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.

Rocky intertidal fishes

some fish out of water do just fine.

A trip at low tide to one or our rocky intertidal sites always yields an array of pleasures and some treasures. Maybe I’ll see my favorite bright red hermit crab! Or find an Aristotle’s lantern—the feeding apparatus of sea urchins, which may be all that’s left of the innards of a hapless urchin demolished by a crow. Or maybe I’ll catch a whelk in the act of laying eggs. Always fun.

Lurking under rocks and rockweed, I’ll find small, slender fishes. Sometimes called eels, or blennies, they are neither: in our area, they are usually gunnels or pricklebacks, and I will focus here on some that reside in the upper portion of the intertidal zone. They spend their entire lives in the intertidal zone, which means that they are not submerged in sea water for a significant portion of each day. Of course, the higher up in the intertidal zone they are, the longer the non-submerged period, which happens twice a day. Most fish can’t handle that; we even have an expression “like a fish out of water” to describe someone completely out of his or her element.

Gunnels and pricklebacks, however, deal with low tides just fine. They (and many other species of fishes, of many different kinds) are able to breathe air. Air-breathing fishes around the world accomplish this feat in lots of different ways: for example, some use their swim bladders, or various parts of the digestive tract, or special chambers above the gills. Gunnels and pricklebacks can breathe air, using both gills and skin, as they do in water. Their respiration is reported to be just as effective in air as it is in water, although prolonged stress might alter that.

To begin this discussion, let me present some basics about respiration (in either air or water). Respiration is all about 1) getting oxygen into the body and then to the cells where mini-organs called mitochondria do the work of oxidizing carbohydrates and creating energy to run the whole body, and 2) getting rid of carbon dioxide, which is one of the byproducts of oxidizing those carbos, so that the interior of the cells and of the body don’t become too acidic (which interferes with lots of processes). Both gills and skin perform these functions, but the relative roles of those organs differ among species.

Gills of most fishes are long, thin, and delicate, so as to expose lots of surface area for uptake of oxygen and elimination of carbon dioxide. But such gills tend to collapse when out of water. Intertidal fishes make what is called a ‘trade-off: they have gills that are shorter and not quite so delicate, thus reducing their tendency to collapse, but they sacrifice some of the surface area for diffusion of respiratory gases. Shorter, stouter gills also reduce the risk of desiccation in air.

Both gills and skin need to be kept moist in order for oxygen to diffuse in and carbon dioxide to diffuse out. So when the tide is out, these intertidal residents may dip in and out of tiny pools or roll in wet places, for example.

High cockscomb prickleback. Photo by Bob Armstrong

A common prickleback in our upper intertidal zone is known as the high cockscomb prickleback—named for the prominent ridge on top of its head. That ridge tends to lie flat, however, when this dark fish is not submerged, making confident identification difficult for non-experts, in most field conditions. In this species, females tend to be larger than males (at equal ages), and males compete for mating privileges with females. Large females are especially worth competing for, because they lay more eggs than small females. Eggs are laid under rocks, where the female takes care of them for about a month: coiling around the ball of stuck-together eggs, fanning them to increase flow of oxygenated water, and guarding.

We also see crescent gunnels in the upper intertidal zone. These are sometimes readily identified by the light-colored marks along the sides, but I’m told that some individuals are dark, so discriminating them from other dark species may not be easy in the field. Crescent gunnels have apparently been studied less than high cockscomb pricklebacks, but both parents (but sometimes one or none) often tend the eggs, which are laid under rocks. Most of the other gunnels and pricklebacks in our region are either relatively rare or occupy lower parts of the intertidal zone, and in some of these species, parental care is by the males.

Another small fish is common in the upper parts of the intertidal zone: the tidepool sculpin. As the name tells us, it typically lives in tidepools left by the receding tide. It’s an air-breather too, using the gills, mouth lining, and skin. Sometimes conditions in its home tidepool become low in oxygen or too acidic; this could happen especially at night when all organisms continue to respire and produce carbon dioxide but there is no photosynthesis to use that carbon dioxide. Or sunlight might make the pool too warm. Then these little sculpins often choose to leave their pools, either partially—just exposing the head to air—or fully, resting on nearby weeds or rocks or, occasionally, crawling to another tidepool. They are said to be quite good at homing…returning to their home pond if they are displaced.

This fish is unusual in that males and females copulate and the males’ sperm are deposited inside the female, but the eggs are actually fertilized after they are laid. This is obviously a contrast with most other fishes, in which males and females spawn by releasing sperm and eggs into the water. There is no parental care.

Crow with a prickleback. Photo by Bob Armstrong

These intertidal fishes face many risks in addition to desiccation and respiratory difficulties. Even though they have escaped the many predators in the open sea, there are opportunistic land-based predators that can find them. For example, ravens and crows fossick about in the rockweed and poke under rocks, sometimes coming up with a prize; mink delve into tidepools or turn over rocks. And we who love to explore the rocky intertidal inevitably do more damage than we would like.

Thanks to Dr. K. L. Martin, Pepperdine University, for helpful references and consultation.