Similarity in morphological characters of diverse species makes complication in taxonomy to identify / determine what the species is. Many times, the identification would be misleading.

When collecting the echinoderm species in Gulf of Mannar, the question arises why these beautiful creatures are more complicated to identify. One of my colleagues suggested that this species will be either Pentaceraster mammillatus or less chances to be Protoreaster species. Can anyone please suggest the key identification between Genus Pentaceraster and Genus Protoreaster in Phylum Echinodermata?