BARNACLES

*Barnacles (Cirripedia) – Overview*

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Introduction
*Barnacles* are marine crustaceans belonging to the subclass *Cirripedia*. Though they resemble tiny mollusk shells, they are more closely related to crabs, lobsters, and shrimp. Adults are sessile (permanently attached) and filter‑feed using feathery appendages called *cirri*. Their ability to cement themselves to almost any hard surface—ship hulls, rocks, driftwood, whales, sea turtles, and even man‑made structures—makes them one of the most successful fouling organisms in the ocean.

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Taxonomy & Diversity
- *Phylum:* Arthropoda
- *Subclass:* Cirripedia (≈ 1,220 species)
- *Orders:*
- *Thoracica* – true barnacles (e.g., _Balanus_, _Lepas_).
- *Acrothoracica* – burrowing barnacles.
- *Rhizocephala* – parasitic barnacles (e.g., _Sacculina_).

Most familiar are the *stalked goose‑barnacles* (_Lepas spp._) and *acorn barnacles* (_Balanus, Chthamalus_).

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Life Cycle
Stage Description
Nauplius (larva) Free‑swimming planktonic larva, 6 stages, feeds on microscopic algae.
Cyprus (cypris larva) Non‑feeding, seeks a suitable substrate using chemical cues (e.g., biofilm).
Metamorphosis Cypris attaches, secretes a strong proteinaceous cement, and transforms into the adult form.
Adult Sessile, encased in calcareous plates (6 for Balanus, 5 for Lepas), filter‑feed with cirri.
Most barnacles are *hermaphroditic* but still require cross‑fertilization; a long, flexible penis reaches a neighbor for sperm exchange.

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Habitat & Distribution
- *Intertidal zones* (rocky shores, mangroves).
- *Subtidal* (coral reefs, offshore structures).
- *Epibiotic* (whales, sea turtles, crabs).
- *Floating debris* (driftwood, plastic).

They thrive in temperate to tropical waters, tolerating wide salinity and temperature ranges. Some species (e.g., _Chelonibia testudinaria_) are specific to sea turtles, while others (e.g., _Xenobalanus globicipitis_) attach to cetaceans.

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Adaptations
1. *Cement Gland:* Secretes a permanent, water‑insoluble adhesive (protein‑based) that hardens within minutes.
2. *Protective Plates:* Calcite shells (rostrum, scutum, tergum) shield soft tissues from predators and desiccation.
3. *Cirri (Feathery Legs):* Rhythmically extended to capture plankton, detritus, and dissolved organic matter.
4. *Respiratory Efficiency:* Gills absent; gas exchange occurs across thin cuticle and through water currents generated by cirri.

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Ecological Role
- *Filter Feeders:* Help recycle nutrients and improve water clarity.
- *Bioindicators:* Sensitivity to pollutants (heavy metals, oil) makes them useful for environmental monitoring.
- *Fouling Organisms:* Colonize ship hulls → increased drag, higher fuel consumption; also colonize offshore platforms and aquaculture cages.
- *Food Source:* Some fish, sea birds, and even humans (gooseneck barnacles _Pollicipes pollicipes_ are a delicacy in Spain and Portugal).

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Economic & Health Impacts
- *Marine Biofouling:* Costs the shipping industry billions annually in fuel and maintenance. Antifouling paints (historically TBT, now copper‑based) aim to deter settlement.
- *Parasitic Barnacles (Rhizocephala):* _Sacculina_ castrates and alters behavior of host crabs, affecting fisheries.
- *Medical Research:* Barnacle cement is studied for biocompatible adhesives (wound dressings, dental cements).

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Interesting Facts
- *Strength of Adhesive:* 4–5 ×  stronger than most synthetic underwater glues.
- *Longevity:* Some intertidal species survive months of exposure to air, heat, and wave action.
- *Mobility Myth:* Though sessile, barnacles can slightly shift position by extending their cirri and re‑anchoring.

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Sources: _Marine Biology_ (Nybakken & Bertelsen), _Encyclopedia of Marine Invertebrates_, NOAA Fisheries.