By Mary F. Willson
For the Juneau Empire
Good minus tides in late April enticed us out to inspect some intertidal zones. Among the first things we noticed were live cockles on the surface, although they are customarily buried an inch or two in the sand and gravel. We were not the only ones to find them — the common mottled sea stars were happily feasting. These five-armed stars often feed on bivalve molluscs (usually mussels and clams), pulling apart the two shells with their powerful arms. On the underside of those arms are numerous hydraulically operated tube feet that clamp onto the shells while the arms pull them open. (Yes, there are “feet” on these “arms.”) These sea stars feed by everting the stomach, and even a narrow opening allows them to insert that stomach between the two shells to digest the soft body inside.
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A mottled sea star in sole possession of a cockle, with the characteristic hump over the prey. (Courtesy Photo / MaryAnne Slemmons)
Some of the mottled sea stars had a cockle all to themselves, possessively humping up over the cockle with their arms spread out all around. But sometimes, we’d find a cockle attended by three or four or even five stars, crowded together and all of them apparently trying to feed on one cockle.
Mottled sea stars come in a wide range of colors: gray, brownish, pale or dark orange, pale or vivid blue, shades of purple. I found little information on the causes (genetic, dietary, etc.) and consequences of the color differences. However, observers have suggested that, because some colors are more conspicuous than others on certain backgrounds, predators such as gulls might prey more heavily on those in some places.
Many of the sea stars looked unwell — unusually pallid, shrunken, appearing limp and soft. A mysterious wasting disease has struck many West Coast populations of sea stars, some species being more greatly affected than others, but I don’t know if this was the cause of the apparent sickliness we observed. Of course, we also found several healthy looking stars that were lacking one or two arms, perhaps lost to predators; new arms were being regenerated to replace the lost ones.
Other kinds of sea stars were much less common, and they are reported to have somewhat different preferences of prey. There were a few sunflower stars of various colors. These voracious predators can feed on a variety of prey by everting the stomach or by swallowing them whole (even spiny urchins!). We found a spectacular leather star; this species likes anemones but also eats many other things, swallowing them whole. A Stimpson’s sun star was there, a predator that likes sea cucumbers, among other prey species. There was one brilliant red blood star; this species can evert its stomach to feed on bryozoans and sponges, but it can also catch edible floating bits in a mucus layer that covers its body surface and is then carried to the mouth by little hairs.
A pile of at least four sea stars converge on a cockle, all trying to feed; but who wins?? (Courtesy Photo / MaryAnne Slemmons)
We found two kinds of ribbon worms, which — despite the lack of claws or teeth — are predators. They have an eversible proboscis just above the mouth that is used to capture prey. The proboscis of purple ribbon worms is armed with a little spear that injects a neurotoxin to inactivate the prey (often another kind of worm), while the ribbon worm coils around it. The ribbon worm then retracts the proboscis, bringing the prey close to the mouth, to be swallowed whole. Purple ribbon worms often hide under rocks, but they lay a mucus trail when they come out to hunt, and follow that trail back to their hidey-hole. Red ribbon worms lack the little spear, but the proboscis is covered with sticky, possibly toxic, stuff that inactivates the prey. This species sometimes occupies tubes made of hardened mucus, attached to shells or rocks; a male and a female may share a sheltering tube.
Ribbon worms are shape-changers: from a compact, pudgy thing the size of my finger they can elongate to a thin string, in some cases over three feet long. This raises the question of what they accomplish by stretching so thin…it might just let them squeeze into tiny spaces, but perhaps it increases respiration by increasing the surface area through which respiration occurs.
Another notable thing about ribbon worms: if one is broken into pieces, each little piece can regenerate a whole worm!
In addition, there were four kinds of anemones, three kinds of sea cucumbers and three of chitons, some big whelk eggs and several spiral deposits of tiny eggs under rocks (by snail? nudibranch?), and three species of small fish, not to mention green urchins, limpets and so on. Notably absent were crabs! We found only one large hermit crab, rapidly retreating into a huge whelk shell, too fast to identify it. Where are the crabs?
A major highlight was a small reef, its sides festooned with bright orange anemones, while a group of black turnstones foraged on the top.
• Mary F. Willson is a retired professor of ecology.”On the Trails” appears in the Juneau Empire every Wednesday.
