The pelagic red crab, Grimothea planipes, is a species of squat lobster, and possibly one of the most important crustaceans of the eastern Pacific
The taxonomic nomenclature of the pelagic red crab has changed over the years since it was first described in 1821 as Grimothea planipes, by English marine biologist William Elford Leach, then Pleuroncodes planipes by the American scientist Wiliam Stimpson, in 1861.
It seems, however, that in 2022 it was formally (and firmly) recognised as a member of the Grimothea genus, after Pleurocondes was ‘deconstructed’ in a 2022 academic paper. This deconstruction appears to follow a 2010, paper, however, which refers Grimothea as ‘invalid‘.
The morphology of its common name is equally divers, with the pelagic red crab referred to as simply red crab, tuna crab, red tuna crab or, sometimes, langostilla.
Despite being referred to as a crab, this unusual crustacean is actually a type of squat lobster, which, rather ironically, are more closely related to hermit crabs than actual lobsters.
Unconcerned with its human designation, however – and unlike many other crabs or lobsters – the pelagic red crab swarms in vast numbers in the open ocean feeding on plankton caught in the hairs on its legs.
It is said to be a harbinger of the El Niño Southern Oscillation, as it proliferates when nutrient-rich, deep-water currents rise to the surface of the Eastern Pacific, triggering the complex changes in sea currents at the start of an El Niño event.
Its population explosions reach such numbers that the sea turns bright red as metre-thick carpets of the 8cm-long animals extend for dozens of kilometres. Tuna, blue whales, sea lions, gulls, and even bats gorge on the bounty, while loggerhead turtles travel more than 12,000km from Japan to join the feast.
Scientists have only recently solved the mysteries of the vast distances that the pelagic red crab travels during its life cycle. The adult population is only found near the coast of the Americas, but G. planipes‘ larvae are swept thousands of kilometres out to sea by the California Current.
It seems that, as the crustaceans mature, they return to the continental shelf via an opposing, far deeper undercurrent, and are often washed up en masse on the shore, providing another rich bounty for the region’s seabirds.
