Barnacles are small shellfish that attach themselves to hard surfaces like rocks, boats, and even crabs. If you’ve seen a crab covered in tiny conical shells, you may have wondered if those barnacles cause any discomfort.
If you’re short on time, here’s a quick answer: Barnacles generally do not hurt crabs once they’ve attached. However, their presence can make movement and eating more difficult for the crab over time due to the added weight and constriction.
How Barnacles Attach to Crabs
Cirripeds Seeking Hard Surfaces
Barnacles, known scientifically as cirripeds, are small marine crustaceans that anchor themselves to hard surfaces like rocks, ships, whale skin, and even crabs! As larvae, barnacles drift freely in the ocean until they find a suitable place to permanently attach.
Crabs’ tough outer shells provide an ideal attachment spot for young barnacles looking to settle down.
Cirripeds have evolved to detect chemical cues that signal a hard surface is near. When the larvae pick up on compounds like amines on a crab’s shell, they kick into high gear, ready to get cemented in place. It’s like barnacles can smell a perfect home just waiting to be inhabited!
Molting Leads to Attachment Sites
Crabs periodically shed their rigid exoskeletons in a process called molting. While a crab’s new shell hardens, there is a brief window of time when the shell is soft and flexible. It is during this time that barnacles can most easily penetrate into the crab’s exposed skeleton and find sites to anchor themselves.
Barnacles use their strong, quick-drying cement to affix onto specific points along the crab’s shell that have undergone wear and tear from molting. They tend to cluster near joints or crevices for additional support and protection. Once cemented, good luck getting those stubborn barnacles off!
Cementing onto the Exoskeleton
The cement that barnacles produce is perhaps their most incredible feature. It begins as a sticky protein complex that rapidly hardens into a flexible, durable glue. This allows the barnacles to form an incredibly strong attachment to the crab’s shell within just a few hours.
The cement is secreted from barnacle’s antennules and spreads quickly across the attachment surface. Small filaments extend from the cement into microscopic pores in the crab’s exoskeleton, literally intertwining the two organisms. What an intimate and lasting bond!
While the crab continues to molt and grow, the barnacles remain fixed in place, hunkering down on their calcified homes. Their cement keeps them firmly rooted despite the crab’s movements, making them highly resistant to detachment.
This allows barnacles and crabs to coexist in a surprising symbiosis.
Do Barnacles Cause Pain or Harm Crabs?
Barnacles attaching to the shells of crabs generally do not cause them pain or direct harm. Here’s a closer look at the impacts barnacles can have on crabs:
No Nerves Where Barnacles Attach
A crab’s shell is made up of protective plates that do not contain nerves. So when barnacles cement themselves to the shell, crabs cannot feel it. The barnacles are essentially gluing themselves to a non-living surface, rather than the crab’s skin or tissue.
This means they do not directly cause crabs pain or discomfort.
Minor Impacts on Movement and Eating
While barnacles do not hurt crabs, their presence can have some minor indirect impacts. As barnacles accumulate on the shell, they can add weight and drag that very slightly hinders the crab’s movement and flexibility.
They may also get in the way of the crab’s ability to effectively brush food into their mouthparts, making eating a bit more difficult.
However, crabs routinely molt their exoskeleton as they grow. This sheds off any accumulated barnacles and restores mobility. So the impacts are temporary until the next molt.
Increased Vulnerability to Predators
The main harm barnacles pose to crabs is making them more visible and vulnerable to predators. Crabs rely on camouflaging themselves against ocean floors and rocks. But clusters of barnacles on their shells can compromise this, making them easier for predators to spot.
Heavy infestations can also impede their ability to hide in small spaces.
So while barnacles do not directly hurt or cause pain to crabs, their presence can put crabs at somewhat greater risk of attack. But crabs have evolved to periodically molt and shed barnacle accumulations, helping minimize this vulnerability.
Removing Barnacles from Crabs
During and After Molting
Crabs naturally remove barnacles and other epibionts during their molting process. When crabs shed their hard exoskeleton during molting, any barnacles or other organisms attached to the old shell are sloughed off as well.
This provides crabs with a clean slate after molting and allows them to regrow their shell without attached barnacles. Molting frequency varies by species, but crabs may molt several times a year when young and growing. As crabs mature and their growth slows, molting events become less frequent.
This natural cleaning process helps keep crab populations healthy by preventing excessive barnacle buildup between molts.
By Hand
People can also manually remove barnacles from crabs caught recreationally or commercially. This should be done carefully to avoid injuring the crab. Use a dull knife or scraper to gently pry barnacles off the crab’s shell. Take care not to crack or puncture the shell, as this could harm the crab.
Focus on larger clusters of barnacles and leave small lone barnacles in place. Excessive cleaning could stress the crab. Rinse the crab afterward to remove barnacle debris. Manual cleaning allows crabs to conserve energy for growth and reproduction rather than constantly re-growing their shell after molting.
Proper cleaning technique creates a mutually beneficial relationship between humans and crabs.
Natural Cleaning through Commensalism
Some fish and invertebrates have formed commensal relationships with crabs to naturally clean them of epibionts like barnacles. The best studied example is the association between some tropical shrimp species like Periclimenes and host crabs.
The shrimp pick parasites, bacteria, fungi and debris off crabs in a cleaning symbiosis. The shrimp get an easy meal, while the crabs get cleaned. This removes barnacles and other encrusting organisms. Other creatures like cleaner wrasses perform similar services for reef fish.
Such cleaning interactions enhance the fitness of both partners. Natural cleaning by commensal organisms supplements molting to help crabs stay healthy in barnacle-prone coastal habitats.
Conclusion
While barnacles generally do not directly hurt crabs during attachment or once settled, over time their presence can impede movement and eating. By understanding how barnacles colonize crabs, the minimal pain caused, and methods for removing them, we can better address any issues they may cause for both crabs we encounter and those in commercial harvesting.
