The First Star Of The Cowlitz Cacophony…
The winter in the Cowlitz Bay subtidal habitats is a time when nothing much appears to be happening, at least down to around a depth of 18m (60 ft) or so. If large is bigger than a golf ball, then a lot of such large critters are visible; however, most of them, such as Pachycerianthus fimbriatus, the large cerianthid tube anemone, and the weathervane scallop, Patinopecten caurinus, while quite attractive and morphologically interesting, are sessile, and observing their behavioral array takes special skills or goals.
While both species do play noteworthy roles in the natural history drama of Cowlitz Bay, their version of a one dive’s act needs some serious augmentation to keep someone’s interest. Individuals of neither species do much – at least overtly. The anemone can … wait for it … retract down into its tube; … rapidly. Wow!! Golly gee, be still, my beating heart! Woo… Woo… Impressive!!
And the scallop… well, now. It may close its shells, an event truly worth a negative number on the excitement scale. However, if one has been blessed by the fates, a scallop may actually swim (!) by rapidly clapping its valves together a few times. This actually IS exciting. Of course, probably the main reason for any excitement is that the behavior happens so rarely and it is compared to all of the other apparently non-interesting things happening in the vacinity. Normally, Patinopecten scallops are the epitome of dull. An individual spends its life in its little mud depression filtering water to obtain the phytoplankton it eats. Of course, a fair-sized clam such as fully-grown weathervane scallop contains a large mass of delicious muscles along with other nutritious innards. Consequently, the scallop is desirable prey item for any number of predators, including sea stars. Presumably as a result, natural selection has given the scallop its rather spectacular swimming escape response. If its mantle edge is contacted by a single tube foot from a sunflower sea star (Pycnopodia helianthoides), the scallop will usually start to clap its valves together rapidly and repeatedly, forcefully blowing water from between the closing valves forcing the clam up into the overlying water where it is blown away by the current. In a way, calling this “swimming” is overstating the activity, it has no direction and a very limited extent. However, currents in the area are often relatively strong, and the behavior can work to move the scallop away from the star. And that is truly worth the show. And then, after the scallop is done, it can be collected and become dinner for an altogether more lethal predator.
Normally, though, to see some interesting action in this habitat in the winter – and actually, through the rest of the year, as well – it is necessary to look for other predators at work. Fortunately, the array of active predators on the surface, in the sediments and in the waters above the soft-sediment areas of Cowlitz Bay is rich, diverse, and impressive resulting in a lot of opportunities to see “ecology in action”. The variety of predators ranges from diving ducks to dogfish and various other fishes from sculpins to flatfishes to sepiolid squids, nudibranchs, moon snails, and crabs. However, perhaps the most commonly seen, abundant, and continuously active predators in the region are sea stars.
The most commonly seen stars are Pycnopodia helianthoides, the sunflower star, and Luidia foliolata, the snakeskin star, both of which may attain large sizes. Sunflower star individuals have reportedly been measured at 1.5 m (5 feet) in diameter, while I have measured the average size in some Vancouver Island populations to be about 81 cm (32 inches) in diameter. While Luidia foliolata individuals don’t commonly exceed 1 m (39 inches), they are often around 80 cm (31.5 inches) in diameter. Pycnopodia helianthoides has been the object of a lot research, undoubtedly because of their large size and ubiquitous nature. They are probably the most frequently encountered, relatively large, subtidal sea star in the region, and given the demonstrated importance of asteroids in ecologically controlling marine communities, they justifiably have attracted a lot of interest. Luidia foliolata, hasn’t been investigated anywhere nearly as much, and as I spent more than a bit of time watching Cowlitz Bay’s L. foliolata, I thought this post would be a good place to introduce them.
The Mouth That Roared, Wetly
Asteroids are one of the most common educational poster children for invertebrates. Back in those ancient days when I was in high school, some time in a biology class was spent dissecting and examining a poor, rather pathetic, pickled Asterias individual shipped in from the New England coast to Montana where it spent the last of its cohesive existence boring some kids who had never seen a body of water much larger than a small farm pond and who didn’t really care for any animals without fur, fins or feathers. For those few of us who had a bit more on the ball (or so we thought), the asteroid’s pentaradiality along with its implied strangeness was really a pretty good introduction to invertebrate weirdness.
