Sea stars – we used to call them starfish – are truly alien beings, at least to us.
We and all other vertebrates, arthropods, worms, most mollusks and sundry others, are all bilaterally symmetrical, with a head and a brain and a bunch of sense organs at the front end. But, except when they are minute planktonic larvae, this isn’t true for sea stars and the rest of the echinoderms.
They are radially symmetrical, meaning they are more or less the same on every radius from their midpoint. They don’t have a head at one end and a tail at the other. They haven’t got either. They have a mouth in the middle on their ventral side, which is the side they attach to things with.
On that same side of each radial arm they have hundreds of little extendable and sticky tube feet, supported by a hydraulic system that also keeps their bodies relatively firm but quickly leaks out if they are removed from water. A sea star feeds mostly on bivalve mollusks which it opens by pulling on the two shells with its tube feet until the exhausted bivalve can no longer stay shut, and then it pushes its stomach out through its mouth and digests the bivalve in its shell.
It has a ring of nervous tissue around its mouth, not a central brain at all. It has simple vision using a light sensitive eye spot at the end of each radial arm. If a healthy sea star loses a few arms to a predator, it just grows new ones, tube feet, eye spots and all.
Could anything be much more alien to us as bilateral vertebrates? If we ever find complex life on another planet, it could be just as alien, quite a challenge to interspecies communication.
Sea stars have yet another very dramatic feature. If a sea star is unhealthy, gets infected with a pathogen of some sort, it turns to mush, and its various legs walk off in different directions, tearing it apart. Just now we’re seeing a lot of this.
A disease, or a bunch of diseases, has hit most sea star species on the Pacific coast of North America from Southern California to British Columbia, and something similar has hit some of the sea stars on the Atlantic Coast as well – in Maine, New Jersey and Florida.
Disease has hit sea stars before, just as it has other echinoderms such as sea urchins, and their populations have recovered after a while. But nothing like what has happened on the Pacific Coast has ever been seen before. There are a lot of sea star species there. The mortality of five species is huge, resulting in local extinctions. Some mortality has been documented in seven more. And seven further species are probably infected, but we lack documentation.
Dead sea stars on the Pacific coast are hard to miss, and the press has covered this event well. A particularly useful web site keeps everyone updated and allows them to add their observation of the growing mass of mush. It’s worth visiting.
The disease has a name: Sea Star Wasting Disease. But that doesn’t mean anything. We have no idea what is killing the sea stars. It could be a virus, or it could be bacteria, or a fungus, or a ciliated parasite, or pollution, or all of these, or none of these.
We simply haven’t got a clue.
A few labs are looking, but the search is slow at best. In any case, experience tells us that if pathogens are eventually identified, it will still be uncertain, and far too late.
Have we somehow caused this? We will probably never know.
Meanwhile, we can only hope that the sea stars recover, returning to their keystone status in their communities, and reminding us again how radically different a living being can be.