Re: Do the darts on a sea anemone actually detach from the tentacles?

Hi Steven,

Sea anemones and their relatives use special stinging cells called 
cnidocytes (pronounced "ny'-do-sites") to catch their food.  The animal's 
tentacles are loaded with batteries of these cnidocytes, and snag/harpoon/
poison small animals that swim or blunder into them.  Each cnidocyte can be 
used only once, and here's an explanation for why:

Inside the cnidocyte is a capsule called a cnida.  Inside the cnida is a 
long, hollow, coiled thread called a nematocyst ("nee-ma'-toe-sist").  The 
nematocyst is the actual "dart" you asked about.  The business end of the 
nematocyst can be armed with hooks, spines, or barbs, and some can inject 
toxins into the prey animal.  Anyway, when the cnida is triggered to fire, 
the nematocyst is rapidly everted (turned inside out) and shot into the 
flesh of the prey.  In some cases, the nematocyst actually leaves the body 
of the anemone, and remains dug into the flesh of the prey -- ouch!  Once 
the thread is everted, it cannot be retrieved and recoiled back into the 
capsule, so nematocysts are definitely one-use-only structures.

Fortunately, sea anemones make lots of cnidocytes, and many different kinds 
of nematocysts.  Even if most of the cnidocytes are blown out during a 
feeding bout, the anemone can probably replace its arsenal within a day or 
so.

Thanks for the question!  

Allison J. Gong
Mad Scientist