Building Science Concepts: Life between the tides

Explore the science concepts that underpin knowledge and understanding about life between the tides.

The New Zealand Ministry of Education’s Building Science Concepts (BSC) series presents sets of interlinking concepts that build stage by stage towards overarching science concepts or big ideas. A big idea shows how a fully developed understanding of the concept might look but recognises that such an understanding might not be achieved until New Zealand Curriculum level 7 or 8.

This resource is a partial replication of Building Science Concepts Book 21 Life between the Tides: Sandy Shores, Mudflats, and Rocky Shores. The background information on this page, combined with the information in the interactive, covers the science notes provided in the original BSC book. The overarching science concepts (big ideas) and how they may be scaffolded in sequence are illustrated in the interactive below.

Introduction to life between the tides

Most students in New Zealand have experience of or access to some form of coastal beach (tātahi): rocky shores (ākau), sandy and shingle shores (onepū, one kirikiri), mudflats (oneparu), lagoons (hāpua) or estuaries (wahapū). These places offer a wide variety of microhabitats where different communities of living things can get the food and shelter they need. Studying this environment gives students opportunities to explore concepts of:

• the variety and interdependence of living things
• the effects of change, both short term and long term, on an environment
• the relationship between living and non-living elements in an ecosystem.

Te ao Māori connections

The topic is also a context for incorporating an awareness of values associated with kaimoana and kaitiakitanga. Within te ao Māori, seashores occur at the border of the realms of Tangaroa (god of the sea) and Tāne (god of the forest). Many pūrākau are set at this boundary. Similarly, there are many tikanga that are observed there and that support the survival and management of this area of environmental richness.

Beaches are places of great importance to Māori as sources of food. Each iwi has its own particular protocols to observe when visiting coastal sites or gathering any sea-based resources. Prior to a visit, arrange for someone from your local iwi to visit to discuss these protocols with students. Introduction of kupu Māori will encourage development of dual perspectives on understanding this environment.

Beach types, characteristics and zones

The term ‘beach’ is used to refer to a range of coastal zones, including rocky shores, mudflats and estuaries, and flat sandy/shingly beaches. All beaches include land for the shore, salty and/or brackish water and tides.

Beaches have tidal zones – distinct parts named according to how much the water level varies within it:

• Above the high-tide line – the land beyond the highest point that the tide reaches in normal conditions.
• The splash zone – the area just above the high-tide line that water doesn’t cover but can get splashed by waves, especially if they are big or at high tide.
• The intertidal zone – the area between the high-tide and low-tide lines, covered at high tide but exposed as the tide goes out.
• Below the low-tide line – the area beyond the lowest point that the tide goes out to in normal conditions that is always underwater.

Plants

The largest group of plants found between tides at the beach are the algae (pūkohu wai). Seaweeds (rimurimu) are a type of algae, and large numbers are grazed off the rocks before they are even big enough for us to see. Herbivores such as limpets – apparently living on bare rock – are eating these tiny algae.

The most numerous algae at the beach are the phytoplankton (meroiti tipu). These tiny, usually single-celled organisms float about, often in films among rocks and on mudflats or attached to floating objects or even the skins of living things such as whales. The seagrasses (pātītī tai), sedges (pūrei) and rushes (wīwī) of estuaries and the mangroves (mānawa) of tidal mudflats are the other most common kinds of plants in the tidal zone (paetai).

Animals

The zooplankton (kōurangi) are tiny organisms that are found at and near the surface of the water and are the most numerous of the animals living in the tidal zone. They include tiny adult animals such as shrimps and krill and the larvae and young of fish and shellfish. Smaller fish (ika) and jellyfish (petipeti) depend on this food source and also live in the shallow waters.

Crabs (pāpaka) and starfish (pātangatanga) move underwater over the hard and soft surfaces of the shore. Other animals such as pāua, limpets, sea anemones and mussels cling to the hard surfaces. Various other shellfish live buried in the sand and mud of soft shores.

Animals that breathe with lungs but depend on the sea for their food such as penguins, seals and seabirds (manutai) live on or near the shore.

Sandhoppers (mōwhiti) and spiders (pūngāwerewere) live in the sand and shore debris above the high-tide line.

Challenges of intertidal life

Living in the intertidal zone offers a range of challenges to the organisms that live there, and these environmental challenges are not constant but are shifting and changing all the time. The organisms that live in this rapidly changing ecosystem have a range of adaptations to enable them to survive.

Organisms face being covered and uncovered by water. For example, shellfish partially open in order to filter food from the water when it covers them and close up for protection against sun and air during low tide. Oysters (tio), mussels (kuku), barnacles (pātitotito) and tube worms seal themselves plus a supply of water inside their external skeleton by excreting a slimy protection.

There are temperature and salinity changes. Animals and plants living in rock pools (hāroto) must be able to tolerate the water becoming quite warm on a sunny day or more salty through evaporation. Crabs sheltering in rock pools in heavy rain have gills in protected pouches that can hold undiluted seawater.

Wave action is another challenge. Limpets, barnacles (pātitotito), seastars (pātangatanga) and pāua have flattened conical forms with wide attachment areas to withstand wave action. Barnacles and rock oysters cement their skeletons to the rock to avoid getting swept away.

The plants of the intertidal zone must also deal with wave action. Brown kelp and sea tulips have flexible, leathery bodies with tough attachments to avoid being damaged by breaking waves, while pipi, tuatua and toheroa avoid wave action by burrowing into the sand or mud.

The best intertidal real estate is near or just below the low-tide line where the greatest variety and abundance of living things can be found. The higher up the intertidal zone, the harsher the conditions and the more extreme the changes. In the middle of the intertidal zone on sandy beaches, there are no producers as there are no rocks for seaweeds to anchor themselves to.

Any change in the living or non-living environment may affect members of the beach community because of their interdependence. Gradual change may be tolerated, but if the change is widespread or rapid, there is a greater chance that it will result in changes in the biodiversity and sustainability of the ecosystem.

Alternative conceptions

Students often overlook the role of small species in a beach community – for example, the plants and tiny insects living there – and focus instead on larger animals such as birds and fish. This topic provides an opportunity to draw their attention to the variety of species in a beach community and the importance and interdependence of all the members of that community.

Students may think of a beach as a hostile environment, exposed to the constant action of waves, tide, wind and weather. However, closer inspection reveals that living organisms have adapted to and colonised every available niche.