The externally visible structures of a sessile barnacle include the shell and the operculum which together form a receptacle that encloses the body of the animal (Figure 1) (Darwin 1854). The shell includes a membranous or calcareous basis that cements onto the substratum, and the compartments (i.e. wall plates) that can either be 8, 6, or 4 in number or in some cases, all fused together into one piece (Pérez-Losada et al. 2014).

The plate at the posterior end is called the carina whereas the opposite plate is called the rostrum (Figure 2) (Darwin 1854). The ones at the sides are the lateral plates with the pair located closest to the rostrum referred to as the rostro-laterals, and the pair located closest to the carina referred to as the carino-laterals (Darwin 1854). Each plate is made up of a paries (i.e. wall of the plate) with alae or radii lining the edges. The ala is the side of the plate that is overlapped by the radius of the adjoining plate (Figure 1) (Darwin 1854).

The operculum sits in the opening of the shell and it is attached to the shell via an opercular membrane (Darwin 1854). It consists of the left and right opercular valves with each valve consisting of a scutum at the rostral-end and a tergum at the carinal-end, that works together as a functional unit (Figure 2) (Darwin 1854). The cirri which is encompassed within the shell can protrude out into the water column to filter feed when the opercular valves are open but when the valves are closed, the shell becomes watertight which prevents desiccation when the animal is exposed at low tides and protects the animal from predators (Crisp and Bourget 1985). 

Amphibalanus amphitrite

A. amphitrite has a calcareous basis and its cone-shaped shell is made up of 6 smooth wall plates (Darwin 1854). This is because the rostro-laterals have fused with the rostrum forming a single plate (Figure 3). The shell of this barnacle can either be steep or depressed and the orifice can range from being diamond-shaped to rounded-trigonal in shape (Figure 4A). This barnacle is generally white or light grey in colour with purple or pinkish-brown longitudinal stripes that can either be at equal distances from one another or arranged in a way that leaves large white spaces between the stripes (Darwin 1854). A. amphitrite is small, and it usually grows up to about 1cm in height and 2cm in width (Newman and Abbott 1980). It is, however, often mistaken with A. reticulatus as they both show very similar longitudinal stripes, but unlike the former species (Newman and Abbott 1980), the latter one has a net-like appearance as it also has transverse stripes that crosses the longitudinal stripes (Figure 4).

Coronula diadema

The crown-shaped shell of C. diadema also consists of the same 6 wall plates (Hayashi 2011). It has transverse ridges on the surface of its plates, especially at the lower portion, and spaced out groups of raised, convex longitudinal ribs (Figure 5) (Darwin 1854). The orifice of this barnacle is hexagonal in shape and it is larger than the membranous basis (Hayashi 2011). The scuta are located at the rostral end but the terga are either absent or rudimentary (Darwin 1854). Due to constant submersion in water unlike the intertidal barnacles, the operculum has become reduced as it does not need to shut them tightly for desiccation prevention (Hayashi et al. 2013). This barnacle can grow up to about 6cm in height and 8.5cm wide (Newman and Abbott 1980).

A. amphitrite is most widely found in the intertidal zone of coastal habitats but it can also occur down to a depth of 18m in estuarine and bay environments (Newman and Abbott 1980). It generally attaches to hard substrates such as rocks, mangroves, shells, and various man-made structures (e.g. ship hulls, jetty pilings, sea walls) (Kim et al. 2019). Albeit rare and unusual, A. amphitrite can sometimes be an epibiont and it has recently been reported for the first time to occur on a few American crocodiles (Crocodylus acutus) in Mexico (Quijano-Scheggia et al. 2012).

C. diadema is most commonly associated with the humpback whales (Megaptera novaeangliae) and it can be found attached to the head, ventral pleates, flippers, genital slits and the tail of almost all humpbacks (Fertl and Newman 2009; Hayashi 2011). However, it has also been found on blue whales (Balaenoptera musculus), fin whales (Balaenoptera physalus), right whales (Eubalaena australis and Eubalaena glacialis), sperm whales (Physeter macrocephalus), and northern bottlenose whales (Hyperoodon ampullatus) (Hayashi 2012).

