Porifera and the Benthos-Pelagic zone power couple (Benthic-Pelagic coupling)

admin Invertebrate Zoology

Benthic-Pelagic coupling describes the exchange of energy, mass, nutrients, etc. from the benthos to the pelagic zone and vice versa. The benthos and the pelagic zone are areas or habitats in aquatic ecosystems. The benthos is the sea floor, the pelagic zone is the water above the seafloor. The pelagic zone is divided into many different zones dependent on depth. The filter feeders of the benthos take in anything that is suspended in the water column, organic matter, nutrients, etc. One of the more abundant and important of the filter feeders are the porifera, or sea sponges. These and other filter feeding benthic biota act like kidneys that filter the sea water to remove the nutrients they are after, but also other particles in the water column. Some of these particles could be hazardous to the organisms.

Porifera and other filter feeders are crucial for the cycle of nutrients in these ecosystems. They take in nutrients that have been dissolved into the water column, from the seafloor or elsewhere. Additionally, poriferans take in organic matter from deceased organisms such as plankton. According to Amanda Kahn and Sally Leys in their article,

Suspension feeders transfer carbon to the benthos by capturing and concentrating plankton from the water column, but exactly how carbon is transferred is still not clear. Traditional work has focused on detrital pathways  which include direct fallout of particulate organic material (POM), filtration of POM and excretion of wastes, and use of dissolved organic matter (DOM) mediated by microbes (microbial loop)” [Kahn].

Pelagic, and benthic, biota then feed on the porifera or the matter that they bring in continuing the cycle by bringing the nutrient and matter back to the pelagic zone.

Poriferans filter many gallons of seawater, dependent on size and type of poriferan. Filtering occurs constantly, if not near constantly. So the cells used to extract nutrients, choanocytes, need to be replaced constantly and rapidly. It is estimated that they replace about a third of their body carbon per day!




A basic sponge structure, including choanocyte structures, that are the third label down from the top.








How do poriferans replace cells so rapidly? They are actually replaced by stem cells. The stem cells form into choanocytes and find their way to a choanocyte chamber where it moves in and begins its functions. These choanocyte chambers are part of the ostia, which brings in the water to filter. The new choanocytes move along the mesohyl until they find a chamber to go into.



The figure shows a choanocyte locating and inserting itself into a choanocyte chamber.






Porifera, and other filter feeders, help to prevent the saturation of nutrients, carbon, and other material in the water column. Keeping these materials in check help to prevent algal blooms and ocean acidification, via the degradation of biomass that produces carbon dioxide.




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