Amphibalanus amphitrite is sessile, and has a shell that is described as cone shaped or subcylindrical shaped. The shell consists of 6 calcareous plates, made from a calcium-rich substance that the barnacle produces from its body surface and can grow up to 1.9 cm in diameter (Poore & Syme 2009). The plates provide protection against predators, waves and dessication when the barnacle might be exposed during low tides. The plates are usually white with clear stripes or bands of purple, pink, brown purple-pink-brown. Plates are arranged differently for different genera of barnacles. (Anderson 1994)  The barnacle is at its widest at the base, where it attaches to the substratum and has an opening that is in the shape of a diamond. This opening is covered by an operculum that is movable and is in fact, 2 triangular calcareous plates (Masterson 2007). Each plate has 2 halves called the tergum and scutum (Masterson 2007).

The body of the barnacle resides within the shell and has no body segmentation except in the form of paired limbs. An oral cone surround the mouth and it and has lips (labrum and labium, in front and back respectively) as well as maxillules, maxillae, mandibles and mandibular palps which aid in feeding (Anderson 1994). The main feeding appendages, known as cirri (singular: cirrus), are modified thoracic appendages or legs. Setae covers each cirrus and helps the barnacle sweep food particles into its mouth (Beatty et al. 2004).

The shell of the A. amphitrite consists of 6 calcareous plates, but the number varies among different genera. The arrangement of the calcareous plates also vary between different genera, which can be useful in identifying a particular species.

The plates have different names with the plate in front of the barnacles being known as the rostrum, the paired side plates known as laterals & carinolaterals and the plate at the back is known as the carina (Figure 2). Each side plate has 2 regions, the central region called the parietal region and the edges that have two names: Radii and Alae. The radii refer to the edge of the plate that overlap the next plate and is exposed to the external environment and the alae is the edge of the plate that is overlapped and not exposed (Figure 3) (Poore & Syme 2009).  A. amphitrite also has a basal plate on the underside of the body, but this is not visible when the barnacle is attached to a surface (Wendt et al. 2006).

The overlapping arrangement of the plates is ideal for the growth of the barnacle while it stays attached to the substrate. These features, however, are not very visible and even harder to observe should the barnacle be eroded (Poore & Syme 2009).

The cirri are feeding appendages of the barnacle, and are modified thoracic appendages. There are 6 different pairs of cirri, known as cirri I (1) through to cirri VI (6), all of which work together when the barnacle feeds. Cirri VI to VI (Figure 4) are long cirri that look very similar to one another, protrude out of the operculum during feeding, and as a whole form a cirral fan. Cirri I to III (Figure 3) have been modified and are now short and specialized as maxillipeds (Anderson 1994). Together, they have a complex interaction in which they transfer food captured by the moving cirral fan to the mouthparts. The movement of the cirral fan, which can be described as a ‘beating action’, creates a current of water that enters the mantle cavity and improves respiration as well as allowing the cirri I to III to capture any food particles that might have been missed by the cirral fan (Anderson 1994). 

Introduction

It has been mentioned in previous research that there is variation in the setae number found in the larvae of A. amphitrite, and is further supported by other research that indicates some barnacles have variations in their number of setae within their own species (Egan & Anderson 1986; Barker 1976). This variation was observed during the naupliar stages V (5) and VI (6) with a slight increase in variation between the 2 stages (Egan & Anderson 1986). An experiment was conducted to determine if there would be a variation in the number of setae between adult barnacles and if so, would there be an increase in barnacle size increase the number of setae.

Materials & Methods

3 rocks, each with approximately 50 A. amphitrite on them, were collected from a shoreline with rocky outcrops that had an abundance of A. amphitrite. It was ensured that the collected barnacles were of a range of sizes and were alive. Great care was also taken to ensure that there were no gastropods or other predators on the rocks there were chosen. The barnacles were collected in the morning and were brought into an aquarium by the afternoon to ensure that they endured as little stress as possible during this period of handling and transporting. Water flow was constant and food was given to the barnacles to ensure their survival during the course of the experiment.