Amphibalanus amphitrite: Zonation

The zonation pattern of intertidal organisms at the jetty pilings of Dunwich has been previously described by Eertman and Hailstone (1988). In their study, A. amphitrite were mostly found in the supralittoral fringe and upper midlittoral zone while various gastropod species (limpets, conniwinks, clusterwinks, whelks) and chitons were found from the midlittoral zone to the infralittoral fringe. Hairy mussels were the bottommost species and rock oysters were found covering the entire pile surface (Eertman and Hailstone 1988). These patterns were thought to be shaped by the physicochemical conditions (e.g. light, temperature, wave action) at that region although biotic pressures such as interspecific competition for space and predation by predatory gastropods such as whelks may also play a role in the distribution of those species (Tomanek 2002; Fairweather 1988).

 

C. diadema- Symbiosis

C. diadema are barnacles that form a symbiosis with cetaceans, particularly the humpback whales (Fertl and Newman 2009). Although they are sometimes referred to as ectoparasites (Hermosilla et al. 2015), they are commensal as they do not obtain any nutrients from their host nor do they appear to harm them (Félix et al. 2006). That said, they attach to the whales by gripping tightly onto their epidermis (Newman and Abbott 1980; Anderson 1994) and thus severe infestations by these barnacles may result in irritations of host skin (Carwardine 2020). They hold on so firmly that pitted scars are generally left on the skin of the host after the barnacles are shed or removed from it (Hermosilla et al. 2015).

Although C. diadema do not feed on whales, they benefit by getting a free ride on these hosts as they filter feed on the plankton in the water column (Carwardine 2020). As the humpback whales swim from a plankton-laden patch to another for feeding, these epibiotic barnacles that travel with them are exploiting those patches for feeding as well (Newman and Abbott 1980). Predator avoidance is another benefit conferred upon those barnacles as whales are not often attacked by predators (Ford and Reeves 2008). Nonetheless, living on a nektonic animal probably also makes them a difficult target to be preyed upon by potential predators.

The swimming efficiency of humpback whales can be impacted by heavy infestations of C. diadema (Carwardine 2020). Interestingly, a humpback whale can have as much as 450kg of Coronula barnacles attached on them (Newman and Abbott 1980). These sessile barnacles often become the substrate for various stalked barnacles which further affects the hydrodynamics of the whales (Newman and Abbott 1980; Félix et al. 2006). However, whales can remove some of these barnacles from their body through intensive breaching and by repeatedly slapping their flippers and tail at the surface of water (Félix et al. 2006). These sessile barnacles can also benefit the humpback whales as they are often used as armours or weapons (“brass-knuckles”) in male competitions and to defend themselves against attacks by their predators such as sharks and killer whales (Ford and Reeves 2008).

As crustaceans, barnacles carry out copulation and internal fertilisation but distinct from the others, barnacles are sessile as adults and thus cannot move around to seek mates (Kelly and Sanford 2010). Consequently, they have evolved extremely long penes, each capable of extending up to eight times its body width (Darwin 1854). The penis length, however, is phenotypically plastic and barnacles have been found to possess longer penes at lower densities (Neufeld 2011).

A. amphitrite and C. diadema are both cross-fertilising simultaneous hermaphrodites (Callan 1941; De Gregoris et al. 2011). During mating, hydraulic pressure is generated to extend the penis of a functional male (Neufeld and Rankine 2012). The extended penis then moves around and begin to search for a functional female starting from the carinal end and slowly moving 360° around its body (Figure 6). When the searching penis touches a ‘female’, it penetrates the operculum and seminal fluid is then ejaculated into the mantle cavity of the ‘female’ (Anderson 1994).

