Based on your knowledge of signal amplification, which type of chemical messenger can have relatively small concentrations within the bloodstream and still get a large response?

Lipophobic is not soluble in non-polar solvents or  in lipids of chemical substances or any parts of molecules. It is the type of chemical messenger that still get a large response despite of having a relatively small concentration within the bloodstream. It is in contrast of the lipophilic.

Allow large amino acids through

Although i don't know what is the simulated system, starfish are predators that eat mostly  microalgae,  snails, sponges, bivalves, mussels and other small marine animals. removing starfish from any system would definitely increase this animals that they prey.