Barnacle (Cirripedia: "Curl-footed")




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1. Classification/Diagnostic characteristics:

The geological history of barnacles can be traced back to animals from the Middle Cambrian (on the order of 510-500 million years ago) during the Cambrian Explosion, a period in paleontological history when a large rise in the number of diverse animals started to form, eventually creating the complex fauna that spreads all over the world.

Barnacles are classified as this:

Domain: Eukarya
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Maxillopoda
Subclass: Thecostraca
Infraclass: Cirripedia

Barnacles are eukaryotic organisms that share similar morphology and recent common ancestors with its crustacean relatives such as Lobsters, Crayfishes, Crabs, Shrimps, Woodlice, etc. Their physical characteristics and relationships with others are nicely supported by their genetic code and developmental patterns, respectively. Just like all of their distant relatives, barnacles share these given features: Bilateral symmetry, three embryonic layers (endoderm, mesoderm, and ectoderm), distinct organ systems, special cell junctions, and connective tissue that produces primary glycoproteins such as collagen, elastin, integrins, etc.

Adult barnacles look more like mollusks, but Zoologist Louis Agassiz remarked that a barnacle is "nothing more than a little shrimp-like animal, standing on its head in a limestone house and kicking food into its mouth".
There are 1220 species of barnacles currently known.


2. Relationship to humans:

Barnacles are not very well associated with humans. For one fact, Barnacles are protostomes, while humans are deuterostomes. Phylogenetic studies have shown that they are both eumetazoans through their common ancestor, Vernanimalcula, which also gave indications of the first bilaterally symmetric, triploblastic (three embryonic germ layers) organism.

Barnacles can attach themselves permanently to ships, causing problems for humans. On the hull of ships, barnacles increase friction and reduce the speed of the ship so people either dry or scrape them off. To prevent barnacles from clinging to ships, the hulls of ships are treated with toxic paint containing tin or copper or covered in plastic. Copper paint forms a toxic film that keeps young barnacles away from the hull. It cost ship owners $125 million dollars a year to get rid of barnacles. (2) (3) (LK)

Doctors are interested in barnacle cement, the substance that barnacles utilize to attach to rocks and the undersides of ships. Barnacle cement is amazingly strong and resilient. One layer of it three ten-thousandths of an inch thick spread out over one square inch can support seven thousand pounds of weight. It will not crack or melt at extreme temperatures, and it does not dissolve in water or in most strong acids. Due to those properties, barnacle cement would be highly useful in repairing broken bones or in teeth fillings if scientists could figure out how to manufacture it artificially. (JM 17)


3. Habitat and niche:

Barnacles will tend to hang around the rocky shorelines of aquatic environments. These organisms will tend to be around the shallow and intertidal zones of marine-based habitats, typically in erosive settings. they will be exposed to aquatic and terrestrial environmental pressures. For example: Rock Barnacles (Semibalanus balanoides) will live around the rocky shores of the North atlantic Ocean. They will hang attach themselves to the rocks and stay there and filter out upcoming water and filter food particles, especially phytoplankton & zooplankton, microscopic plants & animals.

The intertidal pools that barnacles typically are fairly shallow, with 75% of barnacles species living in water less than 100 meters deep. However, barnacles have been found at depths up to six times lower. The intertidal pools in which barnacles live periodically dry up at different points during the tidal cycles, and consequently barnacles have evolved to be very conservative of water. In intertidal pools, different species of barnacles usually group together in precise order at different heights above water level, which allows researchers to accurately determine the location of certain groups of barnacles under the surface. (8) (YR)


Barnacle on rocks (14 RG)
Barnacle on rocks (14 RG)
Barnacles on a boat hull (15 RG)
Barnacles on a boat hull (15 RG)
Barnacles on a whale (16 RG)
Barnacles on a whale (16 RG)


Barnacles not only live on hard surfaces like rocks, they also can live on turtle shells, hulls of ships and even whales (14). RG





4. Predator avoidance:

In order to protect themselves, Barnacles develop a shell structure that is nicely hardened with calcites (which is composed of solid Calcium carbonate) and with the help of their rigid exoskeleton that is hardened by chitin and calcium carbonate stones. In order to remain stable, barnacles attach to a substrate by their muscular stalks and keep themselves situated at those spots for their rest of their lives (during their adult stage)

Barnacles will avoid predators by hiding and grouping together. For example, the northern rock barnacle, S. balanoides, will generally hide when there is a perceived threat. However, there is a balance created between the time spent hiding and time spent foraging by living in groups. Foraging refers to the process during which barnacles are searching for food. The S. balanoides spends less time hiding when there is a perceived threat and is quicker to return to foraging when it is living in a group, yet there is greater competition for resources. As a result, there is an equilibrium between the number of solitary barnacles and group barnacles at which the barnacles will react in this simple binary manner. (9) (RS)

