Some divers swim over patches of sand in California as if they are trying to escape from something. The faster they go, the better. I understand the magnetic appeal of the adjacent kelp forest and rocky reef communities, but experience has taught me to appreciate, rather than avoid, the sand biome. At first glance, some sandy areas appear to be vast wastelands of nothing but sand. But I invite you to look again. I have learned that if I slow down and take a good, hard look, I usually encounter a completely different set of animals that are as fascinating as those found in nearby kelp forests and reefs. 

In many respects the expanses of sand resemble terrestrial deserts, and it is easy to get the mistaken impression that areas where the sea floor is primarily composed of sand are devoid of life. During daylight hours the sand patches often appear barren, but at night the sand biome really comes alive. On some nights, almost everywhere you look you’ll find a pair of eyes staring back at you. A variety of bony fishes, octopuses, eels, crabs, shrimps, rays, and even some sharks are all over the place. The same is true of sea pens, sea pansies, sea stars, tube anemones and the occasional spiny lobster. 
Success in the Sand
Not all sandy areas support the same amount of life. The most interesting sand-based areas tend to be in places where decomposing debris accumulates and continuous supplies of phytoplankton are available to provide both food and cover. In California, in the sand biome there is a distinct lack of plant life, thus the food web is primarily founded upon plankton and decaying detritus that drifts into the area. Some of the best “sand dives” are located some distance from shore in water deep enough that the effect of surge is lessened, and just down current from a kelp forest.
With very few, if any, rocks and only some drifting debris being available, there are relatively few places for animals to hide other than in or on the sand. Interestingly, a variety of polychaete worms make up the largest biomass of any sandy plains animal. While divers only rarely see these worms as they spend the vast majority of their lives buried within the sand as opposed to living on top of it, their presence is not overlooked by the many species of rays, bony fishes and other creatures that prey on them. 
As for the animals that we do see in the sand biome, most have a very low body profile. In addition, in order to survive sand-dwelling animals must be (1) excellent burrowers, (2) able to rebury themselves very quickly after getting exposed, (3) able to stabilize the substrate around them, or (4) a master in the art of camouflage. 
Let’s look at some examples. Able to quickly dig their way into soft substrate, clams are expert burrowers. These mollusks spend a considerable amount of time buried in the sand with only their paired siphons exposed. The siphons enable clams to take in water that contains both oxygen and food, and eliminate waste without having to expose themselves to danger. The colorful California mantis shrimp provides another example of a first-class burrower. It’s actually a crustacean, classified as a stomatopod, as opposed to a shrimp as its common name suggests. This immaculate housekeeper lives the vast majority of its life tucked away in its burrow. California mantis shrimp usually leave the safe confines of their burrows only to hunt, defend their turf or seek a mate.
Sea pens provide excellent examples of creatures that are able to quickly rebury themselves. These colonial cnidarians usually spend their days buried beneath sand, and they emerge at night to feed in the currents. However, if threatened or uncovered during the day, they can quickly re-bury for protection.
In many sites aggregations of sand dollars and worms are so dense that the association serves to stabilize the sand around them. This allows these animals to hold their ground instead of getting exposed or being tumbled along in the current. Flatfishes such as halibut, sole, turbot and sanddabs use the splotchy pattern of their skin and their ability to match the color of the surrounding sand to camouflage themselves. Studies have demonstrated that flatfishes depend on their vision to accomplish this rather remarkable feat. These fishes alter the shape of special black pigments in the cells of their skin to create the match. 
Even much larger sand dwellers such as angelsharks are very adept at changing their color and pattern to help them blend in with the sand. And, like so many sand residents angelsharks routinely bury themselves so that little more than their eyes and spiracles are exposed. Observant divers most often discover these hidden creatures by noting their outlines in the sand. 
Squid Runs
Of all of the events that occur in the sand, the phenomenon commonly known as a squid run is perhaps the most fascinating. Common squid (aka market squid, Loligo opalescens) usually inhabit the waters of the open sea. But during late fall and winter they venture into shallower, nearshore areas to mate. During heavy runs, events that often last from several weeks to more than a month, uncountable numbers of the roughly six-inch long adults gather in coastal canyons, along steep sand drop-offs, and in some coastal plains at the Channel Islands. After successful fertilization the females plant their egg casings and the adults die.
The presence of so many squid attracts numerous scavengers and predators that come to feast on the dead and dying squid. Bat rays, thornback rays, round rays and horn sharks are often seen, as are rockfishes, cabezon, a variety of sculpins and the occasional black seabass. Invertebrates including crabs, California spiny lobster and sea stars are also quick to take advantage of the food. 
Higher in the water column other predators devour the squid. California sea lions, harbor seals, dolphins and the occasional blue shark can be seen as they prey upon still living squid. 
The next time you find yourself swimming over the sand, go slow and take a close look. While many of the animals that inhabit the sand are not as flamboyant or easy to find as the creatures that inhabit the ecosystems of the kelp forests and rocky reef communities, they are equally fascinating. 
One To Look Out For
One member of the sand community to keep a watchful eye out for is the Pacific electric ray, Torpedo californica. They have a grayish blue to yellowish brown upper body that often displays a number of dark spots. Their flattened body is disc-shaped and the lobed tail is large and fleshy. Adults are typically 2 to 3.5 feet long. 
Pacific electric rays are generally encountered as they hover or slowly swim a few feet above the sea floor. These rays also spend time resting on the bottom while partially or completely buried in sand or mud with only their eyes and spiracles protruding. Pacific electric rays are quite capable of ambushing prey from this position.
The Pacific electric ray is equipped with a pair of powerful kidney-shaped electric organs that are embedded under the skin of the head. The jolts produced by these organs are normally used to stun prey and ward off potential threats, and they are easily powerful enough to give a diver considerably more than just a mildly painful zap.