#grapsidae

A Sally Lightfoot crab (Grapsus grapsus) on Galapagos Island, Ecuador

Sally lightfoot crab (Grapsus grapsus) at the Galápagos Islands, Ecuador

After Pohokura we spent a couple of days on the west coast of North Island - specifically, in the vicinity of Taranaki/Mt. Egmont, a young stratovolcano that is the most recent volcano in a long sequence of slowly migrating volcanism in the area. The hills to the northwest, and the plugs at the coast at New Plymouth, are all that remain of its predecessors. In fact, the entire ring of flattish and highly fertile land in Taranaki is the result of the repeated catastrophic collapse of the volcanoes over the last 1.75 million years.

The photo below was taken from Cape Egmont 30 kilometers from the volcano. Even out here there are layers of fridge-sized boulders deposited by the giant volcanic landslides.

The plugs at New Plymouth, 1.75 myo.

Historically, the area consisted of a narrow coastal plain covered by bracken, tutu, rewarewa and karaka trees, with anywhere not close to the coast covered in dense forest.

From about 1823 the Māori began having contact with European whalers and flax traders. English settlers were first dropped here in 1841, and within a year were trying to deal with plagues of the rats they brought with them.

The stuff we saw on the volcano itself I'll cover seperately, but there was no shortage of species in New Plymouth, along the coast, at Lake Mangamahoe, and where we were staying.

The Superb Sally Lightfoot Crab

Also known as the red rock crab, the Sally Lightfoot Crab (Grapsus grapsus) is found along the Pacific and Atlantic tropical coasts of North, Central, and South America. Specifically, they are most commonly seen in California, Mexico, Costa Rica, the Caribbean islands, and the Galapagos islands. They prefer rocky shores near shallow waters or pools, where they can easily feed and mate.

Red rock crabs feed primarily on algae and sponges. However, they will also consume sponges, mollusks, other crustaceans, fish, carrion, young sea turtles, and animal droppings. Most of their foot is obtained on the shoreline, where it can be easily scavenged, although they also catch live fish and mollusks from shallow waters and tide pools. They are able to go after such a large variety of food due to their high dexterity; true to their name, sally lightfoot crabs are able to run in all four directions and can leap considerable distances. Not only does this help them hunt, it makes it easier to avoid their primary predator the chain moray eel. In addition to their quick agility and claws, sally lightfoot crabs can spit a stream of saltwater at persistent threats, and can grow back limbs as needed.

Although they spend most of their time on land, G. grapsus enters the water to mate and breed. Males battle for females using their claws, attempting to break each other’s limbs off. The victor mates with the female, who then suspends fertilized eggs under her belly for safekeeping. Excess sperm is stored in the female’s body for later use; once it is used completely, the female will use again. Eggs take up to three weeks to hatch, which typically occurs with stimulation from their waving their bodies in shallow water. After hatching, red rock crab larvae swim to deeper waters to feed on phytoplankton until they molt into juvenile crabs and make their way back to shore. Red rock crabs breed year-round, and females typically go through a cycle of hatching eggs and molting once a month.

The role of sally lightfoot crabs in the ecosystem is primarily one of population and detritus control. They have been shown to control bird populations by eating eggs, and perform a critical predation role in sea turtle hatching season. In addition, they can quickly consume dead bodies or washed-up kelp and seaweed that would otherwise contaminate the beach. Perhaps most interestingly, on the Galapagos islands, red rock crabs have been observed picking ticks from marine iguanas, suggesting a mutualistic relationship similar to that of oxpeckers and zebras.

Sally Lightfoot crab (Grapsus grapsus)

Sally Lightfoot Crab or Red Rock Crab (juvenile) Grapsus grapsus

A lightfoot crab (Grapsus adscensionis) is hit by a wave in Fuertaventura, Canary Islands

A striped shore crab (Pachygrapsus crassipes) in Montana de Oro State Park, CA, USA