Harlequin shrimp
a. control starfish populations
b. clean parasites from select fish species
c. control tubeworm populations
d. hide within coral using their special camouflage

Harlequin shrimp (a.) control starfish populations. Harlequin shrimp are very picky about what they eat but they do eat starfish. 

p.s. I took a test with the same answer and (a.) was correct 

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature