At low tide, the rocky intertidal regions of coastlines are fascinating areas for the study of marine life. Exposed rocky shores are challenging places to live, with organisms pounded by surf and scoured by sand at high-water and exposed to high light levels and desiccation at low water. The physical forces placed upon algae and animals due to waves far exceed those in terrestrial and oceanic environments. Despite the maelstrom, rocky shores support diverse plant and animal communities that are adapted to grow and reproduce in this environment.
The rocky intertidal is kept clear of sand and mud by the continuous direct action of ocean waves. Wave shock and forces are therefore a continuous challenge for life in the intertidal. Adaptations by these animals to these conditions include hard external skeletons, and low, rounded body profiles that have least resistance to the forces of breaking waves. Animals without these features may seek protection in rock crevices or cracks. Many intertidal animals and all of the intertidal seaweed species are very firmly attached to the substrate. The flexible stipes of the seaweeds allows them to bend and absorb wave forces.
The pattern and vertical range of the tide define the rocky intertidal region. Exposure to both the air and submergence for extended periods twice per day establishes rigorous and well defined zones. Intertidal organisms have adapted to the absence of seawater at low tide. Desiccation becomes a problem at low tide for algae and animals located high up on the shoreline. Unattached animals may avoid desiccation by inhabiting tide pools as the tide recedes. Many animals with hard shells simply close up at low tide to avoid drying out.
Biological processes also influence the distribution of organisms in the intertidal. Most intertidal animals can be classified as one or a combination of the following feeding types: (1) predators, (2) scavengers, (3) algal grazers, and (4) suspension feeders. Competition for space is high and most organisms are only successful competitors within narrow ranges of physical and biological conditions. This can lead to the formation of definite zones characterised by particular groups of plants and animals arranged in a vertical sequence from the low tide mark.
This field trip will introduce you to the physical forces which drive plant and animal community compositions as well as the morphological adaptations and behavioural strategies that organisms have evolved to cope with life in the harsh rocky intertidal environment. (
On Saturday, we split into our groups and proceeded to the rock platform. We were given sheets to help us identify the organisms that we would find on the rock platform. We were in groups of 2, and were randomly assigned points within a 50m by 10m area to put a quadrat down (about a 1 foot by 1 foot metal square) and then count what organisms - seaweed, snails, chitons, limpets, were within the quadrat and how many there were/what percentage of the quadrat they covered (if seaweed). We also deployed light and temperature loggers and dynamometers, which measure wave velocity (there's a picture of this below).
On Sunday, we tested four different hypotheses. The hypotheses were as follows:
-H1: Intertidal organisms are more susceptible to lift rather than drag forces.
-Limpets: Attach the high-tech clamps to the limpets and pull the limpets vertically or at 45 degrees at a constant force of 10 kg on the 25 kg spring scales. Measure the time taken for limpets to dislodge. Do 3 limpets vertically, 3 limpets at 45 degrees. Replace the limpets after they have dislodged and provide some moisture. They should re-attach after a while.
-Hormosira: Make a noose from the light fishing line provided. Try to noose the stipe of Hormosira with fishing line. Lift vertically (lift force) or horizontally (drag force). Pull with a slowly increasing force. Watch the spring scale and record the force required for the stipe to break or dislodge (record break or dislodge also). Do 5-10 plants vertically and horizontally. Use the smaller spring scales for this task.
-Turbo: (Turbo are snails). Noose turbo with fishing line underneath the edge of the shell. Repeat the same procedure as for Hormosira. Use the smaller spring scales for this task.
-H2: Exposure to different temperatures affects the force required to dislodge organisms.
-Limpets: Find two limpets of similar size at roughly the same place on the rock platform. Fill a plastic bag with ice, then add sea water and seal. Fill another plastic bag with sea water of ambient temperature. The idea here is to change the temperature of limpets but not to get them wet. Apply the different bags to the different limpets for 5-10 minutes. Attach the clamps to the limpets, pull at a constant force of 10 kg. Record the time taken for limpets to dislodge. Do 3-5 chilled and 3-5 normal temperature limpets.
-H3: Exposure to desiccation affects the force required to dislodge organisms.
-Turbo: Collect 20 Turbo from the lower intertidal. Find a dry area of rock platform in the sun with a shallow pool nearby. Place 10 Turbo in the shallow pool, 10 on the dry rock platform. Place them so that they can re-attach. Leave for 1 hour so the turbo on the dry rock desiccate. Noose the turbo with fishing line underneath the edge of the shell and measure the lift force required to dislodge them. Put the turbo back where you found them (roughly) when finished. Use the smaller spring scales for this task.
-H4: Organisms are oriented on the rock platform to minimize wave force exposure.
Limpets are oval in shape. Determine if their orientation with respect to the oncoming wave direction is random or non-random. Estimate and record whether limpets are aligned so that they have the smallest amount of shell exposed to the oncoming waves (write 0), or if they are attached at a 45 degree or a 90 degree angle to the oncoming waves. Replicate 100 times.
I am interested in why would you hypothesise that intertidal organisms are more susceptible to lift rather than drag? Thank you and have a great day
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