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Biology

Detach claw & flee: Strategies for porcelain crabs to evade immediate death from different predators

Paper: Knope, M.L., Larson, R.J., (2014). Autotomy in porcelain crabs is an effective escape mechanism from rockfish predation. Marine Ecology. 35:471–477. doi: 10.1111/maec.12103

Imagine you are being chased by a serial killer. Your legs aren’t working as fast as you’d like, and your lungs are screaming for air. Too slow, he’s closing in, and now he grabs your arm. However, you have a trick up your sleeve.  You detach your arm and flee, escaping with your life–armless, but alive. This might sound like a scene out of a sci-fi novel, but for some humble marine critters, this is their reality.

Predation is a strong selective pressure that has shaped the evolution of behaviour, adaptations and life history traits in many life forms. In an attempt to avoid predation, many animals have developed the useful ability to drop appendages such as a limb or tail in order to distract their predator and dart away unnoticed. Porcelain crabs (actually akin to squat lobsters rather than true crabs) have evolved a hair-trigger response for autotomy—quickly shedding their chelipeds or claws in response to a small stimulus. Although autotomy can prevent instant death, the ability to survive without the lost appendage can be tricky. Autonomy can lower animal fitness by impairing mobility, foraging, and reproduction.  As well, obtaining extra energy needed to re-grow a limb may accelerate the time between escape and kicking the bucket. Larger crabs prey on porcelain crabs by clasping onto parts of their victim, then slowly consuming them. It has been well documented that detaching a claw and fleeing, is an effective strategy for porcelain crabs to escape certain death.

Image 1: Rockfish in study.  Clockwise from top left: Gopher rockfish, Black-and-Yellow rockfish, Kelp rockfish (Credits: Kevin Lee) and Copper rockfish (Credit: Clinton Bauder).

Rockfish in study. Clockwise from top left: Gopher rockfish, Black-and-Yellow rockfish, Kelp rockfish (Credits: Kevin Lee) and Copper rockfish (Credit: Clinton Bauder).

This paper looked at whether or not autotomy in porcelain crabs would be an effective defense against rockfish that have different methods of attack. Instead of slowly picking apart their victims, rockfish lie patiently still, then ambush their prey in different ways.  There is the lunge and crab grab, or they can create a vacuum with their mouths to suck up unwilling porcelain prey.

Knope & Larson examined stomach content data from 100 specimens each of four common kelp-forest rockfish species including: gopher rockfish (Sebastes carnatus), black-and-yellow rockfish (Sebastes chrysomelas), kelp rockfish (Sebastes atrovirens) and copper rockfish (Sebastes caurinus)—see figure on left.

Image 2: Crabs in study. Clockwise from top left: Chocolate porcelain crab (Petrolisthes manimaculis) - credit: Dr. Jonathan Stillman, thickclaw porcelain crab (Pachycheles rudis) - credit: Dr. Jim Nestler, blackclawed crab (Lophopanopeus bellus) - credit: Dr. Thomas M. Niesen, and red rock crab (Cancer productus) - credit: Dr. Jim Nestler.

Crabs in study. Clockwise from top left: Chocolate porcelain crab (Petrolisthes manimaculis) – credit: Dr. Jonathan Stillman, thickclaw porcelain crab (Pachycheles rudis) – credit: Dr. Jim Nestler, blackclawed crab (Lophopanopeus bellus) – credit: Dr. Thomas M. Niesen, and red rock crab (Cancer productus) – credit: Dr. Jim Nestler.

All specimens were acquired by spear fishing among rocky reefs and kelp forests of Southern California over the course of 34 trips from November 1969 to April 1972 . Only adult rockfish over a standard length of 100mm were used in the study. The stomachs of the rockfish were removed, and specimens were placed into 10% formalin solution. Later, the specimens were rinsed with fresh water and preserved in 45% ethanol. Stomach contents were then searched for frequency of disembodied claws of porcelain crabs as well as other common prey crabs with large claws, but not known for autotomy—see figure on right.

The results showed that the frequency of disembodied claws of porcelain crabs was six times as common (statistically significant) when compared with other crabs.  About 35.7% of porcelain crab claws were dissociated versus only 5.7% of dissociated claws from true crabs. Surprisingly, even though rockfish are able to engulf their prey whole, autotomy allowed porcelain crabs to escape immediate mortality in one third of rockfish strikes!

Although the claw joint in porcelain crabs may be weak in comparison with other crabs, this study shows that rockfish were not forcibly removing the crabs’ claws, but rather autotomy  was taking place. Crabs facing attack will often hold their claws wide apart to appear larger, or maybe even as an attempt to offer a sacrificial claw to avoid being completely consumed.  However, rockfish being several times larger may not be swayed.  Still, there are advantages to the wide-claw tactic.  Handling time which is the time spent by a predator to maneuver prey into a consumable position may play a role. A study by Van Syoc found that handling time of crabs by one species of rockfish exponentially increased with the size of its crab prey. Therefore, the wide-claw tactic combined with autotomy during handling time can provide a window of opportunity for porcelain crabs to escape, especially for larger crabs. If autotomy works for porcelain crabs, why haven’t true crabs evolved to do the same? One reason is that true crabs need their claws to forage for survival. Porcelain crabs on the other hand, filter feed, so the absence of a claw does not drastically impede feeding. All in all, this paper is the first to investigate the effectiveness of porcelain crab autotomy against a suite of predators, including rockfish. And on another note, publishing a paper on old data reminds us that data collected decades ago can still be relevant and lead to new discoveries that further the understanding of the world around us.

Now, all we have to work on is dislocating, detaching and re-growing human limbs seamlessly if needed… but then, 127 Hours would be a much less dramatic film.

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