Over the next 3 weeks, barnacles that were of a range of sizes were dissected, and the number of setae they had on a particular cirrus was observed and recorded. Cirri VI, which is part of the cirral fan, was used in the experiment. Out of the pair, a single cirrus was picked at random and used for the experiment. The length and width of the base of the barnacle was also measured (before dissection) and the area was calculated to determine the size of the barnacle.

A comparison between the size of the barnacles and the number of setae on the cirrus was then compared between each other by plotting them on a scatter chart in Microsoft Excel. A regression value was then obtained to determine if there is a correlation between sizes of the barnacles and the number of setae.

Results

The results (Figure 5) show that while there is a correlation between increased sizes of the barnacles with an increased number of cirri, it is not a strong relationship. The R² value stands at 0.1415, which signifies a poor correlation. From the graph, a number of obvious outliers are observed, with some much smaller barnacles (about half the size of others) recording 20-40 more setae on the cirrus.

Discussion

The results show that in general, there is an increase in the number of setae with barnacle sizes. This relationship, however, is not strong and there are instances when smaller barnacles have a higher number of setae on their limbs. While previous research have all been conducted on the nauplii and cyprid larvae of the barnacles and thus cannot directly support the results, they lend some credibility to them, in that there is a high possibility that variation in setae number would be reflected in the adults as well. This, however, goes slightly against a technical report published in the past that states that the number of setae in a nauplis can, to a certain extent, serve as a distinguishing factor in identifying a species of barnacle (Lang 1979). While this does not directly disagree with the results of the experiment as it is in relation to the larvae, it does lean towards the idea that the adult would not have variations in setae number within a species. All in all, the results are preliminary and more replicates have to conducted, and possibly with the total number of setae on a barnacle as opposed to the number of setae on a single cirrus. 

As with most acorn barnacles, A. amphitrite is hermaphroditic and harbour both male and female reproductive organs. A. amphitrite is usually able to produce both eggs and sperm simultaneously and the fertilization is internal (Anderson 1994). However, the general method for reproduction among A. amphitrite is via the exchanging of sperms between individuals that are situated next to each other on the substrate. This is performed by one individual depositing its sperm into the mantle cavity of the barnacle beside it with the use of a very long penis (Masterson 2007). Apart from the general cross-fertilization, self-fertilization has been observed in several instances and has been well recorded (Anderson, D.T 1994; Charnov, E.L 1987; Desai et al 2006; Masterson 2007). 

For a number of months, fertilized eggs are brooded in the mantle cavity before the release of planktotrophic, swimming larvae known as nauplii (Figure 6) (Ruppert et al. 2004). Each nauplii has antennae, 3 pairs of jointed limbs called the antennules, antennae and mandibles, and a single eye spot which helps the nauplii respond to light (Anil et al. 2012). The nauplii consists of 6 naupliar stages where it undergoes 5 moulting phases to pass from one stage to another. As the nauplii develops and progresses through the 6 stages of growth, there is generally not much change with regard to its form and functions (Anderson 1994). The limbs perform the same motion and function, which is to swim, collect food and feeding throughout all the stages.

There are 3 main areas whereby there are major changes to the body features of the nauplii. The first is the increase in size of the thoraco-abdominal process at each moult or stage, and an increase in the number of paired limbs. This increase is attributed to the development and production of post-naupliar segments (Anderson 1994). The second change during larvae development is an increasing number and types of setae on their limbs. Long setae develop on parts of certain limbs such as exopods and tips of endopods of antennules and mandibles where the setae are usually plumose or simple. The proximal region of the antennules and mandibles develop a variety of shorter setae (Anderson 1994). During the final 3 naupliar phases, 2 compound eyes are developed alongside the current eye spot, which increase the animal’s sensitivity to light fluctuations (Walker et al. 1987).

All 3 pairs of limbs contribute to the locomotion of the nauplii, but the antennae are the primary limbs used by the nauplii in movement. The antennae move in a wide arc, much like a person swimming breaststroke. The movement of the antennae however, are vigorous and rapid but not smooth, resulting in a jerky forward movements (Anderson 1994). The nauplii swim in intervals, alternating between a burst of swimming activity and rest (Yule 1984).

The planktotrophic nauplii, as with most other species consume a variety of microscopic particles, including phytoplankton and diatoms (Al-Aidaroos & Satheesh 2014; Anderson 1994; Dineen & Hines 1992; Lee & Kim 1991; Stone 1989). This is performed using 2 of their paired limbs, the antennae and the mandibles; the antennules do not contribute to feeding. The antennae are responsible for collecting food particles and the mandibles move the particles to the mouth (Walker et al. 1987). 

The growth of the nauplii terminates at the 6^th naupliar stage, where moulting and feeding do not occur any more. This 6^th naupliar stage is known as the metanauplius although different authors have use the term to describe multiple naupliar stages as well (Harzsch et al. 2012; Karande 1974; Walker et al. 1987). This 6^th naupliar stage metamorphoses into a pre-settlement stage known as the cyprid. The cyprid is a single, larval stage that will eventually attach to a substrate and metamorphosize into a juvenile barnacle. The cyprid does not feed and obtains energy from lipids that have been stored away during the naupliar stages.

The cyprid (Figure 7) has a markedly different appearance as compared to the nauplius, as it now resembles more like a free-swimming bivalve but still of a rather small size, with an average length of approximately 600µm (Anderson 1994). The cyprid has a carapace that can open and close, and is controlled by an adductor muscle, and a mantle cavity lies just underneath the carapace.  Antennules and six pairs of biramous thoracic limbs (that have replaced the naupliar limbs) as well as a pair of uniramous caudal appendages are held within the mantle cavity (Bernard & Lane 1962; Walker & Lee 1976). The carapace, caudal appendages and thoracic limbs all harbor setae that are thought to act as basic mechanoreceptors although some might be chemoreceptors as well (Walker & Lee 1976). The eyespots that were present in the nauplii are retained. The cyprid also produces the calcareous shell plates that the adult uses, in a ‘continuous but erratic’ manner (Walley & Rees 1969).

The streamlined, almost tear-drop shape of the cyprid along with the 6 pairs of thoracic limbs  enable the cyprid to swim at greater speeds and at a higher level of agility comparative to the nauplii (Anderson 1994). The cyprid swims in a generally smooth movement, but upon closer inspection, is made up of many short bursts. With a better ability to control its limbs, and angle of movement, the cyprid has far better manoeuvrability in the water than the nauplii does. The cyprid swims in intervals of between 10 and 40 limb movements and rest (Anderson 1994). Swimming is key to the cyprid in the completion of its main responsibility/role, which is to find and attach to a suitable settlement location. As the cyprid swims, it uses a range of sensory systems such as sensory cells to detect changes in water current, light and pressure and help it determine the depth it should be at. (Anderson 1994; Walker & Lee 1976).

The cyprid requires a solid substrate for it to settle and metamorphose into an adult. However, not every substrate is suitable for it to settle and thus it has to search for one. Upon coming into contact with a substrate, the antennules play a significant role in both temporary attachment to the substrate and exploration of the substrate (Anderson 1994; Walker & Lee 1976). It has been found that antennule have sense organs that detect certain stimuli that might influence the settlement of the cyprid (Walker & Lee, 1976). The antennule also has an attachment disc, which is crucial for temporary and permanent settlement on a substratum. Exploring the substrate is separated into 2 phases, the ‘wide search’ and the ‘close search’. In the wide search, the cyprid uses its antennules to walk across the substrate, making a few directional changes as it goes. If the conditions are favoruable, it proceeds to the ‘close search’ where it makes many changes in direction as it searches for the right spot (Anderson 1994; Walker et al. 1987). This can lead to focusing on a tiny area and possibly fixation, where the cyprid permanently attaches itself to the substratum. If the conditions are found to be unsuitable to the cyprid as it explores the substrate, it will swim away. Various factors have been found to influence and affect the settlement of cyprids and include both physical and chemical factors. The factors include light, speed of water flow, amount of disturbance of the water, surface texture of the substrate, topography of the substrate and various chemical cues and complexes (Anderson 1994; Walker et al. 1987). It has also been found the cyprids prefer to settle on a substrate that other barnacles of the same species have already settled on, or barnacle shells that still remain (Knight-Jones & Stevenson 1950; Gabbott & Larman 1987; Kato-Yoshinaga et al. 2000). Current research suggest that species specific settlement-inducing proteins are responsible for these settlement patterns (Kato-Yosihinaga et al. 2000).

