Bryozoan growth and environmental reconstruction by zooid size variation

Abstract

The modular growth of cheilostome bryozoans combined with temperature-induced variation in module (zooid) size has enabled the development of a unique proxy for deducing seasonal temperature regimes. The approach is based on measures of intracolonial variation in zooid size that can be used to infer the mean annual range of temperature (MART) experienced by a bryozoan colony as predicted by a model of this relationship that was developed primarily to infer palaeoseasonal regimes. Using the model predictions effectively requires a highly strategic approach to characterise the relative amount of within-colony zooid size variation (by adopting random or very systematic measurements of zooids that meet a stringent set of criteria) to gain insights on temperature variation. The method provides an indication of absolute temperature range but not the actual temperatures experienced. Here we review the development of, support for and applications of the zooid size MART approach. In particular, we consider the general issue of why body size may vary with temperature, studies that validate the zooid size-temperature relationship and insights that have been gained by application of the zooid size MART approach. We emphasise the potential limitations of the approach, including the influence of confounding factors, and highlight its advantages relative to other proxies for palaeotemperature inferences. Of prime importance is that it is relatively inexpensive and quick and allows a direct estimate of temperature variation experienced by an individual colony. Our review demonstrates a strong and growing body of evidence that the application of the zooid size MART approach enables robust interpretations for palaeoclimates and merits broad recognition by environmental and evolutionary biologists and climate modellers.

Evidence of El Niño/La Niña–Southern Oscillation Variability in the Neogene-Pleistocene of Panama Revealed by a New Bryozoan Assemblage-Based Proxy

“…For colonial animals like bryozoans, the relationship is expressed at the zooid level, and there is a reasonable body of evidence that zooid size is indeed inversely related to temperature. This evidence comes from diverse laboratory-reared material (e.g., Menon 1972, Hunter and Hughes 1994, Atkinson et al 2006, Amui-Vedel et al 2007, O'Dea et al 2007b, diverse field-collected material (e.g., Okamura 1987, O'Dea and Okamura 2000b, 2000c, O'Dea and Jackson 2002, O'Dea 2005, Lombardi et al 2006, and the growth of bryozoans in the field (O'Dea and Okamura 1999; see Okamura et al 2011 for further review) There are other factors that influence the ultimate size of zooids, the most obvious being species-specific colonial growth rules, such as the production of annual growth check lines (O'Dea andOkamura 2000b, O'Dea 2005, Okamura et al unpubl data), substrate irregularities, predation, and competition from other sessile encrusters, the influence of which can be minimized with the judicious rejection of randomly selected zooids as the approach requires (O'Dea andOkamura 2000a, O'Dea 2005). Salinity and food availability (Hageman et al 2009) have also been shown to influence zooid size, yet the effects are minimal in comparison with those of temperature (Okamura 1987, Hunter and Hughes 1994, O'Dea and Okamura 1999, 2000b, O'Dea 2005, O'Dea et al 2007b.…”

“…Although the mechanistic basis is subject to debate (see Okamura et al 2011 for review), the generality of this inverse relationship enables a means of inferring temperatures based on body size. For colonial animals like bryozoans, the relationship is expressed at the zooid level, and there is a reasonable body of evidence that zooid size is indeed inversely related to temperature.…”

Palaeogeographical patterns in Late Ordovician bryozoan morphology as proxies for temperature

“…For example, according to Hughes & Jackson (1990) and Kuklinski & Taylor (2008), avicularia, generally regarded as having a defensive role, are smaller and fewer in number in high latitude congeneric species, suggesting that predator pressure is relatively less important than physical stresses in these environments. In addition, there is an inverse intraspecific relationship between the size of the zooids in living cheilostome bryozoan colonies and the ambient temperature at the time of budding (see Okamura et al 2011). Limited evidence suggests that this relationship is also true between congeneric species: Kuklinski & Taylor (2008) found that the size of the autozooecia in cheilostome species belonging to six of the eight genera they studied was significantly greater in the Arctic than in congeneric species from lower latitude, warmer water sites.…”

History of upwelling in the Tropical Eastern Pacific and the paleogeography of the Isthmus of Panama

“…Achieving reliable estimates of MART requires strict adherence to several rules described in O'Dea and Okamura (2000b) and Okamura et al (2011). These aim to minimize the known effects of water current activity (Okamura and Partridge, 1999;Berning, 2007) …”