A plague of stars

And I don’t mean plague as in “too many stars to know what to do with,” but as in “disastrous sickness that you don’t want to catch.” Some of the stars in my seawater table have been succumbing to some awful disease lately. A week ago today I noticed that many stars had been busy cannibalizing one of their compadres. Sometimes this just happens, and it doesn’t necessarily indicate that things are about to go south. But when I looked more closely I noticed that the victim, instead of just being eaten, had autotomized its arms. Autotomy occurs in most sea stars and other invertebrates, and in fact is used as a method of clonal replication in some stars and many cnidarians. The species of star that is being affected by this plague (Pisaster ochraceus, the common ochre star) isn’t one that readily autotomizes except in response to some external stress, such as a predator pulling on an arm.

So something was going on in this table. On Monday (Labor Day) I popped in for a quick check and although nobody had lost any arms I couldn’t be absolutely sure that everything was okay. Some of the Pisasters were a little squishy and had arms that were a little twisted. On Tuesday morning there was no autotomy but in the afternoon a star had lost an arm, greatly disturbing the student lab assistant who discovered it. On Wednesday the table looked like an asteroid battlefield:

Many of the other Pisasters were also showing signs of sickness: curly arms (visible in the yellow star in the lower right corner of the photo above. Another ominous sign is that some of the apparently sickly stars were kind of squishy, indicating that the water vascular systems were somehow compromised.

Severed arms littered the table. The autotomized arms retain mobility for quite a while after being cast off–they literally don’t know that they’re dead.

After removing the corpses and cleaning the table as best I could I was able to take a closer look at the survivors. I noticed that most of the remaining Pisaster stars had twisty or crossed arms, and some showed pretty severe stretching in the interambulacral area (“armpit” between adjacent rays), which I think is the first stage of autotomy.

The disease progresses very rapidly, and within an hour a star in this condition had pulled off one arm and was working on another.

Unfortunately, this disease also affects other species. My Orthasterias koehleri (rainbow star) decided to join the fun. When I arrived Wednesday morning it was intact. It dropped an arm. I went away for about 40 minutes to take care of tasks in a different building, and when I returned it had lost two more arms:

Alas, my one and only Orthasterias succumbed later in the day and was dead on Thursday. Interestingly, the disease does not seem to affect either Patiria miniata (bat stars) or Dermasterias imbricata (leather stars). In fact, the Patiria have been eating pretty well over the past week, scavenging on the carcasses of the plague victims. I don’t know if eating the diseased tissue will cause problems later on.

On Friday I lost two more Pisasters and isolated the Patiria and Dermasterias into tanks. A colleague of mine calls this the Molokai treatment, and I probably should have done it sooner, but I figured that at this point all the stars in the table were exposed to whatever pathogen is causing this disease so at that point why bother? However, I will need to sequester the healthy stars in order to disinfect the table once the disease has run its course, so into tanks they went.

After checking on the stars Saturday morning I am cautiously optimistic that the plague may have run its course. One more Pisaster, that was looking sickly the day before, had died, but my last two appeared healthy. Their arms were not curly, I didn’t see any interambulacral stretching, and they felt nice and hard when I poked at them. All of these are good signs, but I will continue to keep close watch on them. If they make it to Monday we just might be out of the woods.

As of today, one week after I noticed the first severe symptoms, I have lost 80% of my Pisaster collection. To put that in to context, this mortality rate is every bit as bad as some villages that were virtually wiped out by the medieval Black Death.