The cyprid possesses 2 large cement glands that are positioned posterior to the compound eyes which merge into a single cement duct via collecting ducts. The cement duct runs the length of the antennule and splits out into several points on the surface of the attachment disc (Walley & Rees 1969). As its name suggests, the cement glands produce a cement that the barnacles uses for permanent attachment. The cement glands contain two types of cells; proteins and phenol and phenoloxidase. Only one of the glands contain both types of cells; the other secretes only proteins (Yule & Walker 1987; Anderson 1994; Walker 1981). The cement is secreted when the barnacle decides to settle permanently and encompasses? The end of the antennules. The cement cannot spread very far as it begins to set and harden as soon as it is secreted; although it does fill in empty spaces on the surface of the substrate (Yule & Walker 1987, Walker 1981). The time taken for the cement to harden fully ranges from 1-3 hours, during which the adhesive strength of the cement gradually increases. As the cement sets, it becomes biochemically stable, and strong acids and alkalis have been shown to have no effect on it (Walker 1971). The cement can still persist years after the barnacle has been removed forcefully and is considered to be non-biodegrable (Yule & Walker 1987). 

After attachment to the substratum, the cyprid will undergo a period of morphological changes for about 24 hours before it moults into a young adult. The cyprid becomes slightly flattened and gradually opaque. The 2 antennules that have attached to the substratum become shorter and lie perpendicularly to the attached surface. The cyprid also undergoes a dramatic 90° rotation of its body whereby its dorsal and ventral surface are now in a position parallel to the substrate (Anderson 1994). The thoracic limbs straighten and lie perpendicular to the thorax’s ventral surface while the mouth moves posteriorly. The mantle cavity now extends almost to the ventral side of the cyprid, and surrounds most of the inner body, leaving just a small region whereby the yet to develop prosoma and mantle tissue can connect through (Walley & Rees 1969).

During & After Moulting

During the moult, contractions of certain muscles and differences in body fluid pressure cause the body of the barnacle to rotate another 90° degrees as it starts to become an adult. Part of the cyprid mantle cavity forms the mantle cavity of the adult, while the other part becomes part of the basal membrane. The antennules now are merely remnants, but still remain with the barnacle underneath the basal membrane. The six pairs of swimming limbs now develop into the feeding appendages, or cirri, that the adult uses (Walley & Rees 1987). The juvenile mouth part and oral cone seen in the adult are developed as well and the cyprid adductor muscle is redeveloped elsewhere. The cyprid compound eyes are now degenerating and will eventually not found in the adult. The animal now sheds the cyprid carapace via contractions and various movements (Maruzzo et al. 2012). The cyprid cement glands also start to break down but the cells are redifferentiated into the development of adult cement glands which releases more cement as the barnacle grows (Walker 1981). The most important feature that is developed shortly after the moulting is the calcareous shell plates. The scutum and tergum of the operculum are laid down fairly early in the development process, and a shortened form of the carina is set in place as well. The other shell plates, the rostrum and the laterals continue to develop and will eventually take their place in the shell (Anderson 1994). 

