Organisms That Live in the Lower & Middle Intertidal Zones

Four Zones

• There are four intertidal zones: the spray zone and the high, middle and lower intertidal zones. The abundance and diversity of organisms is low in the spray and high intertidal zones. In the middle intertidal zone, where the land is uncovered for only short periods of time during low tide, life forms are more plentiful and diversified. In the lower intertidal zone, land is uncovered very rarely during the lowest of seasonal tides, and life is abundant and diverse. The reason for the increasing diversity and abundance of life as the intertidal zones deepen is that the extremity of environmental change is less and for shorter times than in the higher intertidal zones.

Plants

• Plant life in the middle and lower intertidal zones serves as a food source and shelter for the animals of these regions. Plants provide moisture when low tide leaves animals exposed to air, and also shelter them from predators. Marine macroalgae, or seaweed, are multicellular algae that are structured like plants but function differently. The roots of this plant keep it anchored but do not draw nutrition. Eight hundred species of marine animals are supported by brown algae kelp forests of the middle and lower intertidal regions. Red algae absorbs blue light, allowing it to survive in deeper water, where it contributes to the formation of coral reefs.

Animals

• There is no strict boundary between middle and lower intertidal zone animals, although some are better suited for only short exposure and tend to stay in lower regions with minimal exposure. Abalone, chitons, sea hares, purple sea urchins, smooth turban snails and ochre sea stars are lower intertidal species. Animals of the mid-littoral zone are adapted to being exposed and emerged several times daily. Species found in these regions include blue mussels, shore crabs, turban and dog whelk snails, and thatched and gooseneck barnacles. Their habitats vary from rocky regions and tide pools to forests of algae and seaweed.

Adaptations

• Different creatures are adapted to surviving air exposure and wave action in similar ways. Animals retain water in shell chambers during low tides to avoid drying out during air exposure. Sea urchins, anemones, starfish, snails and limpets produce mucus to prevent dessication. They also use mucus in conjunction with vacuum suction to stay anchored. Barnacles, tube building snails, tube worms and bivalves secrete cementing elements onto rocks or the shells of other animals. Some clams, chitons and sea urchins bore into rocks by using mechanics or by secreting acids. Arthropods may hide in crevices or under rocks.