Seminal fluid within the mantle cavity of the ‘female’ stimulates the release of eggs into the oviducal sacs where fertilisation occurs, and the sacs are then moved to each side of the prosoma forming the egg lamellae where embryos develop (Anderson 1994). At the end of embryonic development, the embryos have developed into nauplii, but they are still held within the egg membrane. When hatching occurs, the membrane ruptures and nauplii larvae are released into the water column (Anderson 1994).

The breeding of A. amphitrite is lowest in autumn (April to June) and peaks in spring and summer (September to February) (Anderson and Egan 1986). Unlike A. amphitrite which breeds all year long, the breeding of C. diadema is thought to be synchronised with the breeding season of the whales (Newman and Abbott 1980). As whales migrate to warmer waters each year for breeding, the increase in temperature causes the adult barnacles to die and drop off from the whales and they are then replaced by new barnacle recruits (Félix et al. 2006). By releasing their larvae when all whales aggregate to mate, the barnacles can also increase their chance of settlement as their basibionts (i.e. host substrates) are rather patchy in the open ocean (Thiel 2015).

The larvae of both A. amphitrite and C. diadema released into the water column undergo five moults and develop through six planktotrophic naupliar stages before metamorphosing into a single lecithotrophic cyprid stage (Anderson and Egan 1986; Thiel 2015). The nauplius, however, only starts to feed after stage II (Anderson 1994).

A nauplius has a pair of antennules, antennae and mandibles that are lined by setae and their locomotion is achieved through movements of these limbs and beatings of setae (Figure 7). Apart from the antennules, the other two pairs of limbs are used to capture and transfer food such as flagellates and diatoms to the mouth for ingestion (Anderson 1994).

The final nauplius then moults into a bivalved cyprid larva which then settles and undergo post-settlement metamorphosis into a juvenile (Anderson 1994). The general form and function of the nauplii remain similar throughout the six stages but cyprids are morphologically and functionally different from the nauplii (Figure 8). The naupliar stages are generally the dispersal stages whereas the cyprid larva is more important in locating a suitable substrate for settlement and metamorphosis (Anderson 1994).

The swimming cyprid larva then settles onto the substrate and walks around on its antennules to search for a suitable spot for permanent attachment (Anderson 1994). A. amphitrite often settle in places where conspecific adults are present as the adults release a protein complex that induces the settlement of cyprid (Nogata and Matsumura 2005). For C. diadema, however, a cue released from the host tissue induces them to settle on the whale epidermis (Nogata and Matsumura 2005).

When a suitable spot is found, cement is secreted from the cement glands through the tips of the antennules to adhere the cyprid larva permanently onto the substrate (Anderson 1994). Following that, a single highly complex post-settlement metamorphosis occurs which rapidly transform the cyprid larva into a juvenile barnacle (Anderson 1994).

During metamorphosis, the epidermis separates from the carapace and the muscles of the antennules start to contract which pushes the compound eyes and the antennules against the substrate (Maruzzo et al. 2012). The body is then raised upwards (i.e. rotates 90°) and the carapace, thorax, compound eyes and cuticle of the antennules are then shed off, transforming the larva into a bag-shaped juvenile (Figure 9). The shell plates then begin to form, and the juvenile eventually takes on the shape of an adult barnacle (Maruzzo et al. 2012). The juvenile then grows into an adult through several moults, but the calcareous shell is not shed during moulting and it gets incrementally larger in size overtime (Pérez-Losada et al. 2008)

The feeding appendages of all cirripedes include six pairs of biramous cirri but in both A. amphitrite and C. diadema, cirri I to III have been modified into short maxillipeds (Anderson 1994). Therefore, only three pairs of the posterior-most cirri (cirri IV to VI) have retained the long rami which form the cup-shaped cirral fan that protrude out into the water column (Figure 10). The cirral fan functions to capture larger food particles (e.g. phytoplankton, zooplankton and meroplanktonic larvae) which are then manipulated and transferred to the mouthparts by the maxillipeds in a complex way (Video 1) (Crisp and Southward 1961). The rhythmic beating of the cirral fan creates a water flow through the mantle cavity and smaller food particles (e.g. bacteria) that are not caught by the cirral fan can then be captured by the setae of cirri I to III and transferred to the mouth for ingestion (Crisp and Southward 1961). 

