LAUSR

Massive swarms of the red crab Pleuroncodes planipes (Stimpson, 1860), a species of squat lobster, are a dominant functional component of the upwelling ecosystem in the eastern Pacific Ocean (Boyd, 1967; Smith et al., 1975). These swarms can wash ashore on the coast, creating mass depositions of crustacean carcasses, a striking phenomenon that has been long documented in Baja California and California (Aurioles-Gamboa et al., 1994; Boyd, 1967). However, little is known about the fate of crab swarms transported offshore by oceanic currents. In May 2015, using an autonomous deep-sea robot, we discovered an unexpectedly large fall of red crab carcasses (>1000 carcasses ha ) at a depth of 4050 m on the abyssal Pacific seafloor (Figure 1), almost 1500 km from their spawning areas off the northwest American coast. Several questions arise from this unexpected finding that may help unveil additional close linkages in nutritional transport between processes at the sea surface and the remote abyssal seafloor. While carrying out an extensive visual survey of the abyssal seafloor in the Pacific, we were surprised by the high abundance of squat lobsters. These crustaceans can be encountered from the poles to the tropics and from intertidal rockpools to the greatest depths of the ocean. However, in the abyss, 4000 m deep, it would be very unusual for them to be the most abundant organism observed. On closer inspection, it became clear that our squat lobsters were all dead, often lying on their back with their abdomen extended, a very unnatural pose for a living squat lobster (Figure 1). Further research suggested we were dealing with a mass fall of P. planipes. Carcasses exhibited a surprisingly low state of decomposition, still brilliant red colored in all of the almost intact appendages, though fading in the abdomen. Relatively rapid sinking rates and the large numbers falling might have minimized the scavenging of carcasses during their descent through the water column, as occurs in other massive deep-sea food falls, like those in jellyfish or Received: 29 July 2022 Revised: 2 September 2022 Accepted: 9 September 2022