ABSTRACT
Gigantism refers to a condition in organisms with a larger body size than in phylogenetically related organisms. It can be caused by disease (e.g. pathologic gigantism) or special growth conditions that result in rapid growth and/or longevity (e.g. island gigantism, polar gigantism, abyssal gigantism, or symbiotic gigantism). In particular, elevated dissolved oxygen levels are most commonly cited as the source of gigantism due to special growth conditions (Chapelle & Peck 1999). Symbiotic gigantism results from the presence of a wide range of photosynthesizing endosymbiotic organisms, but most common are the ‘zooxanthellae’ algae which are single-celled autotrophic plants (dinoflagellates) that live symbiotically in the tissues of their heterotrophic animal hosts (Trench 1993). Symbiotic algae photosynthesize and release photosynthetic products (sugars and O2), into
the tissues of their host. The host metabolizes the photosynthetic products and releases CO2 and waste. The symbiotic algae absorb the CO2 and extract the nutrients from the waste (phosphate and nitrate) during photosynthesis. This allows organisms to live in nutrient poor environments (Stanley 2003). Additionally, the symbiotic algae receive protection from predators by living in their host’s tissues. This symbiosis with its efficient recycling of nutrients promotes calcification (Pearse and Muscatine 1971) sometimes at ‘staggering’ rates which may lead to skeletal gigantism (Hallock 1981, 1996, Cowen 1983). This is primarily achieved in two ways: removal of CO2 from the host’s tissues helps convert bicarbonate to carbonate and the addition of energy allows for calorically expensive calcification. Algae-bearing hosts are generally an order of magnitude larger than their non-symbiont-bearing relatives (Cowen 1983). Algal symbiosis has resulted in gigantism in a wide variety of
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Marcus M. Key, Jr. Department of Geology, Dickinson College, Carlisle, USA
Patrick N. Wyse Jackson Department of Geology, Trinity College, Dublin, Ireland
Eckart Håkansson Institute of Geology, Øster Voldgade, København, Denmark
William P. Patterson Department of Geological Sciences, Univ. of Saskatchewan, Saskatoon, Canada
M. Dustin Moore Department of Geology, Dickinson College, Carlisle, USA
ABSTRACT: Photosynthesizing endosymbiotic algae can result in gigantism in their hosts, but this has never been unequivocally documented in extant bryozoans. Unusually large colonies (up to 7 cm branch diameters) of the trepostome bryozoan Tabulipora sp. have been recovered from the Kungurian (Early Permian) Kim Fjelde Formation in eastern North Greenland. Håkansson & Madsen (1991) used carbon and oxygen isotope values from skeletal carbonate to test the hypothesis that the gigantism was caused by algal symbiosis. In this study, a more precise test of their hypothesis was conducted using a finer sampling protocol on a colony from the same formation and location. Skeletal carbonate reveals mean 13C and 18O values of 3.9‰ VPDB and 6.5‰ VPDB, respectively. Diagenetic effects were evaluated by discretely recovering cements contained within zooecial chambers; skeletal values are significantly higher than the surrounding cements. In consideration of the isotope value of the Permian ocean, it is concluded that this isotopic evidence is largely negative. We reject the algal symbiosis hypothesis based on the combined isotopic, morphologic, and paleoenvironmental evidence.
