Barnacles

Barnacles are one of commonest creatures to be found on rocky shores. However due to their size they are often overlooked by the beach goer, noticed only when they are walked over in bare feet when they make their presence known. As barnacles live in shells it is a common misconception that they are molluscs, this is not so they are crustaceans.

In Cornwall there can be found species of two orders of barnacles, the Pedunculata and Sessilia. The Pedunculata represented by goose barnacles and Sessilia by acorn barnacles. Acorn barnacles are the small “limpet like” creatures encrusting rocks, while goose barnacles are “long necked barnacles” found adhering to flotsam washed ashore.

Finally there is a parasitic barnacle Sacculina carcini this is unlike any of the other barnacles previously described.

Reproduction

Barnacles are hermaphroditic however it is normal at time of mating that barnacles will take on the role of either male or female. The eggs are stored in the female until they hatch and are released as free swimming larvae known as nauplii, which join other plankton, floating with the currents and feed on smaller plankton.

The nauplii go through several moults until eventually they metamorphose into a cyprid, at this stage it is still free swimming but it cannot feed. It is at this stage it looks for a suitable place to fix itself which it does, head first. This will be its final resting place but it may be far away from where they originated. This allows the species the possibility to expand its range. After fixing the barnacle metamorphoses again into a small adult.

Acorn Barnacles

Acorn barnacles have a conical shell comprised of plates. The number of wall plates depends on the species.

At the top of the barnacle is an opening which when uncovered by the tide is closed by the operculum. The operculum is comprised of two pairs of plates. The shape of the plates, vary between acorn barnacle species so providing a good means of identification.

The two pairs of plates of the operculum open allowing feathery limbs to extend to feed. Adult acorn barnacles are sessile suspension feeders. If you immerse barnacles in sea water you may be lucky to see them extend jointed feathery limbs. When barnacles are covered by the tide, they continually sweep their limbs in and out of the shell, in the process netting small edible particles.

Barnacles that settle high up the shoreline have to be able to withstand desiccation as they are uncovered by the tide for much of the time. To do this they pull their top plates together so reducing evaporation. This technique is not fool proof as in hot summers even mature adults will die.

Identification of Acorn Barnacles

The following tables attempt to aid identification of the barnacles commonly found on Cornish shores. However it is only an aid to identification. The shore zone in which they are found depend on physical factors such as exposure and salinity. Variations in such factors can provide advantages or disadvantages to each species to a differing degree. Thus leading species to out perform others at certain locations limiting the “opposition” to a smaller slice of the shore or even completely replacing them.

More important in Cornwall is its geographical position, this providing barnacle species with a southern distribution an ecological edge. With global warming this is more than likely to exacerbate unfavorable conditions to species with a northern distribution. Such is highlighted by Semibalanus balanoides which is extremely common in the rest of Britain but not so in this part of Cornwall, where it has been replaced by Chthamalus montagui.

Chthamalus montagui

Chthamalus stellatus  

Semibalanus balanoides

  Wall Plates

6

6

6 greyish

  Operculum

Kite shaped

Oval shaped

Diamond shaped

  Zone

High to mid eulittoral zone on exposed to moderately exposed shores.

Mid to low eulittoral zone on exposed shores.

Mid eulittoral zone on sheltered shores and higher on exposed shores.

  Shape

Conical in shape, but becomes columnar when crowded. 

Conical in shape, but becomes columnar when crowded.

Low conical in shape, but columnar when crowded.

  Size

12 mm in diameter

15 mm in diameter 

12 mm in diameter

  Additional   Information

It is the commonest barnacle to be found in Cornwall. It has a southern distribution.

Chthamalus stellatus is a barnacle with a southern distribution.

A northern species extremely common in the rest of Britain but not so in Cornwall.

Balanus
perforatus

Balanus
crenatus

Elminius
modestus

  Wall Plates

6 purplish vertically ridged plates

6 greyish

4

  Operculum

Oval shaped but recessed

Wide diamond shaped

Diamond shaped

  Zone

Mid eulittoral to shallow sublittoral on exposed shores.

Lower littoral zone to shallow sublittoral.

Range is expanding. Has strongholds in very sheltered shores. Mid littoral to shallow sublittoral.

  Shape

Steeply inclined volcano like.

Conical with one side slightly curved.

Low and conical.

  Size

30 mm in diameter

20 mm in diameter 

10 mm in diameter

  Additional   Information

Balanus perforatus is a barnacle with a southern distribution.

An Australian invader first noticed just after World War II. Thought to have been brought in by shipping or flying boats.

Goose Barnacles

Goose barnacles are similar to acorn barnacles in as such as they have a protective shell. The main difference being that they have a long flexible tubular muscular stalk which is unprotected. They are pelagic, living in the open seas adhering to flotsam.

With their long stalks they hang down from their floating object. They are filter feeders using their legs to ensnare plankton, which then pass them to their mouth.

There are two species commonly washed ashore, the Goose Barnacle Lepas anserifa and the Buoy Barnacle Dosima fascicularis.

Goose Barnacle Lepas anserifa

Goose barnacles fix themselves to any item that floats. This may be singly on a cork or thousands on a telegraph pole. They hang down by their long flexible tubular muscular stalk. They can grow large for barnacles, the shell reaching 2 inches. The shell is comprised of five plates.

Goose barnacles are unusual creatures surrounded by myth. It was once thought that the barnacle goose rather than migrating in winter changed into goose barnacles.

Buoy Barnacle Dosima fascicularis

The buoy barnacle is a warmer water species, their name derives from the float that the animal secretes for itself as it grows.

They can also be found attached to flotsam.

Parasitic Barnacle Sacculina carcini

The parasitic barnacle Sacculina carcini is unlike any of the other barnacles previously described. It has no hard body parts. It parasitizes crabs, most commonly the shore crab. It interferes with the growth processes of the crab, male crabs changing body form and behaviour to that of female over the following moults. The female form is needed by the barnacle. Ironically the barnacle’s produces a sponge like reproductive mass in the same place where the crab would have its own eggs, the crab caring for them as if it was their own.

To prevent removal from the crab during moulting of the shell, it later interferes with the crab’s natural cycle by stopping the crab moulting. An indicator of the parasite are crabs that have barnacles or worm encrustations on their shell. This is a good indicator that the crab has been parasitized. Healthy crabs regularly moult, not providing enough time for barnacles or worms to become established.