Video 1. This video shows how a barnacle uses its cirral fan to feed.

The alimentary canal of A. amphitrite and C. diadema consists of a cuticular-lined foregut, a midgut, and a cuticular-lined hindgut (Anderson 1994). Pancreatic glands and midgut caeca are often found attached to the anterior region of the midgut (Figure 11) (Anderson 1994). The midgut is also lined by a peritrophic membrane which acts as a barrier to protect its epithelium from abrasion by food (Eisemann and Binnington 1994).

Before the food is chewed and swallowed by a barnacle, it gets mixed with secretions that are released from suboesophageal and labial glands which bind and lubricate the food (Rainbow 1984). After being swallowed, the food passes through the foregut but remains relatively unchanged as it enters the midgut (Anderson 1994). When food enters the midgut, it gets broken down by digestive enzymes that are released from the pancreatic glands (Rainbow 1984). The nutrients are then absorbed along the midgut and the at the midgut caeca (Rainbow 1984). The indigestible food forms the faecal pellets that are passed to the hindgut upon contraction of the posterior midgut and they are then discharged from the anus (Anderson 1994).

Barnacles possess a pair of excretory organs known as the maxillary glands that are situated on either side of the foregut (Figure 12) (White and Walker 1981). These glands open to the outside of the body through pores that are found at the base of the second maxillae (Rainbow 1984). Each of those glands consists of an end-sac that lies within the haemolymph sinus, an efferent duct, and a terminal duct that leads to the outside (Figure 12) (Anderson 1994).

The epithelium of the end-sac filters the haemolymph, leading to the formation of primary urine (White and Walker 1981). This primary urine or ultrafiltrate is then modified by the end-sac cells and then passed through to the efferent duct where it becomes further modified. Proteins within the ultrafiltrate are broken down by end-sac cells and those breakdown products are then reabsorbed by the efferent duct cells and sent back to the haemolymph (White and Walker 1981). The efferent duct also functions to store urine as well (like a bladder), and contractions of muscles surrounding the efferent duct help to expel the urine through the terminal duct to the outside of the body (White and Walker 1981).

All barnacles can respire through the whole surface of their body, including their cirri and lining of the mantle cavity as these parts are well-circulated by haemolymph (Anderson 1994). However, balanomorphs also have a pair of highly vascularised branchiae (‘gills’) formed from simple folds of the mantle wall to increase the surface area for gas exchange (Figure 13) (Rainbow 1984). The rhythmic beating of the cirri creates water flow into the mantle cavity and that helps to ventilate the branchiae as well as other surfaces within the mantle cavity (Video 2) (Anderson 1994).

Video 2. Whilst feeding, the beating movements by the cirri create water flow into the mantle cavity.

As an intertidal balanoid, A. amphitrite have a specialised adaptation to enable them to survive emersion during low tides (Anderson 1994). During low tides, they expel water from their mantle cavity and replace that with a bubble of air to facilitate gas exchange between respiratory surfaces and the surrounding air (Anderson 1994). A small opening called the micropyle is held open to allow for the diffusion of gas to occur (Rainbow 1984). During a very hot or windy day, however, the micropyle is closed to prevent desiccation and they will then carry out anaerobic respiration until they are re-immersed again (Rainbow 1984). 

C. diadema, on the other hand, have the largest branchiae of all cirripedes (Cornwall 1924). It probably evolved to meet their high oxygen demand as they travel with the whales during their migration across the warm tropical oceans (Cornwall 1926). Warm waters generally contain less oxygen than cold waters, but increased metabolism associated with higher temperatures causes the barnacles to need more oxygen (Cornwall 1926). Thus, the large branchiae could have evolved as an adaptation to increase their oxygen uptake (Cornwall 1926). 

