I used dry media brushes for this, particularly an oil pastel set. I can't decide which set I love best. I love overlaying several layers of color over my grayscale artwork. I normally tint the grayscale , but this time leaving it gray got the underwater effect I wanted.
As to what manner of creature this is, I have the rest of the text in my blog if you're curious. I had to omit a few paragraphs for brevity and formality. When these shellfish reach the age of thirty, it gains a monstrous form, as well as various shape-shifting abilities. It’s specific behavior is a bit of a mystery, at least to me. Some writings only indicate its desire to ride the waves under the light of the moon. Another describes it as predatory with a penchant for mischief. It prowls the coasts at night looking for prey, it’s known to break in to seaside homes and eat the residents. But perhaps a folktale involving er… let’s just say frustrated pirates made the Sazae Oni gain notoriety.