Yet another way barnacles protect themselves is by simply growing large quickly. Some predators, like snails, will attempt to drill through the shell structure of the barnacle, but the larger the barnacle, the longer it takes to do the drilling. If a barnacle is large enough, the time will exceed tidal changes, meaning that the predator must be exposed to low tides. During low tides, organisms like snails run the risk of drying out, and will therefore generally avoid preying on larger barnacles. Another type of barnacle avoids predator drilling through its operculum- an opening in the shell- by growing in a bent fashion. This shifts the manner in which the operculum is oriented and exposed, making it more difficult for the predator-called Acanthina angelica- to eat it. However, this structural change also makes it harder for the barnacle to reproduce, as it removes part of the gonad. This trade off between reproductive potential and survival advantage is the reason why all barnacles have not adopted the feature. (S.S.-13)

Bent Structure of Barnacle
Bent Structure of Barnacle








5. Nutrient acquisition:

After the barnacles attach themselves to a substrate, they use their feeding appendages that stretch our from the cephalothorax (mixture of both the head and the thorax due to lower number in segments in those areas) in order to filter out the dissolved food particles from the water and into their mouths. These feeding appendages also help the barnacles catch certain microscopic algae, phytoplankton, and zooplankton.

The body of the barnacle is folded up in its shell so that its appendages, or legs, can extend out of the openings when the tide comes in to cover the barnacle. To feed, the barnacle opens its shell plates so that its six cirri, feathery appendages, can protrude out to collect food for the barnacle by whipping around rhythmically so that they can catch phytoplankton and other particles of food. Once the food is collected, the appendages bring it to the mouth of the barnacle for consumption. Barnacles are considered filter feeders because they filter phytoplankton and other food particles from the water surrounding them. Because the ocean currents are always bringing new water with them, the barnacle is able to obtain a sufficient amount of nutrients using this method. (6) (CM)

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One type of barnacle, the Gooseneck barnacle, is able to bend with the ocean currents as the barnacle opens its shell plates and extends its cirri to collect phytoplankton. (6) (CM)



6. Reproduction and life cycle:

Since these organism have the ability to form gametes, they will produce sexually (interesting fact: barnacles have the largest penis to body size ratio of the animal kingdom). Most barnacles are hermaphroditic. Since barnacles are sessile, for them to reproduce sexually is pretty difficult; but this ability to grow male and female gametes compensates for that problem. Also, in order to facilitate genetic transfer between isolated individuals, barnacles have extraordinarily long penises.
Barnacles can also reproduce through a method called spermcasting, in which the male barnacle releases his sperm into the water and females pick it up and fertilise their eggs.
These crustaceans undergo two metamorphoses- first from a feeding planktonic larva to a non feeding swimming larva, and then to a sessile adult that forms a "shell" composed of four to eight plates cemented to a hard substrate.

Barnacles larvae have two stages before they mature. The two stages are the nauplius and cypris stages. During the nauplius stage the barnacle larvae swim freely in the water and they molt many times before becoming cypris. During the cypris stage the barnacle larvae do not eat, but instead the barnacle larvae find a good surface to settle down on and cement itself on. Cypris attach itself to the new spot using its antennas and within twelve hours of attachment it becomes a mature barnacle. The mature barnacle has a shell. Barnacles are unable to go and find a mate due to the fact that they are attached to the rock they cemented themselves too and are stationary. To reproduce the barnacle must mate with its neighbor. To be successful in reproduction the barnacle must have a strong stretchable penis, so all the barnacles must be hermaphrodites: both male and female at the same time. Once the mating season ends, the penis is thrown away, and a new one is developed each year. Months before fertilization, the eggs and the sperm are stored in two different cavities. After fertilization happens, the eggs hatch. The larva stay with their mother until the spring when there is enough plankton (1). (SM)
Barnacle in Nauplius larvae stage (SM) (4)
Barnacle in Nauplius larvae stage (SM) (4)
Barnacle in Cypris larvae stage (5) (SM)
Barnacle in Cypris larvae stage (5) (SM)




7. Growth and development:

During early development in many animals, in a process known as gastrulation (after the formation of a ball of cells called a blastula/blastocyst from a constant cellular division process known as cleavage, where then the individual cells are called blastomeres, and the ball forms a fluid-filled cavity called blastocoel and the outer cells make up what is called the trophoblast (which forms the placenta connecting the fetus to the nurturing mother's uteran wall)), a hollow ball one cell thick indents to form a cup-shaped structure. The opening of the cavity formed by this indentation is called the blastopore. After that formation, there is development of the differentiated structures that give certain cells their unique fates (decrease in differential potential) and grow into the external body structures that the world can see and the internal organs that all work to maintain the physiological balance of the organism. For barnacles, at the beginning of gastrulation, there is the derivation of the mouth from the blastopore, making them protostomes, while other organisms such as echinoderms, chordates, hemichordates, etc. derive their anus from the blastopore, while the mouth forms on the opposite end of the blastula, making them deuterostomes.
During the barnacles development, like all other crustaceans' development, there is the formation of three disitnct regions: the head, thorax, and abdomen. The segments of the head are fused together, and the head bears five pairs of appendages. Each of the multiple thoracic and abdominal segments usually bears one pair of appendages. The appendages on different parts of the body are specialized for different functions, such as gas exchange, chewing, capturing food, sensing, walking, and swimming.