To even the most literate of us, a sea star was a pretty exotic critter; most of us had never seen a living one. Had the specimen been remotely like a living animal, it really would have been a neat thing to examine, I think. Unfortunately, the specimens reeked of formalin, and had a semi-slushy consistency resulting from much of the ossicular skeleton having dissolved in the acidic formaldehyde solution in which they had been stored. Finally, to top everything else off, their normal purplish color had turned to a gawd-awful pale diarrhea brown. Although the dissection wasn’t too hard, determining one tan glob from another was uninspiring to say the least. Still… the effort was made to show us sea stars, and point out some pertinent typical features of their anatomy and biology, such as their complete gut, part of which, the so-called “cardiac stomach” could be extended into the clams that they ate, and the suckered tube feet which used suction to hang on to anything.
Long years later, I was trying to teach many of the same things to my students. Fortunately, we were using specimens more freshly murdered “for the cause”, which weren’t a pasty mess. I hoped the students were able to more carefully examine and “understand” what they were seeing in their specimens than I had been in mine so many years before. Remembering my travails, I tried a number of different ways to make the important points. One of these was that they got to examine other sea stars, to become aware of a bit of the diversity in this awesome group. I always tried to have a live Luidia foliolata available in this exercise, as this was the first of a number of examples I used in my survey course to show the students that the “typical” animals they were learning about were, perhaps, not that “typical” after all.
To a careful observer, even one unfamiliar with sea stars, Luidia specimens are a bit weird. When first observed, there is just something about them that seems “odd”; perhaps it is the spines on edges of the rays, or the odd “scale-like” pattern of plates on the top of the arms, or the non-descript brownish-grey color of the aboral surface, but they leave the impression that they are somehow “different”. And, of course, they are (otherwise the “wily” instructor, aka “the old fart”, would not have put it out for them to examine).
Close examination shows that these animals lack the “suckers” or, more correctly, the “adhesive pads”, on their tube feet. Nonetheless, they are still able to stick to surfaces and hang on to prey. As is now known, sea stars attach themselves to substrata by a duo-gland adhesive system, not suction. Duo-gland adhesive systems were first discovered using Luidia, in part because the stars were seen crawling up the sides of aquaria by some students who flashed on the fact that this is a star that lacks “suckers” on its tube feet, and it shouldn’t be able to climb up a vertical aquarium wall. And if that weren’t odd enough, Luidia do not have a complete gut and do not (actually, cannot) extend their stomach into any clams that they eat. In Cowlitz bay their primary prey are sea cucumbers, although small clams are also on the menu. And all of the prey items are ingested.
Individuals of Luidia foliolata move over the substrate in Cowlitz Bay at a fairly good pace. Although a big one can move along at about a meter per minute when it is, for some reason, in a hurry, normally their pace is more leisurely. When they decide to feed, they stop, and start to burrow into the substrate. The tube feet move sediments from beneath the arms and central disk out to beside the animal and the whole critter just slowly descends into the substrate, taking a day or so to disappear completely. This activity leaves a large Luidia-sized star-shaped pattern on the sediment surface. Presumably, as it descends, any potential prey items, such as individuals of sea cucumbers in the genus Pentamera, or small bivalves such as Macoma carlottensis are transported to the mouth and ingested. I suspect they stop descending into the sediments when the tube feet have not encountered sufficient numbers of appropriate sea cucumbers for a while. They spend some time, probably no more than a couple of days below the surface, feeding and digesting their meal. When they are done, they rise up, emerge from the sand, regurgitate the indigestible remains of their meal, and mosey off looking for another place to feed.
In Cowlitz Bay, the aboral surface of Luidia foliolata individuals is sometimes covered with a layer of the large caprellid amphipods, Caprella gracilior. More than several hundred may be found on the back of a large star. When the star buries in the sediment, these amphipods will be seen filling the star‑shaped pattern with a layer of pink skeleton shrimp. The amphipods remain in place and when the asteroid rises from the sediments, they climb on their host and continue their ride. What these are doing on the back of the asteroid is unclear. This relationship has not been commonly reported, and I have never seen it elsewhere, although it was fairly commonly seen on my dives in Cowlitz Bay. It was just one more thing about this place that made it a worthwhile place to work.
The tale of Cowlitz Bay will continue in the future…