This is the Spiny Tops seaweed, Turbinaria ornatum, photographed at Cape Tribulation in Queensland, Australia. Superficially, it looks like the seaweed Sargassum crassifolium featured in a previous Posting. However, if you look closely, you can see that here the ‘fronds’ are much thicker and rounder. They are actually the expanded ends of small branchlets. They look a bit like small cookies that have been cut out of thin dough with a serrated-edge cookie cutter.
A. B. Cribb in his book Seaweeds of Queensland, A naturalist’s guide (ISBN 0-9595607-1-8) says about this species that:
The axis bears closely placed top-shaped branchlets with rigidly spiny margins. Eventually a gas-filled cavity develops in these branchlets and the buoyancy keeps the plant erect when submerged. This species is restricted mainly to the tropics where it is common on coral reefs.
Damon Ramsey in his book Ecosystem Guides, Tropical Seashores of Australia (ISBN 978-0-9757470-6-3) tells us that in these very distinctive seaweeds:
Along the top half of the stalk many smaller branchlets grow off to form a dome shape, and each has flat, star-shaped spikes at the end. They can be common in the murkier sandy shallows, and sometimes wash up on tropical beaches.
Compared with the coast of Great Britain there were relatively few seaweeds on the beaches I visited on the Queensland coast. However, the red seaweed illustrated here was one of the most unusual in appearance. I found it washed up on the sandy beach at Cape Tribulation. It resembled a mass of molluscan or fish eggs – a bit like glistening caviar.
I photographed it from every angle and concluded from the way the swollen ‘leaves’ or ‘eggs’ were attached to branching stems that it was indeed a type of marine alga. It wasn’t till I obtained a copy of the book Seaweeds of Queensland – A Naturalist’s Guide(by A. B. Cribb of the Queensland Naturalists’ Club: Handbook No. 2, 1996, ISBN 0 9595607 1 8) that I could verify its identification. In fact, it is the same species that is featured on the front cover of the book – Botryocladia leptopoda (J. Agardh) Kylin.
The specimen in my own photographs has been affected by the hot sun and most of the ‘leaves’ have dimples in them where they have begun to shrivel out of water. Apparently, the common name is Grape Weed and to quote from the above book,
the densely placed vesicles clothing the branches make this one of the most beautiful red algae in Queensland. Large specimens may reach 40 cm in length. The generic name is derived from the Greek botrys – bunch of grapes, and clados – branch.
Its habitat is mainly in the sub-tidal region in sheltered and semi-exposed areas but also occasionally in shaded pools. Intriguingly, this lovely seaweed is thought to contain chemicals with important medicinal properties. Research is being conducted into these properties in connection with the treatment of lymphatic filarial parasites.
We are used to thinking of rocks as ancient structures that have been in place for millions of years but, of course, rocks are in the continual process of being formed. An example might be the way rivers carry erosion sediments downstream to form layers on the beds of seas, lakes, and lagoons. Or erupting volcanic lava solidifying on contact with air or water. On the coastline of Queensland in Australia the most easily visible type of present-day rock formation is that of cay sandstone, commonly called “beach rock”.
Beach rock forms very rapidly. It happens in warm shallow water close to coral reefs, where the combination of heat and evaporation, an abundance of dissolved calcium from pieces of coral and seashells, and the addition of phosphates from bird guano, lead to a cementing of all the loose fragments together to form hard concretions of rock. This is such a rapid way of rock building that it is sometimes possible to see man-made objects included in the concretion – apparently soft drinks cans have been recorded. More commonly seen are pieces of coral (sometimes still coloured), sea shells, and the impressions of plant remains such as Pandanus fruits.
The photographs in this post were taken mostly on the sheltered shore of Normanby Island where the beach rock layers form an almost continuous link and low-tide walkway to the neighbouring island. Large slabs of beach rock are prone to break off from the layers and rest on the shore. In other places deep deposits of algal-covered rock have started to wear into depressions and hollows that form new habitats for marine gastropods and crustaceans.
Soldier Crabs are a phenomenon. They are not very big or very special to look at but the behaviour that they exhibit is spectacular. For some reason that scientists do not fully understand, these tiny crabs (about 1 centimetre across the carapace) will suddenly emerge from the wet sand of the lower shore in specific tidal conditions, gather in vast numbers, and march across the beach. Millions of them. Armies of them. Not scuttling randomly sideways like normal crabs but walking forwards. Then they disappear again just as quickly by cork-screwing themselves down into the wet muddy sand. If you stop and stare at the surface sediments, you can see the sand grains heaving as the little creatures excavate burrows below, presumably extracting microscopic food particles from sand they eat, and bringing new deposits or casts to the top.
While I was walking along Myall Beach near Cape Tribulation in Queensland, I was thinking that the vast sandy shore was a bit disappointing from the marine invertebrate point of view. Then I suddenly became aware of extensive dark shadows moving across the surface – looking rather like wind-blown sand. As my eyes focused on these erratically-moving darker patches, I realised that they were battalions of moving crabs. If I stood still long enough, more small crabs would emerge from the wet sediments at my feet, as if on some unknown cue, congregate together and march away, joining the horde and gathering new recruits on the way.
The crabs probably belong to the Family Myctyridae but I don’t think they are Myctyris longicarpus on the basis that the carapace colouring and markings are different in the specimens I photographed. The Soldier Crabs are abundant in this location because the tidal flats are enriched with nutrients brought down from the mountains by a river that flows through the tropical rain forest – reaching the beach as Myall Creek (complete with crocodiles).
The short video clips attached to this post – best viewed without sound because the wind blowing over the microphone is a bit intrusive – demonstrate some of these Soldier Crab habits.