The sea urchin Pseudechinus magellanicus is a dominant species in coastal habitats of southern Argentina and Chile. This study investigates its size distribution, abundance, and spatial arrangement in relation to coastal environmental gradients and habitat complexity along a depth gradient in central Patagonia (Argentina) during the austral spring. P. magellanicus exhibited a unimodal response to depth, with lower densities observed at intertidal levels and depths greater than 12 m. Size distribution showed depth-related patterns, with larger individuals prevalent at subtidal levels, intermediate sizes more common in intertidal and shallow depths, and recruits and juveniles most abundant at the infralittoral fringe and subtidal zones. A positive relationship between structural complexity and sea urchin densities was found, especially for smaller size classes. Conversely, larger individuals tended to inhabit areas with lower structural complexity and higher food availability. Sedimentation impacted the vertical distribution, particularly affecting recruits, juveniles, and young adults. Spatial arrangement analysis showed that aggregation is the predominant pattern along the coastal depth gradient. However, at depths of 3–5 m, where the kelp forest (Macrocystis pyrifera) dominates, recruits, juveniles, and intermediate-sized sea urchins displayed a less aggregated, more random distribution. These findings reveal the critical role of habitat complexity and depth in shaping the population dynamics of P. magellanicus and highlight the adaptability of this species to varying habitat conditions and its potential as an indicator of coastal ecosystem health.
Echinodermata, Patagonia, Echinoidea, SW Atlantic Ocean