The blood (haemolymph) circulation of balanomorph barnacles is highly complex and it involves the flow of haemolymph through various sinuses and vessels (Rainbow 1984). From the basal vessel, the haemolymph first circulates through the mantle and the shell (Burnett 1977). It is then passed from all parts of the mantle to the branchiae to ventilate them. Following that, the haemolymph enters the body and is supplied to all body parts through a distributive circulation (Burnett 1977). In the return or peripheral-collecting circulation, the haemolymph then collects in the rostral sinus and finally returns to the basal vessel (Figure 14) (Burnett 1977).

The rostral sinus acts as the functional heart (or ‘blood pump’) of the barnacles (Southward 1987). However, it was suggested that the rostral muscles are too weak for this pumping action and instead, it is the regular contractions of prosomal muscles that drives the circulation (Southward 1987). Haemolymph of barnacles is extremely important as it has replaced the use of muscles for the extension of opercular valves, cirri, thorax and penis (Anderson 1994). Nonetheless, barnacles do possess many muscles that are highly crucial such as those that are used for opercular clamp-down (to close the operculum) and movements of various body parts (Anderson 1994).

The first thoracicans were the stalked barnacles and they originated in the Early Carboniferous period (~340Ma) (Pérez-Losada et al. 2008). These barnacles possess a capitulum in which the body and cirri are held, and a peduncle (i.e. stalk) that attaches the capitulum to the substrate (Newman and Abbott 1980). The sessile barnacles evolved from stalked barnacles in the Upper Jurassic (~161.8Ma) (Pérez-Losada et al. 2014). They have lost their peduncle and their capitulum becomes directly attached though a broad basis to the substrate (Newman and Abbott 1980). During the evolution, the sessile barnacles have also acquired a rigid calcareous shell and it was hypothesised that the change from stalked to sessile forms were due to an increase in predation pressure (Southward 1987). The lost of peduncle, however, happened twice, rendering the Sessilia non-monophyletic (Figure 17) (Pérez-Losada et al. 2008).

Although some suggest that the most speciose Balanomorpha evolved after the Cretaceous-Tertiary extinction event (~65Ma) (Buckeridge 2012), many studies suggest that balanomorphs have already appeared in the Lower Cretaceous (139.6Ma) (Pérez-Losada et al. 2008; Pérez-Losada et al. 2014). By the Paleocene (~60.9Ma), most balanomorphans (Chthamaloidea, Tetraclitoidea, Coronuloidea, Balanoidea) had already appeared, with the Chthamaloidea being the oldest and the Balanoidea being the youngest (Pérez-Losada et al. 2014).

Coronuloidea is monophyletic and that suggests that the evolution of epizoic barnacles that attach on marine vertebrates happened only once (Hayashi et al. 2013). The evolution of these, however, is not well-studied. Nonetheless, molecular studies have confirmed the basal position of Chelonibia within Coronuloidea (Hayashi et al. 2013). The relatively older Chelonibia species have been found attached on other crustaceans, turtles, sirenians and even on mooring ropes and hulls of ships (Frazier and Margaritoulis 1990). Collareta et al. (2016) suggested that the Coronulidae (whale barnacles) evolved from Chelonibia. They proposed that Chelonibia widened its host habits from turtle shells to the callosities of cetaceans in the Pliocene and adaptation to this new substrate (whale skin) has led to the evolution and radiation of the Coronulidae (Collareta et al. 2016).

It is important to note that the systematic relationships within thoracican barnacles remain unresolved and most nodes of a recently constructed thecostracan phylogeny (‘Barnacle Tree of Life’) are polytomous (Ewers-Saucedo et al. 2019). Despite that, the currently used systematics and taxonomy for A. amphitrite and C. diadema are summarised in Figure 18.

I would like to thank Sandie Degnan, Bernie Degnan, tutors and lab staff of BIOL3211 at the University of Queensland for making this project possible. Photos of Amphibalanus species used and videos of barnacle feeding were all taken by Sandie, Bernie and the tutors. I am also deeply grateful for the help with species identification by Andrew Hosie, the curator of crustacea and worms from Western Australian Museum.

