For centuries, biology has been preoccupied with the size of organisms; however, the true key to evolutionary adaptation may be hidden within life’s smallest units: cells. In fish, erythrocytes (red blood cells) are not merely hemoglobin containers; they are biological sensors reflecting how an animal respires and survives in environments ranging from polar abysses to tropical rivers.
Unlike mammals, whose red blood cells lack a nucleus, fish possess nucleated, oval, and biconvex erythrocytes—a feature that makes them ideal models for understanding the evolution of “cold-blooded” vertebrates. Despite their metabolic significance, this information was previously scattered across thousands of technical papers, often in languages other than English.
To bridge this knowledge gap, an international team led by Félix P. Leiva from the Alfred Wegener Institute has developed ErythroCite, the most exhaustive systematic mapping effort to date regarding fish cytomorphology.
Key Points
- Unprecedented Diversity: The ErythroCite project compiles 1,764 records from 660 fish species, spanning lineages from sharks to bony and lungfish.
- Extreme Variability: Data reveals up to a 414-fold range of variation in cell volume across the studied species.
- Biological Thermometer: Erythrocyte size serves as a critical indicator of how ectothermic (cold-blooded) species respond to rising global temperatures.
- Open Source Tool: The database integrates phylogeny and ecological data, and is freely available to propel macroecology and comparative physiology.
ErythroCite: A Multilingual Map of Aquatic Life
The creation of this database was no mere Google search. The team implemented a multilingual systematic mapping approach, recognizing that a vast amount of valuable scientific knowledge is produced outside the Anglophone sphere.
The Scientific “Tower of Babel” Challenge
To ensure global representativeness, researchers utilized search engines such as ISI Web of Science, Scopus, and Google Scholar, analyzing literature in seven languages: English, Spanish, Italian, Portuguese, German, French, and Polish. This strategy allowed for the recovery of studies that might otherwise have been overlooked due to the linguistic biases common in science.
Filtering Methodology and Precision
From an initial pool of 8,979 records, the team applied rigorous eligibility criteria:
- Primary Research Articles Only: Original data sourced directly from primary literature.
- Mature Cells: Immature erythrocytes were excluded to maintain consistency.
- Diploid Organisms: Polyploid individuals (those with extra chromosome sets) were discarded to avoid distortions in standard cell size.
- Taxonomic Harmonization: Species names were verified using databases such as NCBI, ITIS, and GBIF, ensuring each record corresponded to a valid and updated species.
Findings: From Actinopterygii to Dipnoi
The ErythroCite database classifies fish into four major lineages, revealing a fascinating cellular architecture:
| Lineage | Common Description | Database Representation |
| Actinopterygii | Bony fish (the vast majority) | 90.2% (595 species) |
| Chondrichthyes | Cartilaginous fish (sharks and rays) | 8.6% (57 species) |
| Cyclostomata | Jawless fish (lampreys) | 0.75% (5 species) |
| Dipnoi | Lungfish | 0.45% (3 species) |
The most striking discovery is the 414-fold variation in cell volume detected across species. While some bony fish possess extremely compact cells, others, such as lungfish (Dipnoi), exhibit the largest erythrocytes recorded in the database, with areas and volumes far exceeding the average of other lineages.
Actinopterygii: Masters of the Bony World
Representing 90.2% of the database (595 species), this group includes the vast majority of well-known fish, from salmon to tuna. Their erythrocytes tend to be smaller and more compact—an adaptation that facilitates rapid gas exchange in highly dynamic environments.
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Chondrichthyes: The Legacy of Sharks and Rays
With 57 registered species (8.6% of the total), cartilaginous fish show significantly larger cells than bony fish. Their red blood cells must efficiently transport oxygen in animals that are, in many cases, large predators with high energy demands.
Dipnoi and Cyclostomata: Giants and Ancestors
Lungfish (Dipnoi), despite representing only 0.45% of the species in the database, possess the most voluminous erythrocytes on record. Conversely, Cyclostomata (jawless fish such as lampreys) provide a window into the evolutionary past with 5 documented species that help clarify the origins of the vertebrate circulatory system.
Global Impact: Climate Change and Metabolism
Why is knowing the size of a fish cell vital in 2026? The answer lies in Ecophysiology. Studies suggest that species with larger cells typically inhabit colder regions, such as polar areas. However, these same large cells may pose a disadvantage in the face of global warming.
The Optimal Cell Size Theory
ErythroCite seeks to strengthen the optimal cell size theory, which links cellular dimensions to metabolic efficiency and oxygen transport. In a world with increasingly warmer and deoxygenated oceans (hypoxia), understanding these cellular limitations is fundamental to predicting which species will survive and which are destined for extinction.
Integration with Biological Big Data
One of ErythroCite’s greatest values is its interoperability. By including data from FishBase, researchers can cross-reference red blood cell size with body weight, habitat (marine, freshwater, or brackish), and phylogenetic position. This enables global-scale meta-analyses that were previously impossible to execute systematically.
Limitations and the Path Forward
Despite being the world’s largest catalog, the authors acknowledge that geographic and taxonomic biases still exist. Most data stem from adult fish and species of commercial interest or easy access. There is a notable scarcity of information regarding early life stages (larvae and juveniles) and deep-sea species. Moving forward, the team proposes the use of phylogenetic imputation—a statistical technique that utilizes the tree of life to predict traits in species where data are not yet available, thus intelligently filling database gaps.
ErythroCite is not a static project. It is hosted on GitHub and Zenodo under a Creative Commons license, allowing any scientist worldwide to contribute new measurements or correct existing data, keeping the flame of open knowledge alive.
Contact
Félix P. Leiva
Integrative Ecophysiology, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research
27570, Bremerhaven, Germany
Department of Environmental Science, Radboud Institute for Biological and Environmental Sciences, Radboud University Nijmegen
6500 GL, Nijmegen, The Netherlands
Email: felixpleiva@gmail.com
Reference (open access)
Leiva, F.P., Molina-Venegas, R., Alter, K. et al. ErythroCite: a database on red blood cell size of fishes. Sci Data 13, 307 (2026). https://doi.org/10.1038/s41597-026-06904-1
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
