February 11, 2011 
Ciliated Mantis Shrimp (Pseudosquilla ciliata)
During the survey on Wednesday morning, I noticed a Mantis shrimp hanging out right next to the transect line. I quickly snapped a picture before the shrimp darted into a hole.
Mantis shrimp, also know as stomatopods, are crustaceans. They have modified forearms that are shaped like stout clubs or serrated swords, which they keep folded alongside their bodies. They can be harmful to handle because they can thrust forward their forearms at lightning-speed, almost sounding like a loud ‘snap’. This technique is used in crushing (smashers) or impaling (spearers) prey. It’s so fast that smashers are capable of cracking aquarium glass and spearers can cause serious injury to both human or critter. They like to burrow into the holes and crevices of rocks on shallow reefs.
Reef Watch Waikīkī 
That’s the crackling you sound you hear when snorkeling no?
Yep, these (and other) shrimp contribute to much of the crackling you’re hearing.
I thought I saw one of those not long ago, but it quickly boogied into a hole as I approached and I could not make a positive ID. I’ll have to start looking out for them.
I’m so glad you posted this. I’ve seen these in the lagoon at Magic Island, and wondered what they were. Now I know!