Anderson, D., and Egan, E. (1986). Larval development of Balanus amphitrite Darwin and Balanus variegatus Darwin (Cirripedia, Balanidae) from New South Wales, Australia. Crustaceana 51, 188-207.

Anderson, D. (1994). ‘Barnacles.’ 1st Edn. (Chapman & Hall: London.)

Bhargava, S., Lee, S.S.C., Ying, L.S.M., Neo, M.L., Lay-Ming Teo, S., and Valiyaveettil, S. (2018) Fate of nanoplastics in marine larvae: a case study using barnacles, Amphibalanus Amphitrite. ACS Sustainable Chemistry & Engineering 6, 6932-6940.

Brusca, R., Moore, W., and Shuster, S. (2016). ‘Invertebrates.’ 3rd Edn. (Sinauer Associates Inc.: Sunderland, MA.)

Buckeridge, J. (2012). Opportunism and the resilience of barnacles (Cirripedia: Thoracica) to environmental change. Integrative Zoology 7, 137-146.

Buckeridge, J., and Newman, W. (2006). A revision of the Iblidae and the stalked barnacles (Crustacea: Cirripedia: Thoracica), including new ordinal, familial and generic taxa, and two new species from New Zealand and Tasmanian Waters. Zootaxa 1136, 1-38.

Burnett, B. (1977). Blood circulation in the balanomorph barnacle, Megabalanus californicus (Pilsbry). Journal of Morphology 153, 299-306.

Callan, H. (1941). Determination of sex in Scalpellum. Nature 148, 258-258.

Carlton, J., and Newman, W. (2009). Reply to Clare And Høeg 2008. Balanus amphitrite Or Amphibalanus amphitrite? A note on barnacle nomenclature. Biofouling 25, 77-80.

Carwardine, M. (2020). ‘Handbook of whales, dolphins and porpoises.’ (Bloomsbury Wildlife: London.)

Collareta, A., Bosselaers, M., and Bianucci, G. (2016). Jumping from turtles to whales: a Pliocene fossil record depicts an ancient dispersal of Chelonibia on Mysticetes. Rivista Italiana di Paleontologia e Stratigrafia 122, 35-44.

Cooke, J.G. 2018. ‘Megaptera novaeangliae. The IUCN Red List of Threatened Species 2018: e.T13006A50362794.’ Available at https://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T13006A50362794.en. [accessed 27 May 2020].

Cornwall, I. (1924). Notes on West American whale barnacles. Proceedings of the California Academy of Sciences 13, 421-431.

Cornwall, F.G.S. I. (1926). No. 23: Some North Pacific whale barnacles. Contributions to Canadian Biology and Fisheries 3, 501-517.

Crisp, D., and Southward, A. (1961). Different types of cirral activity of barnacles. Philosophical Transactions of the Royal Society of London Series B 243, 271-307.

Crisp, D, and Bourget, E. (1985). Growth in barnacles. Advances in Marine Biology 22, 199-244.

Darwin, C. (1851). ‘A monograph on the sub-class Cirripedia.’ (Ray Society Pubications: London.)

Darwin, C. (1854). ‘A monograph on the sub-class cirripedia.’ (Ray Society Publications: New York.)

De Gregoris, T., Rupp, O., Klages, S., Knaust, F., Bekel, T., Kube, M., Burgess, J., Arnone, M., Goesmann, A., Reinhardt, R., and Clare, A. (2011). Deep sequencing of naupliar-, cyprid- and adult-specific normalised expressed sequence tag (EST) libraries of the acorn barnacle Balanus amphitrite. Biofouling 27, 367-374.

Eertman, R., and Hailstone, T. (1988). Zonation of intertidal epifauna on jetty piles in Moreton Bay, Queensland. Journal of the Malacological Society of Australia 9, 11-18.

Eisemann, C., and Binnington, K. (1994). The peritrophic membrane: its formation, structure, chemical composition and permeability in relation to vaccination against ectoparasitic arthropods. International Journal for Parasitology 24, 15-26.

