Low tide explorations

a plethora of crabs, siphoning sponges, and other intertidal delights

June brought some good low tides, lower than minus four feet, so out we went to look at the rocky intertidal zone. This is always a little voyage of discovery, seeing some old ‘friends’ and finding some new ones. But I sure wish that some nice intertidal biologist would come with us, to answer our many questions!

We went out on two consecutive days, to two different sites. These two places differed greatly in the composition of the invertebrate community. For instance, at one site, the false white sea cucumber was overwhelmingly abundant, but at the other site, it was very rare…although the big orange sea cucumber was common there. The little black ‘tar-spot’ cucumbers were extremely plentiful at the first site, but they were not common at the second one. Crabs were hard to find at the first site but common, and of several different species, at the other. Such differences might be due, in part, to the difference in the amount of exposed bedrock habitat vs cobbles, but a real intertidal biologist could probably suggest some other possible explanations for the differences.

I picked up a big whelk shell and had a brief look at a bright red hermit crab rapidly retreating into the top of the shell’s spiral. This Pacific red hermit had found a shell so big that it could disappear entirely from view. Nice for protection, but the shell was so big compared to the crab that there was no way the crab could move its big house from one place to another.

Carefully turning over rocks, we often exposed small fish called pricklebacks. People erroneously or confusedly call them all sorts of names: blennies, eels, snakes, worms, wigglers, flippers, and whatnot. Pricklebacks, and the much less common crescent gunnels, can survive a low tide in damp, protected spots (such as under rocks, where ravens and gulls can’t grab them), because they can breathe through their skin.

Among the several kinds of crab were some brown ones totally covered with short bristles (helmet crabs, I think). We picked up one, only to find that it was an empty shell—the crab was gone. The shell was undamaged, but it was easy to lift open the back of the shell and view the interior. This crab had molted its old shell (along with the barnacles clinging to the legs), pulled its legs and eye and other parts free, and backed out. Now it had to produce a new hard shell, which must take some time, leaving the soft-bodied crab very vulnerable to predators. And the cost of making a new hard shell must be considerable. But a critter with a hard external skeleton can only grow by shedding the old one and making a new one.

Perched among the stubby fronds of an alga were several hairy snails—not very big, with very bristly shells. Something new for us! The usual kind of hairy snail appears to have a short life. After a larval stage, a young snail is male for the first year of its adult life. In its second year, it becomes female (while retaining the penis, just behind one eye), mates with a young male, lays eggs, and dies. An interesting way to go about things—I have to wonder what factors favored the evolution of this life history.

At one site, we found several crumb-of-bread sponges, yellowish in color, encrusting the rock. They break apart easily, hence their common name. I’ve read that a good diagnostic feature is their odor, said to smell like exploded gunpowder. Sponges are multicellular animals that have a very long fossil record, back to the earliest evolution of animals. They feed by pumping in water through small pores and passing water currents though the body, filtering out microorganisms. Several volcano-like structures are scattered over the body of this sponge; water currents and undigested food exit the sponge via these openings. The body is laced with glassy spicules made of silica but, nevertheless, it is eaten by various molluscs and other things. It simply can spread over the rock, but it can also reproduce sexually. However, unlike most sponges, this one has separate sexes.

Under a few flat rocks, there were several small, grub-like animals with no visible appendages that we’d not seen before. With long-distance help from an expert, these were identified as sandpeanuts, a kind of polychaete worm. It usually lives in the sediment below the mid-tide level and feeds on small bits of debris caught by its little tentacles. Without a hand-lens, we could not see the numerous tiny bumps and hairs that are distributed all over the body, nor did we discern the many ill-defined body segments. The feeding apparatus and gills at the front end can be retracted, so they are not visible when the animal is exposed by a low tide. No wonder we had a hard time figuring out what it might be!

I came upon a charming scene—a father with two small children (and a friendly dog) had found a small rock crab. The father carefully picked it up, showing the kids how to avoid the big pincers at the front end. After they all looked at the crab, it was gently placed back where they found it, and it wedged itself tightly between two rocks. What a good example of how to teach exploring kids.


Tidepooling in May

treasures of the seasonal low tide

Mid-May brought some fairly low tides during the daylight hours, so naturally I had to go prowling in the intertidal zone to see what I could see. I like to go with a companion, because the extra eyes greatly help the search. We found lots of things, as expected, but here are a few highlights.

–A gumboot chiton, about five inches long. I seldom see this species, which is often heavily harvested by humans. It’s an unusual chiton in that its dorsal surface is completely covered by the granular mantle, so none of the eight plates or shells is visible. An herbivore, it eats mostly red algae. It’s the largest chiton in the world, sometimes over a foot long, although in our area half that size is more usual. Beginning its life as a larva, after just a few days it settles on a rock and begins to look like a chiton. It can live for forty years or so, growing a few millimeters a year.

–Clinging to the underside of a rock, we found (I think) two long-armed brittlestars (called serpent stars in one field guide). These are said to be quite uncommon around here. The arms are very long relative to the size of the disc or body; they break easily but regenerate. This species eats detritus and can burrow into sediments.

Long-armed brittlestar. Photo by Kerry Howard

–The small sea cucumber known here as the white cucumber is actually the so-called false white cucumber, to distinguish it from the ‘true’ white sea cucumber of the outer coast. This species is extremely numerous in some places, so finding it was no surprise. But it was an occasion for remembering something weird about these critters. Many sea cucumbers breathe by taking in water through the anus into respiratory trees comprised of narrow tubules. Some cucumbers can eviscerate themselves if they are molested, spewing out both the gut and the respiratory trees through the anus, and regenerating them later. We found ejected guts of several false white cucumbers. The thought of regenerating body parts is exciting (and not very common in the animal kingdom); sea stars can regenerate their broken arms, and we often see evidence of this, but regenerating internal organs??!! How long does it take to re-grow these parts and what do the animals do before the missing pieces are replaced? We also found two giant black cucumbers, which were apparently near the upper limit of their tidal range.

–Cucumber suckers are tiny snails, just a few millimeters long, that feed on sea cucumbers. Strangely, they do not have the rasping, file-like radula by which many snails scrape their living. Instead, the somehow stick to a host cucumber, penetrate the skin, and suck its body juices. Although that’s how they feed, and there were many potential hosts close by, I found a cluster of this species just perched under a rock.

–We love to find baby king crabs tucked under the edge of a rock or a sea star but this time we found only three of them. Starting life as larvae in the plankton, they transform into little crabs and move to the intertidal zone. They molt their exoskeleton as they grow and move to deeper waters, eventually maturing at age five or six years.

–Hermit crabs may be everyone’s favorite intertidal beasties, and there are several kinds here. We found a few very tiny hermits wedged nicely into really small periwinkle shells—the ‘big’ claws of these individuals were less than two millimeters long. A small hermit was startled when we picked up a three-inch whelk shell, and it scuttled directly up into the top spirals of that shell and out of sight—clearly, this shell was not to be carried about on the crab’s abdomen! One old, barnacle-covered whelk shell housed a beautifully colorful widehand hermit, whose extra-wide big claw can close the ‘door’ when the hermit retreats into its protective shell.

–Some small rock crabs were inconspicuous in the sediments under loose rocks. I pried one out of its niche, to look more closely. It protested vigorously, nipping me several times—which I ignored. But then it got serious and its pincers opened up the end of my finger, staining red the rocks at my feet. That worked—it was soon back among the cobbles where it belonged.

There were other enjoyable observations, too: a gorgeous, six-inch crescent gunnel, crows harvesting and toting food to their nests, a flotilla of bright harlequin ducks, a whale blowing in the channel. A partially visible sparrow in the bushes above the beach sang an unfamiliar song repeatedly, making it hard to identify, but then it sang the normal song of a song sparrow. Was it just practicing, or do such song variants have another purpose?

Intertidal discoveries…

…include kleptoparisitism

On the next-to-the-lowest tide in late April, some friends and I explored the intertidal zone not far from town. We soon catalogued all the usual beasts, but several observations really caught my fancy.

Three tiny, bright red hermit crabs (very cute!) guarded their protective periwinkle shells with their big claws. We counted over seventy small king crabs, all less than an inch in carapace width. There were lots of five-armed sea stars, but one had only three arms and one had only two arms, with no signs of regeneration. So we wondered about the feeding success of these crippled stars, with so few arms for pulling open clams; how does their diet differ from that of intact stars, and how long does it take to get enough nutrition to regenerate those missing arms?

