Supplementation of a diatom diet with cholesterol can enhance copepod egg‐production rates

Abstract

Cholesterol is required for animal growth, yet it cannot be biosynthesized de novo by crustaceans. Dietary sterols are thus necessary for the growth and reproduction of copepods. Sterols vary widely in both composition and concentration in phytoplankton. To explore the potential for sterol limitation of copepod growth, I did experiments with adult copepods feeding on diatoms with and without cholesterol supplementation. Cholesterol was added to the diet by (1) homogenizing the insoluble cholesterol with an ultrasonic disintegrator or (2) preparing gelatin/acacia microcapsules with and without cholesterol using triolein as a carrier. egg‐production rates were measured after 2–4 d on the experimental diets, except in one experiment, in which an 18‐d feeding period was used. Eggproduction rates increased 1.5–2‐fold for Acartia hudsonica, Acartia tonsa, and Calanus finamrchicus when the diatom Thalassiosira weissflogii was supplemented with 5–20 µg L−1 cholesterol; supplementation at 1 µg L−1 had no effect. Nearly three‐fold higher rates of egg production were observed with C. finmarchicus after feeding for 18 d on the supplemented diet. Cholesterol had a positive effect on egg‐production rates when added by either method. Egg hatch rates for A. hudsonica were also higher when they were fed T. weissflogii supplemented with cholesterol (91% hatched) than when they were fed an unsupplemented diet (40% hatched). Egg production of Centropages hamatus feeding on T. weissflogii was unaffected by supplementation. egg‐production rates of A. hudsonica were unaffected when Chaetoceros affinis, Phaeodactylum tricornutum, or Thalassiosira rotula were supplemented. Higher egg‐production rates in supplemented treatments were caused by a decrease in the frequency of small clutches during spawning events rather than an increase in maximum clutch size. These experiments demonstrate the potential for limitation of egg production by dietary sterols when copepods feed exclusively on diatoms.