Tidepooling

an invertebrate extravaganza (with some vertebrates too)

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When the tide goes out, it’s time for a natural history treasure hunt. This year, both May and June brought really low tides (more than minus-four feet) at more or less reasonable hours of the day. Of course, we had to go looking for weird and wonderful creatures that might be visible. We went to two likely spots, one in May and the other in June. At one of them, we were supervised closely by a pair of watchful black oystercatchers.

Here are a few of our ‘treasures’. All of them were viewed in place and sometimes photographed, or they were carefully replaced where they were found.

Numerous tiny sculpins scuttled for cover as soon as their pools were disturbed or even at the approach of the terrestrial monsters that cast long shadows. Hiding under small boulders were cockscomb pricklebacks, crescent gunnels, and a pale, thin fish called a gravel-diver. These little fishes are sometimes called eels, which they are not, or blennies, although they are not even very closely related to true blennies. They are reported to eat a variety of small invertebrates.

Sunflower sea stars, with their soft surfaces and multiple arms, were plentiful at one site; they came in all sizes from about four inches to perhaps twenty inches in diameter. They usually feed by swallowing their prey whole, and they eat almost anything, including sea urchins, clams, snails, other sea stars, and mussels. The common five-armed sea stars displayed an astounding array of colors: gray, olive, bright and dull orange, brilliant purple, turquoise, and tan. There were lots of little ones of this species, about half an inch across, and these didn’t seem to show such a wide variety of color. These sea stars can use their tube feet to pry molluscs open or lift them off the rocks. They typically feed by everting part of the stomach over the prey and digesting it in place. Despite their very crunchy nature, they are preyed upon by large gulls, big sunflower sea stars, and large crabs. A special treat was finding a few brittle stars, mottled in maroon and green. They are detritus feeders, preyed upon by some fishes and diving ducks.

There were at least five kinds of sea cucumbers, a big purple one, medium orange ones, small white ones, smaller translucent ones, and thousands of the very small black ‘tar spot’ cucumbers. Sea cucumbers typically breathe through their hind ends—pulling sea water through the anus into a set of branched respiratory tubules connected to the hind gut. They feed on organic detritus mopped up from the substrate or captured in the water column. I presume there is a mechanism for keeping digestive products out of the respiratory system! Their predators include several kinds of sea stars, some fishes, and sea otters.

Worms came in several guises. My favorite, one I’d never seen before, was an intertidal gillworm, buried in mud under a rock: bright red, with feeding tentacles at the front end and many thin filaments along the side that serve as gills for breathing. Of course, there also were other polychaete (meaning ‘many-bristled’) worms of several types, with their numerous body extensions containing various kinds of stiff bristles that may help in locomotion, and the extensions also assist in respiration. Many polychaetes feed by extending a tubular, muscular proboscis, usually armed with teeth, to grab their invertebrate prey. Polychaetes are the favored prey of ribbon worms, which can change their shape from elongate, skinny ribbons to stubby slug-like forms. Ribbon worms subdue their prey by stabbing with a sharp stylet and injecting a neurotoxin, then pulling it in to digest.

The common ‘black katy’ chitons came in all sizes, and so did the more colorful lined chiton. We found one hairy chiton, with its frill of ‘hairs’ all around the edge. Chitons are basically grazers, eating algae and little invertebrates that are stuck to the rocks (baby barnacles, sponges, and so on). Chitons are prey of sea urchins, some sea stars, black oystercatchers, harlequin ducks, and river otters, among others. I once found a pile of plates from a chiton on top of one of the mountain ridges, perhaps indicating that a raven had pilfered one from another predator.

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Lined chiton. Photo by Pam Bergeson

There were hermit crabs of all sizes, hundreds of urchins, the usual big green and red anemones, and the smaller green burrowing anemones that somehow squeezed themselves into impossibly small crevices. Small periwinkle snails abounded in some places; they are grazers. And there were a few larger, carnivorous snails known as whelks, which can drill into other shelled creatures and slurp out the innards. Just imagine being a blue mussel and feeling that big snail rasping through your shell! Ah, but sometimes even those sedentary mussels can fight back, by ensnaring the attacking whelk in byssal threads, which are usually used to attach mussels to the rocks but can be diverted to repel invaders.

A nice find was an alga that turned out, upon investigation, to be two algae. A dark, filamentous alga bore odd, warty, oval bubbles or sacs on its fronds. Those sacs didn’t belong to that alga; they were another alga altogether, one that lives epiphytically, attached to other kinds of algae. It is called ‘studded sea balloons.’ A new one for me!

A treasure hunt, indeed. I’m basically a terrestrial ecologist, so a visit to the intertidal zone is always both fun and educational.

June tidepooling…

…and a bear story

A minus 4.5-foot tide drew us out to check the intertidal zone. As we strolled across a wide sandy stretch at the water’s edge, we were startled to see a frond of rockweed steadily disappearing into the sediments. A few steps later, another algal frond went quickly down and was gone from our view. Then it happened again. Very mysterious. Some buried critter was pulling in algae, but who could it be? The most likely perpetrators were horse clams, withdrawing their siphons as our steps shook the sands and thus pulling down algae that were attached to the siphons.

We found several Aristotle’s lanterns, the grazing apparatus of sea urchins, left behind by birds that had cleaned out the urchins’ soft innards. I was interested to see that every lantern had a ribbon worm wedged into the intricate structure.

The big whorl of Neptunea whelk egg cases that we found in May was still there, with the presumed parent still nearby. But now the egg cases were open. We had thought, in May, that the whelk was just-then laying the eggs, but a month is too short a time for them to have hatched, so some predator may have got them.

