Imagine if you could fend off would-be attackers by surrounding yourself in a cocoon of snot. Well, the humble slime star (or Pteraster tesselatus) can do just that. When exposed to stressful conditions such as rising temperatures or wanton man-handling, this slippery little fellow secretes copious amounts of clear, stringy slime, which completely envelopes it in a protective bed of ooze. This has got to be one of the coolest and grossest defenses in the animal kingdom!
The source of their mucous power lies within their skin. Completely covering their dorsal side are small openings called spiracula, which connect to spaces called nidamental cavities. The nidamental cavities are lined with tall, columnar cells which are actually unicellular glands that secrete the snot in question. These glands can also be found in the supradorsal membrane and body wall, but it’s the glands of the nidamental cavities that are patched in to the slime stars unique form of respiration. By inflating and deflating itself by way of the spiracula and osculum (a large hole in the center of the dorsal side), P. tesselatus maintains a constant flow of water through it’s tissues. When at rest, it will inflate and deflate about four times per minute, but when stressed, inflation rate will often increase, becoming deep and convulsive. This water movement in conjunction with expression of the columnar glands, allows the slime star to move large quantities of slime in a very short amount of time.
You may wonder why P. tesselatus has developed such a snotty method of defense. After all, so many other seastars seem to defend themselves just fine without the use of overabundant mucous. Well, one possible explanation might involve the fact that they lack pedicellariae. Pedicellariae are very small pincer-like structures found on most echinoderms that help keep them free of larva and other small encrusting organisms. Without pedicellariae, a seastar would be prone to overgrowth and possible consumption by a host of smaller animals. By having a perpetual layer of goo on their skin, the slime star is effectively protected from settlement by these parasitic party-crashers. The sheer volume of slime that they produce can also deter larger predators such as crustaceans and fish.
If you want to get a closer look at a slime star ‘in the flesh’, stop by Lab 3 where we are currently housing two of these nifty little critters, one of which we caught in the trawl returning from Sucia Island. By observing them at rest, you may get a glimpse of their wide open osculum venting water from their body cavity. But don’t get them too riled, an irritated slime star could potentially clog the drainage system in their tank!