Ecophysiology of glass sponge reefs

• Author / Creator

  Kahn, Amanda S

• Suspension feeders are an important component of carbon exchange between the water column and the seafloor, a process called pelagic-benthic coupling. Sponges (Phylum Porifera) are filter feeders that consume especially small particles. They eat bacteria, which are inaccessible to most other filter feeders but which make up 10-30% of primary productivity. Feeding by sponges in shallow waters may have considerable effects on energy flow through ecosystems, especially in food-poor environments. This dissertation focuses on the ecophysiology of glass sponge (Porifera, Hexactinellida) reefs, one of the densest aggregations of sponges known in deep water, to determine the flow of energy through reefs and its underlying physiological mechanisms from the scale of the cell up to the ecosystem. Combined, the results explore the ecosystem functions of an important habitat on the northeast Pacific continental shelf and lend insight into the ecology of hexactinellids elsewhere in the world and in past oceans including the ancient sponge reefs in the Tethys Sea. Paired water samples collected in situ before and after passing through a sponge showed that reef sponges remove bacterial carbon and release ammonium into the water column. Stable carbon and nitrogen isotope signatures of reef sponges indicate that bacterioplankton came from both terrestrial and oceanic sources to differing degrees at different reefs, and possibly from bacteria associated with sediments. Microscopical investigation showed that reef sponges also released fecal pellets – aggregates that were 100 to 1000 times larger than the particles they consumed – thereby moving microbial food energy to the benthos as well. Glass sponge reefs have the highest grazing rate of any benthic suspension feeding community measured to date because of the high volumes of water they filter, their efficient removal of bacteria, and their sheer density and size. 13C-labeled bacteria fed to the sponges remained in the tissue for at least two weeks, suggesting that sponges retain and sequester carbon as biomass as well. Repeat visits to the same reef sites and same individual sponges over three years showed that reef-forming glass sponges have similar growth rates, recovery after damage, and recruitment rates to those of shallower water demosponge species. Study of cell and tissue production however showed conservative processes for tissue maintenance. Pieces of the reef species Aphrocallistes vastus were collected and newly forming nuclei labelled with the cell proliferation marker EdU. Very little proliferation occurred in mature regions of the body; most labeling occurred in growing (tip) regions of the sponge. Cell turnover rates were similar to those found in non-growing, mature regions of three shallow temperate sponge species (Sycon coactum, Spongilla lacustris, and Haliclona mollis). In general, sponges were found to vary rates of cell turnover depending on season, taxon, and life history stage suggesting an ability to modify energetic investment in tissue maintenance depending on environmental conditions. Most importantly, for demosponges at least, mature choanocytes – the pumping and feeding cell of sponges – were replaced not by direct replication as in colonial flagellates, but by immigration and differentiation of stem cells as in other animals. Light and electron microscopy showed that tissue and skeletal growth was localized to the growing regions at the tips of the glass sponge Aphrocallistes vastus. Choanoblasts – founder cells for flagellated chambers – first divided to form clusters, then produced enucleate collar bodies that expanded the flagellated chambers to their full size. Combined, the results presented here contribute to an understanding of the flow of energy through glass sponge reefs and the energetic requirements of reef sponges. Glass sponge reefs transfer from microbial food energy to the water column and the benthos through pelagic-benthic coupling, with food sources that can sustain their intense feeding and can fuel comparable growth rates to those of shallower species in food-rich habitats. Other glass sponge communities throughout the deep sea may share similar roles of pelagic-benthic coupling and, in their food-poor environment, act as localized oases of food energy. The tissue structure and tissue maintenance of syncytial glass sponges reflects adaptations to a low-food environment, and could also reflect the conditions in which sponges and other early animals evolved.

• Subjects / Keywords

  • Reef
  • Energetics
  • Suspension feeding
  • Porifera
  • Sponge
  • EdU
  • Pelagic-benthic coupling
  • Hexactinellida

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3FX74C5V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.