Locomotion on Cobble Beach       Hemigrapsus nudus in motion near a hermit crab and a cheliped of Petrolisthes sp.

1. The relationship between habitat and locomotion. The landscape of Cobble Beach is composed of many different sized rocks, most ranging from about 20 cm2 - 40 cm2. Rocks are frequently stacked more than two layers deep. Under the larger rocks, one may find pebble-sized rocks, shell fragments, and sand. It is interesting to think about the kinds of ways that animals may move around this particular environment, and why they might move from one place over another. The tops of rocks are more exposed to the sun, wind, and wave action. In turn, the areas under rocks tend to hold moisture and afford some protection from desiccation. Which animals are best suited to move around the environment? Which animals spend most or all of their time in one place? What are some consequences of moving around this habitat in differen ways? 2. Different types of movement used by the animals at Cobble beach
	Limpets , like Lottia sp. (pictured), generally live on the rocks above the algae zone in Cobble Beach. Limpets move through muscular undulations of the foot (the pale oval photographed at the left through aquarium glass; the mouth is orange, and the edge of the shell is banded). A gland in the foot secretes mucus on which foot will glide. Other gastropods, like Littorina sp. and Nucella sp. also move around on the muscular foot.
	Sea Stars pull themselves across surfaces through the contractions of muscles in of hundreds of ventral podia (tube feet) on each arm.
	Sponges, like the bright orange Ophlitaspongia penatta and the burrowing yellow Cliona sp. are sessile, meaning they do not move around. Bryozoans and barnacles are other sessile organisms found at Cobble Beach.

Among the most agile animals at Cobble Beach are the decapod crabs, which can move quickly when disturbed (for example, by lifting up the rock they are under). Crabs walk on their paired jointed legs by means of muscular contractions, and their tough chitin exoskeleton provides excellent body support for rapid movement. The most common crabs at Cobble Beach have  very differ legs: the shore crabs ( Hemigrapsus) have four pairs of large walking legs behind the clawed chelipeds, while the porcelain crabs ( Petrolisthes) have just three (the fourth pair are small and tucked up underneath the dorsal carapace). One notable exception to the motile crab trend at Cobble Beach is the hermit crab, Discorsopagurus schmitti , which lives in abandoned polychaete worm tubes and (so far as anyone knows) does not venture out of the tubes frequently.

3. Locomotion case studies

The study of movement in more closely related animals is an interesting way to learn about more subtle differences and the possible relationship between animal form and function. The two common  crabs, Hemigrapsus nudus and Petrolisthes sp. might be good species to look at in this manner because they sometimes are found under the same rocks on Cobble Beach.

Hemigrapsus nudus

Petrolisthes sp.

There are some major structural difference between these two species, including the number of walking legs (noted above) and the size and shape of the chelipeds (see the photo above). Thus, one could postulate that H. nudus is the more agile species because it has more legs to work with, or because its center of gravity is closer to the middle of its body. We found, in informal tests, that when both crabs are placed on their backs (i.e. dorsal side), Petrolisthes sp. does have a much harder time flipping back over than does H. nudus. Several tests could help show whether the difference in cheliped size or in number of legs is the cause of the difference in agility:

• one could remove the last pair of legs on Hemigrapsus, and compare their righting ability to undamaged shore crabs and to porcelain crabs (this is a simple and humane experiment: shore crabs often lose their legs naturally in fights with other crabs, and can naturally regenerate their lost legs)
• one could remove the chelipeds of Petrolisthes and test the righting ability of individual crabs as they gradually grow back normal-sized claws to see whether righting ability gets worse as the chelipeds become larger
  
• one could examine other species of porcelain crabs (or other members of this group, the Anomura, in which the fourth walking legs are small) and other shore crabs (the family Grapsidae, in the Brachyura or true crabs), and ask whether there is a general tendency for greater agility in crabs with more pairs of walking legs
  

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