Pelagic Red  Crab, Grimothea planipes

Pelagic Red  Crab, Grimothea planipes. Crab collected with a bait net off the surface in coastal waters of the greater Cabo San Lucas area, Baja California Sur, March 2005. Lenth: 1.9 cm (0.75 inches).

 Phylogeny: The Pelagic Red Crab, Grimothea planipes (Stimpson, 1860), is member of the Munidae Family of  Squat Lobsters. The Grimothea genus is one of thirty-three genera in the Munidae Family, and it contains sixteen species. The genus name Grimothea is a Latinization of the German word Grim and the Greek goddess Thea and roughly means fierce or savage goddess. The species name planipes comes from the Latin word meaning barefooted. They are also known as Lobster Krill, the Red Crab, and the Tuna Crab. In Mexico they are called Langostilla and Langosta Pelagica. Somehow, Barefooted Savage Goddess didn’t make the name list.

Morphology: Pelagic Red Crabs have an elongated and dorsoventally flattened body, similar to a lobster.  Their abdomen is well developed, but shorter than most lobsters. They have three pairs of long thin legs (pereiopods) and a pair of pincers. Their pincer legs are somewhat long and slender, but less than other species in this family. The carapace and legs lack obvious spines. The legs do have dense, short setae (“hairs”). They have a long, sharp rostrum. The eyes are large and obvious. They are red-orange to reddish brown in color, often with darker lines on the carapace. This species shows sexual dimorphism, with males larger and more robust than females. The Pelagic Red Crabs reach a maximum of 13 cm (5.1 inches) in length, though most are much smaller.

Habitat and Distribution: Pelagic Red Crabs spend their larval phase and the first year of their life as plankton. They ride the California Current far out to sea and then return to the continental shelf by riding a deeper counter-current. As adults they settle to the bottom and spend much of their lives on sand or mud, at depths of 50 m (164 feet) to 366 m (1,800 feet). They swim well enough to be able to vertically migrate to swarm, mate, and feed in the water column or at the surface. If the wind blows the surface water on-shore, the Pelagic Red Crabs may be pushed ashore and stranded. Sometimes they become stranded in groups numbering into the hundreds of thousands. They are a subtropical to tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean with the exception that they are absent from south of Mazatlán, Sinaloa along the central and southwest coasts of the mainland. During El Niño events, they are found along the entire west coast of Mexico.

Diet: Pelagic Red Crabs are omnivores that feed on microscopic algae, copepods, diatoms, and detritus. They use the hairs on their legs to filter food from the water.

Predators: Pelagic Red Crabs are an abundant and critical food source for fish, baleen whales, pinnipeds, squid, octopuses, sea birds, Green Sea Turtles, and Hawksbill Sea Turtles. At times, they comprise 85% of the diet of Yellowfin Tuna. There is even an endemic species of bat, the Fish-eating Bat, Myotis vivesi, that feeds on these animals.

Reproduction: Pelagic Red Crabs are gonochoric (male or female for life). Reproduction is sexual with external fertilization. Mating is accomplished by the transfer of a sperm packet from the male to the abdomen of the female. The female scratches open the sperm packet as she releases her eggs. The female carries the fertilized eggs on her abdomen until they hatch after a few weeks. The eggs hatch into planktonic larvae. Reproduction generally occurs during spring and summer.

Ecosystem Interactions: Any form of engagement in commensal, parasitic or symbiotic relationships b the Pelagic Red Crab has not been formally documented.

Human Interactions: Pelagic Red Crabs are not normally eaten by humans but they are important to commercial and recreational fisheries. They are harvested commercially for fishmeal production and as fish bait. The fishmeal is used in aquaculture. From a conservation perspective they have not been evaluated, but they are common and widespread and should be considered to be of Least Concern.

Synonyms: Pleuroncodes planipes.