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The Anatomy of an Adult Barnacle (SL 19)


8. Integument:


In many species of the barnacles, just life all other arthropods, there is the formation of an external covering called the cuticle, which is secreted by the underlying epidermis (the outermost cell layer). This cuticle provides these organisms with protection and support. In barnacles, these cuticles function as external skeleton, or exoskeleton. Exoskeletons contain rigid and resistant components (such as chitin, a strong waterproof polysaccharide) that fulfill a set of functional roles including protection, excretion, sensing, support, feeding and acting as a barrier against desiccation in terrestrial organisms. Exoskeletons have a role in defense from pests and predators, support, and in providing an attachment framework for musculature.

Barnacles secrete calcium-hard plates that completely encase them. There are six plates that form a circle around the crustacean, four more of those calcium-hard plates form a "door" which the barnacle can control by either opening or closing it depending on what the tide is like, and how much moisture they need. When the tide is low, barnacles will keep these "doors" closed tight so as to conserve moisture, but when the tide is high a muscle opens the "doors" and allows the barnacle to sift through the water. This hard exterior helps the barnacle survive in its surrounding by taking advantage of the water when it needs it, but protecting it from the environment when it would be detrimental. (AG) (7)


9. Movement:

Barnacles are sessile animals at the adult stage, but are swimming during the larva stage. Sessile animals are fixed in one place, immobile. During the larva stage, they search for a suitable site to settle down by pulling themselves along by the adhesive tips of their antennae. Barnacles may take days to find a satisfactory spot. They investigate one area, then allow themselves to be drifted by the current to another location. Upon selecting a spot, the barnacle secures itself head first to the chosen surface with its brown glue. This glue is so strong, the barnacle's cone base is left behind long after the organism has died. After releasing its glue, it become sessile (7). (SP)




10. Sensing the environment:


With the help of their external appendages covered by exoskeleton and their antennae, barnacles can sense the environment around them.

Barnacles begin to sense the environment around them and establish a location to settle down in when they are in the final larval stage, the cyprid. The cyprid only begins to undergo metamorphosis into an adult barnacle after it has found an appropriate place to do so in, and it uses chemo- and neuro-receptors to perceive environmental cues. Cyprids cannot make a second move after its initial settlement, so this process is crucial to the organism's survival. Using chemo- and neuro-receptors, cyprids can determine one of the most important good cues of an environment, the presence of other individuals of the same species. If others of the same species are able to thrive in the environment, it is safely assumed that the cyprid will also do so, as it will seek the same resources as the individuals. Settling in a successful, populated area also heightens the cyprid's chance of reproduction once it fully matures. The small hairs on the cyprid also assist it in detecting the consistency of an environment's habitat, helping it to determine other cues, such as amount of bacterial film and surface texture, as to whether it is a suitable place to settle down. (AC) (11)

Adult barnacles tend to attach to rocks at the edge of the ocean. They are covered by water at high tide and and are exposed to hours of sun and wind during low tide because the water surroundings disappear. For this reason, the organism closes up its hard outer plates, trapping some water inside in order to keep water in their cells. In this way, a barnacle can keep its body moist until the next high tide. (18) (MT)




11. Gas exchange:

Although the barnacle lacks gills, it can breathe just fine. Gas exchange is accomplished by diffusion through the thin exoskeleton of the carapace, located in the mantle cavity.


A barnacle’s respiratory system is primarily composed of the mantle cavity and several feeding appendages called cirri. The cirri create a “beat” or regular rhythmic movements in an effort to draw food into the organism. In doing this the barnacle also draws in a flow of water which passes over a thin exoskeleton located in the carapace of the mantle cavity which act like gills, taking in and expunging gases. The beat of a barnacle can depend on several factors such as nutrients in the water and the speed at which the water generally moves (a faster beat for stiller water). Barnacles can also sometimes attach to moving hosts which, when the hosts moves, transfers large amounts of water through the mantle cavity. (SF)(10)



12. Waste removal:


Barnacles primarily release ammonia as their nitrogenous waste. Ammonia is highly soluble in water and diffuses rapidly, so its excretion is relatively simple for many water-breathing animals; they continuously lose it from their blood to environment by diffusion across the carapace (a fold of the exoskeleton that covers the cephalothorax).



