SHORT COMMUNICATION

Abstract. Hydroacoustic data indicate that the benthic red crab Pleuroncodes planipes (Stimpson, 1860) rises from the bottom towards the surface at dusk from very compacted patches settled on the continental shelf. The behaviour is fast and abrupt, lasting <60 min. The red crab Pleuroncodes planipes (Stimpson, 1860) (Figure 1) is an important component of the pelagic and benthic ecosystem along the west coast of Baja California, Mexico. This crustacean of the Galatheidae family, langostilla as it is known commonly in Mexico, is a very abundant grazing organism in the coastal upwelling at the southern part of the California Current System (Longhurst et at., 1967; Blackburn and Thorne, 1974), and a common prey for some marine verte-brates and invertebrates (Walsh et al., 1974; Galvan, 1988). In its benthic phase, this crab is detritophagous in low-oxygen-concentration environments (Rowe et al., 1985). Juvenile and young adults may be found at the bottom as well as in the water column. However, when they reach their second year,>32 mm in standard carapace length, they become strictly benthic (Boyd, 1967). The seasonal inshore-offshore movements of benthic P.planipes are well studied (Aurioles