realize es a marine animal very resistant that lives in or very close to water. It is normally confused with a mollusk due to its thick shell, but it is actually a crustacean that is related to the crabs and lobsters.

Barnacles are estimated to be the oldest species on the planet, as they are believed to have originated millions of years ago. In all that time there have been adaptations to the changes of the planet, but it is thought that not much has changed in all this time.

Species

There are more than 1.000 known species of barnacles. Some species are parasitic and the vast majority are harmless as they are filter feeders. In this way they do not need to harm another animal to feed, since they obtain the necessary nutrients by filtering the sea water. They are so harmless that many animals do not realize that they have a barnacle attached to them.

Many species are really small, reaching about 7 cm, but there are also larger barnacles. They usually live between 5 and 10 years, but certain large species are known that can live much longer.

Features

The barnacle when it is young floats freely, but the moment comes that they have to adhere to any rock, shell or other nearby object and remain there for the rest of their lives. Sometimes it is possible to see them attached to crabs, whales, boats, rocks and sea turtle shells.

Most are sessile invertebrates, that is, they do not move and remain immobile, attached to the substrate in which they live. To join the substrate they make use of cement glands that form the base of the first pair of antennae, in effect, the animal is fixed upside down by means of its forehead. However, some species adhere by means of a long, muscular stem, but in most cases they are part of a flat membrane or calcified plaque.

A plaque ring surrounds its body, homonymous with the shell of other crustaceans. It is made up of calcite. It is created once you stick to a rock and will protect you from the elements and predators. in sessile barnacles, the apex of the ring of plates is covered by an operculum, which can be embedded in the shell. Plates are joined by various means, varying by species, in many cases solidly fused.

Barnacle attached to a colony

Barnacle attached to a colony

Inside the shell, the animal lies face up with the limbs facing up. Segmentation is generally imprecise, and the body is divided more or less evenly between the head and the thorax, with little or no abdomen. Adults have very few head appendages, with only one pair of vestigial antennae attached to the cementum gland. The six pairs of thoracic limbs are called "cirri," which are feathery and very long, and are used to filter food.

They do not have a true heart, although a sinus close to the esophagus performs a similar function, as blood is pumped through it by a series of muscles. The blood vascular system is minimal. They also do not have gills and extract oxygen from the water through their extremities and the inner membrane of their shells. The excretory organs of barnacles are the maxillary glands.

The main sense is believed to be touch, with the hairs on their limbs being highly sensitive. The adult has three photoreceptors (ocelli), one medium and two lateral. These photoreceptors register the stimulus for the shadow reflex, where a sudden decrease in light causes the cessation of the hunting rhythm and, therefore, the closure of the opercular plates. Photoreceptors are likely only able to perceive the difference between light and dark. This eye is derived from the primary naupliar eye.

Habitat

The barnacle tends to live exclusively in marine environments in shallow waters and high tides. 70% of the species inhabit depths less than 100 m. However, there are always exceptions and barnacles have been found at a depth of 600 m.

Distribution

The barnacle is distributed throughout the planet.

Food

Barnacles are filter feeders that feed on nutrients that they extract from the water. The shell is made up of a series of plates (usually 6) with extremities in the form of feather legs that draw water towards it. To feed, it is enough to slide two of its six plates so that the water begins to enter, and then close it when it is satisfied. This way of feeding prevents the barnacle from being exposed too much to dirty water.

Predators

The barnacle must fight against the greatest number of predators in its larval stage, when its shell does not protect it. The larvae are very small and in their first stages they float in the sea with plankton. When the barnacle is adult and has its shell, few predators can eat it. The greatest predator of the barnacle is humans, but only the European barnacles found in Spain and Portugal are edible.

Reproduction

Most barnacles are hermaphrodites, therefore, they have male and female reproductive organs. Some species are gonochoric. The ovaries are located at the base or stem, and can extend into the mantle. The testicles are located at the back of the head, sometimes extending to the thorax. They can self-fertilize, although this is very rare and most of the time they choose for the eggs produced by one barnacle to be fertilized by another.

The sessile lifestyle makes reproduction difficult as they cannot let their shells mate. To facilitate reproduction between different isolated individuals, they have extremely long penises. It is so long, that it probably has the largest penis in the animal kingdom if we consider the size in proportion to its body. They can also reproduce by releasing sperm into the sea and a female will collect it to fertilize her eggs.

The Rhizocephala superorder was considered hermaphroditic but it was discovered that males injected themselves into the female's body, degrading to the condition of being nothing more than sperm-producing cells.

The larvae take more than 6 months to fully develop into adults. When they are young and in the larval stage of their life, they look for a place to adhere to and once they have done so, a thin layer of flesh surrounds it, which will gradually harden until it forms the hard shell characteristic of the adult barnacle.

Lifecycle

When the ovum is fertilized and hatches, a small larva called nauplii emerges. It has only one eye and consists of a head and a telson, it lacks a thorax and an abdomen. Before moving to the cypress stage, it goes through five stages, a process that lasts about six months. They are reared by the parents and released in the first molt as free-swimming larvae using the mushrooms (rigid structure similar to a hair or a bristle, present in many invertebrates).

Cypress larvae are the last larval stage before becoming an adult barnacle. It is not a stage to feed, its function is to find a good place to settle. This stage can last from days to weeks, depending on your energy reserves that will make them less selective as you consume them. To explore it has the help of modified antennas to identify the ideal surface. The larvae assesses the surface based on surface texture, chemistry, relative wettability, color, and the presence or absence and composition of a surface biofilm. Species that create colonies are more likely to adhere near other barnacles. Once the place is found, it attaches itself upside down using its antennae and a secreted glycoprotein substance. It permanently adheres to the substrate with another protein compound, and then metamorphoses into a juvenile barnacle.

At the beginning of metamofsosis you will develop the six hard calcareous plates to surround and protect your bodies. Once the metamorphosis has finished and reached their adult form, they will continue to grow adding new material to their plates.

Colony of the white barnacle

Colony of the white barnacle

State of conservation

Although the levels of contamination of the water have increased, it does not seem that the barnacles have been affected by the changes that other animals have suffered. Currently barnacles are out of danger.

Relationship with humans

The barnacle by adhering to different human structures can cause economic losses. For example, it is quite common to see barnacles attached to boats. A large number of them can cause poor navigation.

Some species of barnacle are edible to humans. Like the Japanese barnacle (Capitulum mitella) and the barnacle (Pollicipes pollicipes) which is a delicacy in Spain and Portugal. The picorocó barnacle is used in Chilean cuisine and is one of the ingredients of curanto.

Plate of edible barnacle in Madrid (Spain)

Plate of edible barnacle in Madrid (Spain)

In English, the word "barnacle" is spelled "Barnacle." Originally Barnacle referred to the Goose barnacle / Barnacle goose (Branta leucopsis), since the fleshy stem of the barnacle species (Pollicipes pollicipes) is very similar to the neck of the goose and led to the thought that in ancient times that species of Geese, which lived in the sea, literally grew from the barnacle.

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