Of the many striking creatures that live between the tides of Cattle Point, the decorator crab (Oregonia gracilis) is perhaps the most elaborate. With a penchant for collecting animals and plants near its home, the decorator crab is the magpie of Cattle Point. But instead of using the hydroids, polychaete tubes, bryozoans, wood chips, and alga it gathers to line a nest, this fashion-conscious crustacean wears other organisms like a prom dress. Although decorator crabs are stunning when removed from their environment, in the proper setting, they're hardly noticeable at all. The crabs use hooked setae, or bristles, as holdfasts to anchor animals and algae to their exoskeleton.
Cattle Point: Where Fashion Is the Best Defense
Protection. As any supermodel could tell you, a killer dress is much more important than food, and young decorator crabs would be the last to disagree. Marine biologists working in North Carolina found that decorator crabs, in the presence of enemies, will decorate themselves before they'll feed. When we placed a young decorator crab in a collecting jar with a sliver of red sponge, the crab used its front claws, or chelipeds, and mouth parts to cut pieces from the sponge. Then, using its rear walking legs, it flipped the plasterboard of sponge onto its rostrum, and carefully balanced it there--like an adolescent learning good posture by balancing a book on her head.
These costumes, worthy of a carnival or invertebrate ball, are more than just visual camouflage. Seaweed and invertebrates such as sponges and polychaetes can provide chemical defenses against fish and other predators, which find these organisms toxic or distasteful.
Like all crustaceans, decorator crabs must shed their exoskeleton in order to grow. At the time they shed the skeleton, they are vulnerable to predators, and some crabs, conservative by nature, will wear the same old organisms they just shed, by gathering up their tattered old accessories. As they age, decorator crabs lose interest in fashion, perhaps because as they grow larger they have fewer predators.
Locomotion. Although decorator crabs tend to remain in one place, they do have to move to feed and reproduce. The characteristic sideways movement of crabs is accomplished by using striped skeletal muscles attached to a chitonous exoskeleton. Decorator crabs are decapods (meaning "ten legs") with long, spindly legs. The chelipeds, or pinchers, are modified for feeding; the other four pairs of legs are for walking. Each leg has five segments, with a thin cuticle between segments that forms a flexible joint. Each joint can move in specific directions and angles, allowing for fine-scale movement. Muscle fibers originate in the inner cuticle and end on apodemes, tendon-like flat sheets of cuticle located on an adjacent segment. The movement of muscles works as a lever system. When the closer muscle contracts, the joint flexes and the dactylus moves down. There is usually one axon, or nerve cell, per muscle, and coordination of muscles is accomplished by nerve impulses from the central nervous system in response to stimulation.
Feeding. The decorating crab generally feeds in a manner conserved throughout the Decopoda order of the class Malacostraca. The mechanics are amazing. The five most anterior segments of the crabs body are fused into what we recognize as a head, although five pairs of appendages still mark the position of these segments. These head appendages have become highly modified to aid in feeding. Decorator crabs posses six pairs of mouth parts. The most posterior three, the maxillipeds, are not head appendages but in fact are the first three appendages of the thorax. They are used to cover and protect the inner most mouth parts as well as to manipulate large pieces of food. Once food has been obtained via the large pincers, or chelipeds, it is passed to the posterior-most maxilliped, which forces the food item into the other mouth parts. The inner- most mouth part, the mandible, a structure analogous to a jaw, is the main agent of food maceration. The other mouth parts, the maxillae, are used to rip food while it's held by the mandible. Finally, once the food has been reduced to an appropriate size, it is passed into the mouth. The mouth leads to the esophagus, which in turn opens to a specialized stomach. The stomach is lined with contiguous exoskeleton shaped like teeth that line an area of the stomach known as the gastric mill. This muscular stomach can grind food into a messy pulp using the teeth and strong muscle contractions. It is also interesting to note that this exoskeleton covering the fore gut and a similar piece aft is molted with the rest of the exoskeleton during the crab's cyclic periods of growth.
The decorator crab is not a mollusc predator or probably much of a carnivore. The long spindle pincers don't have the mechanical advantage need to crush bivalve shells or other crabs exoskeleton's. In fact, the decorator crab, a type of spider crab, is more of an omnivore, using the longer pincers to pluck small animals from crevices and scavenging on the sea floor. Tropical species of decorator crab have a diet consisting mostly of algae.
Reproduction. The decorator crab lives singly during most of the year, until the arrival of mating season. Courtship behaviors have been seen between mates, but it is believed that chemical cues play an important role in acquiring a mate.
The male pleopods, or abdominal appendages, are highly modified for clasping the female and transfering sperm. The sperm lack flagella, so the male packages them into spermatophores, which are inserted into the female's oviduct. The fertilized eggs are transported and attached to bristles on the abdomen. The abdomen plate covers the eggs, often bulging outward. Eggs may form grape-like clusters, and a gravid female may be referred to being berried. During the brooding stage, maternal care is given to the eggs by frequent aeration and removal of foreign bodies. When the eggs hatch, the larvae become planktonic in the water column. For more information and pictures on the decorator crabs larvae, go to the Plankton.