Climate change has caused marine animals to shrink for millions of years

22 07 2026

For hundreds of millions of years, ectothermic animals—from mussels and crustaceans to fish—have responded to environmental crises by decreasing in body size. Until now, however, it has remained unclear how widespread this phenomenon is and which factors are responsible for it.

An international team of researchers, including Prof. Kenneth De Baets from the Institute of Evolutionary Biology at the Faculty of Biology, University of Warsaw, analysed the so-called Lilliput effect, which refers to the reduction in organism body size during environmental crises. The scientists examined nearly 9,000 body-size changes described in fossil, historical and modern studies. This made it possible to compare the responses of marine animals over approximately 450 million years.

The analysis confirmed that body-size reduction is a widespread response of marine animals to environmental crises. The phenomenon occurs across a wide range of groups—from dwarfing within individual species to an increasing proportion of smaller species in entire communities. It may therefore serve as a clear indicator of the pressure experienced by ecosystems.

Fig 1. Accumulation of belemnites (cephalopods from the Mesozoic Era) pictured here come from Peniche, Portugal. They lived during a warming interval approximately 183 million years ago and are only half the size of their counterparts from just before and after that episode (Pliensbachian-Toarcian crisis). (Photo: Kenneth De Baets)

The changes were particularly pronounced during periods of intense global warming. Although reductions in body size within communities were observed during all the environmental crises analysed, warming-related crises led to considerably stronger and more variable changes directly within species. On average, these effects were approximately twice as strong as those associated with other types of crises.

The results also revealed a relationship between the magnitude of warming and the degree of body-size reduction: the greater the increase in temperature, the more pronounced the decrease in body size. Earth’s history therefore provides an important warning signal about the future of ocean ecosystems.

Fig. 2. Fossils of belemnite cephalopod species Catateuthis longiforma that exhibited body size reduction during the Pliensbachian–Toarcian warming crisis and were included in one our datasets (Kenneth De Baets; https://doi.org/10.6084/m9.figshare.32236524)

The data indicate that the currently observed reduction in the size of marine fish and invertebrates is part of a long-term pattern rather than a short-lived phenomenon. If global warming continues unabated, smaller body sizes may become increasingly common throughout the world’s oceans, with potentially far-reaching consequences for marine food webs and commercial fisheries.

The article entitled “Unique fingerprint of marine ectotherm body size change during hyperthermal crises” was published in Proceedings of the National Academy of Sciences (PNAS). The author from the Faculty of Biology at the University of Warsaw is Prof. Kenneth De Baets. The co-authors of the publication are Paulina S. Nätscher and Wolfgang Kiessling from Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany (PSN and WK), as well as Université de Lille in France (PSN).

Congratulations to the authors on the publication!

Link to the publication: https://doi.org/10.1073/pnas.2505564123