Did this for some fun times. I know there are some problems with how this would actually physically exist, but hey, I'm not a marine biologist.
The Craytin is a marine creature that roams the coastal waters, reefs, and shallows; the common denominator of these habitats being large populations of minnows and juvenile fish. The Craytin preys upon these small fish in much the same manner as the jellyfish - ensnaring them in its poisonous thread-like tentacles (11). The venom that the Craytin utilizes is a unique and potent and blend of neurotoxins and hemotoxins that can kill a small fish within seconds of injection. This venom is produced in the sacs that lie to either side of the 'head' of the Craytin, and is pumped to the tentacles via a large vein running the length of the Craytin's sail (10). The Craytin produces such large quantities of this highly potent venom that it is estimated that the life-time output of a single Craytin would be enough to eliminate the entire population of Madagascar.
The Craytin has developed a great degree of locomotion. It uses both its swimmerets (8) and its pectoral fins (6) combined with the flexing and contraction of its body to swim. The gills located on the 'head' of the Craytin (4) indicate that it must be in relatively constant movement to filter oxygen from the water. This is constitutes a large expenditure of energy, especially when compared to other venom-ed and tentacled marine creatures such as the jellyfish. As a result, the Craytin has also developed an extremely sophisticated method of predation, which is highly successful.
The Craytin's primary sensory organ is the horn-like structure protruding from its head (1). The filaments extending from this horn can detect the slightest changes in pressure, temperature, and direction of the water. When the Craytin detects prey within its vicinity, it will release large amounts of its neurotoxin out of the pores in its head (3) into the water, with the directionality of the current pushing this small cloud of neurotoxin towards the prey. All it takes is the ingestion of just a few molecules of this neurotoxin to slow the movement of a small fish. This allows the Craytin to swim around the fish, ensnaring it in the Craytin's tentacles. The Craytin uses its long sensory antennae (2) to aid in this maneuver.
Upone entanglement, the Craytin will then use its posterior and anterior maxillipeds (9,5) (appendages modified to assist in feeding) to transfer the food from the tentacles to the mouth, which is located on the underside of the Craytin (7).