Amazingly it was quite a horrible day, with clouds, murky water and blustery conditions. But we still had a very profitable morning and it just became a summer beach sweep.
We were very lucky again to have Dr Anne Brearley volunteer her time again, and she started the children off eagerly digging for ghost crabs. Fortunately the crabs were a metre or two below, so we did not disturb them. Then we collected interesting bits of seaweeds, seagrasses, shells and other assorted animals and plants. After about an hour, we spread out a tarpaulin and assembled our finds in the various groups like we always do and Anne kindly went through all the groups.
Adult and Young Naturalists looking at their finds on Cottesloe Beach
There was a great deal of diversity in all the algae we saw. Brown Algae was plentiful. We saw quite a few Sargassum seaweeds with their round air bladders that keep them vertical in the water while still attached to a rock and if broken off the rock they will float to the surface.
Anne told us about the giant kelp in Antarctica which can be 40 m long where the seals and penguins swim through. David Attenborough has shown this in his episodes with the Killer Whales stalking various penguins. Anne also explained about the holdfast, which is not a root, but can itself be a medium for other organisms to also settle on, like worms, crabs and snails. Anne explained that when the kelp rots it is a favourite of amphipods and sea lice for decomposition and thus passed on through the various food chains.
We also saw the brown funnel weed which had some calcium carbonate deposits on it. The red algae were not so well represented. Quite a few of the red algae had the red pigment bleached out of them and consequently were green in colour. Quite a few pieces of algae also had calcium carbonate which is known as coralline algae. The green algae was made up of sea lettuce which is a good indicator of nutrients in the water. A build-up of filamentous algae are an indicator of too much nutrient, such as occurred in Mandurah in the 1970’s with massive amounts of eutrophication which eventually resulted in the construction of the Dawesville Channel. There was some Caulerpa, which looks like bunches of grapes. In Asia some species of Caulerpa are grown for food.
We then came to the sea grasses which are a flowering plant and really just like grass. The seagrass has a true root and takes up nutrients from the sediments. It has male and female flowers on the same plant.
First of all we saw some ribbon weeds (Posidonia). Anne did show us a seed. It looked a little like a piece of apple. This seed can float for a couple of days before settling.
She also had some wire weed (Amphibolus) which had some brown nodes which indicated where a leaf was attached. It is possible to estimate the age of the shoot as it grows one leaf per month. The wire weed can put out a little comb anchor which can attach itself to any weed or rock and eventually the shoot will grow and put down roots. This group has both sexes on the one plant, and apparently self-fertilizes.
Next we saw the paddle weeds (Halophila). Anne explained that if the seagrasses grow some algae on their leaves, the fish and molluscs like to eat it and also hide in it. Unfortunately if too much algae grows it smothers the seagrasses and they can die.
A lace coral or Bryozoan could be seen growing on one of the sea grasses. This is an example of the various epiphyte communities that can be seen in the water and in nature generally.
We also had a couple of terrestrial plants from the sand dunes behind. One was a Cakile from South Africa and the other was a Spinifex , both of which are dune stabilizing plants.
Next we had a few sponges. Basically these had several inhalant pores and often larger common exhalant pores. The sponges filter bacteria and other materials from the water.
We had a few molluscs like cuttlefish shells, which showed evidence of nibbling by fish. You could also see that they had successive layers of bone laid down over time.
The abalone were interesting as they scrape algae off the rocks and apparently at night abalone have been observed raising one end up off the rock and using their pincers to gather bigger pieces of algae for eating. I had certainly never heard of this before. It makes you wonder if any other grazing mollusc does this at night. The abalone takes in water over its gills and the water then goes out the lateral exhalant holes.
Other molluscs were various bivalves and a limpet.
Of great interest was a large egg case of the Spindle shell. Anne pulled this egg case apart and produced a small immature form of the shell.
Anne explains reproduction in the Spindle shell, Syrinx spp
The last group were the ascidians and mostly these were sea tulips which are single sessile animals, although there are colonial versions with an inhalant and exhalant pore. There were also a few sea squirts which could be squeezed to demonstrate the exhalant pore quite effectively. The ascidians are part of the Chordate group which includes vertebrates. The ascidians have a larval tadpole which has a notochord in common with other vertebrates.
Mike Gregson was showing us printed versions of the Cottesloe Coast Care website. You might like to click on the two links below, as it shows many of the plants and animals we saw today and more that we didn’t see.
Many thanks to Steve Page for organising a very successful Snorkelling session and Mike Gregson who is now stepping down from organising the Young Nats after several very energetic years.
We had at least 8 adults as well as several Young Nats families which was great to see and I am quite sure we all enjoyed ourselves.