Zooids of Cyclostome Bryozoans Became Progressively Smaller Over 200 Million Years

In evolutionary biology, body size is often one of the most direct and informative traits for understanding how organisms function and interact with their environments. Size influences respiration, feeding strategies, and locomotion, and it also reflects an organism's ecological role and competitive ability. In colonial organisms composed of many small repeating units, such as bryozoans, the concept of body size becomes more complex. Each colony consists of numerous modules called zooids, allowing researchers to examine size at two hierarchical levels: the overall colony and the individual zooids themselves.

Cheilostome bryozoan Electra pilosa showing individual zooids（感謝 Andrej Ernst 提供）

Previous studies on the bryozoan order Cheilostomatida revealed that zooid size remained largely stable across approximately 150 million years of evolutionary history. Because of this long-term stability, researchers expected that another bryozoan order, Cyclostomatida, might display a similar pattern. However, a recent investigation revealed a very different evolutionary trajectory. Cyclostome bryozoans belonging to the “Berenicea” morphotype show a clear and persistent reduction in zooid size beginning in the Late Triassic and continuing to the present day—an evolutionary pattern that runs counter to Cope's rule, which predicts that body size tends to increase over evolutionary time.

Cheilostome bryozoan Reteporella grimaldii（圖片來源：Parent Géry，CC0 1.0 公共領域）

Cyclostome bryozoan Disporella novaezelandiae（圖片來源：Alison M. Fortunato - Gall L (2019)，CC0 1.0 公共領域）

To examine this pattern, researchers analyzed 200 well-preserved fossil and modern specimens of cyclostome bryozoans. For each specimen, they measured the maximum zooid width, selecting five zooids per colony to obtain representative values. This metric was chosen because it is the most stable and easily recognizable size parameter in cyclostome bryozoans.

Colonies of the “Berenicea” morphotype typically grow as thin circular sheets that encrust shells or rocky surfaces. New zooids bud continuously along the colony margin, allowing the colony to expand outward over the substrate. Mature zooids possess a calcified frontal wall perforated by small pores, and each zooid bears a circular or elliptical opening through which a lophophore—a crown of tentacles used for suspension feeding—can extend to capture microscopic food particles from the water.

Because “Berenicea” is a form-taxonomic category rather than a true evolutionary lineage, the specimens analyzed likely belong to many different genera and families. This makes reconstructing precise phylogenetic relationships difficult. Instead of attempting a phylogenetic reconstruction, the researchers treated these morphologically similar specimens as a functionally consistent ecological group in order to track long-term trends in zooid size through geological time.

The results revealed a clear long-term decline in zooid size from the Late Triassic to the present. Importantly, this decrease did not occur as a gradual continuous trend. Instead, the data suggest step-like reductions at particular intervals, with notable shifts occurring around 165 million years ago and again approximately 78 million years ago.

Zooid width of five zooids per sample plotted against age across 200 samples（感謝 Ma J et al. (2025). 提供）

What forces could have driven this two-hundred-million-year trend toward smaller zooids? One obvious possibility is environmental change, particularly fluctuations in oxygen levels. Smaller organisms typically possess higher surface-area-to-volume ratios, which can be advantageous in low-oxygen environments. However, time-series analyses comparing zooid size with reconstructed atmospheric oxygen levels showed no convincing causal relationship. Although weak correlations occasionally appeared, the statistical analyses provided no evidence that oxygen changes directly drove the size reduction.

Another possible explanation involves biological competition. Cheilostome bryozoans first appeared in the Late Jurassic and underwent a major evolutionary radiation during the Cretaceous, eventually becoming the dominant bryozoans in modern marine ecosystems. Their zooids are generally larger and capable of generating stronger feeding currents, giving them an advantage when competing for space and food. If competition from cheilostomes drove cyclostome size reduction, one would expect zooid size changes to coincide with the rise of cheilostomes. Yet statistical tests again failed to reveal a clear causal relationship between the diversification of cheilostomes and changes in cyclostome zooid size.

Temperature is another factor known to influence zooid size in cheilostome bryozoans. In those species, zooids tend to be smaller in warmer waters and larger in colder environments. However, when researchers examined the relationship between zooid size and latitude—a proxy for temperature—they found no statistically significant correlation in the “Berenicea” cyclostomes. Moreover, fossil records indicate that occurrences of these bryozoans at low latitudes actually declined through time, making temperature an unlikely explanation for the observed size reduction.

With environmental drivers largely ruled out, attention turns to biological characteristics intrinsic to cyclostomes themselves. Cyclostome zooids tend to be slender and tubular, and the connections between neighboring zooids are relatively weak compared with those in cheilostomes. Smaller zooids may therefore offer metabolic advantages, potentially reaching functional maturity more rapidly and beginning feeding sooner after formation. This ability could allow cyclostome colonies to establish themselves quickly on newly available substrates.

Another important consequence of smaller zooids is a reduction in mouth and lophophore size. Because feeding structures scale with zooid size, this trend suggests that cyclostomes of the “Berenicea” morphotype gradually shifted toward feeding on smaller food particles. Such a shift would place them in a distinct ecological niche within marine suspension-feeding communities. Instead of competing directly with cheilostomes for the same food resources, these bryozoans may have adapted to exploit a finer particle size range within the planktonic food web.

Naturally, the study includes several uncertainties. Fossil ages depend on geological dating methods and sometimes lack precise stratigraphic information, especially for specimens collected decades ago. In addition, the “Berenicea” morphotype likely represents multiple unrelated evolutionary lineages. Finally, maximum zooid width is only a single linear measurement and cannot fully capture overall body volume or biomass. Despite these limitations, the data consistently reveal a strong signal: zooid size in these cyclostome bryozoans has steadily decreased over the past 200 million years.

This finding highlights an important evolutionary insight. Even organisms that appear morphologically conservative over long periods can undergo significant changes at subtle biological scales. In this case, the long-term reduction in zooid size may reflect a deep ecological strategy. Rather than evolving toward larger or more complex forms, these bryozoans gradually adopted a strategy of miniaturization, allowing them to occupy a distinct ecological niche and persist quietly across vast spans of geological time.

Author: Shui Ye-You

Reference:

Ma J et al. (2025). Zooid size reduction in cyclostome bryozoans from the Late Triassic to the present-day. Palaeontology.