Tide pools, as their name implies, are the pools of seawater that remain in the intertidal zone (i.e., the area between high and low tide) when the tide has receded. They are a common feature of rocky shorelines because the nooks between boulders and the cracks and depressions in bedrock effectively trap and hold water. In these dynamic and constantly changing pockets of ocean, a host of specially adapted plants and animals make a home.

To the marine biologist, tide pools are mini-ecosystems where the processes of competition, herbivory, predation, and biological invasions can be well studied within the context of a harsh, wave battered environment. To a family on vacation, they are fascinating windows on ocean life. Unless you are willing to try snorkeling or scuba diving, there is no better place to observe marine critters going about their business in a natural environment. For those in search of a meal (once they obtain the appropriate permits, of course), tide pools contain edible shellfish and seaweeds, as well as juveniles of many other tasty species. To the artists, the intertidal rock pools of New England are a constantly changing palette of different colors and textures: fronds of olive, brown, and purple seaweeds waving in the water; crusts of bright red, pink, and brown seaweeds looking like paint splashes on the rocks; green, red, and white sponges; spiky sea urchins; and chalky barnacles.

The interplay of physical factors and biological interactions determine the characteristics of individual tide pools. The location of the pool in relation to low and high tides is a key physical factor, determining the amount of time a tide pool remains submerged by ocean waters or exposed to air during the daily tidal cycle. This exposure time affects the stability of environmental conditions and consequently the types of organisms that can survive. Farthest from the sea are the high tide pools, some of which are flooded only during extreme high tides while others are covered with ocean water daily, but for only an hour or two each day. During the heat of a summer day, direct sunlight elevates temperatures in high tide pools to extreme levels. If it rains when the tide is out, salinities in these high tide pools may decline rapidly. These extreme fluctuations mean that to survive in a high tide pool, organisms must be highly tolerant of temperature and salinity fluctuations. As a result, high tide pools tend to be lower in species diversity than those closer to the sea. Many are dominated by green algae that are tolerant of temperature and salinity extremes and thrive where conditions are too unstable for the animals that eat them.

At the other end of the spectrum are the low tide pools, which are directly connected to the ocean except for a few hours around the time of low tide, or for those at the extreme low limit, are exposed to air only on a few days each month during the lowest tides. Tide pool aficionados are keenly attuned to the lunar cycle of spring (extreme) and neap (weak) tides so they know when they can reach some low tide pools that are only accessible during spring tides.

Because they are covered with water most of the time, low tide pools are relatively stable compared to those at mid and high elevations. Low tide pools closely resemble the adjacent shallow subtidal waters and, all else being equal, contain the greatest diversity of organisms. This is the place you are most likely to find the superstars of intertidal life, such as seastars, brittle stars, sea cucumbers, anemones, nudibranchs, sponges, and kelps. The relatively stable physical conditions allow biological interactions to play a more prominent role in determining who lives in low tide pools than in the high tide pools. As an example, sea urchins, which cannot survive the changeable physical conditions in high tide pools, are voracious herbivores in low tide pools, often consuming all seaweeds except for those that are hard inedible crusts. Thus smaller marine creatures that depend on upright seaweeds for hiding from predators are out of luck when sea urchins are abundant.

Size and depth are other key factors that influence tide pool life. A particularly large, deep pool at mid to high tide could harbor creatures that would normally be found only in a low tide pool, since the larger the volume of water, the less significant the temperature and salinity fluctuations. Tide pools with a complex structure, such as a bottom covered by jagged rocks, tend to harbor more organisms than pools with smooth bottoms. Complexity provides more surface area for seaweeds and for sessile, filter-feeding animals like mussels and sea squirts to attach. It also provides hiding places from predation. Another major factor is wave exposure. Limpets, chitons, and barnacles are well adapted to handle harsh waves, whereas periwinkle snails are not.

Because of their small size and definite boundaries, tide pools of Massachusetts have been wonderful outdoor laboratories to study ecological interactions between organisms. Ecologists can, for example, remove one component of the biological community and then examine the responses of other organisms. In the 1970s, Jane Lubchenco, a well-known marine ecologist who was then a graduate student at Harvard, found that when she removed the dominant herbivorous snail, the European periwinkle (Littorina littorea), from mid-tide pools in Nahant, the pools became overgrown with a monoculture of sea lettuce and other green algae. The pools without snails contained fewer species of seaweeds and sessile (stationary) animals because the competitively dominant green algae excluded other species. By preferentially feeding upon green algae, the snails gave other algae, such as the slow growing pink crusts, a better chance to grow. Thus, more snails equal more diversity in the tide pool. Young snails, however, were preyed upon by European green crabs (Carcinus maenas), thus the crabs could prevent recruitment of new snails to the tide pools and foster the dominance by green algae. Gulls, which feed on the crabs, provide another level of control on the community.

The tide pools of Massachusetts are not the same today as when colonists first arrived from Europe due to the invasion by non-native species. The aforementioned green crab is a European native that arrived on the East Coast of the United States in the 1800s. The Asian shore crab (Hemigrapsus sanguineus) has been in the Bay State for less than 10 years. Both are voracious predators and conceivably have had a host of impacts on tide pool ecology. The European periwinkle, so abundant in our tide pools today, showed up in New England only after 1850. Recent genetic evidence suggests that this may be a range expansion by a species native to North America that survived in an unglaciated part of the northeastern Canada during the last ice age rather than an invasion by a European snail. Dumont's red weed (Dumontia incrassate), a common seaweed in mid-tide pools in early summer, came from Europe and was unknown in New England before 1900.

Tide pools provide one of the most accessible and enjoyable ways to learn about the natural history of coastal habitats. Show a child and a parent a tide pool, and without any particular enticement, they will start looking to see what treasures are there. The Massachusetts Audubon Society and Salem Sound Coastwatch have taken advantage of the public's natural affinity for tide pools by recruiting volunteers to monitor them so that we can learn about invasives and other possible long-term changes. To become a volunteer tidepool monitor, see http://www.salemsound.org/.