As with most crustaceans, the barnacle has a complete gut; a foregut, midgut and hindgut (Figure 8). The foregut is lined with a thick cuticle and contains muscles that aid in receiving and swallowing food. The foregut contains the oesophagus which has longtitudinal, circular and radian muscles, used for peristalsis during feeding. The cuticle remains thick at the start of the oesophagus but starts to become thinner further down the oesophagus. Eventually, the oesophagus meets the midgut at a junction. The midgut is split into 2 segments: the anterior and posterior segments, where the anterior is thicker than the posterior. The anterior of the midgut contains caeca that increase the length of the midgut. The midgut of the barnacles is looped and is rather long and intricately folded. The anterior section of the midgut contains a pair of midgut glands that produces enzymes to aid digestion (Anderson 1994). The wall of both anterior and posterior midgut are made of epithelial cells that are constantly replaced, with new cells coming about as a result of cell division. The hindgut is short but is still split into 2 sections. The first section serves as a ‘sphincter’ that controls waste to the next hindgut section and has a muscular wall and convoluted lining. The next section of the hindgut is an anal chamber which connects to the anus (Anderson 1994).

Food ingested passes through the foregut and comes to the midgut without any changes apart from being lubricated from saliva in the mouth, which is located behind the labrum. In the midgut, the epithelial cells secrete a peritrophic membrane which wraps the food in a bolus and separates it from the epithelium. The peritrophic membrane ensures that the epithelium does not suffer damage from food or infected with pathogens and improves digestion (Lehane 1997; Bolognesi et al. 2008). The midgut epithelium absorbs nutrients during digestion and also stores some nutrients such as glycogen and lipids (Anderson 1994; Rainbow & Walker 1978). The food is digested with the help of digestive enzymes produced by the midgut glands. The undigested food is enclosed in some peritrophic membrane and is known as a faecal pellet. The pellet is travels to the end of the posterior midgut where it is lubricated and rapidly pushed through the hindgut and out the anus. 

The barnacles’ circulatory system is with the use of haemocoelic fluid which is used in place of muscles for certain parts of its body, mainly the valves of the operculum, thorax, cirri and the penis (Anderson 1994). Controlling the flow the fluid is key for some the barnacles movements and activities. They play a role in opening the operculum valves as well as extending the thorax, cirri and penis (Anderson 1994). Mantle tissue in the mantle cavity forces haemolymph into the tissue of the operculum valves and causes the movement of the valves (opening). Most of the haemolymph is pushed into paired branchiae which are used in respiration (Darwin, 1859). Vessels known as scutal vessels branch out from the branchiae and lead into the body of the barnacle and distributed to the oral cone, cirri, penis and the thorax. From here, the haemolymph is returned to the mantle via a returning circulation known as the collecting circulation (Burnett 1977). The haemolymph is brought to a rostral sinus, which is a pumping organ, with a rostral valve. From this valve, a basal vessel brings the haemolymph back to the mantle to be recirculated (Anderson 1994).

The barnacle excretory organs are a pair of structures that are situated on either side of the foregut and are known as maxillary glands (Figure 9). The maxillary gland develops during metamorphosis although the nauplii do exhibit a more basic state known as antennal glands (Walley & Rees 1969). It is thought that the maxillary glands also play a role in excreting nitrogenous materials and heavy metals from the body, apart from its normal functions. The maxillary glands can be divided into 3 parts: end sac, efferent duct and the terminal duct (White & Walker 1981; Anderson 1994). The end sac contains epithelial cells with vacuoles and also has a layer of external elastic fibers. The epithelial cells are responsible for ultrafiltration of haemocoelic fluid into the end sac itself. The end sac and the efferent duct are joined via a funnel (Figure 10) that is created with 4 modified cells of the end sac, although everywhere else is separated by a layer of parenchyma cells (White & Walker 1981; Anderson 1994). The efferent duct has an epithelium that is covered with a layer of elastic fibres and allows the duct to expand and have a far greater volume. Current knowledge suggests that the efferent duct functions as a storage area or bladder for urine. The terminal duct is lined by a layer of cuticle and leads out of the body. 