13. Environmental physiology (temperature, water and salt regulation):

Barnacles are osmoconformers as they live in sea water. Sea water is abundant in salt, so they simply allow their extra-cellular fluid to equilibrate with sea water. Barnacles, unlike osmoregulators, keep extra-cellular fluid osmolarity equal to that of the sea water environment. This prevents it from living in extremely salty environments or too dilute environments. Yet, in very extreme, they will have to still eventually osmoregulate in order to maintain homeostatic conditions for themselves.

Barnacles use cues from the environment to distinguish good sites from bad. A good cue is one that reliably predicts locations on the shore where individuals are likely to survive and reproduce. One of the most reliable cues is the presence of other individuals of the same species; if a site already has barnacles, it is a strong indication that survivorship and mating opportunities are good. Other cues that influence the selection of a settlement site include bacterial films, surface texture, and even the presence of predators.
Many studies have shown that once a barnacle settles on the shore, it must deal with a suite of physical and biological factors in order to survive, grow, and reproduce. (12) (NU)




14. Internal circulation:


Arthropods come from a lineage of animals that have a coelom, a membrane-lined cavity between the gut and the body wall that accommodates the internal organs and is covered by peritoneum that is derived from the mesoderm. The strong, segmented limbs of arthropods eliminate the need for one of the coelom's main ancestral functions, as a hydrostatic skeleton, which muscles compress in order to change the animal's shape and thus enable it to move. Hence the coelom of the arthropod is reduced to small areas around the reproductive and excretory systems. Its place is largely taken by a hemocoel, a cavity that runs most of the length of the body and through which blood flows.
Barnacles have open circulatory systems, a system in which a fluid in a cavity called the hemocoel bathes the organs directly with oxygen and nutrients and there is no distinction between blood and interstitial fluid; this combined fluid is called hemolymph. In barnacles, the oxygen is passed to the areas of the body primarily through a series of muscles since they have no true heart andthe inner membrane of the carapace. (20)


15. Chemical control (i.e. endocrine system):

Osmoconformers are ionic regulators meaning barnacles regulate the ionic composition of their extracellular fluid; however, barnacles may not regulate the overall concentration of osmolarity. Barnacles usually regulate the concentration of certain ions through the help of excretory organs such as the antennal gland and the maxillary gland.

Review Questions:


1) Explain the habitat of barnacles. Where would one see barnacles? How does a barnacle's habitat assist it with osmoregulation? (SL)
2) Barnacles can reproduce sexually, but the sexual reproduction process is inefficient for what reason? Explain the alternative and more effective method barnacles use to reproduce. (SL)
3) How will a regular barnacle protect itself from possible predators when it remains sessile during its adult life?
4) What aspects of Barnacle characteristics make it related to humans?
5) Describe (with the use of integument, sensing the environment, and chemical control) how barnacles are able to stay within their specific respective habitats.


References:
1. "Barnacles." Barnacles. N.p., n.d. Web. 24 Nov. 2013.
2. http://animal.discovery.com/marine-life/barnacle-info.htm
3. http://seagrant.gso.uri.edu/factsheets/597barnacle.html
4. Nauplius larvae picture
5. Cypris Larvae
6. http://homeschool-academy.com/MBook/Chapter%2010-%20Crustaceans.pdf
7."Barnacles." Barnacles. N.p., n.d. Web. 17 Nov. 2013.
8.http://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.1996.tb01659.x/abstract;jsessionid=88D347D3CF847586D9656911B8C26437.f01t04
9. Mauck, R. A., and K.C. Harkless. "The Effect of Group Membership on Hiding Behaviour in the Northern Rock Barnacle, Semibalanus Balanoides." Animal Behavior 62.4 (2001): 743. Kenyon College, 12 Mar. 2012. Web. 17 Nov. 2013.
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13. http://bio.research.ucsc.edu/people/carr/Raimondi%20Reprints/Barnacle%20chapter%20proofs.pdf
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15."Dive Services." Destin Yacht Cleaning. N.p., n.d. Web. 24 Nov. 2013.
16. "How Do Barnacles Attach to Whales?" » Scienceline. N.p., n.d. Web. 24 Nov. 2013.
17. Stout, Prentice. "BARNACLE." //Seagrant.gso.uri.edu//. University of Rhode Island, n.d. Web. 25 Nov. 2013.
18. //Organisms: Growth, Reproduction, Homeostasis, Recycling Waste//. Flora, MS: Madison Schools, n.d. PPT.
19. http://www.mesa.edu.au/friends/seashores/images/barnacle.gif
20. http://en.wikipedia.org/wiki/Barnacle