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Behavior

Costume Age: some crabs are too young to start dressing up

 

 

Article:

Hein, Steven R., and Molly W. Jacobs. “Decorating behavior begins immediately after metamorphosis in the decorator crab Oregonia gracilis.” Marine Ecology Progress Series 555 (2016): 141-150.

Background:   

Humans aren’t the only species that dress in costume. Many marine animals find ways to adorn themselves. Ok so maybe these animals aren’t getting treats, but dressing up can help animals avoid being eaten. Some of the best known costume wearers in the ocean are decorator crabs (Fig. 1). These crabs use their pincer claws (chelae) to rub material from their surroundings (things like sponges, seaweed, and debris) on their exoskeletons to decorate themselves (Fig. 2). The material they use sticks to them because their shells have hooked setae, which are these hooked hard structures that grow as part of the shell (Fig. 3). The more hooked setae a crab has, the more elaborate their “costume” can be. Decorator crabs use these costumes as camouflage and chemical deterrents, whereby adorning themselves with certain materials they can blend in with their background or put something on themselves that their predators want to avoid (Fig. 4 – VIDEO). But scientists have yet to figure out when these crabs start exhibiting this behavior, due in part to the vast differences in life stages of marine invertebrates.

Fig.1: A decorator crab, Oregonia gracilis, costumed by seaweeds, sponges, bryozoans, and probably a lot else! (Photo: Flickr user Rick)

Fig.1: A decorator crab, Oregonia gracilis, costumed by seaweeds, sponges, bryozoans, and probably a lot else! (Photo: Flickr user Rick)

Fig. 3: Hooked setae, the presence of these hooks allows decorator crabs to successfully cover themselves in different materials. (Photo: McDaniel College)

Fig. 3: Hooked setae, the presence of these hooks allows decorator crabs to successfully cover themselves in different materials. (Photo: McDaniel College)

Fig. 2: Some common items that decorator crabs might use to costume themselves. (Photo: Scientific American, Inc.)

Fig. 2: Some common items that decorator crabs might use to costume themselves. (Photo: Scientific American, Inc.)

 

Decorator crabs have a pretty typical life history for a crab (Fig. 5). When they hatch from eggs they enter into a free swimming life stage called zoea. There are usually a number of zoea stages before they transition into the megalopa stage. It’s during the megalopa stage where crabs move from swimming in the water column to settling on the ocean floor. From the megalopa, crabs develop into juvenile phases before eventually transitioning into adults. While it has yet to be determined when decorator crabs start “dressing up,” it is believed to occur at the earliest in the megalopa stage. It wouldn’t make sense for a zoea crab to be swimming around with a bunch of extra weight, therefore decorator crabs must start doing this when they transition from the water column to the sea floor. Very little research has focused on how certain behaviors correlate with development, and researchers wanted to pinpoint exactly when these crabs start exhibiting this behavior. In addition, adult decorator crabs have been shown to be selective in what they decorate themselves with, which means they are actively picking their “costume!” Do young, juvenile decorator crabs also have preferences, and are they different from what the adults choose?

Fig. 5: A "typical" crab life cycle. (Photo: asnailsodyssey.com)

Fig. 5: A “typical” crab life cycle. (Photo: asnailsodyssey.com)

 

The Study:

To answer these questions, researchers set up a series of lab based experiments. Decorator crabs (Oregonia gracilis), individuals from both megalopa and juvenile stages, were collected from coastal waters off the coast of Washington state. A small subset of individuals at each stage were sacrificed so their carapace (or the body of their exoskeleton) could be photographed using scanning electron microscopy (SEM). The SEM images could be analyzed for the presence and abundance of hooked setae, which are the necessary structures for holding their “costume” in place.

To determine the preferred type of adornment, a series of choice experiments were run for both megalopae and juveniles. Megalopae were placed in a chamber with an open (and clean water) section in the middle (Fig. 6). There were four separate chambers the megalopae could enter that contained sponges, bryozoans, red algae, and a mix of the three. All of these are common items used by decorator crabs to costume themselves as well as good refuge habitat. Megalopae were monitored for their movement and habitat preference. The same experiment was run using juvenile decorator crabs, allowing them the same four habitats and monitoring their movement and preference.

Fig. 6: The tank design for testing preferred habitat and preferred decorating material.

Fig. 6: The tank design for testing preferred habitat and preferred decorating material.

From analyzing SEM images, researchers found that megalopae lacked hooked setae, indicating that decorating behaviors do not develop in this life stage (Fig. 7). They did find, however, that juveniles (the next life stage of decorator crabs) had significantly different carapace morphology (or structure), which included the presence of a number of hooked setae. But does the presence of hooked setae mean that juvenile crabs start exhibiting decorating behavior?

Fig. 7: Panels A and B show the carapace of a megalopa, where no hooked setae are present. In contrast, panels C and D show a juvenile carapace covered in hooked setae.

Fig. 7: Panels A and B show the carapace of a megalopa, where no hooked setae are present. In contrast, panels C and D show a juvenile carapace covered in hooked setae.

Researchers found that juvenile decorator crabs DO exhibit this behavior as all juveniles tested had accumulated a significant amount of material. To ensure that this was indeed done on purpose (instead of things just sticking to the crabs randomly) the researchers ran an experiment in which the front claws were removed from juveniles (yes, it might be a little cruel!) and found that crabs without claws were not covered with material (Fig. 8). So yes, the decorating behavior was occurring in juveniles. Now, while the juveniles were given multiple choices for their “costumes,” they actually didn’t preferentially choose any of the given items, rather, they adorned themselves with the debris of all of the items (Fig. 9)!

Fig. 8: By removing chelae, researchers showed that decorating behavior was practiced and not just a random accumulation of floating material as evidence by the higher material load with clawed crabs.

Fig. 8: By removing chelae, researchers showed that decorating behavior was practiced and not just a random accumulation of floating material as evidence by the higher material load with clawed crabs.

Fig. 9: Juvenile decorator crabs don't adorn themselves with the same things as adults, and prefer to cover themselves in debris.

Fig. 9: Juvenile decorator crabs don’t adorn themselves with the same things as adults, and prefer to cover themselves in debris.

 

 

In addition to characterizing decorating behavior, researchers were able to determine that both megalopa and juvenile decorator crabs preferred the same habitat, which was among the bryozoans.

 

The Significance:

This research marks one of the first instances in which a behavior is tied with a specific life stage development. While megalopae lack the hooked setae necessary for decorating themselves, it’s possible they don’t develop them due to energetic costs. For a smaller crab, decorating may require a lot of energy, which is also needed to continue to grow and forage. What’s interesting is that juveniles don’t choose to adorn themselves with the same things that adults do, which might actually make sense. Juveniles are smaller and are likely to be amongst the debris when feeding.

Studies such as this provide an important glimpse into the development of unique behaviors and show a link between development, morphology, and behavior. Who knows, maybe even these crabs can inspire some last-minute costume ideas, just head to the beach and cover yourself in seaweed and other washed up debris – now you’re a decorator crab!

 

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