Can you offer a reasoned guess behind the Dark secondary typing to Crawdaunt & the star-shaped crest included in its design? Bulbapedia suggests “Crawdaunt’s shell may be based on batesian mimicry in association with Sharpedo, as it resembles an open shark mouth with a star on the nose.”
Well, the Dark type has to do with Crawdaunt’s violent, aggressive nature and its habit of driving other Pokémon away from its territory. Dark Pokémon are linked with malice, so fighting purely for the sake of violence (contrast Fighting-types, who fight to improve their skills) seems like a reasonable Dark-type trait. You can compare here Tyranitar, Hydreigon, and Incineroar. As for the star thing, I don’t think I’d read too much into it. It looks like a starfish, and Crawdaunt is an aquatic Pokémon, so the motif is on-theme. It looks a little like a crown, which expresses Crawdaunt’s higher status compared to Corphish. It adds a little more variety to the colours of the design, and some interest to the contours of Crawdaunt’s head. I don’t think it needs to be more than that.