The barnacle nervous system is a much simple system as compared to the typical crustacean nervous system. The barnacle has a reduced brain that does not resemble anything like a typical crustacean brain, as it lacks many features that might indicate so. With the lack of compound eyes and antennae, parts of the brain have become vestigial, namely the protocerebrum and tritocerebrum, which are the associated brain sections (Anderson 1994; Gwilliam 1987; Gwilliam & Cole 1979). The brain receives its main inputs via photoreceptive ocelli. A ventral nerve cord is present, which is actually a ventral ganglionic chain that has been fused together to form a single mass although segmentation may be observed internally (Gwilliam 1987).

A. amphitrite, as with other barnacles, have a variety of sense organs. As mentioned before, they have ocelli, that are photoreceptors and help the barnacle respond to light. The ocelli are involved in an activity known as the shadow reflex. A sudden drop in light levels when their cirri are extended will cause the barnacles to retract their cirri and close up (Gwilliam 1987; Anderson 1994). Photoreceptors have also been found within their cirri (Clark & Dorsett 1978). They also have chemoreceptors as well as mechanoreceptors located on various parts of their body. Chemoreceptors are found on the valves of the operculum, the lining of the mantle and on the cirri (Anderson 1994).  Setae on the penis have also been found to have mechano and chemoreceptive functions as well. 

Respiratory exchange in A. amphitrite, as in all barnacles occurs via the surface of their body and limbs, as well as the lining of the mantle cavity. Circulation of the haemolymph throughout the body coupled with these surfaces are mostly responsible for maintaining oxygen levels within the body as there is an absence of respiratory pigments within the haemolymph of the barnacle (Anderson 1994; Waite & Walker 1988). A. amphitrite, along with other balanomorph barnacles have possess paired branchaie within their mantle that aids in respiratory exchange by increasing the surface area through which respiratory exchange can occur. The paired branchaie (Figure 11) have folded surface areas, which greatly increase their surface areas (Anderson 1994; Darwin 1859). Respiratory exchange across surfaces within the mantle are attributed by the water flowing by when the barnacle extends and moves its cirri in rhythmic movements (Anderson 1994).

A. amphitrite, and all other intertidal barnacles, have developed a method that is adapted to respiratory exchange when the water level falls and they are exposed to air. The barnacles do not fully shut their operculum when exposed to air and leave a small, diamond-shaped opening, which functions as a micropyle (Grainger & Newell 1965). The barnacles expels water from the mantle cavity and replaces it with a bubble of air. The air bubble allows the barnacle to perform aerial respiration as it acts as a medium through which gas exchange between the surfaces within mantle and the external air can take place. The presence of the air bubbles also allows the respiratory exchange to take place with minimum loss of water. (Grainger & Newell 1965; Anderson 1994). This occurs multiple times over the course of the barnacle being exposed to air. Should the barnacle be subject to dessication, it closes the operculum tightly and aerobic respiration takes place. When the barnacle is re-submerged in water after being exposed to the air for a period of time, it expels any gas bubbles that are found within its mantle cavity, and normal activity resumes (Grainger & Newell 1965; Anderson 1994).

A. amphitrite, as with all barnacles and crustaceans, have complex and varied forms of muscles. Muscles are found from the thorax to cirri, mouthparts and even the penis. The operculum has different muscles that help it open and shut (Figure 12). The scutal (scutum) adductor muscle is the main muscle that is involved in the opening of the operculum valves and is aided by opercular depressor muscles (Anderson 1994). Large tergal (tergum) and the scutal depressor muscles firmly pull on the operculum to close it tightly, an action that can be easily seen when the barnacle is disturbed. The scutal depressor muscles are also key in opening the operculum.

The barnacle contains a number of muscles within its prosoma. With their body being underneath the operculum, main suspensor muscles, which suspend the barnacle within its shell lie at a slanted angle parallel to the apico-basal direction.  Muscles that are responsible in the extension of the cirri are found here. The prosoma lifting muscles are the largest muscles in the prosoma and lift the thorax towards the operculum when it is open. Following that, compression of the prosomal muscles result in the thrusting of the thorax into the opening (Anderson 1994).