Among all the small fishes that lurked under boulders were two large crescent gunnels, at least eight inches long. They were so dark that the crescent marks on the backs were hard to see; these individuals were reddish on the lower parts of head and body and were probably males. The heads of two flatfish lay on a low ledge. The bodies of both fish behind the gills had been gnawed off, leaving a ragged margin. Someone had feasted well—probably otters, which commonly eat these fishes tail-first, and crabs may also have picked off a few bits.

The biggest puzzle was a heap of fuzzy-looking snails piled up on a mound of mostly indistinguishable material. We had no idea what this was, so I asked an expert and did some reading. Here is what I learned: These snails are known as Hairysnails (currently classified as Trichotropis cancellata). The shell bristles with hairs, and experiments have shown that the hairs deter some, but not all, predators. This snail lives in the North Pacific, usually sub-tidally, so the very low tide on this day was fortuitous for us.

Photo by Kerry Howard

Hairy snails are sequential (protandrous) hermaphrodites: as they mature they are male first and then gradually turn into females as they grow. Bigger snails produce larger clutches of eggs, increasing their reproductive output, while size is less important to male reproductive success, so for this species it pays to start reproducing (as a male) when small and producing eggs, lots of them, when large. During the transition time, they can sometimes function as both sexes at the same time—researchers have seen pairs of snails in which both snails are being male and female simultaneously.

The feeding habits of Hairysnails are fascinating. They can feed on small organisms suspended in the water but, more interestingly, they are kleptoparasites—stealing food particles right out of the mouths of tube worms (they are rarely seen parasitizing other potential hosts). The tube worms feed by catching plankton on their waving tentacles and funneling the prey to their mouths, and the snails just cling to the side of the tube and reach out with an extended lower ‘lip’ to snap up incoming prey. The snails that we saw were all clinging to a conglomeration of tube worms, ready to snatch food particles when the tide came up again.

Worms thereby suffer a reduced growth rate, but the sneaky, thieving snails grow much better—up to eighteen time faster than when simply suspension feeding. Small, immature snails, including those that are only a millimeter in size, gain the most in growth from their kleptoparasitism, but larger ones are also able to reproduce better. In most places, there is only one kleptoparasitic snail on a given tube worm, but occasionally there are two or even three, potentially in competition for stolen prey.

Hairysnails mate and lay eggs in winter, and in that season the adults generally leave their hosts and occupy themselves with reproductive activities. Males come back to stealing from tube worms when they are done mating, but females tend their batches of eggs for a while, and thus return to their hosts later than the males do. Small snails, less than about five millimeters in size, do not leave their hosts seasonally, but continue to thieve and grow.

Kleptoparasitism, in reference to stealing food from another animal, is widespread in the animal kingdom. Birds are perhaps the best studied: frigatebirds, jaegers and skuas, and some terns are well-known for harassing other birds that have caught a fish, making the victim drop the fish for the pirate to grab. In Berners Bay, we watched young glaucous-winged gulls, acting like teen-age hoodlums, chase other gulls until the prey was dropped. We sometimes see eagles forcing another eagle or an osprey to drop its fish. Among insects, cuckoo-bees lay their eggs on the pollen balls made by queen bumblebees for their own larvae and thus are stealing food from the bumblebee larvae. Web-building spiders may lose their prey to other spiders or to flies. Hyenas sometimes harry lions at a kill until the lions depart, letting the hyenas take much of the lions’ prey; and sometimes lions steal from hyenas. Those are just a few examples, but researchers say that kleptoparasitism occurs in every major taxonomic group of animals. Humans are not exceptions!

Sometimes the concept of kleptoparasitism is broadened to include the stealing of any useful item from another animal. So male bowerbirds that swipe alluring objects from each other, in order to decorate their own bowers, are kleptoparasites. So, too, are penguins robbing each other of stones used in their nests or tree swallows forcing a chickadee to give up its nest for the swallows’ use. Again, humans are not exceptions!

Thanks to Dr. Aaron Baldwin, ADFG, for sharing some of his extensive knowledge of marine invertebrates and providing literature references.