Ewers-Saucedo, C., Owen, C., Pérez-Losada, M., Høeg, J., Glenner, H., Chan, B., and Crandall, K. (2019). Towards a Barnacle Tree of Life: integrating diverse phylogenetic efforts into a comprehensive hypothesis of thecostracan evolution. PeerJ 7 e7387.

Fairweather, P. (1988). Correlations of predatory whelks with intertidal prey at several scales of space and time. Marine Ecology Progress Series 45, 237-243.

Félix, F., Bearson, B., and Falconí, J. (2006). Epizoic barnacles removed from the skin of a humpback whale after a period of intense surface activity. Marine Mammal Science 22, 979-984.

Fertl, D., and Newman, W.A. (2009). Barnacles. In ‘Encyclopedia of marine mammals’. (Eds W. Perrin, B. Würsig, and J. Thewissen.) pp. 75-77. (Academic: London, UK.)

Ford, J., and Reeves, R. (2008). Fight or flight: antipredator strategies of baleen whales. Mammal Review 38, 50-86.

GBIF (2019). ‘Amphibalanus amphitrite Darwin, 1854 in GBIF Secretariat. GBIF Backbone Taxonomy.’ Available at https://doi.org/10.15468/39omei [accessed 28 May 2020].

GBIF (2019). ‘Coronula diadema (Linnaeus, 1767) in GBIF Secretariat. GBIF Backbone Taxonomy.’ Available at https://doi.org/10.15468/39omei [accessed 28 May 2020].

Frazier, J., and Margaritoulis, D. (1990). The occurrence of the barnacle, Chelonibia patula (Ranzani, 1818), on an inanimate substratum (Cirripedia, Thoracica). Crustaceana 59, 213-218.

Harris, D. J., Maxson, L., Braithwaite, L., and Crandall, K. (2000). Phylogeny of the thoracican barnacles based on 18s rDNA sequences. Journal of Crustacean Biology 20, 393-398.

Hayashi, R. (2011). Atlas of the barnacles on marine vertebrates in japanese waters including taxonomic review of superfamily Coronuloidea (Cirripedia: Thoracica). Journal of the Marine Biological Association of the United Kingdom 92, 107-127.

Hayashi, R. (2012). A checklist of turtle and whale barnacles (Cirripedia: Thoracica: Coronuloidea). Journal of the Marine Biological Association of the United Kingdom 93, 143-182.

Hayashi, R., Chan, B., Simon-Blecher, N., Watanabe, H., Guy-Haim, T., Yonezawa, T., Levy, Y., Shuto, T., and Achituv, Y. (2013). Phylogenetic position and evolutionary history of the turtle and whale barnacles (Cirripedia: Balanomorpha: Coronuloidea). Molecular Phylogenetics and Evolution 67, 9-14.

Hermosilla, C., Silva, L., Prieto, R., Kleinertz, S., Taubert, A., and Silva, M. (2015). Endo- and ectoparasites of large whales (Cetartiodactyla: Balaenopteridae, Physeteridae): overcoming difficulties in obtaining appropriate samples by non- and minimally- invasive methods. International Journal for Parasitology: Parasites and Wildlife 4, 414-420.

Kelly, M., and Sanford, E. (2010). The evolution of mating systems in barnacles. Journal of Experimental Marine Biology and Ecology 392, 37-45.

Kim, J., Kim, H., Kim, H., Chan, B., Kang, S., and Kim, W. (2019). Draft genome assembly of a fouling barnacle, Amphibalanus amphitrite (Darwin, 1854): the first reference genome for Thecostraca. Frontiers in Ecology and Evolution 7, e00465.

Li, H., Getzinger, G., Ferguson, P., Orihuela, B., Zhu, M., and Rittschof, D. (2015). Effects of toxic leachate from commercial plastics on larval survival and settlement of the barnacle Amphibalanus amphitrite. Environmental Science & Technology 50, 924-931.