The cirri contains muscles, a large flexor muscle that is made up of a number of striated muscles, which are only used to curl the cirri back from their extended positions (Pfeiffer & Lowe 1989). Extension of the cirri are brought about by hydraulic activity (Anderson 1994). The curling back of the cirri is known as furling and brought about by the extension of these flexor muscles. Movement of the maxillipeds are brought on by the contraction of both extrinsic cirral muscles that are found on the wall of the thorax and intrinsic cirral muscles (Anderson 1994).

The oral cone and mouthparts have their movements attributed to muscles as well. The labrum retractor muscle is a thick muscle that helps pull to labrum towards the wall. A basal muscle that connects from a wall of the palp to the base of the mandible is responsible in the abduction of the palp, which is caused by the contraction of said muscle (Anderson 1994). The mandibles are controlled by the mandibular abductors and adductors, which are the largest muscles within the oral cone, and mediate the movement of the mandibles chopping motions. The maxillule has 4 muscles attached to its base and also to the oral cone itself. The maxillae has a few muscles attached to it as well, with a transverse muscle and 2 longitudinal muscles present (Anderson 1994). 

The phylogeny of the Cirripedia continues to be disputed and questioned, with new groupings and evolutionary histories being suggested (Tsang et al. 2015; Gale 2014). In the past, morphological features were the characteristics that helped in grouping and the cirripedia, but has been found to be rather unreliable in the modern day, with evolutions of barnacles that completely lack shells (Rhizocephala & Acrothoracica). As such, DNA sequences have been used in recent times to better analyse the phylogeny of the cirripedia.

Recent research that investigated the cirripedia phylogeny as a whole suggested that there was a need to rearrange the current phylogenetic table to properly reflect the evolutionary route the cirripedia has undertaken (Figure 14) (Pérez-Losada 2008). For a start, the researchers found that barnacles without shells had ancestors with shells and lost them in the process of evolution as opposed to the idea that it was a pleisomorphic trait. They also found that shell plate numbers could not be used as a characteristic to sufficiently explain the evolutionary history of the cirripedia and could not answer and represent the basic phylogenetic patterns that are currently observed within the currently accepted phylogeny of barnacles (Figure 13).

Barnacle fossils are well represented, with publications of fossils that date back to the Silurian period (Newman & Abbott 1980; Wolfe 2013). However, barnacle fossils are not usually complete shells, or even plates that are adjacent to one another, with most fossils found being single valves. However, there are enough complete fossilized shells and valves to determine that the stalked and not stalked feature seen in the modern barnacles have been around for a long time (Foster & Buckeridge 1987). Unfortunately, cirripedes without a shell do not have a substantial fossil record to determine their evolutionary history from. The earliest literature on barnacle fossils can be traced back to 1670 but it was only in the 1800s that barnacle fossils were properly described, with Darwin famously describing most of these (Foster & Buckeridge 1987). Over the years, it has been recognized that barnacles with a tougher, stronger operculums had a better fossil record. The same can be said for barnacles that settled on more benthic habitats.

For the Mesozoic period, there is a huge variation in the fossils as there was massive radiation of forms. It also helped that back in this time, barnacles either had no stalks or had extremely armoured stalks, possibly as protection against predators (Foster & Buckeridge 1987). The Cretaceous period produced far lesser numbers of complete fossils, especially for verrucids and brachylepadids. In fact, the best brachylepadid shell from this era does not have any operculum valves (Foster & Buckeridge 1987). In the tertiary era, Balanomorpha fossil records are very much limited to certain species but rich nonetheless. It is thought that the lack of information on many species could be due to the habitat at which they settled on, most of which being littoral habitats and opportunities for fossilization do not occur often (Foster & Buckeridge 1987). 

Apart from normal predation from gastropods, crabs and starfish, the only other discernible threat to barnacles is the increase in ocean acidification (McDonald et al. 2009; Buschbaum 2002; Connell 1961). Increased levels of carbon dioxide in the atmosphere has been suggested to decrease the pH of seawater (McDonald et al. 2009). This impending decrease in the pH of seawater has been modelled and hypothesized to weaken barnacle shell walls. It is also thought that longer exposure to decreased pH could eventually lead to lowered levels of larvae recruitment and settlement (McDonald et al. 2009). Ocean acidification does not appear to affect growth rates and in fact, increases the strength needed to remove the barnacles from the substrate. However, their shells suffer a higher level of dissolution as they grow and thus will be significantly weaker and thus lesser strength is needed to break the shell (McDonald et al. 2009). This means that the barnacles exposed to ocean acidification will be more susceptible and vulnerable to predators (Buschbaum 2002; Connell 1961). 

