is a trade-off between larval feeding/growth kinetics and larval dispersal.

Natural selection may favor that some meroplanktonic larvae, such as P. ciliata, present low filtration efficiency and low growth rates despite inhabiting environments with high food availability. This larval performance allows a planktonic development sufficiently long to ensure efficient larval dispersion.