Linnaeus, C. (1767). ‘Systema naturae.’ (Cramer, Weinheim.)

Lozano-Cortés, D., and Londoño-Cruz, E. (2013). Checklist of barnacles (Crustacea; Cirripedia: Thoracica) from the Colombian Pacific. Marine Biodiversity 43, 463-471.

Maruzzo, D., Aldred, N., Clare, A., and Høeg, J. (2012). Metamorphosis in the cirripede crustacean Balanus amphitrite. PLoS ONE 7, e37408.

McDonald, M., McClintock, J., Amsler, C., Rittschof, D., Angus, R., Orihuela, B., and Lutostanski, K. (2009). Effects of ocean acidification over the life history of the barnacle Amphibalanus amphitrite. Marine Ecology Progress Series 385, 179-187.

Neufeld, C. (2011). Modular phenotypic plasticity: divergent responses of barnacle penis and feeding leg form to variation in density and wave-exposure. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 316B, 254-262.

Neufeld, C., and Rankine, C. (2012). Cuticle and muscle variation underlying phenotypic plasticity in barnacle feeding leg and penis form. Invertebrate Biology 131, 96-109.

Newman, W.A., and Abbott, D.P. (1980). Cirripedia: the barnacle. In ‘Intertidal invertebrates of California’. (Eds R. Morris, D. Abbott, and E. Haderlie.) pp. 504-534. (Stanford University Press: Stanford, California.)

Nogata, Y., and Matsumura, K. (2005). Larval development and settlement of a whale barnacle. Biology Letters 2, 92-93.

Pérez-Losada, M., Harp, M., Høeg, J., Achituv, Y., Jones, D., Watanabe, H., and Crandall, K. (2008). The tempo and mode of barnacle evolution. Molecular Phylogenetics and Evolution 46, 328-346.

Pérez-Losada, M., Høeg, J., Simon-Blecher, N., Achituv, Y., Jones, D., and Crandall, K. (2014). Molecular phylogeny, systematics and morphological evolution of the acorn barnacles (Thoracica: Sessilia: Balanomorpha). Molecular Phylogenetics and Evolution 81, 147-158.

Petrunina, A., Neretina, T., Mugue, N., and Kolbasov, G. (2013). Tantulocarida Versus Thecostraca: inside or outside? First attempts to resolve phylogenetic position of tantulocarida using gene sequences. Journal of Zoological Systematics and Evolutionary Research 52, 100-108.

Pitombo, F. (2004). Phylogenetic analysis of the Balanidae (Cirripedia, Balanomorpha). Zoologica Scripta 33, 261-276.

Quijano-Scheggia, S., Reyes-Herrera, E., Gaviño-Rodríguez, J., García-García, M., and Escobedo-Galván, A. (2012). Occurrence of Amphibalanus amphitrite (Darwin, 1854) (Cirripedia, Balanidae) on Crocodylus acutus (Reptilia, Crocodylia) in Colima, Mexico. Crustaceana 85, 1145-1148.

Rainbow, P. (1984). An introduction to the biology of British littoral barnacles. Field Studies 6, 1-51.

Rainbow, P. (2011). Charles Darwin and marine biology. Marine Ecology 32, 130-134.

Ramsay, D., Dickinson, G., Orihuela, B., Rittschof, D., and Wahl, K. (2008). Base plate mechanics of the barnacle Balanus amphitrite (=Amphibalanus Amphitrite). Biofouling 24, 109-118.

Southward, A. (1987). ‘Barnacle biology.’ (A. A. Balkema Publishers: Rotterdam.)

Thiel, M. (2015). ‘Lifestyles and feeding biology.’ (Oxford University Press: Oxford.)

Tomanek, L. (2002). Physiological ecology of rocky intertidal organisms: a synergy of concepts. Integrative and Comparative Biology 42, 771-775.

White, K., and Walker, G. (1981). The barnacle excretory organ. Journal of the Marine Biological Association of the United Kingdom 61, 529-547.