Symbiosis between the holothurian Scotoplanes sp. A and the lithodid crab Neolithodes diomedeae on a featureless bathyal sediment plain

Vulnerability to predation may be high for many megafaunal taxa in deep‐sea sedimentary habitats where physical heterogeneity is low. During ROV observations in a bathyal sediment plain off Central California, juveniles of the lithodid crab Neolithodes diomedeae were frequently observed on or under the holothurian (sea cucumber) Scotoplanes sp. A, and are hypothesized to benefit from this association as a nursery or refugium from predation. Ninety‐six percent (n = 574 of 599) of the juvenile N. diomedeae observed (density varied from 0.02–0.75/m² among sites and seasons) in the study area were associated with Scotoplanes sp. A. Of the 2596 Scotoplanes sp. A observed (density varied from 0.48 to 25.90/m²), 22% were attended by at least one juvenile crab, and rarely two crabs (n = 4). Solitary N. diomedeae were rarely observed. This decapod–holothurian symbiosis appears to be largely commensal, with juvenile crabs (carapace width = 0.03–0.31 ×  holothurian length) observed on or beneath Scotoplanes sp. A in a habitat with few refugia from epibenthic predators. Other hypotheses may explain or enhance the potential benefits of the association for N. diomedeae, such as elevated food availability due to the activities of Scotoplanes sp. A. The relationship may be mutualistic if there is a benefit for the holothurian, including the removal of epizoic parasites. Ultimately, the nursery or other effects on the population dynamics of N. diomedeae may be minimal in low‐relief, sediment‐dominated habitats, as very few sub‐adult crabs were observed in the study area and were likely consumed upon outgrowing their refugia. While sedimentary habitats may be a sink for N. diomedeae populations, growth of juvenile crabs during their association with Scotoplanes sp. A should increase energy flow to its predator populations. This association has not been reported previously but may be expected in sediment‐dominated habitats where these species overlap.