Creepy things Mike plays with - Part 2
Wednesday, July 16 2008 | Comments (0)
Barnacles in tide water.
Last week Mike took us to a cave and then out to sea. We learned about cave crickets and crawfish and about hag fish. This week we continue our exploration of the creepy-crawly things he has come across during his adventures.
In this week’s episode, Mike helped the Coast Guard clean barnacles off a whistle buoy. The word barnacle is common to most vocabularies thanks to pirates – “rotate me barnacles” – but many people don’t really know what one is. Thanks, in part, to Wikipedia you’re about to find out.
A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea. Arthropods hae a segmented body with appendages on at least one segment. They have a dorsal heart and a ventral nervous system. Arthropods are covered by an exoskeleton. They grow by shedding this, called molting.
The barnacle is distantly related to crabs and lobsters. They are exclusively marine and tend to live in shallow and tidal waters, and on buoys. There are around 1,220 known species.
Barnacles are encrusters, which means the attach permanently to a hard substrate, the substance found at the bottom of marine environments – sand, gravel, rocks, etc . . . The most common type of barnacle is the acorn barnacle. They are sessile, meaning they grow their shells directly onto the substrate. Other types attach themselves by means of a stalk.
Most barnacles are suspension feeders. They stay in their shell and reach into the water with their modified legs. These appendages beat rhythmically to draw plankton and detritus in for dinner.
Although they have been found at water depths up to 600m, most barnacles inhabit shallow waters – 75 percent live in water less than 100 meters deep.
Barnacles are well adapted against water loss. Their calcite shells are impermeable, and they possess two plates, which they can slide across their aperture when not feeding. These plates also protect against predators.
Besides their numerous predators, barnacles have to compete with limpets and mussels for space. They employ two strategies to overwhelm their competitors. The first is called swamping. In this method, vast numbers of barnacles settle in the same place at once, covering a large patch of substrate, allowing at least some to survive in the balance of probabilities. The second is called fast growth. This method allows them to access higher levels of the water column then their competitors, and to be large enough to resist displacement – species employing this response can reach 7 cm in length and other species may grow larger still.
Most barnacles are hermaphroditic, meaning they have both sex organs. Typically, recently molted hermaphroditics are receptive as females. Self-fertilization, although theoretically possible, has been experimentally shown to be rare.
Sexual reproduction is difficult for barnacles because they cannot leave their shells to mate. To facilitate genetic transfer between isolated individuals, barnacles have extraordinarily long penises, up to 15cm in length – the largest penis to body size ratio of the animal kingdom.
Barnacles have two larval stages - the nauplius and the cyprid – before developing into a mature adult.
During the nauplius stage a fertilized egg hatches, leaving behind a one eyed larva comprising of a head and a telson, without a thorax or abdomen. It undergoes six molts before transforming into the cyprid stage. Nauplii are initially brooded by the parent and then released as free-swimming larvae after the first molt.
The cyprid stage lasts from a few days to a few weeks. During this part of the life cycle, the barnacle searches for a place to settle. Once it has found a potentially suitable spot, it attaches head-first using its antennules, and a secrets an adhesive substance.
As the larva exhausts its finite energy reserves, it becomes less selective in the sites it selects. If the spot is to its liking it cements down permanently with another adhesive compound. Once it finds its home, it undergoes metamorphosis into a juvenile barnacle.
The adult barnacle generally develop six hard calcareous plates to surround and protect their bodies. For the rest of their lives they are cemented to the ground.
Once metamorphosis is over and they have reached their adult form, barnacles will continue to grow by adding new material to their heavily calcified plates. These plates are not molted.
The geological history of barnacles can be traced back to the early Palaeozoic, 400 – 500 million years ago, although they do not become common in the fossil record until the last 20 million years.
Let's do some evolutin.
Barnacles were first fully studied and classified by Charles Darwin who published a series of monographs in 1851 and 1854. Darwin undertook this study at the suggestion of his friend Joseph Dalton Hooker in order to thoroughly understand at least one species before making the generalizations needed for his theory of evolution.
Historian of science and novelist Rebecca Stott has published a detailed account of Darwin's eight year study in a book called Darwin and the Barnacle, which challenges the supposition that Darwin was using the barnacle project as a way of delaying writing the book on species which would become Origin of Species by Natural Selection.
Barnacles cause problems as well. They are blamed from some corrosion. They are also of economic consequence, as they often attach themselves to man-made structures, like whistle buoys and ships, and cause detrimental damage.
Some barnacles are edible by humans, and goose barnacles are treasured as a delicacy in many Mediterranean countries.
Another critter down. Next week is shark week, so look forward to a little danger in the pet department.
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