Saturday, 10 October 2020

BARNACLES: CUVIER TO DARWIN

Barnacles all closed up for protection.
One of the most interesting and enigmatic little critters we find at the seashore are barnacles. They cling to rocks at the waters' edge, closed to our curiosity, their domed mounds like little closed beaks shut to the water and the world.

They choose their permanent homes as larvae, sticking to hard substrates that will become their permanent homes for the rest of their lives. It has taken us a long time to find how they actually stick or what kind of "glue" they were using.

A clever fellow from Duke University's Marine Laboratory in Durhan, North Carolina finally cracked that puzzle. Instead of chopping up barnacles to see what makes them stick, he observed and collected the oozing glue from some Amphibalanus amphitrite as they secreted it.

Remarkably, the barnacle glue sticks to rocks in a similar way to how red cells bind together. Red blood cells bind and clot with a little help from some enzymes. These work to create long protein fibres that first blind, clot then form a scab. The mechanism barnacles use, right down to the enzyme, is very similar. That's especially interesting as about a billion years separate our evolutionary path from theirs.

So, with the help of their clever enzymes they can affix to most anything – ship hulls, rocks, and even the skin of whales. If you find them in tidepools, you begin to see their true nature as they open up, their delicate feathery finger-like projections flowing back and forth in the surf.

Barnacle Cirri seeking tasty plankton
Those wee feather-like bits you see are called cirri. Eight pairs of these thoracic limbs help barnacles to filter tasty bits of plankton from the surrounding water into their mouths.

Barnacles are cirripedes, a kind of crustacean that is covered with hard plates of calcium carbonate. Named for their cirri, they live stuck to hard surfaces in and around our world's oceans. While they do not look like crustaceans, they are definitely part of this taxonomic grouping that includes crab, lobster, crayfish, prawn, krill, and woodlice.

They have an old history. Their ancestors can be traced back to animals such as Priscansermarinus that lived during the Middle Cambrian – some 510 to 500 million years ago. I found my first barnacle fossil at a fossil site called Muir Creek on the south end of Vancouver Island. The fossil exposures at Muir are Oligocene, 20-25 million years old. This is about the time that barnacles can be found more readily as skeletal remains.

One of the reasons for the limited number of barnacle remains in the fossil record is their preferred habitat – high energy, shallow ocean environments. These tend to see a lot of tidal action that leads to erosion and barnacles being broken apart, slowly eroded down to bits too small to recognize for what they are.

One of the fossil remains we do find are not the barnacles themselves, but trace fossils of acrothoracican barnacle borings from Rogerella. These are commonly found in the fossil record beginning in the Devonian right up to today. Rogerella is a small pouch-shaped boring (a type of trace fossil) with a slit-like aperture currently produced by acrothoracican barnacles. These crustaceans extrude their legs upwards through the opening for filter-feeding (Seilacher, 1969; Lambers and Boekschoten, 1986). They are known in the fossil record as borings in carbonate substrates (shells and hardgrounds) from the Devonian to the Recent (Taylor and Wilson, 2003).

Barnacles' ancestry can be traced back to the Middle Cambrian
Barnacles were originally classified by Linnaeus and Cuvier as Mollusca, but in 1830 John Vaughan Thompson published observations showing the metamorphosis of the nauplius and cypris larvae into adult barnacles. He noted how these larvae were similar to those of crustaceans.

In 1834 Hermann Burmeister published further information, reinterpreting these findings. The effect was to move barnacles from the phylum of Mollusca to Articulata, showing naturalists that detailed study was needed to reevaluate their taxonomy.

Charles Darwin took up this challenge in 1846 and developed his initial interest in a major study published as a series of monographs in 1851 and 1854. Darwin undertook this study, at the suggestion of his friend Joseph Dalton Hooker, to thoroughly understand at least one species before making the generalizations needed for his theory of evolution by natural selection.

Barnacles are suspension feeders, sweeping small food into their mouth with their curved 'feet'. They are cemented to rock (usually), and covered with hard calcareous plates, which they shut firmly when the tide goes out. The barnacles reproduce sexually and produce little nauplius larvae that disperse in the plankton. Eventually, the larvae change into cypris form and attach on other hard surfaces to form new barnacles.