Bryozoan growth mirrors contrasting seasonal regimes across the Isthmus of Panama

Two highly environmentally contrasting localities on either side of the Isthmus of Panama were chosen to investigate the response of zooid size to differing seasonal regimes within colonies of cupuladriid bryozoans. The Bocas del Toro region in the southern Caribbean experiences a very low mean annual range of temperature (MART) while, as a result of seasonal upwelling, the Gulf of Panama in the eastern Pacific experiences a high MART. Profiles of zooid size, from ancestrula to colony margins, were made on colonies from both localities. Results reveal that in colonies from the Gulf of Panama zooid size varies cyclically, while in colonies from the Bocas del Toro region no strong trends are observed, hence paralleling the respective seasonal temperature regimes. These data support previous evidence that zooid size varies inversely with temperature. We propose that given good taxonomic understanding, profiling zooid size in this way could provide important information on aspects of life histories in both fossil and Recent cupuladriids. We were also able to show that the amount of variation in zooid size within cupuladriid colonies correlates well with the MART which the colony experiences. This approach may be used to provide data on absolute MARTs over geological time by analysing zooid sizes in fossil cupuladriids. The abundance of cupuladriids in the Neogene of Central America suggests that this technique could provide important corroborative data on regional environmental changes associated with the closure of the Central American Isthmus.