The reef at Woody Head has a lot of live molluscs. You may not see them all at once, but the more times you go out to look, the more you’ll see. The water at the southern part is good for snorkelling and live molluscs, too – not to mention wobbegongs, nudibranchs and sea hares (the last two are molluscs). I’ll talk about some of the molluscs I have found there. They are typical of molluscs on the subtropical eastern seaboard of Australia.
Barnacles are filter feeders – when the tide comes in and they are covered, they open up and poke out their cirri, resembling long, jointed fingers, with which they grasp anything edible floating by and drag it into their mouths. When the tide is out and they are uncovered, they shut their valves tight to minimise water loss and shield their soft bodies against the sun.
Most barnacles are hermaphroditic, but they do not shed sperm and eggs like many other sea dwellers. They cannot move once the larval form has settled. The barnacle has a very long penis that it extends into a neighbour to fertilise it. The eggs are brooded within the shell until they develop into a larval form called a nauplius, which are then shed into the water. These moult into other stages, and when they turn into a cyprid larva, they attach themselves to rocks in the vicinity of other barnacles of the same species. The valves and fixed plates are then secreted.
Whelks, mussels and some starfish feed on barnacles. The whelks drill into the barnacles and eat the flesh. It’s horror movie stuff, as the barnacle cannot move and takes days to be eaten alive. I’ve read that, on the east coast of the USA, the sea slug Onchidoris bilamellata has been seen to eat barnacles (http://slugsite.us/bow/nudiwkps/nudiwk34.html; http://northislandexplorer.com/molluscs/barnacleeatingnudibranch.htm), although I can’t imagine how it gets through the barnacle’s defenses. I don’t think nudis drill.
Limpets live in similar circumstances to barnacles. They move about when the tide covers them, scraping away at the algae growing on the rock surface, and go back to the same spot to rest. That spot gets deeper over time until noticeable depressions form over generations of limpets. They lay their eggs on the rock surface, too. Oystercatchers eat limpets, prising them from the rock with their strong beaks. Carnivorous snails eat them in the usual way – by drilling into the shell and scraping the flesh.
On rock platforms, you can see chitons on the surface of fixed rocks and on the undersides of loose ones – make sure you put the rocks back exactly where they came from – and, like limpets, move around (generally at night) scraping off algae, small worms, crustaceans and bryozoans to eat. Their soft bodies are shielded by eight plates or ‘valves’, surrounded by a muscular ‘girdle’. Sometimes you find the valves on their own after the animal has died and broken up. Crabs, fish, anemones, starfish and octopuses (or octopi, if you prefer) have been reported to eat them.
In the centre of the photo on the left, the mulberry whelk (Morula marginalba), a carnivorous snail, may be trying to drill through the valves of the chiton.
A chiton is either male or female: ‘Males always release their sperm into the sea. The sperm is carried on the ocean currents to the eggs. Depending on the species, females either release their eggs singly or in strings into the water or keep them inside the special groove that separates the girdle and muscular foot. Eggs fertilized in the water usually develop into free-swimming, unsegmented larvae covered with tiny, hair-like structures called cilia. Eggs that develop inside the groove remain with the adult female until they become well-developed young chitons (from http://animals.jrank.org/pages/1886/Chitons-Polyplacophora-BEHAVIOR-REPRODUCTION.html).
The Australian Museum says that Australia has about 150 species of chitons, about 90% of which are endemic (found only here).
A follow-up post will tell about the gastropods, another form of mollusc – the typical spiral shell form that most people think of when they think of molluscs.