Final answer:
Sea cucumbers use their respiratory trees and cloacal muscles to pump water in and out, regulating their internal pressure and allowing osmoregulatory mechanisms to maintain salt and water balance.
Step-by-step explanation:
The question 'What regulates pressure within the system—a reservoir in sea cucumbers?' pertains to the biological mechanisms that sea cucumbers use to maintain internal pressure. Unlike cephalopods that possess a siphuncle, a tube that runs between the chambers to adjust internal pressure and buoyancy, sea cucumbers have different anatomical features to regulate their pressure. They utilize their respiratory trees, which are branched gill-like structures from the posterior gut, to pump water in and out with the help of muscles around the cloaca. This system helps in maintaining pressure equalization within the sea cucumber similar to how scuba divers have to be mindful of buoyancy and pressure equalization when diving. Additionally, these creatures have adapted osmoregulatory mechanisms that allow them to survive in various aquatic environments, regulating their internal salt and water balance.