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This article is a contribution from the Marine Zooplankton Ecology Lab at the ICM, CSIC.-- 13 pages, 6 figures, supporting information https://doi.org/10.1111/1365-2435.14356.-- The datasets generated for this study can be found in the Digital CSIC repository https://doi.org/10.20350/digitalCSIC/14754. (Calbet & Saiz, 2023), 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, This research was funded by Grant PID2020-118645RB-I00 by Ministerio de Ciencia e Innovación (MCIN)/AEI/10.13039/501100011033. With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S). The open access publication fee was covered by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI), Peer reviewed

ASLO Aquatic Sciences Meeting, Resilience and Recovery in Aquatic Systems, 4-9 June 2023, Palma de Mallorca, Spain, The temperature-size rule proposes 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 protozoans because in these unicellular organisms body volume is directly related to the consumption of prey and, on most occasions, the true volume of the cell is unknown. In our opinion, to approach this question, the actual size of the protozoan should be measured when prey are fully digested. To prove our arguments, we designed a series of experiments with the heterotrophic dinoflagellate <em>Oxyrrhis marina</em>, including functional and numerical responses, estimation of the protozoan volume during starvation, and time-dependent acclimation responses. We found that after the digestion of the prey, the size of <em>Oxyrrhis marina</em> was the same regardless of the temperature. We believe that the previous reports on the temperature-size rule using protozoans that show important cellular volume changes because of feeding are biased by imbalances between ingestion and digestion of prey. These imbalances disappear after sufficient acclimation, Peer reviewed