Water fleas require microbiota for survival, growth and reproduction

Abstract

Microbiota have diverse roles in the functioning of their hosts; experiments using model organisms have enabled investigations into these functions. In the model crustacean Daphnia, little knowledge exists about the effect of microbiota on host well being. We assessed the effect of microbiota on Daphnia magna by experimentally depriving animals of their microbiota and comparing their growth, survival and fecundity to that of their bacteria-bearing counterparts. We tested Daphnia coming from both lab-reared parthenogenetic eggs of a single genotype and from genetically diverse field-collected resting eggs. We showed that bacteria-free hosts are smaller, less fecund and have higher mortality than those with microbiota. We also manipulated the association by exposing bacteria-free Daphnia to a single bacterial strain of Aeromonas sp., and to laboratory environmental bacteria. These experiments further demonstrated that the Daphnia-microbiota system is amenable to manipulation under various experimental conditions. The results of this study have implications for studies of D. magna in ecotoxicology, ecology and environmental genomics.

Figure 1

Size comparisons of Daphnia from (A) parthenogenetic and (B) ephippial eggs with and without microbiota. (A) Bacterial treatment has a significant effect on sizes of parthenogenetic Daphnia at day 4 and 10 (one-way analysis of variance, P<0.0001). Group means were compared with Tukey HSD test. (b) Bacterial treatment has a significant effect on sizes of ex-ephippial Daphnia at day 6 (Kruskal–Wallis test, P<0.008). Steel–Dwass test was used for pairwise comparisons of groups. Groups not connected by same letter are significantly different (P<0.05). Means and s.e. are shown.

Figure 2

Fecundity of Daphnia raised from parthenogenetic eggs under different treatments. (A) Percentage of egg-bearing Daphnia between days 11 and 25 for the four treatment groups. (B) Size of first clutch (median, lower and upper quartile, range) in four treatment groups. Bacterial treatment has a significant effect on the number of first clutch eggs produced (Kruskal–Wallis test, P<0.0001) and comparison of the groups show that Bac-Suppl produced significantly higher number of eggs than untreated, BacFree and BacFree+AB (Steel–Dwass tests, all P<0.01). Groups not connected by same letter are significantly different.

Figure 3

Survival curves of bacteria-supplemented (E-Bac-Suppl), untreated (E-untreated) and bacteria-free (E-BacFree) Daphnia hatched from ephippial eggs. Mantel Cox log-rank test indicates that bacteria-supplemented Daphnia lived longer than untreated and bacteria-free Daphnia (P<0.001).

Figure 4

Body size at day 6 of Daphnia treated with different sources of bacteria. (A) Daphnia from parthenogenetic eggs associating either with a supplement containing diverse bacteria from the mother (Bac-Suppl), a single bacterium (Aeromonas sp.) or bacteria free (BacFree). Bac-Suppl and Aeromonas treatments do not differ significantly, but they significantly differ from the BacFree treatment (Steel–Dwass test, P<0.002). (B) Daphnia from ephippial eggs exposed to bacteria from a non-Daphnia source (E-Bac-ADaM; non-sterile ADaM) had the same body size at day 6 as Daphnia exposed to bacteria from a Daphnia source (E-Bac-Suppl). Both Daphnia groups are significantly bigger than E-BacFree Daphnia (P<0.02). Groups not connected by same letter are significantly different.