Down in the grim intertidal zone

By Mark Brazil | Aug 19, 2004


A coffee-shop friend of mine recently summed up his appreciation of our local lowland forest just outside Sapporo, saying: “You know, it’s wonderful here; every season is the best season.” And, you know, he has a powerful point.

I believe, though, that it is his underlying interest in things natural that makes each season so special for him. I have spent a considerable amount of time in countries where summers are too cool, winters too wet, spring too short and autumn uncolorful. In such places, I have found myself longing for other times of year.

Here in Japan, the seasons — and I’m a proponent of Japan having six, since we have the rainy season and the typhoon season as well as the usual four — are rich, reflecting strong climatic shifts, and there’s extraordinary biodiversity to boot.

Each season carries within it a hint of the next to come, or the one that has just passed. In the high mountains, summer’s slow-melting snow patches are like time bubbles, holding the small area surrounding them in an encapsulated, delayed spring. In the lowlands, where greens have deepened to that dark mid-summer hue, the plethora of hawker dragonflies is a hint of the autumn dragonfly season ahead, while here and there an errant spray of leaves has already turned orange, presaging the colorful delights of fall.

Given the extraordinary heat that this summer has blasted us with, thoughts of cooling sea water are likely to be on many people’s minds. However, this is a time of year when pottering along a seashore makes sense from a natural history point of view as well as for thermoregulation.

By now, most birds have finished breeding and are less vocal, while dense summer foliage effectively hides them and the native mammals that might otherwise make inland forays exciting. On the coast, though, shallow waters are refreshing, and if you can find a rocky shoreline or intertidal area that is accessible, then you have a marvelous opportunity for fossicking.

I am as guilty as the next person when it comes to having a far too strong vertebrate bias. Since childhood I’ve reveled in mammals and birds, and I bet that worldwide, for every 10 vertebrate biologists/naturalists, there is only one hooked on invertebrates.

However, if you are ever fortunate enough to be in Homer, Alaska, and can look up Conrad and Carmen Field — or better still, join them on a field trip (pun intended) — then you may find your inherent vertebrate bias weakening under their enthusiastic tutelage.

Irresistibly fascinating

This husband-and-wife team of naturalist-educators makes the marine invertebrate world so irresistibly fascinating (and, with remarkable modesty, they are so multitalented that they can do the same for plants, birds, marine and terrestrial mammals, too) that spending time with them in the field has even made me wonder whether I made the right choice in following avian pursuits.

Alas, their kind are too few and far between, and here we must be content with self-guided forays. Nevertheless, armed only with books, patience and a willingness to get your feet wet, the intertidal zone welcomes explorers. This is no African savannah with charismatic megafauna on parade, but dramas of death, destruction, passion and procreation are all played out here where saltwater ebbs and flows. Though it presents challenges well beyond humans’ capacity for survival, the intertidal environment is inhabited by an astonishing array of organisms.

Consider first the physical battering that this zone experiences; it is open to the cyclic rise and fall of the tides, and to the constant wave action that wears and washes at the shore. Any organism living here must be able to hold on tight, or somehow withstand this battering by hiding away in cracks and crevices. If that wasn’t hard enough, consider the change in temperature and salinity as a sometimes submerged home site becomes first exposed to air and is then baked or boiled under the sun.

In addition to all that, an added challenge for inhabitants of northern shores is the effect of winter icing. Around northern Japan, exposed shorelines freeze; Even as the tide recedes, ice forms on the shore, and in some areas solid masses of floating drift ice bulldoze their way ashore. These forces of change shape both the coastal environment and the organisms that can live there.

A foray along the tide’s edge is one thing, but if you can time your visit to coincide with low tide and make a foray down from the high-tide line to the water’s edge, you will encounter the widest range of zonation. Down near the low-tide line live creatures that are barely able to withstand even a short spell out of saltwater, while those living up near the high-tide line are able to withstand long periods exposed.

Active shores, exposed to strong waves and surf, are the most dramatic to take a long relaxing walk along. With their sand and shingle beaches they are the archetypal summer-holiday strands. I have a favorite near Dunedin, New Zealand, where massive rollers crash surf onto the beach and where yellow-eyed penguins and Hooker’s sea lions come ashore — but there I go again, reflecting my vertebrate bias.

Along more sheltered shores, muds and sands are more stable and form a soft substrate, providing another habitat entirely, with intertidal mudflats home to a very different array of organisms. In Japan’s southern islands from Amami Oshima to Iriomote-jima, these same substrates even support mangrove forests. Not surprisingly, many of the species living in this kind of soft habitat are burrowers, delving to differing depths to avoid the fluctuations that come with the daily tides.

Burrowing is out of the question for species making rocky shores their home, unless they can find a crack, or crevice, or a rock to hide beneath. Here, many creatures take a medieval knight’s approach — heavy, protective armor in which they face up to the worst the environment throws at them. My anthropomorphistic mind conjures images of a wayward adolescent conical limpet clamping its shell on an exposed rock, its massive foot sucking itself down hard against the wave action, and repeating over and over to itself, “I’m tough; I’m tough” — until, of course, the big wave crashes in and, whoooops! . . . It’s the limpet’s last thought.

Limpets, of course, like other marine invertebrates, reveal their natures not only through their appearance, but also through their behavior. Most of the immense array of mollusks secrete some sort of protective shell, whether a conical one like the limpet, a spiral one like the whelk, or one in two halves such as bivalve mussels and clams. In each case, though, their behavior is adaptive and appropriate to their morphology.

That previously mentioned limpet would have been safe at home in a protected rock crevice, or snug in its well-worn socket on a less exposed rock, whereas a clamor cockle would find safety by burrowing into sand or mud. Among the usually well-armored mollusks, some are surprisingly shell-less — witness the nudibranch and the octopus that have taken protective action along a very different track.

But more of that next time.