Why do I sometimes see a single limpet in a large open area on a rock?

Limpets are herbivores in that they feed on various types of algae. They spend their days and nights slowly moving around rocks scraping the algae of the surface. Owl limpets are known to stay in a single location for long period of time essentially “farming” an area on a rock.

Sea star in pool of water

Why does it seem there are more plants and animals that live in small cracks

Marine life that lives in a tidepool is continually struggling to survive. One of the main limiting factors when living in a tidepool is having enough water to keep the plant or algae alive. Animals that live part of the time exposed to water must have adaptations that allow them to trap water and keep it until they are submerged again. The water is used for breathing and feeding.

The small cracks frequently found in tide pool areas will trap water and keep it longer. The crack in the rock allows water to collect at a lower place but also has reduced surface area that limits evaporation. This increase in the availability of water allows animals the need more water to survive in these cracks. These cracks essentially allow animals found lower in the tidal zones to live higher in the tidal zones as long as they live in a crack.


Tide Pool Biological Factors

Plants and animals living in tidepool areas are also subjected to a variety of biological related pressure. These include predation from other animals, competition for living space, competition for food and human impacts. These are referred to as biological or biotic factors.

  • Food availbility
  • Predation
  • Competition for living space
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    Tide Pool Physical Factors

    Marine life that lives in this environment is subjected to very harsh physical conditions. These conditions include huge waves that continually pound the plants and animals, constantly being submerged in sea water and then exposed to air, rapidly changing temperatures both in the water and air and quickly changing saltiness or salinity of the water. These are called abiotic factors or non-biological factors.

  • Physical force like wave action, logs, debris
  • Chemical factors – PH, Salinity
  • Water – drying out, having water to use
  • Temperature – too hot, too cold
  • Sun light – too bright, too dark
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    Smaller animals higher in tide zone

    Tide pool animals like all animals need three basic elements to survive. Food, water and shelter are the primary factors that will determine where a plant or animal can live. Tide pool animals are animals that live primarily in the ocean that have adapted to live exposed to air during parts of the day. The ocean provides food and water to most all tide pool animals. The more time an animal is exposed to air, the less food and water is available and the harder it is for the animal to thrive.

    The limitation of food and water the higher up in the tidal area is the primary reason animals are smaller in higher tidal zones. Plants and animals that live in the upper tide zones have adaptations that allow them to live in these harsh conditions where there is limited time for feeding. Barnacles are a good example of an animal that can thrive where few other animals can live. They have specific adaptations that allow them to manage the limitations related to food, water and protection.

    The upper tidal zone or splash zone where these small barnacles live is covered by water only a few hours each day. This means very little ocean borne food is available. Larger animals that require more food simply can’t survive. The small acorn barnacle is only about ¼ inch when adult. This smaller size allows the animal to survive even with this extremely limed food supply.

    Another factor that barnacles must manage is desiccation or drying out. Barnacles have a hard outer shell and mouth plates that allows them to close very tightly. They can close tight enough to keep the sea water in for hours at a time.

    The barnacle’s hard outer shell also has another function. That is protection from predation. Barnacles live exposed to air more than they are exposed to water. This means that they can be eaten by terrestrial animals or animals that live on land. The hard outer shell is a means to prevent most predation.

    It is interesting to note that these smaller barnacles also live in lower parts of the tidal zone. They are frequently found attached to mussels and bare patches of rock. They are such good colonizers that they will attached to any substrate at varying depths.

    Sand Castle worms covering the lower part of the channels

    Why do Sand Castle worms seem to live only in certain areas

    Sand castle worms provide another common patter seen in the tidepool areas. They seem to only grow in certain areas. They tend to thrive in the lower parts of the tidepool area and in places protected from direct wave action but high water flow. Sand Castle worms are filter feeders in that they extend feeding parts into the water column to capture food. They live in tubes built out of sand that they use for protection.

    The most obvious physical factor is the availability of sand. If there is no sand, there is no material for the animal to build its home. These animals also cannot move around to collect their own sand. Sand must be provided for them. Where in the tidepool environment can these worms capture sand without moving around?

    Fast moving water is the mechanism these animals use to get the sand they need. Water moving at a sufficient velocity can carry sand particles a limited distance from the source. Sand Castle worms will live in places where there is fast moving water that is close to a source of sand. This explanation however is only partially true. If these animals require fast moving water near sand, why are they not seen in exposed places like mussels and barnacles?

    The reason lies in the strength of their home. These tubes are strong enough to withstand some physical pressures but these tubes are not strong enough to withstand the power and force of direct wave action unlike mussels and barnacles. These animals therefore must live in places not exposed to direct wave action but still have plenty of water to carry the necessary sand and food. These animals are typically found in surge channels near sandy areas in the lower tide zone.


    Why do mussels and barnacles live in exposed areas

    Another similar pattern commonly seen with mussels is they seem to only thrive where they are exposed to high wave action. Take for example a large pool located behind a large reef or barrier with only a small opening to the ocean. The water will cover this area most if not all the time. We know that mussels feed on small food items in water, the more water, the more food, the more food the more mussels. It would seem logical that mussels would thrive in an area where they would be covered by sea water all the time.

    However, in situations like this, there are no mussels. There are no barnacles and noticeably less diversity of marine animals. The reason for this pattern is again related to food. There is plenty of food available in the water and there is plenty of water. The problem is that the water is not moving or moving too slowly. Mussels need water that is constantly flowing to allow them to consume greater amounts of food. Water that is not moving swiftly will not provide the required amount of food.
    Large mussel bed on exposed rock

    Why do mussels only live in a narrow vertical band

    Visitors will often notice that mussels seem to only live in a narrow vertical band typically within just a few feet. They do not live high up in the intertidal zone and also are not normally seen below a certain level. This distribution of marine life can be defined as a pattern in tide pool ecology.

    Too understand this pattern more and find out why this pattern occurs we must first understand a little more about the ecology of mussels. Mussels feed by capturing small food particles from the water. Mussels will pump water through their body cavities and filter out the small bits of food and plankton. Mussels are also sessile in they cannot move ( or only move a small distance ) once they have settled. Mussels need a constant supply of water where they can pump large volumes of water that contain food. The higher up in the intertidal zone, the less water is available. The less water means less food. The reason mussels don’t survive well in the high intertidal zone is this limited supply of food. This explains the reasons for the upper boundary of this pattern but what about the lower boundary.

    Mussels prefer to live submerged as much as possible. The lower they can live leads to more water and more food. The availability of food is clearly not a factor. Drying out by lack of water is also not an issue the lower they live in the water column. Wave action does not seem to be a factor in that the wave action would generally be less the lower in the water column. Other physical factors also don’t seem to be the reason. The reason must be a biological factor that prevents mussels from living lower in the tidal zone. Sea stars are the primary predators for mussels. Sea stars can devour vast numbers of mussels in a short amount of time. Sea stars are the reason why mussels don’t survive for long below a certain level because they are eaten by sea stars and a few other animals.

    This is a good explanation but there is one problem. Why wouldn’t the Sea stars eat all the mussels? The reason is related to the Ecology of Sea Stars. Sea stars however do not like to venture too far away from water or they will dry out and die. Sea stars will only climb high enough where they can safely return to water or where there is enough water and moisture for them to survive. Sea Stars will only consume mussels up to a certain point and then stop because there is not enough water for them to survive.