Meeting Abstract
The appearance of multicellular animals during the late Proterozoic Era is thought to have coincided with oxygenation of the oceans, however we know little about the oxygen needs of early animals. Extant sponges are our best modern links to the theoretical bacterivorous, filter-feeding ancestors of animals. The apparent simplicity of the sponge body plan commonly leads to the assumption they are readily ventilated and have a low oxygen demand. However recent data suggest that resistance through the sponge can make filtration cost up to 30% of metabolism. To understand how modern sponges cope with the costs of filtration we studied the metabolism of two groups of deep water sponges in situ and in tanks. Glass sponges have thin tissues and large canals that are highly adapted to use induced current to reduce their costs of pumping. Whereas the glass sponge Aphrocallistes vastus removes only 0.5-2 µM of dissolved oxygen from the water it filters, in contrast, with dense tissues packed with microbes and narrow canals, the HMA demosponge Geodia barretti removes 20-40 µM of dissolved oxygen from the water it filters, presumably due to efficient metabolism of DOM by its symbionts. We used flow-through chambers to determine how tolerant G. barretti might be to hypoxic conditions. G. barretti quickly becomes anoxic if pumping ceases. The sponges continued to filter, though at a reduced rate, at 40 µM ambient oxygen (14% present atmospheric levels, PAL) and ceased pumping at 4-7% PAL. Recovery was rapid even after 48 hours in 20 µM (7% PAL) oxygen. While some sponges tolerate periods of hypoxia, normal feeding is energetically expensive. If the first multicellular animals were filter feeders then oxygen availability could have been a driver for the evolution of early body plans.
