Water, as we all know, is a liquid, and moves freely in response to the force of the Earth’s gravity. It also moves in response to other gravitational forces, specifically those of the Moon and the Sun. But it is only in the enormous oceans that we see this movement.
These movements of the oceans are known as tides, and they follow an enormously complex pattern. The primary influence is the gravitational pull of the moon, so the tidal cycle follows very closely the movement of the moon in its orbit.
But this major effect can be compounded by the tilt of the Earth, so that there are higher tides in areas that are tilted more closely to the Moon; this results in seasonal differences in the tides. The tides are also affected by the lesser gravitational pull of the much more distant Sun, so that the highest tides occur on the New Moon, when the Moon and the Sun are on the same side of the Earth. A second smaller tide occurs because the landmasses on the side of the Earth opposite the Moon are pulled towards the Moon more than the water. The whole is further complicated by local geographic eccentricities on land and on the sea floor. The daily tidal fluctuation in British Columbia ranges from about two meters in the south to over seven on the North Coast. Despite all the complications, however, the tides follow quite predictable patterns, and a tide book is a worthwhile investment on any visit to the coast.
The tides have been having their way on the planet for eons, and thus have had their effects on the evolution of life. Just as there are plants and animals which have adapted to live beneath the waves, and in the many diverse habitats on land, so, too, there are thousands of species which live in the life zone which is affected twice daily by the tides.
As the tide recedes to its daily low point, this life zone is exposed to the elements, and the plants and animals there must survive until the next high tide. They are also exposed to the curious eyes of the intertidal explorer. Even an average tide will reveal wondrous things along the shore, or clinging to dock pilings, but it is the lowest tides of the year, in the summer, which expose the richest diversity of life.
All intertidal habitats have their species of interest, from muddy bays to wave-washed beaches, and rugged rocky outcrops. But it is these rocky shores that offer the best viewing opportunities, with relatively easy access, and many small pools left behind by the receding tide.
Marine organisms in the intertidal area fall into a few basic groups. While there are a few grasses to represent the flowering plants, most “seaweeds” are algae, and are grouped generally as either red, or green, or brown algae. The long whip-like stipes of Bull Kelp (Nereocystis luetkeana), a brown alga, with their hollow bulbous tips, can be seen in masses in the water, and often cast up on the beach. If you think it looks funny, don’t be too critical; you have probably eaten an extract from kelp that keeps your ice cream smooth and creamy.
Held fast to the rocky substrate are masses of rockweed (Fucus species), whose mucous covered fronds retain moisture, and provide cover for small marine animals. The bright green seaweed you see may be sea lettuce (Ulva species). The occasional translucence of this alga is understandable, because each frond is only two cells thick.
In the tidepools, small sculpins dart into protected places. The most common is the Tidepool Sculpin (Oligocottus maculosus), whose colour is best described as “camouflaged”. This and a few other small fish are the only vertebrate animals likely to be encountered in the intertidal zone.
By far the vast majority of the life in the intertidal zone consists of species of invertebrate animals. These “spineless” wonders range from the well-armoured crabs to the limp-wristed but beautiful sea slugs, or nudibranchs.
Some of the most obvious of the invertebrates do not look much like animals at all. Almost any tidepool will reveal numbers of Giant Green Anemones (Anthopleura xanthogrammica). The waving fronds are minutely raspy, and help to guide prey items toward the animal’s central mouth. The tentacles then close around the prey, and it is digested. The Aggregating Anemone (A. elegantissima) appears like a collection of olivaceous jelly-like blobs. When they open, however, they reveal tentacles of exquisite pink and blue. Their “aggregation” results from their reproduction by cloning. The Frilled Anemone (Metridium senile) prefers deeper water, but it’s prominent white “flowers” are often seen on dock pilings.
Abundant in some areas is the Ochre Sea Star (Pisaster ochraceus), which, true to its name, is orange. Except when it is purple. Sea stars (they are not fish) move along the bottom until they find suitable prey like limpets or mussels. Their strong suction feet will open the toughest shell, and the stomach is then inserted into the prey, and digests the prey. These animals are in turn sometimes the prey of Glaucous-winged Gulls, which will patiently sit while a sea star in the mouth relaxes sufficiently to be swallowed whole.
In the tidepools, and under beach wrack and rocks, are found dozens of the little Purple Shore Crab (Hemigrapsus nudus). They scurry sideways in search of cover, and although small, are capable of inflicting some pain with their little pincers. The tidepools are home, too, to Hermit Crabs (Pagurus species). Unlike their cousins, who shed their shells and grow new ones, the Hermit Crabs borrow the shells of various deceased snails, moving into larger ones as their growth dictates.
Exposed rocks are festooned with mussels. California Mussels (Mytilus californiensis) are more often found on exposed shores, while the smaller Blue Mussels (M. edulis) prefer more protected waters, often with lower salinity. More difficult to pick out on the rocks are the chitons, molluscs with segmented shells that look a little like oversized sowbugs. The various species range in size from 5 to 30 cm.
Barnacles look like molluscs, but in fact they are crustaceans, like shrimp. When submerged by the tide, their hard shells open, and delicate fan-like fronds sieve the water for food. At low tide, they shut tightly to preserve moisture.
On a very low tide, it may be possible to find the amazingly soft-bodied sea cucumbers, and sea slugs, or nudibranchs. Some of these are the most exotically coloured animals in these waters. Many are quite small, growing to 6 or 8 cm, but a careful search of tidepools can be rewarding.
Sandy beaches are not devoid of life, but much of it is buried below the surface. There are many species of clams, some native, and some introduced, and most make their existence by filtering nutrients from the water in the sandy safety of their burrows. The granddaddy of all the clams is the Geoduck (Panopea abrupta), which is pronounced “Gooey-duck”. It may reach twelve pounds in weight after a life span of 150 years. Despite living 1 or 2 meters under the sand, they are harvested commercially.
The taking of clams and mussels and other shellfish for human consumption is legal, but there are regulations about the numbers and sizes of each species that may be taken. WARNING: It is also imperative that shellfish lovers watch for Red Tide warnings. Red Tide is an algal bloom that produces toxins in filter-feeding organisms like shellfish, and these toxins are concentrated and passed on to humans, causing the potentially fatal Paralytic Shellfish Poisoning.
The beach is also home to beach fleas, which are not fleas at all, but would give them a jump for the money. On exposed beaches, the California Beach Flea (Megalorchestia californiana), with its striking red antennae, is common, while on more protected beaches, the smaller Beach Hopper (Traskorchestia traskiana) replaces it. Both feed on decaying kelp and other beach wrack, and retreat to burrows during most of the day.
A walk along the beach may also turn up the shells of Sand Dollars (Dendraster excentricus). The living animal, related to the sea urchins, moves just under the sand, filtering detritus out of the water.
Exploring the intertidal zone can be done anywhere there is access to the shore, and at any time of the year. Daytime low tides occur only from spring through fall, however. And a word of caution here:
Remember that the water that receded with the tide, will return. Be extremely careful not to become stranded offshore by an incoming tide; the next low tide could be 24 hours away. Be especially careful, too, on exposed coasts. Many people have lost their lives when swept off rocks and beaches by large waves. Occasional “rogue” waves are not a maritime myth; they can and do occur.
Lastly, show some respect for the creatures you have discovered. Avoid stepping on them, even barnacles where possible. Leave the creatures where you have found them. They will not survive “at home”, and sea stars will not dry out aesthetically. Return animals to their tidepools, and replace any rocks that you have lifted.