We often find sea stars that are missing one or two arms, and sometimes they are in the process of regenerating them. But on this day, we found a sea star that had lost four of its five arms, and the regenerating arms were still very small. With only one functional arm, this star would find it hard to pry open clams or mussels or pull big snails off the rocks. I’m guessing that it would have to rely on small prey that requires less manipulation, but regenerating four full arms would take a lot of energy. I wonder if any researcher has ever studied the energetics of regenerating missing arms or the possible changes of diet for sea stars with many missing arms.

I noticed two black oystercatchers, unusually quiet. Then I saw that those two were accompanied by another one—it was back-lit, so colors were not clear, but it was slightly smaller and had a shorter bill. Aha! Parents with a big chick. They moved off without fuss to another rocky point.

And now a bear story: Sometime in May, I came home and looked out my front window. There was something large and black in the spruce tree across the pond—right where the pulley line for the seed feeders is attached. The feeders hang over the pond, where the spill is relished by a bunch of mallards. The large black creature tugged repeatedly on the pulley line with her claws and then with her teeth, but to no avail. She gave up, came down the tree, and came around the pond into the yard to eat horsetails, then ambled all around the house, sniffed some primroses, and went back to the other side of the pond. There she climbed a small pond-side alder that bent low under her weight, so she was then balancing—and wobbling—over the water on a trunk that was much too narrow for her bulk. Soon she was upside-down under the alder, like a sloth. She let go with her hind feet, so then her butt was in the water, and used her front feet to inch her way sideways along the trunk to shore. She stood quietly on the bank.

Now the pair of mallards that had been watching her shenanigans from the safety of the middle of the pond began to take closer notice. Mr Mallard suddenly and loudly lunged at the peaceable bear on the bank—and she scampered quickly up a tree! After she came down, he did it again—and she scooted up another tree! And then she ambled off into the woods. A friend noted, later, that I have ‘watch ducks’.

Gastineau Channel

abundance of life along Juneau’s busy waterway

In late April and early May, Gastineau Channel is notable for the large aggregations of scoters. They raft up in hundreds and thousands at the mouths of Gold Creek and Sheep Creek. Most are surf scoters, whose males are distinguished by the white patches on the heads. Less common are the white-winged scoters, whose white wing patches are best seen when the wings are spread. Only careful inspection would tell if there are a few black scoters mixed in the flocks.

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Surf scoters in Gastineau Channel. Photo by Bob Armstrong

Scoters are chunky, heavy-bodied sea ducks that winter along the coast. Those that winter to the south of us migrate northward in spring, often stopping to refuel in our area, toward their nesting grounds in the Interior. Canadian researchers have found a tendency for scoter migrations to follow the timing of herring spawning, which is generally later on more northerly coasts. Herring spawn is a favorite food of scoters.

On a low-tide morning in early May, a couple of friends and I walked down the beach on Douglas Island to Ready Bullion Creek. We went to see if dippers occupied their usual territory on the lower part of the creek. And yes, they were there, but they seemed to be nesting in a new site. The new place is one that for several years I thought would be ideal for them, but they had preferred to nest either down close to the intertidal or well upstream in a very narrow canyon. This time the nest site is between the former sites, on a cliff next to a nice waterfall and above a beautiful pool. (I finally got it right!)

As always, there were interesting things to be seen along the beach: two eagles with locked talons, spinning downward and breaking off just before crashing into the beach logs; a couple of migrating golden-crowned sparrows in the brush above the beach, on their way to the Interior and the subalpine habitats around here; male cottonwood trees starting to flower; a greater yellowlegs standing in the shallows; a pair of hooded mergansers flying by.

Small flocks of Barrow’s goldeneye cruised slowly along, in some cases in the company of a few pairs of harlequin ducks. The goldeneyes nest mostly in the Interior, but sometimes they nest in coastal areas—and at least occasionally in the Dredge Lake area. The harlequin females will go up along the coastal streams to nest, and (with any luck) they’ll bring flotillas of ducklings down to the sea later in summer.

The beach was covered with strange little tracks, which we deduced were those of crabs scuttling to and fro. A raven had marched in a straight line for many yards, and a deer had run down the sand. In one area, numerous holes in the sand, many of the surrounded by a tiny turret of slender, cylindrical fecal castings, may have indicated a population of some kind of worm (my ignorance is showing!).

The most interesting part of the beach was a shallow bight whose shore was densely occupied by sea stars. Many of the stars were steeply humped up over cockles or mussels (alive, alive oh!), having breakfast. They will also eat chitons, sea squirts, and limpets; the escape reaction of limpets is worth trying to see – they try to avoid the attacking star by ‘galloping’ away at Olympic speeds (relatively speaking, considering that they lack legs…). We noticed that quite a high proportion of the common five-armed star had only three or four arms, having lost the others to a predator (gulls, king crabs, other stars). Sea stars can regenerate lost arms, in time, but I wonder if there is a loss of efficiency in opening mussels or clams when there are fewer arms to pull open the shells. One sea star had a supernumerary arm, apparently regenerated from the side of a normal arm.

These sea stars displayed a remarkable array of colors—bright orange, dull orange, gray, brown with blue highlights, purple, brown with black bands across the arms, brown with dark blotches (like a rattlesnake, said a friend). It is highly unusual for any species to show such a diversity of colors. In the case of sea stars, it may be due, at least in part, to what they have been eating. And that may explain why an individual star can, reportedly, change color during its lifetime, and why a regenerating arm can be a different color that the rest of the star. A study of another species of sea star showed that diet had a big effect on the color of the star, although other factors must also be involved. I wonder if color has any effect on the risk of predation!