Biofouling is the act of organisms attaching and growing on man-made surfaces that are underwater (Holm 2012). Barnacles have been for many years, part of the marine biofouling community. Their sessile way of life, huge numbers and fantastic ability to hold on to a surface make them extremely successful biofoulers (Christie & Dalley 1987). They have brought about massive economic costs and scientists have for years, been trying to better understand their biology and to control their fouling. As such, no conservation actions have been taken to preserve barnacles, but instead people have been trying to control their growth and spread (Anil et al. 2012). In the last 20 years, research on barnacle biofouling has rapidly increased, with A. amphitrite being one of two species mostly used in research laboratories around the world as people attempt to mitigate fouling. (Holm 2012).

Barnacles have been previously found to be the most highly reported form of biofouling, comprising almost half of the reported cases (Christie & Dalley 1987). Out of the barnacles, acorn barnacles have a statistically higher fouling percentage as compared to all the other types of barnacles and have been found fouling numerous man-made structures including rigs, buoys, pipelines and shipping vessels (Christie & Dalley 1987).

Barnacle fouling has proved to be extremely costly economically. Barnacle fouling on hulls of ships significantly increases the resistance due to the roughness of the hull and this results in the need for more power and fuel to meet the same speed as a ship without fouling on it (Holm 2012). The increase in fuel consumption could rise to as much as 40% (Christie & Dalley 1987). Removal of barnacles has been shown to be costly as well, with the US Navy previous releasing figures stating that between 180 and 260 million dollars were spent on a combination of fuel, removal of barnacle and repainting the hull (Holm 2012). Barnacle fouling on fixed structures such as oil rigs and pipelines out in the ocean can also incur huge costs. Their presence can hinder inspections, compromise structural integrity and can be costly to remove them and allow the structure to resume normal function (Christie & Dalley 1987). 

Barnacles also play a major role in the corrosion of structures, and have been known to accelerate corrosion. Bacteria that can reduce sulphate grow under a barnacle and this is a major contributor to the acceleration of corrosion, albeit the barnacles being an indirect cause (Christie & Dalley 1987). Barnacle cyprid however, have been found to seek out cracks in surfaces and settle there. Their growth and resulting pushing forces on all sides of the surface can cause significant damage and the resulting repairs can be extremely costly (Christie & Dalley 1987).

The fight against barnacle biofouling has brought about numerous methods to combat the issue. The most widely used method are antifouling coatings. Over the years, different chemicals such as Dichlorodiphenyltrichloroethane (DDT) and Tributyltin (TBT) have been used in these coatings. While they were observed to be effective in controlling biofouling, they were soon banned due their harmful environment effects when they were leached out into the water (Christie & Dalley 1987; Anil et al. 2012; Holm 2012). As of now, many chemicals and compounds are being investigated as a replacement for these harmful chemicals. The current direction research on these coatings has been redirected into reducing the strength of barnacle adhesion rather than to outrightly prevent them from attaching (Holm 2012; Kamino 2013). Other methods of combating barnacle biofouling that are being experimented on and investigated include heat treatment, UV radiation, ultrasound, electric fields and antifoulants produced by other marine organisms (Anil et al. 2012).

Barnacle biofouling has also brought about a worldwide issue, which is the invasion of the barnacles. By attaching on vessels that travel to ports all around the world, the barnacles are transported to a brand new location where they might not previously occur (Carlton et al. 2011; Cohen 2011). Spawning of the barnacles could take place and they can then establish themselves in this new environment. Barnacles could also be transported as larvae by a ships ballast water and movements of buoys (Carlton et al. 2011).