Does the temperature–size rule apply to marine protozoans?

Abstract

Abstract The temperature–size rule suggests that there is a negative relationship between the size (volume) of an ectothermic organism and the environmental temperature experienced during its development We question how to validate this hypothesis for the particular case of some algivorous protozoans because in many of these unicellular organisms, body size is directly related to the consumption of prey and, on most occasions, the true size of the cell is uncertain. In our opinion, to approach this question, the actual size of the protozoan should be measured when the prey are fully digested. To prove our arguments, we designed a series of experiments with the heterotrophic dinoflagellate Oxyrrhis marina, including functional and numerical responses, time‐dependent acclimation responses and the estimation of the protozoan volume during absence of food. We found that after the digestion of the prey Rhodomonas salina, the size of our long‐term thermally acclimated strains of Oxyrrhis marina was the same regardless of the temperature (16 and 22°C). We believe that previous reports showing protozoan size reduction because of temperature might actually reflect imbalances between ingestion and digestion of prey, perhaps amplified by insufficient acclimation. In support of the previous argument, we found evidence that short‐term temperature exposure experiments may suggest a negative relationship between well‐fed protozoan size and temperature, although this was not observed after long‐term acclimation. In light of the results presented here, we suggest that the actual evidence supporting the temperature–size rule in algivorous protozoans may not be sufficiently conclusive because most of the present studies largely ignore the effect of prey presence on the grazer's volume and are usually based on short‐term responses to temperature. Read the free Plain Language Summary for this article on the Journal blog.