Fiend Shrimp:

Behavior: Fiend Shrimp swim through the sea in swarms of uncountable numbers. They show little to no interest towards inorganic matter, but will avidly surround and attack living organisms they encounter. Encased in quite robust plated shells, these nasty crustaceans can withstand a surprising amount of punishment before expiring. Their primary tactic of feeding is using their foremost legs to pierce prey and then utilize their hellish mouths to burrow inside to feed. In small quantities they aren't very dangerous, but as their numbers grow so do their lethality.

Details: Upon closer inspection, their antennae are actually not for perception as was previous thought, but instead a means of neurochemical communication. Upon sighting food or danger, a colorless cloud of hormones is released from their tips to alert and attract others to their location and is estimated to last roughly 2 hours after the initial release.(edited)